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Currently, Mediterranean forests are experiencing the deleterious effects of global warming, which mainly include increased temperatures and decreased precipitation in the region. Relict Abies pinsapo fir forests, endemic in the southern Iberian Peninsula, are especially sensitive to these recent environmental disturbances, and identifying the genes involved in the response of this endangered tree species to climate-driven stresses is of paramount importance for mitigating their effects. Genomic resources for A. pinsapo allow for the analysis of candidate genes reacting to warming and aridity in their natural habitats. Several members of the complex gene families encoding late embryogenesis abundant proteins (LEAs) and heat shock proteins (HSPs) have been found to exhibit differential expression patterns between wet and dry seasons when samples from distinct geographical locations and dissimilar exposures to the effects of climate change were analyzed. The observed changes were more perceptible in the roots of trees, particularly in declining forests distributed at lower altitudes in the more vulnerable mountains. These findings align with previous studies and lay the groundwork for further research on the molecular level. Molecular and genomic approaches offer valuable insights for mitigating climate stress and safeguarding this endangered conifer. [ABSTRACT FROM AUTHOR]

Radiocarbon ages for samples of organic material within and overlying the highest Holocene floodplain and fan terraces in Glen Feshie and Glen Tromie imply sediment aggradation after ∼4.3 cal ka and probably incision after ∼3.7 cal ka, and in the Edendon Valley aggradation after ∼2.8 cal ka, with incision after ∼2.7 cal ka. The timing of sediment aggradation at all three sites postdates the onset of pine forest decline (∼4.8 cal ka) at nearby high-level sites, and coincides with wet periods characterised by high water tables. This coincidence in timing suggests that forest decline may have caused upstream extension of the tributary network, headwater incision and flashier flood responses, and that increased rainfall enhanced sediment discharge from headwater tributaries, with consequent sediment accumulation downstream on low-gradient fans and floodplains. More speculatively, exhaustion of readily entrained sediment from headwater areas may have stimulated subsequent floodplain and fan incision. Our results show that the highest Holocene terrace (the Main Holocene Terrace) is a diachronous feature, even in valleys emanating from the same upland source area, and support the conclusions of simulation models that predict marked increases in sediment discharge when deforestation is succeeded by an episode of increased rainfall. [ABSTRACT FROM AUTHOR]

This study demonstrates a framework for using high-resolution satellite imagery to automatically map and monitor outbreaks of red needle cast (Phytophthora pluvialis) in planted pine forests. This methodology was tested on five WorldView satellite scenes collected over two sites in the Gisborne Region of New Zealand's North Island. All scenes were acquired in September: four scenes were acquired yearly (2018–2020 and 2022) for Wharerata, while one more was obtained in 2019 for Tauwhareparae. Training areas were selected for each scene using manual delineation combined with pixel-level thresholding rules based on band reflectance values and vegetation indices (selected empirically) to produce 'pure' training pixels for the different classes. A leave-one-scene-out, pixel-based random forest classification approach was then used to classify all images into (i) healthy pine forest, (ii) unhealthy pine forest or (iii) background. The overall accuracy of the models on the internal validation dataset ranged between 92.1% and 93.6%. Overall accuracies calculated for the left-out scenes ranged between 76.3% and 91.1% (mean overall accuracy of 83.8%), while user's and producer's accuracies across the three classes were 60.2–99.0% (71.4–91.8% for unhealthy pine forest) and 54.4–100% (71.9–97.2% for unhealthy pine forest), respectively. This work demonstrates the possibility of using a random forest classifier trained on a set of satellite scenes for the classification of healthy and unhealthy pine forest in new and completely independent scenes. This paves the way for a scalable and largely autonomous forest health monitoring system based on annual acquisitions of high-resolution satellite imagery at the time of peak disease expression, while greatly reducing the need for manual interpretation and delineation. [ABSTRACT FROM AUTHOR]

The emergence of new plant diseases is an increasingly important concern. Climate change is likely to be among the factors causing most of the emerging diseases endangering forest and tree heritage around the world. Such diseases may be caused by latent pathogens or microorganisms cryptically associated with plants. The shift from a non-pathogenic to a pathogenic stage may depend on physiological alterations of the host, environmental changes, and/or stress factors. In some woods of the Salento Peninsula (Apulia Region, Italy), sudden declines of holm oak plants (Quercus ilex L.) have been observed since 2016. The morphological and molecular characterization of representative fungal isolates associated with cankers and necrosis in declining plants indicated that these isolates belong to the Botryosphaeriaceae family, and the most frequent species were Diplodia corticola and Diplodia quercivora, followed by Neofusicoccum vitifusiforme. In artificially inoculated young holm oak plants, both D. corticola and D. quercivora species produced intense and severe subcortical and leaf margin necrosis. N. vitifusiforme, although less aggressive, induced the same symptoms. Our research, in addition to confirming the involvement of D. corticola in olm oak decline, represents the first report of D. quercivora as a new pathogen of Q. ilex in Italy. Furthermore, to the best of our knowledge, we also found N. vitifusiforme as a new pathogen of Q. ilex. [ABSTRACT FROM AUTHOR]

Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)—a culturally, ecologically and economically important species—has recently experienced anomalously high mortality rates and partial canopy dieback. We investigated how WRC tree growth and dieback responded to climate variability and drought using tree‐ring methods. Location: Pacific Northwest, USA. Taxon: Western redcedar (Thuja plicata). Methods: We collected tree cores from three tree health status groups (no canopy dieback, partial canopy dieback, and dead trees) at 11 sites in coastal (maritime climate) and interior (continental climate) WRC populations. From growth rates, we computed four growth indices that assessed the resilience to drought and estimated the year of death. Results: Warmer and drier climate conditions in May/June that extended the annual July‐to‐September dry season reduced radial growth in 9 of 11 sites (1975–2020). WRC trees recovered growth to pre‐drought rates within 3 years when post‐drought climate conditions were cooler/wetter than average. However, recovery from drought was slower or absent when warmer/drier conditions occurred during the post‐drought recovery period, possibly leading to the recent and widespread mortality across the coastal population. WRC mortality was portended by 4–5 years of declining growth. Annually‐resolved mortality in coastal populations predominately occurred in 2017–2018 (80% of sampled dead trees), a period that coincided with exceedingly hot temperatures and the longest regionally dry period from May to September (1970–2020). In interior populations, mortality was dispersed among years but associated with warmer and drier conditions from August to September. Main conclusions: Our findings forewarn that a warming climate and more frequent and severe summer droughts, especially in consecutive years, will likely increase the vulnerability of WRC to canopy dieback and mortality and possibly other drought‐sensitive trees in one of the world's largest forest carbon sinks. [ABSTRACT FROM AUTHOR]

Global forests are increasingly being threatened by altered climatic conditions and increased attacks by pests and pathogens. The complex ecological interactions among pathogens, microbial communities, tree hosts and the environment are important drivers of forest dynamics. Little is known about the ecology of forest pathology and related microbial communities in temperate forests of the southern hemisphere. In this study, we used next‐generation sequencing to characterize sapwood‐inhabiting fungal communities in North Patagonian Nothofagus forests and assessed patterns of diversity of taxa and ecological guilds across climatic, site and host variables (health condition and compartment) as a contribution to Nothofagus autecology. The diversity patterns inferred through the metabarcoding analysis were similar to those obtained through culture‐dependent approaches. However, we detected additional heterogeneity and greater richness with culture‐free methods. Host species was the strongest driver of fungal community structure and composition, while host health status was the weakest. The relative impacts of site, season, plant compartment and health status were different for each tree species; these differences can be interpreted as a matter of water availability. For Nothofagus dombeyi, which is distributed across a wide range of climatic conditions, site was the strongest driver of community composition. The microbiome of N. pumilio varied more with season and temperature, a relevant factor for forest conservation in the present climate change scenario. Both species carry a number of potential fungal pathogens in their sapwood, whether they exhibit symptoms or not. Our results provide insight into the diversity of fungi associated with the complex pathobiome of the dominant Nothofagus species in southern South America. [ABSTRACT FROM AUTHOR]

Episodic tree mortality can be caused by various reasons. This study describes climate‐driven tree mortality and tree growth in the Black Forest mountain range in Germany. It is based on a 68‐year consistent data series describing the annual mortality of all trees growing in a forest area of almost 250 thousand ha. The study excludes mortality caused by storm, snow and ice, and fire. The sequence of the remaining mortality, the so‐called "desiccated trees," is analyzed and compared with the sequence of the climatic water balance during the growing season and the annual radial growth of Norway spruce in the Black Forest. The annual radial growth series covers 121 years and the climatic water balance series 140 years. These unique time series enable a quantitative assessment of multidecadal drought and heat impacts on growth and mortality of forest trees on a regional spatial scale. Data compiled here suggest that the mortality of desiccated trees in the Black Forest during the last 68 years is driven by the climatic water balance. Decreasing climatic water balance coincided with an increase in tree mortality and growth decline. Consecutive hot and dry summers enhance mortality and growth decline as a consequence of drought legacies lasting several years. The sensitivity of tree growth and mortality to changes in the climatic water balance increases with the decreasing trend of the climatic water balance. The findings identify the climatic water balance as the main driver of mortality and growth variation during the 68‐year observation period on a landscape‐scale including a variety of different sites. They suggest that bark beetle population dynamics modify mortality rates. They as well provide evidence that the mortality during the last 140 years never was as high as in the most recent years. [ABSTRACT FROM AUTHOR]

Aim: Anticipating when and where changes in species' demographic rates will lead to range shifts in response to changing climate remains a major challenge. Despite evidence of increasing mortality in dry forests across the globe in response to drought and warming temperatures, the overall impacts on the distribution of dry forests are largely unknown because we lack comparable large‐scale data on tree recruitment rates. Here, our aim was to develop range‐wide population models for dry forest tree species (pinyon pine and juniper), quantifying both mortality and recruitment, to better understand where and under what conditions species range contractions are occurring. Location: Western United States. Major taxa studied: Two pinyon pine (Pinus spp.) and three juniper (Juniperus spp.) species. Methods: We developed range‐wide demographic models for five species using forest inventory data from across the western United States and estimated population trends and climate vulnerability. Results: We find that four of the five species are declining in parts of their range, with Pinus edulis having the largest proportion of populations declining (24%). Population vulnerability increases with aridity and temperature, with up to ~50% of populations declining in the warmest and driest conditions. Mortality and recruitment were both essential to explaining where populations are declining. Main conclusions Our results suggest that dry forest species are undergoing an active range shift driven by both changing recruitment and mortality, and that increasing temperatures and drought threaten the long‐term viability of many of these species in their current range. While four of the five species examined were experiencing some declines, P. edulis is currently most vulnerable. Management actions such as reducing tree density may be able to mitigate some of these impacts. The framework we present to estimate range‐wide demographic rates can be applied to other species to determine where range contractions are most likely. [ABSTRACT FROM AUTHOR]

Sweet chestnut forests occupy an important role in Italy and Europe, its resilience is often affected by aggressive pathogen attack. In the last times some chestnut wood colour aberrations have attracted attention suggesting intercorrelation with biotic attacks. The present study analyses chestnut wood from a coppice-with-standards stand in Central Italy aiming to understand the reasons for discolorations in respect to a possible effect on wood physical and mechanical character. Wood specimens with different chromatic aberrations were analysed by mechanical tests, microscopic techniques (SEM), specific density measurements, and ATR-FTIR spectroscopy. Standing trees and logs were investigated by IML-RESI. Results by SEM show as all stained wood zones correspond to mycelium presence and abnormal cell wall morphologies. No real trend was found between specific density, or compressive strength, with coloured samples, infact some test samples with colour aberrations had the highest value for specific gravity. Comparing sound wood with stained wood, the semiquantitative analysis by ATR-FTIR spectroscopy allowed to detect a change in the ratio lignin/cellulose which might be due to a white rot fungus. IML-Resi has showed wood decay symptoms at root collar in coppice standards with a decrease in amplitude at least of 30%. The results find an agreement with similar studies carried out in Spanish chestnut suggesting to indeep the analysis to investigated the geographical distribution and the impact of the damage. [ABSTRACT FROM AUTHOR]

• Summer heat induced growth decline of Pinus taiwanensis at its southern limit. • Intra-annual growth pattern shifted from unimodal to bimodal in recent decades. • Actual decline started several decades earlier than the reductions in ring widths. Warming-induced aridity has caused forest decline and mortality for many sites with water-limiting conditions. However, equatorward rear-edge Pinus taiwanensis trees at the Daiyun Mountains in humid subtropical China are also suffering die-backs and decline, but the roles played by heat or drought stress still remain unclear. Here, we compared the tree-ring radial width, anatomical features, stable carbon isotope (δ13C) and intrinsic water use efficiency (iWUE) between die-back and healthy trees to elucidate potential causes driving the decline. Die-back trees showed sustained growth reductions and produced tracheids with thinner cell walls over the recent decade, indicative of reduced carbon assimilation. The climate response pattern and Vaganov-Shashkin (V-S) model indicated the critical role of summer (June-August) temperature in recent growth decline. Long-term higher wood δ13C and iWUE within die-back trees indicated that actual growth decline already started several decades earlier. This conservative growth strategy was at the cost of low efficiency of photosynthesis due to chronic stomatal closure. When the lethal heatwaves arrived, these weakened trees were not able to access sufficient carbonhydrates to maintain metabolism, causing a distinct decline and mortality. We concluded that recent decline in Pinus taiwanensis trees was mainly caused by long-term carbon starvation. [ABSTRACT FROM AUTHOR]

The global rise in temperature and associated changes in climate have led to decline of forests around the globe, across multiple species and ecosystems. A particularly severe example of this is yellow-cedar (Callitropsis nootkatensis) decline along the coast of British Columbia and Alaska, where anthropogenic climate change has led to reduced insulating snowpack, leaving yellow-cedar roots vulnerable to thaw-freeze events, resulting in freezing damage to fine roots and water stress during the subsequent growing season. This includes abundant evidence of tree decline and mortality on the islands of Haida Gwaii. Yellow-cedar decline is complex, with the potential for freezing injury over multiple years and damage that can accumulate over time. We found trees in various stages of decline, from long dead to currently declining, and multiple growth patterns at each study site. We conducted a principal component analysis and identified patterns of divergent growth and divergent response to climate among yellow-cedars within the same stands, across all sites, including three distinct periods of an onset of growth decline (1960s, 1990s, 2000s). Yellow-cedars affected by decline were decreasing in growth and negatively associated with warmer drier winter conditions, whereas yellow-cedars not affected by decline were increasing in growth and positively associated with warmer growing season temperatures. The limiting factors for declining trees, warm dry winter conditions, are consistent with the hypothesis from the mainland that climate warming has led to root freezing. Our research highlights the need to consider multiple signals within a site that would be masked by a single site-level chronology. This is especially relevant within the context of forest decline, where stressors may have differing effects on individual trees. [ABSTRACT FROM AUTHOR]

Maritime pine (Pinus pinaster) is a Mediterranean forest tree species of great ecological importance within the European Union and the Iberian Peninsula in particular, whose presence is seriously threatened by forest decline. Knowledge of the diversity, abundance, and functionality of rhizospheric microorganisms can provide important information in the design of new strategies for sustainable forest management against forest decline. In this work, rhizospheric samples were collected from representative pine forests in the region of Castilla y León (North-western Spain) (in 10 municipalities of 5 different provinces), analyzing different physicochemical parameters and bacterial and fungal biodiversity (by metabarcoding). In addition, different variables of tree health and climatic conditions were analyzed. The main phylum of microorganisms found in the rhizosphere of P. pinaster were the Ascomycota (54.5 %) and Acidobacteria (16.4 %) in fungi and bacteria, respectively. A clear relationship was found between the presence/absence of certain bacterial and fungal groups (taxonomic and functional) and the presence/absence of healthy/sick trees. Specifically, the fungal genus Umbelopsis and the bacterial genus Paenibacillus were thought as possible control agents of decline, as their presence was related to the absence of disease. Understanding the relationships between rhizosphere microbiota and forest health parameters in Pinus pinaster can be only achieved by exploring the complex 'ecosystem microbiome' and its functioning using focused, integrative microbiological and ecological research performed across multiple habitats. • Pinus pinaster is seriously threatened by forest decline in the Iberian Peninsula. • Metabarcoding analysis of bacteria/fungi can be useful to understand the decline. • There is a relationship between bacteria/fungi diversity and decline. • The genera Umbelopsis and Paenibacillus could be control agents of decline. [ABSTRACT FROM AUTHOR]

Global climate change impacts the functioning and productivity of forest ecosystems at various spatiotemporal scales across a wide range of biomes. Although summer temperatures are considered the main driver of boreal tree growth, the importance of soil moisture availability is likely to rise with decreasing latitude and increasing warming. Here, we combine dendrochronological measurements with evidence from tree growth modeling and remote sensing to quantify the effect of climate on phenology and productivity of Scots pines (Pinus sylvestris L.) in southern Siberia. Between 1960 and 2017, pine ring widths along a latitudinal transect from 53° to 56°N were mainly controlled by the availability of summer soil moisture. This finding challenges the common belief that summer temperatures are the predominant growth control in boreal forests. Moreover, we show that earlier growing season onsets can compensate for warming-induced drought stress. Despite the phenotypic plasticity of Scots pines to adapt to warmer and drier conditions, we speculate that predicted climate change will likely exceed the species' physiological tolerance in much of Eurasia's forest-steppe by the end of the twenty-first century. [ABSTRACT FROM AUTHOR]

The paper presents the results of research on the influence of climatic factors on the occurrence of defoliation and decline of forests as a final outcome. The research was conducted in a mixed beech and fir forest, in the period from 2004 to 2019 at sample plot 415 - Maljen, as a representative example of forest decline, which is part of regular monitoring conducted on the territory of the Republic of Serbia. Due to the fact that defoliation may be due to the influence of various factors, research, analysis and results are focused on long-term trends. Special attention is paid to the analysis of the degree of influence of climatic parameters on the occurrence of defoliation. Data on temperatures and precipitation in the period 2004-2019 were processed and analyzed on a monthly and annual basis, as well as in the vegetation period, for the main meteorological station Valjevo, which is closest to sample plot 415. Also, drought assessment was performed during this research period based on Lang's rain factor, de Martonne aridity index and climate diagram by Walter. In order to confirm these methods, the Standardized Precipitation Index was calculated as one of the method that has been mostly used in drought identification lately. It was stated that the defoliation of trees was initiated by extreme climatic events during three consecutive drought years (2011-2013), after which there was the largest decline of forests in the researched area. [ABSTRACT FROM AUTHOR]

In western Ukraine, forest decline and dieback of several broadleaved tree species have become increasingly evident during recent years, and surveys in some areas have shown symptoms indicative of Phytophthora infections. In this study, we aimed to determine the occurrence and diversity of Phytophthora species associated with several broadleaved tree species (Alnus glutinosa, Betula pendula, Castanea sativa, Fagus sylvatica and Quercus robur) from forest stands where dieback has been observed. Rhizosphere soil samples were collected from 14 forest stands during 2017 and 2018 and tested for the presence of Phytophthora species using morphological and molecular methods. Seven Phytophthora species (P. bilorbang, P. cactorum, P. gallica, P. gonapodyides, P. lacustris, P. plurivora and P. polonica), and two other clade six taxa were detected from the various forest types, several of which are probable agents responsible for decline. Four of the Phytophthora species (P. bilorbang, P. gallica, P. plurivora and P. polonica) have previously never been reported from broadleaf forests in Ukraine. [ABSTRACT FROM AUTHOR]

Trees in a state of decline exhibit a reduced foliage density and accumulate dead branches in their crowns. Consequently, forest decline can markedly affect both the habitats and sources of food for canopy-dwelling insects. The decline-induced increase in canopy openness may also modify the understory, shrub and ground layers, and have cascading effects on associated species. Flight interception traps and green Lindgren traps were used to survey the canopy-dwelling insects in stands of healthy and declining oak trees, in particular two insect orders: Raphidioptera, saproxylic insects associated with canopies, and Mecoptera, necrophagous or opportunistic species associated with the herbaceous or shrub strata. Overall, green Lindgren traps caught more of these insects than flight interception traps. The traps caught fi ve species of Raphidioptera. Three of them, Subilla confinis, Phaeostigma major and, to a lesser extent, Phaeostigma notata, were more abundant in stands or plots with declining trees. However, the other two species of Raphidioptera, Atlantoraphidia maculicollis and Xanthostigma xanthostigma exhibited a reverse trend. Two species of Mecoptera, Panorpa germanica and Panorpa communis, were particularly abundant, but unaffected by the level of decline. Our results show that declining forests can either host more or fewer species of Raphidioptera with saproxylic larvae, whereas Mecoptera with ground-living larvae were unaffected. Seasonal phenology and sex ratio of the species are also discussed. [ABSTRACT FROM AUTHOR]

Declining forests usually face uncertain regeneration dynamics and recovery trajectories, which are challenging to forest management. In this study, we investigated the decline pattern of Castanopsis fargesii and examined the effects on conspecific seedling regeneration. We found that 61.45% of adult individuals were in decline and the smaller DBH size classes of trees (10–40 cm) had a greater probability of decline. Most of the intermediate decline (94.52%) and nondecline individuals (95.23%) did not worsen, and the crowns of 21.91% of the intermediate decline trees were recovered during 2013–2018. Adult tree decline had a negative effect on seed production (mean mature seed density of nondecline, intermediate decline, and high decline individuals was 167.3, 63.3, and 2.1 seeds/m2, respectively), but no effect on key seed traits. The seed survival rate of declining trees was greater than that of nondeclining trees at both the seed production and seed dispersal stages. The seed to seedling transition rates in canopy gaps, decline habitats, and nondecline habitats were 7.94%, 9.47%, and 109.24%, respectively. The survival rate and height growth of newly germinated seedlings were positively correlated with the light condition, which was notably accelerated in the canopy gaps. Taken together, these results indicate that the reduction in seed production of some adult trees had a weakly negative effect on new seedling recruitment, while the improved environmental condition after the decline significantly enhanced the survival and growth of both advanced and new germinated seedlings. Looking at the overall life history, the short‐term defoliation and mortality of some C. fargesii adult trees can be regarded as a natural forest disturbance that favors conspecific seedling regeneration. High‐intensity management measures would be unnecessary in cases of an emerging intermediate decline in this forest. [ABSTRACT FROM AUTHOR]

Knowledge about forest decay dynamics is important for a better understanding of major forest ecosystem processes. The main aim of this study was to describe the dynamics of the natural decline of a mountain spruce (Picea abies) forest caused by bark beetle (Ips typographus) infestation in the Plešné Lake basin. Furthermore, we estimated the dry biomass of the trees to get information about wood storage. The spruce forest decay was described in 10 particular years within the period 2000-2015. Individual trees for every year were delineated from aerial photographs to estimate tree position, vitality (healthy, dead, stump) and age class (sapling, young, adult). LiDAR data were acquired (2008, 2011) to estimate tree height, subsequently DBH and volume biomass for 2011 using allometric relationships. At the beginning, in 2000, more than 20 thousand trees were delineated in the basin, where more than 5.8% were dead or dying trees. The strongest bark beetle outbreak occurred prior to 2009, when 74.6% of all trees in the basin were dead. The resulting number of all dead trees was in 2015 67.7%. The average biomass in the basin was 504 t/ha in 2011. The average biomass of particular tree components were 22.4, 26.7, 8.2, 8.6, 294.2, 25.6 and 120.7 t/ha for needles, branch wood, branch bark, fine branches, stem wood, stem bark and roots, respectively. [ABSTRACT FROM AUTHOR]

The 'Altai Tavan Bogd' National Park, located between 2000 m and 4000 m a.s.l. in the north-western part of the Mongolian Altai, is a forest-steppe ecosystem in Central Asia which is noticeable because of its extreme continental and high-altitude conditions. Its vegetation is very sensitive to environmental changes and impact by nomadic people. To reconstruct the dynamics of this forest-steppe ecosystem, a broad approach was taken employing multi-proxy analyses including the analysis of pollen, spores, non-pollen palynomorphs, charcoal and diatoms, as well as XRF scanning of a lacustrine sediment core of a small and shallow lake located south of Dayan Nuur. Five radiocarbon dates indicate a record spanning the last 4375 years. Between 4310 and 1040 cal yr BP, the vegetation was characterized by a mixture of alpine meadow and moist true steppe communities as well as forested areas with Larix sibirica , Pinus sibirica and Picea obovata stands. Starting around 2350 cal yr BP, a substantially greater and/or denser forest occurrence can be inferred. A marked forest decline after 1040 cal yr BP and the loss of Pinus sibirica and Picea obovata near the study area proceeded in accordance with a significantly higher abundance of large herbivores and a strong indication of nomadic people in the area as inferred from coprophilous fungi. One period of markedly increased fire activity is found from 640 to 550 cal yr BP. For the latest Holocene period, the records of non-pollen palynomorphs and diatoms suggest a rise of the lake water level. These contrasting trends of forest decline despite higher water availability may reflect the considerable human impact of the nomadic population and their cattle on the ecosystem in the 'Altai Tavan Bogd' National Park and Central Asia in general. • First multi-proxy record using a large set of proxies in western Mongolia • Detailed forest-steppe dynamics in the Altai for the last 4310 cal yr BP • Sporormiella record suggests first evidence of nomads in the area at 2350 cal yr BP. • Human impact driving force of forest decline since 1000 cal yr BP in this region • Recent lake water level rise is probably caused climate warming of 21st century. [ABSTRACT FROM AUTHOR]

Abstract With the advent of Sentinel-2, it is now possible to generate large-scale chlorophyll content maps with unprecedented spatial and temporal resolution, suitable for monitoring ecological processes such as vegetative stress and/or decline. However methodological gaps exist for adapting this technology to heterogeneous natural vegetation and for transferring it among vegetation species or plan functional types. In this study, we investigated the use of Sentinel-2A imagery for estimating needle chlorophyll (C a+b) in a sparse pine forest undergoing significant needle loss and tree mortality. Sentinel-2A scenes were acquired under two extreme viewing geometries (June vs. December 2016) coincident with the acquisition of high-spatial resolution hyperspectral imagery, and field measurements of needle chlorophyll content and crown leaf area index. Using the high-resolution hyperspectral scenes acquired over 61 validation sites we found the CI chlorophyll index R 750 /R 710 and Macc index (which uses spectral bands centered at 680 nm, 710 nm and 780 nm) had the strongest relationship with needle chlorophyll content from individual tree crowns (r2 = 0.61 and r2 = 0.59, respectively; p < 0.001), while TCARI and TCARI/OSAVI, originally designed for uniform agricultural canopies, did not perform as well (r2 = 0.21 and r2 = 0.01, respectively). Using lower-resolution Sentinel-2A data validated against hyperspectral estimates and ground truth needle chlorophyll content, the red-edge index CI and the Sentinel-specific chlorophyll indices CI-Gitelson, NDRE1 and NDRE2 had the highest accuracy (with r2 values >0.7 for June and >0.4 for December; p < 0.001). The retrieval of needle chlorophyll content from the entire Sentinel-2A bandset using the radiative transfer model INFORM yielded r2 = 0.71 (RMSE = 8.1 μg/cm2) for June, r2 = 0.42 (RMSE = 12.2 μg/cm2) for December, and r2 = 0.6 (RMSE = 10.5 μg/cm2) as overall performance using the June and December datasets together. This study demonstrates the retrieval of leaf C a+b with Sentinel-2A imagery by red-edge indices and by an inversion method based on a hybrid canopy reflectance model that accounts for tree density, background and shadow components common in sparse forest canopies. Highlights • Chlorophyll indices and model inversion methods assessed with Sentinel-2A data. • Chlorophyll content estimation conducted on open-canopy conifer forest. • Sentinel-2A chlorophyll indices were successful but showed non-linear trends. • Chlorophyll estimation from Sentinel-2A with INFORM yielded r2 = 0.71. • Without red-edge bands performance decreased (r2 = 0.44/RMSE = 22.8 μg/cm2). [ABSTRACT FROM AUTHOR]

Questions: (a) Have Metrosideros polymorpha trees become re‐established in Hawaiian forests previously impacted by canopy dieback in the 1970s? (b) Has canopy dieback expanded since the 1970s? (c) Can spatial patterns from this dieback be correlated with habitat factors to model future dieback in this area? Study Site: An 83,603 ha study area on the eastern slopes of Mauna Loa and Mauna Kea volcanoes on the island of Hawaii, USA. Methods: We analyzed very‐high‐resolution imagery to assess status of Metrosideros polymorpha forests across the eastern side of the island of Hawaii. We generated 1,170 virtual vegetation plots with a 100‐m radius; 541 plots in areas mapped in 1977 with trees dead or mostly defoliated (dieback), and 629 plots in adjacent wet forest habitat, previously mapped as non‐dieback condition. In each plot we estimated the frequency of M. polymorpha trees that were dead or mostly defoliated, and the frequency of trees with healthy crowns. These results were combined with habitat data to produce a spatial model depicting probability of canopy dieback within the study area. Results: Seventy‐nine percent of plots mapped in 1977 in dieback condition recovered their canopy and were now considered in non‐dieback condition. Ninety‐one percent of plots in previous non‐dieback areas were found to still have a healthy M. polymorpha canopy in 2015. A spatial model allowed us to identify areas within the study area with high, medium, and low probability of experiencing this same type of canopy dieback in the future. Conclusions: Most former dieback areas mapped within the study area in 1977 now show recovery of the tree canopy through growth of new cohorts of young M. polymorpha trees. This suggests these forest communities are resilient to this type of canopy loss and tree death so long as other factors do not disrupt the natural regeneration process. Large portions of the native M. polymorpha forests on the windward side of the island of Hawaii experienced severe canopy dieback in the late 1960s and early 1970s. By monitoring very high‐resolution aerial imagery in 2015 we found strong seedling regeneration of M. polymorpha had led to a recovery of the tree canopy in 79% of the impacted sites. [ABSTRACT FROM AUTHOR]

Abstract: The frequently observed forest decline in water‐limited regions may be associated with impaired tree hydraulics, but the precise physiological mechanisms remain poorly understood. We compared hydraulic architecture of Mongolian pine (Pinus sylvestris var. mongolica) trees of different size classes from a plantation and a natural forest site to test whether greater hydraulic limitation with increasing size plays an important role in tree decline observed in the more water‐limited plantation site. We found that trees from plantations overall showed significantly lower stem hydraulic efficiency. More importantly, plantation‐grown trees showed significant declines in stem hydraulic conductivity and hydraulic safety margins as well as syndromes of stronger drought stress with increasing size, whereas no such trends were observed at the natural forest site. Most notably, the leaf to sapwood area ratio (LA/SA) showed a strong linear decline with increasing tree size at the plantation site. Although compensatory adjustments in LA/SA may mitigate the effect of increased water stress in larger trees, they may result in greater risk of carbon imbalance, eventually limiting tree growth at the plantation site. Our results provide a potential mechanistic explanation for the widespread decline of Mongolian pine trees in plantations of Northern China. [ABSTRACT FROM AUTHOR]

The operational monitoring of forest decline requires the development of remote sensing methods that are sensitive to the spatiotemporal variations of pigment degradation and canopy defoliation. In this context, the red-edge spectral region (RESR) was proposed in the past due to its combined sensitivity to chlorophyll content and leaf area variation. In this study, the temporal dimension of the RESR was evaluated as a function of forest decline using a radiative transfer method with the PROSPECT and 3D FLIGHT models. These models were used to generate synthetic pine stands simulating decline and recovery processes over time and explore the temporal rate of change of the red-edge chlorophyll index (CI) as compared to the trajectories obtained for the structure-related Normalized Difference Vegetation Index (NDVI). The temporal trend method proposed here consisted of using synthetic spectra to calculate the theoretical boundaries of the subspace for healthy and declining pine trees in the temporal domain, defined by CI time=n /CI time=n+1 vs. NDVI time=n /NDVI time=n+1 . Within these boundaries, trees undergoing decline and recovery processes showed different trajectories through this subspace. The method was then validated using three high-resolution airborne hyperspectral images acquired at 40 cm resolution and 260 spectral bands of 6.5 nm full-width half-maximum (FWHM) over a forest with widespread tree decline, along with field-based monitoring of chlorosis and defoliation (i.e., ‘decline’ status) in 663 trees between the years 2015 and 2016. The temporal rate of change of chlorophyll vs. structural indices, based on reflectance spectra extracted from the hyperspectral images, was different for trees undergoing decline, and aligned towards the decline baseline established using the radiative transfer models. By contrast, healthy trees over time aligned towards the theoretically obtained healthy baseline . The applicability of this temporal trend method to the red-edge bands of the MultiSpectral Imager (MSI) instrument on board Sentinel-2a for operational forest status monitoring was also explored by comparing the temporal rate of change of the Sentinel-2-derived CI over areas with declining and healthy trees. Results demonstrated that the Sentinel-2a red-edge region was sensitive to the temporal dimension of forest condition, as the relationships obtained for pixels in healthy condition deviated from those of pixels undergoing decline. [ABSTRACT FROM AUTHOR]

Functional studies of tree decline have mainly focused on process inside the plant showing possible alterations of carbon transport, storage and hydraulic functions. However, the processes occurring at the plant-soil interface have been seldom investigated. Our objective is to examine carbon functional alterations in the belowground compartment in the case of a long term beech decline. Soil and nutrient content profiles were characterized under five healthy and five declining trees. Seasonal root growth and rhizodeposition were characterized using, ingrowth cores, and microbial biomass combined with soluble carbon organic content, respectively. Podzolisation associated with soil acidification and deficiencies in Ca, Mg and Mn were observed in the soil under declining trees, but not under healthy trees. Spring fine root growth was higher in declining trees than in healthy trees but there were only minor differences concerning rhizodeposition proxies. In our study, we showed that the tree health status is associated to a marked heterogeneity of soil characteristics. Podzolisation close to declining trees leads to a local mineral deficiency which probably stimulates their fine root growth but without altering their flux of carbon exudates. [ABSTRACT FROM AUTHOR]

Abstract: Insects, diseases, fire and drought and other disturbances associated with global climate change contribute to forest decline and mortality in many parts of the world. Forest decline and mortality related to drought or insect outbreaks have been observed in North American aspen forests. However, little research has been done to partition and estimate their relative contributions to growth declines. In this study, we combined tree‐ring width and basal area increment series from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58°N) in western Canada and attempted to investigate the effect of drought and insect outbreaks on growth decline, and simultaneously partition and quantify their relative contributions. Results indicated that the influence of drought on forest decline was stronger than insect outbreaks, although both had significant effects. Furthermore, the influence of drought and insect outbreaks showed spatiotemporal variability. In addition, our data suggest that insect outbreaks could be triggered by warmer early spring temperature instead of drought, implicating that potentially increased insect outbreaks are expected with continued warming springs, which may further exacerbate growth decline and death in North America aspen mixed forests. [ABSTRACT FROM AUTHOR]

Forest decline is either caused by damage or else by vulnerability due to unfavourable growth conditions or due to unnatural silvicultural systems. Here, we assess forest decline in the Czech Republic (Central Europe) using fuzzy functions, fuzzy sets and fuzzy rating of ecosystem properties over a 1 × 1 km grid. The model was divided into fuzzy functions of the abiotic predictors of growth conditions (F pred including temperature, precipitation, acid deposition, soil data and relative site insolation) and forest biomass receptors (F rec including remote sensing data, density and volume of aboveground biomass, and surface humus chemical data). Fuzzy functions were designed at the limits of unfavourable, undetermined or favourable effects on the forest ecosystem health status. Fuzzy sets were distinguished through similarity in a particular membership of the properties at the limits of the forest status margins. Fuzzy rating was obtained from the least difference of F pred − F rec . Unfavourable F pred within unfavourable F rec indicated chronic damage, favourable F pred within unfavourable F rec indicated acute damage, and unfavourable F pred within favourable F rec indicated vulnerability. The model in the 1 × 1 km grid was validated through spatial intersection with a point field of uniform forest stands. Favourable status was characterised by soil base saturation (BS) > 50%, BCC /Al > 1, C org > 1%, MgO > 6 g/kg, and nitrogen deposition < 1200 mol (H + )/ha·year. Vulnerable forests had BS humus 46–60%, BCC /Al 9–20 and NDVI ≈ 0.42. Chronic forest damage occurs in areas with low temperatures, high nitrogen deposition, and low soil BS and C org levels. In the Czech Republic, 10% of forests were considered non-damaged and 77% vulnerable, with damage considered acute in 7% of forests and chronic in 5%. The fuzzy model used suggests that improvement in forest health will depend on decreasing environmental load and restoration concordance between growth conditions and tree species composition. [ABSTRACT FROM AUTHOR]

Submediterranean forests are considered an ecotone between Mediterranean and Eurosiberian ecosystems, and are very sensitive to global change. A decline of Scots pine (Pinus sylvestris L.) and a related expansion of oak species (Quercus spp.) have been reported in the Spanish Pre-Pyrenees. Although this has been associated with increasing drought stress, the underlying mechanisms are not fully understood, and suitable monitoring protocols are lacking. The aim of this study is to bring insight into the physiological mechanisms anticipating selective decline of the pines, with particular focus on carbon and water relations. For this purpose, we performed a sampling campaign covering two growing seasons in a mixed stand of P. sylvestris and Quercus subpyrenaica E.H del Villar. We sampled seasonally twig xylem and soil for water isotope composition (δ18O and δ2H), leaves for carbon isotope composition (δ13C) and stems to quantify non-structural carbohydrates (NSC) concentration, and measured water potential and leaf gas exchange. The first summer drought was severe for both species, reaching low predawn water potential (-2.2MPa), very low stomatal conductance (12 ± 1.0mmolm-2 s-1) and near-zero or even negative net photosynthesis, particularly in P. sylvestris (-0.6 ± 0.34 μmolm-2 s-1 in oaks, -1.3 ± 0.16 μmolm-2 s-1 in pines). Hence, the tighter stomatal control and more isohydric strategy of P. sylvestris resulted in larger limitations on carbon assimilation, and this was also reflected in carbon storage, showing twofold larger total NSC concentration in oaks than in pines (7.8 ± 2.4% and 4.0 ± 1.3%, respectively). We observed a faster recovery of predawn water potential after summer drought in Q. subpyrenaica than in P. sylvestris (-0.8MPa and -1.1MPa, respectively). As supported by the isotopic data, this was probably associated with a deeper and more reliable water supply in Q. subpyrenaica. In line with these short-term observations, we found a more pronounced negative effect of steadily increasing drought stress on longterm growth in pines compared with oaks. All these observations confer evidence of early warning of P. sylvestris decline and indicate the adaptive advantage of Q. subpyrenaica in the area. [ABSTRACT FROM AUTHOR]

Tree mortality is an important process in forest ecosystems, frequently hypothesized to be highly climate sensitive. Yet, tree death remains one of the least understood processes of forest dynamics. Recently, changes in tree mortality have been observed in forests around the globe, which could profoundly affect ecosystem functioning and services provisioning to society. We describe continental-scale patterns of recent tree mortality from the only consistent pan-European forest monitoring network, identifying recent mortality hotspots in southern and northern Europe. Analyzing 925,462 annual observations of 235,895 trees between 2000 and 2012, we determine the influence of climate variability and tree age on interannual variation in tree mortality using Cox proportional hazard models. Warm summers as well as high seasonal variability in precipitation increased the likelihood of tree death. However, our data also suggest that reduced cold-induced mortality could compensate increased mortality related to peak temperatures in a warming climate. Besides climate variability, age was an important driver of tree mortality, with individual mortality probability decreasing with age over the first century of a trees life. A considerable portion of the observed variation in tree mortality could be explained by satellite-derived net primary productivity, suggesting that widely available remote sensing products can be used as an early warning indicator of widespread tree mortality. Our findings advance the understanding of patterns of large-scale tree mortality by demonstrating the influence of seasonal and diurnal climate variation, and highlight the potential of state-of-the-art remote sensing to anticipate an increased likelihood of tree mortality in space and time. [ABSTRACT FROM AUTHOR]

Ash ( Fraxinus L.) species are currently threatened by the emerald ash borer (EAB; Agrilus planipennis Fairmaire) across a growing area in the eastern US. Accurate mapping of ash species is required to monitor the host resource, predict EAB spread and better understand the short- and long-term effects of EAB on the ash resource. Hyperspectral remote sensing technologies have been used to successfully map forest species, although most efforts are focused on healthy canopies for relatively homogeneous forested stands. This study uses imagery collected by the NASA Goddard LiDAR, Hyperspectral and Thermal (GLiHT) airborne imager to map ash species at the tree level in an EAB infested urban setting. The overall goal of the study is to understand how canopy condition impacts species mapping accuracy and identify data collection and image processing techniques to more accurately map the location of ash species in infested regions. Results indicate that while overall independent validation mapping accuracy of ash and non-ash trees was 81%, correct identification of ash canopies dropped from 62% for vigor 1 trees to 22% for vigor 2 trees. To minimize these errors, we developed a multiple endmember, spectral unmixing technique to overcome challenges presented by a spectrally complicated target in a complex urban environment. This hinges on the use of endmember spectra from trees across a range of canopy condition, including the derivation of vegetation indices to inform the spectral unmixing calibration. This approach was more accurate than calibrations performed using traditional unmixing based only on pure endmember spectra. Implications for this work suggest that urban forest managers may attain more accurate maps by conducting remote sensing data collections prior to infestation while the trees are still healthy. Where this is not possible, mapping efforts must reflect a range of canopy conditions and include vegetation indices concurrent with reflectance data. The resulting ash species maps provide urban forest managers spatially explicit products to help estimate the extent of possible impacts in their communities, guide the implementation of management and monitoring efforts and provide the basis for planning as EAB continues to spread. [ABSTRACT FROM AUTHOR]

Functional studies of tree decline have mainly focused on process inside the plant showing possible alterations of carbon transport, storage and hydraulic functions. However, the processes occurring at the plant-soil interface have been seldom investigated. Our objective is to examine carbon functional alterations in the belowground compartment in the case of a long term beech decline. Soil and nutrient content profiles were characterized under five healthy and five declining trees. Seasonal root growth and rhizodeposition were characterized using, ingrowth cores, and microbial biomass combined with soluble carbon organic content, respectively. Podzolisation associated with soil acidification and deficiencies in Ca, Mg and Mn were observed in the soil under declining trees, but not under healthy trees. Spring fine root growth was higher in declining trees than in healthy trees but there were only minor differences concerning rhizodeposition proxies. In our study, we showed that the tree health status is associated to a marked heterogeneity of soil characteristics. Podzolisation close to declining trees leads to a local mineral deficiency which probably stimulates their fine root growth but without altering their flux of carbon exudates. [ABSTRACT FROM AUTHOR]

Forests worldwide have been recently affected by severe decline and mortality, while our understanding about forest decline across spatial scale is still limited. In this work, we study how Quercus suber trees adjust their physiology, in terms of water use efficiency and secondary growth, to pathogen-induced oak decline at the whole-tree, local and landscape scales. This work was carried out in Mediterranean mixed forests where their dominant key species Q. suber is affected by a severe decline and mortality induced by the exotic soil-borne pathogen Phytophthora cinnamomi. Significant differences were not observed between defoliated and healthy trees, either in terms of water use efficiency or growth at the whole-tree scale. We found that limiting conditions, such as low soil depth and high pathogen abundance, induced trees to higher water use efficiency at local and landscape scales. Overall our findings point out that Q. suber trees subjected to soil drought and root pathogens increase water use efficiency to some extent, while this response might not be enough for the trees to overcome the physiological stress associated with the pathogen-induced dieback. We discuss the complex way by which adult Q. suber trees physiologically respond to P. cinnamomi-induced mortality, improving our understanding of the likely consequences of chronic oak decline in the future. [ABSTRACT FROM AUTHOR]

Plant-soil feedbacks ( PSFs) play a relevant role as drivers of species abundance, coexistence, and succession in plant communities. However, the potential contribution of PSFs to community dynamics in changing forest ecosystems affected by global change drivers is still largely unexplored. We measured the direction, strength and nature (biological vs. chemical) of PSFs experienced by coexisting tree species in two types of declining Quercus suber forests of southwestern Spain (open woodland vs. closed forest) invaded by the exotic soil pathogen Phytophthora cinnamomi. To test PSFs in a realistic community context, we focused not only on individual PSFs (i.e., comparing the growth of a tree species on conspecific vs. heterospecific soil) but also calculated net-pairwise PSFs by comparing performance of coexisting tree species on their own and each other's soils. We hypothesized that the decline and death of Q. suber would alter the direction and strength of individual and net-pairwise PSFs due to the associated changes in soil nutrients and microbial communities, with implications for recruitment dynamics and species coexistence. In support of our hypothesis, we found that the decline of Q. suber translated into substantial alterations of individual and net-pairwise PSFs, which shifted from mostly neutral to significantly positive or negative, depending on the forest type. In both cases however the identified PSFs benefited other species more than Q. suber (i.e., heterospecific positive PSF in the open woodland, conspecific negative PSF in the closed forest). Our results supported PSFs driven by changes in chemical soil properties (mainly phosphorus) and arbuscular mycorrhizal fungi, but not in pathogen abundance. Overall, our study suggests that PSFs might reinforce the loss of dominance of Q. suber in declining forests invaded by P. cinnamomi by promoting the relative performance of non-declining coexisting species. More generally, our results indicate an increase in the strength of net PSFs as natural forests become disturbed by global change drivers (e.g., invasive species), suggesting an increasingly important role of PSFs in forest community dynamics in the near future. [ABSTRACT FROM AUTHOR]

Novel forest decline is increasing due to global environmental change, yet the causal factors and their interactions remain poorly understood. Using tree ring analyses, we show how climate and multiple biotic factors caused the decline of whitebark pine ( Pinus albicaulis) in 16 stands in the southern Canadian Rockies. In our study area, 72% of whitebark pines were dead and 18% had partially dead crowns. Tree mortality peaked in the 1970s; however, the annual basal area increment of disturbed trees began to decline significantly in the late 1940s. Growth decline persisted up to 30 years before trees died from mountain pine beetle ( Dendroctonus ponderosae), Ips spp. bark beetles or non-native blister rust pathogen ( Cronartium ribicola). Climate-growth relations varied over time and differed among the healthy and disturbed subpopulations of whitebark pine. Prior to the 1940s, cool temperatures limited the growth of all subpopulations. Growth of live, healthy trees became limited by drought during the cool phase (1947 -1976) of the Pacific Decadal Oscillation ( PDO) and then reverted to positive correlations with temperature during the subsequent warm PDO phase. In the 1940s, the climate-growth relations of the disturbed subpopulations diverged from the live, healthy trees with trees ultimately killed by mountain pine beetle diverging the most. We propose that multiple factors interacted over several decades to cause unprecedented rates of whitebark pine mortality. Climatic variation during the cool PDO phase caused drought stress that may have predisposed trees to blister rust. Subsequent decline in snowpack and warming temperatures likely incited further climatic stress and with blister rust reduced tree resistance to bark beetles. Ultimately, bark beetles and blister rust contributed to tree death. Our findings suggest the complexity of whitebark pine decline and the importance of considering multiway drought-disease-insect interactions over various timescales when interpreting forest decline. [ABSTRACT FROM AUTHOR]

Combining hydraulic- and carbon-related measurements helps to understand drought-induced plant mortality. Here, we investigated the role that plant respiration ( R) plays in determining carbon budgets under drought., We measured the hydraulic conductivity of stems and roots, and gas exchange and nonstructural carbohydrate ( NSC) concentrations of leaves, stems and roots of seedlings of two resprouting species exposed to drought or well-watered conditions: Ulmus minor (riparian tree) and Quercus ilex (dryland tree)., With increasing water stress (occurring more rapidly in larger U. minor), declines in leaf, stem and root R were less pronounced than that in leaf net photosynthetic CO2 uptake ( Pn). Daytime whole-plant carbon gain was negative below −4 and −6 MPa midday xylem water potential in U. minor and Q. ilex, respectively. Relative to controls, seedlings exhibiting shoot dieback suffered c. 80% loss of hydraulic conductivity in both species, and reductions in NSC concentrations in U. minor. Higher drought-induced depletion of NSC reserves in U. minor was related to higher plant R, faster stomatal closure, and premature leaf-shedding., Differences in drought resistance relied on the ability to maintain hydraulic conductivity during drought, rather than tolerating conductivity loss. Root hydraulic failure elicited shoot dieback and precluded resprouting without root NSC reserves being apparently limiting for R. [ABSTRACT FROM AUTHOR]

Climate change is expected to impact forests worldwide, and yellow-cedar ( Callitropsis nootkatensis (D. Don) Oerst. ex D.P. Little) decline has been used as an example of how changing climate can impact a tree species. However, most previous research has not placed yellow-cedar decline within the context of yellow-cedar overall. We used a 2004-2013 regional inventory of the temperate rainforest of Alaska (671 plots with yellow-cedar) to estimate current attributes and a subset of 564 remeasured plots (established 1995-1998) to estimate recent change. Results show that in unmanaged forests, yellow-cedar live tree basal area recently (1995-1998 to 2004-2013) increased, with a 95% confidence interval of a 0.3% to 3.3% increase per decade. Yellow-cedar has a relatively low mortality rate, 0.41% of trees per year. An analysis of live tree to snag ratios was consistent with elevated mortality of yellow-cedar prior to 1995 but also indicated that little range contraction had occurred. The large numbers and wide geographic range of yellow-cedar trees in Alaska and the recent (1995-2013) stability in the monitored population serve as important contextual information for yellow-cedar decline. This research also illustrates that understanding the spatial and temporal complexities of how tree species respond to climate change will be improved if focused studies are accompanied by regional monitoring. [ABSTRACT FROM AUTHOR]

To accurately estimate stem respiration (RS), measurements of both carbon dioxide (CO2) efflux to the atmosphere (EA) and internal CO2 flux through xylem (FT) are needed because xylem sap transports respired CO2 upward. However, reports of seasonal dynamics of FT and EA are scarce and no studies exist in Mediterranean species under drought stress conditions. Internal and external CO2 fluxes at three stem heights, together with radial stem growth, temperature, sap flow and shoot water potential, were measured in Quercus pyrenaica Willd. in four measurement campaigns during one growing season. Substantial daytime depressions in temperature-normalized EA were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and EA cannot be uniquely ascribed to diversion of CO2 in the transpiration stream. Low internal [CO2] (<0.5%) resulted in low contributions of FT to RS throughout the growing season, and RS was mainly explained by EA (>90%). Internal [CO2] was found to vary vertically along the stems. Seasonality in resistance to radial CO2 diffusion was related to shoot water potential. The low internal [CO2] and FT observed in our study may result from the downregulation of xylem respiration in response to a legacy of coppicing as well as high radial diffusion of CO2 through cambium, phloem and bark tissues, which was related to low water content of stems. Long-term studies analyzing temporal and spatial variation in internal and external CO2 fluxes and their interactions are needed to mechanistically understand and model respiration of woody tissues. [ABSTRACT FROM AUTHOR]

In the continued quest to explain the decline in productivity and vigor with aging forest stands, the most poorly studied area relates to root system change in time. This paper measures the wood production, root and leaf area (and mass) in a chronosequence of fire-origin lodgepole pine ( Pinus contorta Loudon) stands consisting of four age classes (12, 21, 53, and ≥100 years), each replicated ~ five times. Wood productivity was greatest in the 53-year-old stands and then declined in the ≥100-year-old stands. Growth efficiency, the quantity of wood produced per unit leaf mass, steadily declined with age. Leaf mass and fine root mass plateaued between the 53- and ≥100-year-old stands, but leaf area index actually increased in the older stands. An increase in the leaf area index:fine root area ratio supports the idea that older stand are potentially limited by soil resources. Other factors contributing to slower growth in older stands might be lower soil temperatures and increased self-shading due to the clumped nature of crowns. Collectively, the proportionally greater reduction in fine roots in older stands might be the variable that predisposes these forests to be at a potentially greater risk of stress-induced mortality. [ABSTRACT FROM AUTHOR]

• TWs and BAIs were lower for dieback poplar and Mongolian pine than for healthy groups. • δ 13C, δ 18O, and iWUE were higher for dieback poplar; but no differences in Mongolian pine. • Two species adopt different leaf gas-exchange strategies under long-term drought conditions. • iWUE and δ 18O had the highest and lowest relative contributions to TW, respectively. • Compared with healthy groups, dieback trees had less climate sensitivity and greater physiological response. Tree-ring stable isotopes provide insights into the drought-induced eco-physiological mechanisms of forests. We investigated the growth of poplar and Mongolian pine trees decline in the Three-North Shelter Forest, and analyzed the differences in tree-ring width (TW), δ 13C, δ 18O, and intrinsic water-use efficiency (iWUE) between dieback and healthy trees. An R package was used to quantify the relative contributions of climatic and physiological factors to growth. Both TW and basal area increment were lower for the dieback poplar and Mongolian pine than for the healthy trees. δ 13C, δ 18O, and iWUE were higher for the dieback poplar than for the healthy individuals; there were no significant differences in Mongolian pine. The measured iWUE for healthy poplar, healthy and dieback Mongolian pine were consistent with the modeled iWUE for C i / C a scenario, indicating the stomata were open and C i increased along with C a. In contrast, the measured iWUE was higher for the dieback poplar than for the healthy individuals, which was consistent with the constant C i scenario and partial stomatal closure. A significant decreasing C i /C a was detected only for the dieback poplar, reflecting the diversity in the leaf gas-exchange strategies of the two species. Pearson coefficients showed that on the annual timescale, tree growth was significantly negatively correlated with temperature, VPD, and iWUE, but positively correlated with SPEI. Moreover, the relationships between TW and stable isotopes and stomatal regulation, which differed in the two species, were affected by the dieback process. Furthermore, iWUE and δ 18O had the highest and lowest relative contributions to TW, respectively. The relative contribution of meteorological factors (Tem, VPD and SPEI) to the growth was significantly lower than that of physiological factors (iWUE and δ 18O). Compared with the healthy controls of both species, the dieback trees were less sensitive to climate conditions, but the effects of physiological activities on growth were greater. These results provide a better understanding of the climate sensitivity and physiological responses of trees under long-term drought conditions. [ABSTRACT FROM AUTHOR]

Monthly litterfall was monitored over a 3-year period in afforested Aleppo pines in the Mediterranean semiarid SE Spain with the aim of determining the long-term response of pines to reductions in tree competition and how this forest practice might influence stand resilience. Three thinning intensities applied 5 years earlier were evaluated (T75 = 75% of the basal area removed, T60 = 60% and T48 = 48%), both at the stand and at the tree level. On average, the total annual litterfall varied between 1.30 Mg ha −1 yr −1 (±0.24 SE) in T75 and 3.28 Mg ha −1 yr −1 (±0.78 SE) in the unthinned control. At the stand level, monthly differences among the treatments were found over time in the needles ( F = 11.09, df = 3, P = 0.0009) and woody fraction ( F = 4.36, df = 3, P = 0.0269) following the thinning gradient: T0 (control)>T48 > T60 > T75, and for the total amount of needles ( χ 2 = 9.33, P = 0.025) and twigs ( χ 2 = 9.11, P = 0.027) recorded at the end of the study period. High amounts of twig and needle fall were recorded during summer and beginning of autumn, whereas the main miscellanea inputs were registered during the spring, coinciding with the fall of nests and frass from caterpillar outbreaks. At the tree level, the total litterfall fluctuated between 1.5 kg tree yr −1 in T0 (2nd yr) and 7.0 kg tree yr −1 in T75 (3rd yr), although mean annual statistical differences among the treatments were found only for the first year of monitoring. However, needle fall was higher for larger pines (T75) than for the smaller ones in control (T0) when the data were analysed over the 3-year-period ( F = 3.64, df = 3, P = 0.0247), and the same happened for the woody fraction ( F = 3.63, df = 3, P = 0.0250). By contrast, pine trees in the unthinned control registered needle-fall rates (measured as kg m −2 tree −1 ) that were similar to or higher than those of pine trees in thinned stands, suggesting that defoliation processes took place at high tree densities, especially after a severely dry period. We propose thinning as a measure to adapt high-density plantations to alterations due to climate change, in order to prevent forest decline and mortality. [ABSTRACT FROM AUTHOR]

Questions Is mal del ciprés, the widespread decline and death of Austrocedrus chilensis trees, caused by a single pathogen or multiple factors? Using a novel dendrochronological approach, we disentangled the influences of climatic variation on the radial growth decline and death of A. chilensis trees in declining forests. We distinguish possible causes of reduced radial growth and mortality from autogenic processes driven by stand development. We present a conceptual model of forest decline including multiple factors that predispose, incite and contributed to decreased radial growth and death of A. chilensis. Location A. chilensis forests on mesic sites in northern Patagonia, Argentina. Methods We used dendrochronology to determine the years of (1) onset of radial growth decline of 301 living and dead trees stratified by canopy position at decline onset, and (2) mortality of 339 trees stratified by radial growth patterns and canopy position at death. Events were years with low or high numbers of trees initiating decline or dying. We tested the hypothesis that onset of decline and mortality were concurrent with drought for individual trees, using contingency tables, and for events, using superposed epoch analysis. Results Climatic variability acts as an environmental stress inciting and contributing to stand-level forest decline. The onset of radial growth decline and mortality of individual trees were significantly associated with summer moisture deficits. High-magnitude onset-of-decline and mortality events were concurrent with adverse climatic conditions. Conclusions Climatic variation and drought incite and contribute to tree- and stand-level decline and mortality in A. chilensis forests. Deciphering the effects of stand development is critical as autogenic processes independently drive tree mortality and mediate the effects of climatic variability on A. chilensis forest decline. Based on our results, we present a conceptual model within the framework of a forest decline process, and conclude A. chilensis mortality is a forest decline process driven by complex interactions between allogenic abiotic and biotic factors and autogenic stand development processes. Site conditions, genetic variation and sex of trees are predisposing factors that likely interact with the pathogen Phytophthora. [ABSTRACT FROM AUTHOR]

Background and Aims: A worldwide increase in tree decline and mortality has been linked to climate change and, where these represent foundation species, this can have important implications for ecosystem functions. This study tests a combined approach of phylogeographic analysis and species distribution modelling to provide a climate change context for an observed decline in crown health and an increase in mortality in Eucalyptus wandoo, an endemic tree of south-western Australia. Methods: Phylogeographic analyses were undertaken using restriction fragment length polymorphism analysis of chloroplast DNA in 26 populations across the species distribution. Parsimony analysis of haplotype relationships was conducted, a haplotype network was prepared, and haplotype and nucleotide diversity were calculated. Species distribution modelling was undertaken using Maxent models based on extant species occurrences and projected to climate models of the last glacial maximum (LGM). Key Results: A structured pattern of diversity was identified, with the presence of two groups that followed a climatic gradient from mesic to semi-arid regions. Most populations were represented by a single haplotype, but many haplotypes were shared among populations, with some having widespread distributions. A putative refugial area with high haplotype diversity was identified at the centre of the species distribution. Species distribution modelling showed high climatic suitability at the LGM and high climatic stability in the central region where higher genetic diversity was found, and low suitability elsewhere, consistent with a pattern of range contraction. Conclusions: Combination of phylogeography and paleo-distribution modelling can provide an evolutionary context for climate-driven tree decline, as both can be used to cross-validate evidence for refugia and contraction under harsh climatic conditions. This approach identified a central refugial area in the test species E. wandoo, with more recent expansion into peripheral areas from where it had contracted at the LGM. This signature of contraction from lower rainfall areas is consistent with current observations of decline on the semi-arid margin of the range, and indicates low capacity to tolerate forecast climatic change. Identification of a paleo-historical context for current tree decline enables conservation interventions to focus on maintaining genetic diversity, which provides the evolutionary potential for adaptation to climate change. [ABSTRACT FROM AUTHOR]

Forest decline is frequently associated with infection; however, infections habitually affect trees that have been previously debilitated by environmental stress. Nevertheless, the causes and physiology of noninfectious forest decline are not well known. Some Juniperus thurifera L. forests presented severe (noninfectious) declines, with defoliations over 50%. The goal of this study was to determine the causes and characterize the physiology of this noninfectious decline, and we hypothesized that it could be related to environmental stress from increasing interspecific competition resulting from land abandonment. We randomly assigned 60 trees to either a control group or one of two competition-release treatments, i.e., vegetation clearing or soil ploughing. We characterized the physiological state of the trees both before treatment application and 14 months after treatment application. The J. thurifera trees that were declining experienced significantly lower soil nutrient and water availability, which resulted in lower leaf nutrient concentrations, lower photosynthetic rates, higher water stress, and arrested growth and reproduction. We confirmed that competition release increased nutrient availability and acquisition, reduced water deficit, improved photosynthetic rates, and abruptly stopped defoliation. Competition plays an increasingly critical role in forest conservation, particularly with the reported increase in the number of species colonizing previously unfeasible habitats due to global change and the absence of traditional activities that used to buffer competition. [ABSTRACT FROM AUTHOR]

This paper examines the potential of MODIS-NDVI time series for detecting clear-cuts in a coniferous forest stand in the south of France. The proposed approach forms part of a survey monitoring the status of forest health and evaluating the forest decline phenomena observed over the last few decades. One of the prerequisites for this survey was that a rapid and easily reproducible method had to be developed that differentiates between forest clear-cuts and changes in forest health induced by environmental factors such as summer droughts. The proposed approach is based on analysis of the breakpoints detected within NDVI time series, using the "Break for Additive Seasonal and Trend" (BFAST) algorithm. To overcome difficulties detecting small areas on the study site, we chose a probabilistic approach based on the use of a conditional inference tree. For model calibration, clear-cut reference data were produced at MODIS resolution (250 m). According to the magnitude of the detected breakpoints, probability classes for the presence of clear-cuts were defined, from greater than 90% to less than 3% probability of a clear-cut. One of the advantages of the probabilistic model is that it allows end users to choose an acceptable level of uncertainty depending on the application. In addition, the use of BFAST allows events to be dated, thus making it possible to perform a retrospective analysis of decreases in forest vitality in the study area. [ABSTRACT FROM AUTHOR]

Fir and beech trees have been dying in the Tanzawa Mountains, which are located southwest of the Kanto Plain. We have observed fog characteristics since 1988 in Mt. Oyama, which is isolated from others, located southeast of the Mountains. Acid fog has been frequently observed there. The annual mean pH of the fog has remained roughly constant, but the pH distribution of fog has shifted to higher pH recently, which corresponds closely with the improvement of air pollution at the base of Mt. Oyama. Acid fog is still formed, and it may have affected tree species such as fir and beech, which are sensitive to air pollution. We observed rainy periods and foggy periods using a visibility meter at the mountain top. The canopies at the high altitude are confirmed to be wetted for a long period, which might enhance the air pollutant deposition. We also observed air pollution and wet deposition at Mt. Nabewari, located southwest of the Tanzawa Mountains. The acid gas concentrations are about the same level in the two mountains and are in the order of SO 2 > HNO 3 > HCl. Comparison of wet deposition at the summit of Mt. Nabewari to that of Mt. Oyama also revealed that they were of similar levels. In the Tanzawa Mountains, fir trees and beech trees are declining by the damage of acid fog, although the high concentration ozone and the other factors may have also affected them. [ABSTRACT FROM AUTHOR]

1. Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought-induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought-related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir (Abies alba), Scots pine (Pinus sylvestris) and Aleppo pine (Pinus halepensis), to a severe drought event. 2. We compared basal area increment (BAI) trends and responses to climate and drought in declining (very defoliated and dying) vs. non-declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non-structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood-inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death. 3. Declining silver fir and Scots pine trees showed less growth than non-declining trees one to three decades, respectively, before the drought event, whereas Aleppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood-inhabiting fungi were found in post-drought declining trees apart from blue-stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death. 4. Synthesis. Early warning signals of drought-triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for Scots and Aleppo pines. Longer growth and defoliation series, additional vigour parameters and multi-species comparisons are required to understand and predict drought-induced tree death. [ABSTRACT FROM AUTHOR]

We investigated the potential effects of projected climate change on red spruce (Picea rubens Sarg.) growth in the Great Smoky Mountains of Southeastern USA. A model called Annual Radial Increment Model (ARIM) was used to capture ecosystem complexity manifested as direct and indirect effects in multifactorial within- and across-scale interactions. The model was run under different scenarios, including projected climate change under reduced, no change, and increased atmospheric pollution. Modeled red spruce growth at end of 21st century (2080–2099) was compared to modeled growth at end of the 20th century (1980–1999). Red spruce growth at high elevations (≥1700 m) declined by 10.8% when climate change interacted with a 10% increase in air pollution, but red spruce growth increased by 8.4% when air pollution decreased by 10%. In contrast, red spruce growth at low elevations (<1700 m) declined by 11.2% with a 10% increase in air pollution, 8.9% with no change, and 6.4% with a 10% decrease in air pollution. Our results suggest that red spruce populations at high-elevation may grow more rapidly under climate change if air pollution decreases, but populations at low-elevation may decline irrespective of air pollution changes as habitats shrink. [ABSTRACT FROM AUTHOR]

The rate of metabolic processes demanding energy in tree stems changes in relation with prevailing climatic conditions. Tree water availability can affect stem respiration through impacts on growth, phloem transport or maintenance of diverse cellular processes, but little is known on this topic. Here we monitored seasonal changes in stem CO2 efflux (Fs), radial growth, sap flow and non-structural carbohydrates in trees of Quercus ilex in a Mediterranean forest stand subjected since 2003 to either partial (33%) throughfall exclusion (E) or unchanged throughfall (C). Fs increased exponentially during the day by an effect of temperature, although sap flow attenuated the increase in Fs during the day time. Over the year, Fs also increased exponentially with increasing temperatures, but Fs computed at a standard temperature of 15°C (=start_CONVERSION_=Fs15=start_CONVERSION_=) varied by almost 4-fold among dates. =start_CONVERSION_=Fs15=start_CONVERSION_= was the highest after periods of stem growth and decreased as tree water availability decreased, similarly in C and E treatments. The decline in =start_CONVERSION_=Fs15=start_CONVERSION_= was not linked to a depletion of soluble sugars, which increased when water stress was higher. The proportion of ecosystem respiration attributed to the stems was highest following stem growth (23.3%) and lowest during the peak of drought (6.5%). High within-year variability in =start_CONVERSION_=Fs15=start_CONVERSION_= makes unadvisable to pool annual data of Fs vs. temperature to model Fs at short time scales (hours to months) in Mediterranean-type forest ecosystems. We demonstrate that water availability is an important factor governing stem CO2 efflux and suggest that trees in Mediterranean environments acclimate to seasonal drought by reducing stem respiration. Stem respiratory rates do not seem to change after a long-term increase in drought intensity, however. [ABSTRACT FROM AUTHOR]

Previous studies show that Masson pine ( Pinus massoniana L.) stands grown at the industrially-polluted site have experienced unprecedented growth decline, but the causal mechanisms are poorly understood. In this study, to understand the mechanisms of growth decline of Mason pine strands under pollution stresses, we determined the reactive oxygen species levels and chemical composition of the current-year (C) and one-year-old (C + 1) needles, and calculated the needle construction costs (CC) of Masson pine trees grown at an industrially-polluted site and an unpolluted remote site. Pine trees grown at the polluted site had significantly higher levels of hydroxyl radical and superoxide anion in their needles than those grown at the unpolluted site, and the former trees eventually exhibited needle early senescence. The contents of lipids, soluble phenolics and lignins in C and C + 1 needles were significantly higher at the polluted site than at the unpolluted site, but the total amounts of non-construction carbohydrates were lower in non-polluted needles than in polluted needles. Elevated levels of the reactive oxygen species and early senescence in polluted needles together led to significant increases in CC and a longer payback time. We infer that the lengthened payback time and needle early senescence under pollution stress may reduce the Masson pine tree growth and consequently accelerate tree decline. [ABSTRACT FROM AUTHOR]