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Ongoing climatic change is threatening the survival of drought-sensitive tree species, such as silver fir (Abies alba). Drought-induced dieback had been previously explored in this conifer, although the role played by tree-level genetic diversity and its relationship with growth patterns and soil microsite conditions remained elusive. We used double digest restriction-site-associated DNA sequencing (ddRADseq) to describe different genetic characteristics of five silver fir forests in the Spanish Pyrenees, including declining and non-declining trees. Single nucleotide polymorphisms (SNPs) were used to investigate the relationships between genetics, dieback, intraspecific trait variation (functional dendrophenotypic traits and leaf traits), local bioclimatic conditions, and rhizosphere soil properties. While there were no noticeable genetic differences between declining and non-declining trees, genome–environment associations with selection signatures were abundant, suggesting a strong influence of climate, soil physicochemical properties, and soil microbial diversity on local adaptation. These results provide novel insights into how genetics and diverse environmental factors are interrelated and highlight the need to incorporate genetic data into silver fir forest dieback studies to gain a better understanding of local adaptation.

Some tree species have shown to be very vulnerable to drought and heat waves in the Mediterranean Basin, causing a loss of important socio-economic and ecosystem forest services. In this regard, oaks are important but vulnerable species which are showing losses in terms of productivity and growth and rising mortality rates. Dendroecological studies using retrospective analysis of wood anatomical traits and tree rings have demonstrated their potential to assess long-term patterns of growth and vigor in several Mediterranean oak species. Moreover, the long-term reconstruction of wood anatomical traits such as transversal lumen area, allows investigating hydraulic adjustments of trees through time.In this study, we reconstructed changes in wood anatomy for a 38-year long period (1980-2017) to investigate how drought impacted the hydraulic functionality and triggered dieback in five ring-porous oak species from Italy and Spain (Quercus robur, Quercus frainetto, Quercus cerris, Quercus canariensis, Quercus pubescens). We compared non-decaying (ND) and decaying (D) coexisting trees of each species showing low and high defoliation levels, respectively. We analyzed earlywood anatomical traits (vessel area, hydraulic diameter, vessel density, theoretical hydraulic conductivity, etc.) in these species and analysed them considering a ranking of increasing drought tolerance: Q. robur, Q. frainetto, Q. cerris, Q. canariensis, and Q. pubescens.We observed differing growth patterns and xylem conduit area responses in D trees compared with ND trees. The D trees formed narrower EW vessels than ND and the Dh were lower in D trees compared with ND trees. We discuss the relationships between radial growth, changes in wood anatomy and hydraulic functioning of trees focusing on those proved more sensitive to growth decline and mortality in order to highlight the climatic triggers of dieback in ring-porous oak species as related to hydraulic failure.

21 páginas, figuras, 4 tablas, Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ18 O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower δ18 O than ND trees. The uncoupling between iWUE and δ18 O, together with the positive correlations between P and K leaf concentrations and δ18 O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies., This study was financially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) via competitive grant RTI2018-096884-B-C31 (FORMAL project), RTI2018-096884-B-C33 (LESENS project) and by the Government of Aragón via competitive grant LMP242_18. AG acknowledges financial support from the Spanish Ministry of Science and the AEI through the Ramón y Cajal grant (RyC2020- 030647-I). JIQ acknowledges the financial support from Spanish Ministry of Science through the project (PID2019-107382RB-I00). JMI acknowledges financial support from the Project “CLU-2019-05—IRNASA/CSIC Unit of Excellence”, funded by the Junta de Castilla y León and cofinanced by the European Union (ERDF “Europe drives our growth”). RSS and JCL acknowledge the financial support from the Andalusian government (P20_00813 VUERCLIM), (IE19_074 UPO DendroOlavide) and (UPO-1263216 VULBOS), and from the Spanish Ministry of Science trough the project (EQC2018-005303-P)

The Mediterranean region is projected to experience severe drying trends and more extreme hydroclimate events as a consequence of anthropogenic climate change over the next century. In some places this signal may have already emerged from natural variability. Here we provide context for recent and future changes with a new high-resolution (0.5$^o$) spatial reconstruction of the Palmer Drought Severity Index (PDSI) using a tree-ring network that spans much of the last millennium. This network provides new information that extends and enhances the existing Old World Drought Atlas (OWDA), and we identify and attribute the differences between the OWDA and our reconstruction. We also reexamine previous conclusions about the severity of recent drought in the context of earlier centuries. We find that, in both the western Mediterranean and the Levant, recent dry periods are indeed the worst in at least the last 500 years, but our assessment of the significance and uncertainty of this finding is affected by differences in the tree-ring networks used for the reconstructions. Long millennium-length hydroclimate reconstructions in the Mediterranean provide the opportunity to understand variability and trends in the hydroclimate of the region, while extant uncertainties arising from the existing tree-ring chronology network and methodological choices call attention to locations that require further proxy collection and statistical scrutiny.

Tree phenology is sensitive to climate warming and changes in seasonal precipitation. Long xylogenesis records are scarce, thus limiting our ability to analyse how radial growth responds to climate variability. Alternatively, process-based growth models can be used to simulate intra-annual growth dynamics and to better understand why growth bimodality varies along temperature and precipitation gradients. We used the Vaganov-Shashkin (VS) growth model to analyse the main climatic drivers of growth bimodality in eight trees and shrubs conifers (four pines and four junipers) across Spain. We selected eleven sites with different continentality degree and spring/autumn precipitation ratios since we expected to find pronounced bimodal growth in less continental sites with spring and autumn precipitation peaks. The VS model successfully simulated annual growth rates at all sites as a function of daily temperature and soil moisture data. Bimodal growth patterns clustered into less continental sites showing low spring/autumn precipitation ratios. This finding agrees with observed climate-growth associations showing that growth was enhanced by wet-cool winter-to-spring conditions, but also by wet autumn conditions in the most bimodal sites. We observed a stronger growth bimodality in pines compared to junipers. We discuss the spatial variability of climate drivers in bimodality growth pattern and how increasing continentality and shifts in seasonal precipitation could affect growth patterns. Bimodality could be an advantageous response to overcome summer drought in Mediterranean forests. The ability of some species to reactivate growth during autumn might determine their capacity to withstand increasing summer aridity. This work was supported by project RTI2018–096884-B-C31 and by FPI grant (ref. PRE2019–089800) to CV from the Spanish Ministry of Science. RSS was supported by DendrOlavide I (EQC2018–005303-P), Ministry of Science, Innovation and Universities, Spain; DendrOlavide II (IE19_074 UPO), VURECLIM (P20_00813) and VULBOS (UPO-1263216) Junta de Andalucía, Fondos Europeos de Desarrollo Regional. JT received the support from Charles University (UNCE/HUM 018). VS was grateful for the support of RSF project (# 22–14–00048).

Forest plantations with exotic species in the northwestern Peruvian Andes have brought different ecosystem benefits. The wood productivity in this Páramo region is directly related to the great availability of water from abundant rainfall compared to other regions of the semi-arid Andes. To address the lack of information on forest inventories of plantations in the Páramo region, this study used annual growth rings (dendrochronology) to build new models of tree growth and wood productivity and compared 22-year-old Pinus patula plots with thinning and unthinning treatments. Our results show that late thinning, removing 63% of stem density in the 15th year, does not have significant effects on the diameter increase or stand-level productivity. For these plantations, we propose a management rotation of 21 years with a first thinning treatment (35%) at 5 years and a second thinning treatment (50%) at 12 years. Production at 21 years is expected to be between 194.6 m3 ha−1 and 504.6 m3 ha−1 for stands with low and high wood productivity, respectively. Tree-ring studies are potentially useful for monitoring forest plantations and provide an alternative method for forest managers who use allometric equations to predict silvicultural treatments and to propose management guides for plantations.

Identificador de proyecto: Marie Sklodowska-Curie grant agreement No 101029581, Long-term records of tree-ring width (TRW), latewood maximum density (MXD) and blue intensity (BI) measurements on conifers have been largely used to develop high-resolution temperature reconstructions in cool temperate forests. However, the potential of latewood blue intensity (LWBI), less commonly used earlywood blue intensity (EWBI), and delta (difference between EWBI and LWBI, dBI) blue intensity in Mediterranean tree species is still unexplored. Here we developed BI chronologies in moist-elevation limits of the most southwestern European distribution of Pinus nigra subsp. salzmanii Arnold. We tested whether BI variables derived from tree rings of black pine are better proxies than ring-width variables to reconstruct long-term changes in climatic factors and water availability. For this we applied correlations and regression analyses with daily and monthly climate data, a spatial and temporal drought index (Standardized Precipitation-Evapotranspiration Index-SPEI) and Vapour Pressure Deficit (VPD), as well as atmospheric circulation patterns: North Atlantic Oscillation (NAO), Southern Oscillation Index (SOI) and Western Mediterranean Oscillation (WeMO). We found a positive relation between black pine growth (RW) and temperature during the winter preceding the growing season. Among all variables LWBI and dBI were found to be more sensitive than TRW to SPEI at low-elevation site, with EWBI series containing an opposite climatic signal. LWBI and dBI were significantly related to June and September precipitation at high-elevation site. Winter VPD was related with higher EWI and LWI series, whereas dBI and EWBI were related with January SOI and February NAO. We confirm the potential of long-term dBI series to reconstruct climate in drought-prone regions. This novel study in combination with other wood anatomical measurements has wide implications for further use of BI to understand and reconstruct environmental changes in Mediterranean conifer forests., Wageningen University & Research, Universidade de Santiago de Compostela, University of Amsterdam

Tree species inhabiting riparian forests under Mediterranean climate have evolved to face summer water shortage but may fail to cope with future increases in drought severity. Thus, understanding tree growth phenological variations in response to environmental conditions is necessary to assess the impact of seasonal drought in riparian forests. In this study, we investigated the response of stem radial growth to climate in the narrow-leaved ash (Fraxinus angustifolia) over its distribution in southern Europe. We simulated intra- and inter-annual growth patterns using the Vaganov-Shashkin (VS) model considering five sites subjected to summer drought but showing different climate conditions. The growth pattern in this species varied from unimodal in cool-wet sites to facultative bimodal in warm-dry sites. Bimodal patterns were characterized by two growth peaks coinciding with favorable climate conditions in spring and autumn. The spring growth peak occurs earlier (May) in warm-dry sites than in wet-cool sites (June–July). The variation in the season growth length and growth timing suggests different strategies adopted by this species to cope with summer drought. The VS model revealed different growth patterns across which would be relevant in predicting the response of this and other riparian tree species to climate warming and aridification. Differences in the length of the growing season, timings of growth peaks and the shift from unimodal to bimodal growth patterns should be considered when assessing growth adjustments to future climate scenarios.

Droughts chronically alter resource availability in forest ecosystems. The increased frequency and severity of such extreme climate events challenge the acclimation potential of tree species especially across the drought-prone Mediterranean region. Pinus nigra is a widely distributed tree species in the Mediterranean region and considered vulnerable to extreme droughts. We used a 1000 km latitudinal gradient from northern Morocco to north-eastern Spain incorporating four regions (Edge-South, Core-South, Core-North and Edge-North) and including different P. nigra provenances. We aim to identify the climate and forest structure related drivers that influence tree radial growth (BAI, basal area increment). We developed statistical models for BAI by incorporating the potential effects of climate and forest structure (diameter and age distributions). Then, we forecasted the future growth of P. nigra forests during the 21st century considering the emission scenario A2 with an expected increase of +2.7 °C at the end of this century. Our results showed large variability across P. nigra populations in terms of environmental conditions, forest structure, and growth. The northernmost P. nigra populations, subjected to wetter and cooler conditions were those presenting the lowest BAI (4.9 cm2), whereas the southernmost P. nigra populations subjected to drier and warmer conditions presented the highest BAI values (11.5 cm2). Pinus nigra growth was enhanced by high spring precipitation, but this positive effect was probably modulated by forest structure. Temperature explained a higher proportion of the BAI variance than precipitation, with warmer summer conditions decreasing growth. Growth projections forecasted a decline in BAI (from 9.6 to 7.0 cm2) across all ecological regions starting around the mid-21st century but being lower in Edge-North populations compared to the other populations. Our study provides quantitative knowledge related to how P. nigra populations have been growing across four distinctive ecological regions. We also provide a forecasting tool that incorporates both climate and stand structure related information to project dynamics in tree populations.

Riparian ash forests subjected to seasonal drought are among the most endangered ecosystems in Europe. They are threatened by climate warming causing aridification and by land-use changes modifying river flow. To assess the impacts of these two stress factors on riparian forests, we studied radial growth and xylem anatomical traits in five narrow-leaved ash (Fraxinus angustifolia) stands across wide climatic and ecological gradients from northern Italy to southern Portugal. Radial growth rates and earlywood hydraulic diameter (Dh) were directly correlated, whilst earlywood vessel density and growth rates were inversely associated. Ash growth positively responded to precipitation. Higher and lower rates of growth increase in response to precipitation were found in dry (annual precipitation 357–750 mm, annual water balance -39 to -48 mm) and wet (annual precipitation 1030 mm, annual water balance 27 mm) sites, respectively. Wet conditions in autumn and winter of the year prior to tree-ring formation lead to larger Dh values, except in the wet site where warmer conditions from prior autumn to current spring were positively associated to wider vessels. Growth was also enhanced by a higher river flow, reflecting higher soil moisture due to elevated groundwater table levels. Peaks in river flow from late winter to early spring increased Dh in dry-continental sites. Growth and potential hydraulic conductivity in drought-prone riparian ash forests are differently impacted by climate variability and river flow depending on site and hydrological conditions. Nevertheless, covariation between radial growth and the earlywood vessel diameter was found, regardless of site specific differences. Wood production and hydraulic conductivity are coordinated through the production of large earlywood vessels which may allow reaching higher growth rates. © 2021 The Authors