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Dendroclimatic reconstructions, which are a well-known tool for extending records of climatic variability, have recently been expanded by using wood anatomical parameters. However, the relationships between wood cellular structures and large-scale climatic patterns, such as El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), are still not completely understood, hindering the potential for wood anatomy as a paleoclimatic proxy. To better understand the teleconnection between regional and local climate processes in the western United States, our main objective was to assess the value of these emerging tree-ring parameters for reconstructing climate dynamics. Using Confocal Laser Scanning Microscopy, we measured cell lumen diameter and cell wall thickness (CWT) for the period 1966 to 2015 in five Douglas-firs [Pseudotsuga menziesii (Mirb.) Franco] from two sites in eastern Arizona (United States). Dendroclimatic analysis was performed using chronologies developed for 10 equally distributed sectors of the ring and daily climatic records to identify the strongest climatic signal for each sector. We found that lumen diameter in the first ring sector was sensitive to previous fall–winter temperature (September 25th to January 23rd), while a precipitation signal (October 27th to February 13th) persisted for the entire first half of the ring. The lack of synchronous patterns between trees for CWT prevented conducting meaningful climate-response analysis for that anatomical parameter. Time series of lumen diameter showed an anti-phase relationship with the Southern Oscillation Index (a proxy for ENSO) at 10 to 14year periodicity and particularly in 1980–2005, suggesting that chronologies of wood anatomical parameters respond to multidecadal variability of regional climatic modes. Our findings demonstrate the potential of cell structural characteristics of southwestern United States conifers for reconstructing past climatic variability, while also improving our understanding of how large-scale ocean–atmosphere interactions impact local hydroclimatic patterns.

A number of studies have suggested that growing season length determines carbon sequestration of forest ecosystems. Given the possibility that drought-induced growth decline will be caused by a prolonged growing season under a warming climate, we investigated the effect of growth rate and duration of xylem production on annual wood biomass in drought-prone environments. We analyzed the intra-annual dynamics of wood formation in Qilian junipers (Juniperus przewalskii) from the semi-arid north-eastern Tibetan Plateau, China (2009–2014) and in ponderosa pine (Pinus ponderosa) from the hyperarid Mojave Desert in Nevada, USA (2015–2016). Most variability in the number of xylem cells (Ncell) was related to growth rate (rm) rather than duration of cell production (Dcell). At the Tibetan sites, 69.9% and 54.7% of variability in Ncell was attributable to rm for the lower and upper treeline, respectively. Within the Mojave Desert site, 53.9% of the variability in Ncell was related to rm. The growth rate in the Tibetan Plateau forest is affected by minimum temperature and precipitation. Thus, rm is a primary control on wood biomass in conifer species of semi-arid forests. Under warmer and drier conditions, a longer growing season will not benefit xylem formation of conifers, and in turn warming-induced drought could limit carbon sequestration by reducing the rate of cell production.

In this study, a comprehensive analysis of deadwood was conducted in four macro-areas located in two beech forests of public utility in Enciso (La Rioja, Spain). Dendrometric data, as well as qualitative and quantitative characteristics of deadwood, were collected and analysed with respect to the degree of accessibility to the forest to determine the effect of different levels of forest accessibility on deadwood volume and carbon stocks. All decomposition classes were present except the first, highlighting the development of natural degradation dynamics. Deadwood stored 6.9 t/ha of C in the easy accessibility class, 5.7 t/ha of C in the medium accessibility class and 2.2 t/ha of C in the difficult accessibility class. The average volume of deadwood and carbon stored calculated in this study were higher than the values reported in the Spanish and Italian national forest inventories, including one developed for Riojan beech forests. Deadwood volume was on average 22.5 m3/ha, showing an unequal distribution, with the lowest values found far from the access roads, despite forest accessibility generally being considered a factor that facilitates the human collection of deadwood. The distribution patterns of deadwood in beech forests of La Rioja, apparently counterintuitive, were due to a combination of different factors, including slope, cattle grazing, and weather conditions which might have favoured downward movement of the deadwood This research was funded by the Spanish Ministry of Science, Innovation and Universities, AGL2016-76769-C2-2-R SI

Significance Forest trees can live for hundreds to thousands of years, and they play a critical role in mitigating global warming by fixing approximately 15% of anthropogenic CO2 emissions annually by wood formation. However, the environmental factors triggering wood formation onset in springtime and the cellular mechanisms underlying this onset remain poorly understood, since wood forms beneath the bark and is difficult to monitor. We report that the onset of wood formation in Northern Hemisphere conifers is driven primarily by photoperiod and mean annual temperature. Understanding the unique relationships between exogenous factors and wood formation could aid in predicting how forest ecosystems respond and adapt to climate warming, while improving the assessment of long-term and high-resolution observations of global biogeochemical cycles., Wood formation consumes around 15% of the anthropogenic CO2 emissions per year and plays a critical role in long-term sequestration of carbon on Earth. However, the exogenous factors driving wood formation onset and the underlying cellular mechanisms are still poorly understood and quantified, and this hampers an effective assessment of terrestrial forest productivity and carbon budget under global warming. Here, we used an extensive collection of unique datasets of weekly xylem tissue formation (wood formation) from 21 coniferous species across the Northern Hemisphere (latitudes 23 to 67°N) to present a quantitative demonstration that the onset of wood formation in Northern Hemisphere conifers is primarily driven by photoperiod and mean annual temperature (MAT), and only secondarily by spring forcing, winter chilling, and moisture availability. Photoperiod interacts with MAT and plays the dominant role in regulating the onset of secondary meristem growth, contrary to its as-yet-unquantified role in affecting the springtime phenology of primary meristems. The unique relationships between exogenous factors and wood formation could help to predict how forest ecosystems respond and adapt to climate warming and could provide a better understanding of the feedback occurring between vegetation and climate that is mediated by phenology. Our study quantifies the role of major environmental drivers for incorporation into state-of-the-art Earth system models (ESMs), thereby providing an improved assessment of long-term and high-resolution observations of biogeochemical cycles across terrestrial biomes.

Future droughts are expected to become more severe and frequent under future climate change scenarios, likely causing widespread tree mortality in the western USA. Coping with an uncertain future requires an understanding of long-term ecosystem responses in areas where prolonged drought is projected to increase. Tree-ring records are ideally suited for this task. We developed 24 tree-ring chronologies from 20 U.S. Forest Service Forest Inventory and Analysis (FIA) plots in the southwestern USA. Climate variables were derived from the PRISM climate dataset (800-m grid cells) to capture the bimodal precipitation regime of winter snow and summer monsoonal rainfall, as well as warm-season vapor-pressure deficit (VPD) and winter minimum temperature. Based on mixed linear models, radial growth from 1948 to 2013 for four conifer species (Pinus edulis, Juniperus osteosperma, Pinus ponderosa, and Picea engelmannii) responded negatively to warm-season VPD and positively to cold-season precipitation. Pinus spp. benefited from warm-season precipitation linked to the North American monsoon, and Pinus spp. and J. osteosperma radial growth increased with warmer cold-season minimum temperature. However, warmer cold-season minimum temperatures countered the beneficial influence of cold-season precipitation for radial growth in Pinus spp. and J. osteosperma, while P. engelmannii was unaffected. Also, enhanced drying effects of warm-season VPD associated with decreased cold-season precipitation negatively affected radial growth of Pinus spp. and P. engelmannii. Of the four conifer species studied, Pinus spp. are most affected by droughts since 1948, while P. engelmannii and J. osteosperma appear to be more resilient. Investigating seasonal climate responses and interaction effects on radial growth in areas impacted by severe drought helps identify species that may be particularly at risk from climate change impacts in the Anthropocene.

Future seasonal dynamics of wood formation in hyperarid environments are still unclear. Although temperature-driven extension of the growing season and increased forest productivity are expected for boreal and temperate biomes under global warming, a similar trend remains questionable in water-limited regions. We monitored cambial activity in a montane stand of ponderosa pine (Pinus ponderosa) from the Mojave Desert for 2 consecutive years (2015-2016) showing opposite-sign anomalies between warm- and cold-season precipitation. After the wet winter/spring of 2016, xylogenesis started 2 months earlier compared to 2015, characterized by abundant monsoonal (July-August) rainfall and hyperarid spring. Tree size did not influence the onset and ending of wood formation, highlighting a predominant climatic control over xylem phenological processes. Moisture conditions in the previous month, in particular soil water content and dew point, were the main drivers of cambial phenology. Latewood formation started roughly at the same time in both years; however, monsoonal precipitation triggered the formation of more false rings and density fluctuations in 2015. Because of uncertainties in future precipitation patterns simulated by global change models for the Southwestern United States, the dependency of P. ponderosa on seasonal moisture implies a greater conservation challenge than for species that respond mostly to temperature conditions.

Quantitative wood anatomy represents a tool to investigate paleoclimatic signals on a seasonal to sub-seasonal basis.

The seasonal extent of the North American Monsoon (NAM) is highly variable and potentially sensitive to future climate change. Our objective was to determine how regional monsoonal patterns influence mountain precipitation near the NAM northwest boundary. Among the data we analyzed, a unique opportunity was provided by hourly observations collected on the Sheep Range (2300 m asl), in the Mojave Desert of southern Nevada, during 2011&ndash, 2017. Long-term 800-m Parameter-elevation Relationships on Independent Slopes Model (PRISM) precipitation time series showed that the site is representative of mountain areas in the NAM northwest region. Based on in situ observations, we divided the water year into three seasons: cool (1 October through 31 March), early warm (1 April through last day with dewpoint &lt, 9.4 &deg, C), and late warm (first day with dewpoint &ge, C through 30 September). Dewpoint temperature differed by about 8 &deg, C between early warm season (mean of &minus, 6.3 &deg, C) and late warm season (mean of 2.3 &deg, C). According to ANCOVA model results, increasing hourly dewpoint associated with afternoon thunderstorms in the late warm season had the greatest relationship with hourly precipitation (F-value = 237.8, p-value &lt, 0.01). Except for 2016, more precipitation fell at our study site during the late than the early warm season. Late warm season precipitation contributed the most (43&ndash, 56%) to total water-year precipitation during the 2012&ndash, 2015 extended drought. Southwestern USA regional composites of vertically integrated water vapor transport (IVT) suggested that water vapor in the cool and early warm season originated from the Pacific Ocean to the west, while a transition to a NAM-like pattern of northward IVT coincided with the late warm season.

Summary Old-growth studies commonly emphasize structural and age conditions, selecting proxy indicators of long-term ecological processes. Transition dynamics from mature to old-growth status reveal how natural legacies are progressively accumulated in forests after major disturbances, including human ones. In late-successional, multi-aged forests, the chronosequential ranking of developmental stages is a difficult task, as stand age provides little information, and time since last stand-replacing disturbance cannot be easily determined. Canopy age features, disturbance/suppression history, and growth trajectories were reconstructed from ring-width series of canopy trees in a network of 19 old-growth and managed European beech forests in the eastern Alps and central Apennines. A set of tree-ring metrics able to describe the intensity and time distribution of biological and ecological processes (e.g. understorey suppression, canopy accession age), were used to describe the advancement of old-growth status, and compared to established metrics of forest structure. Tree-ring metrics were site dependent, as biogeoclimate affects turnover rates and constrains the onset and recovery rate of old-growth attributes. Under the same environmental conditions (high-mountain, limestone-bedrock beech forests), values of the best indicators (number/duration of growth suppression phases; synchronicity of first release; maximum and range of canopy tree age; canopy accession age of the slowest-growing trees) increased monotonically (2 to 5 times) from managed to secondary and primary old-growth forests. Trees in well-conserved primary old-growth forests experienced several and long suppressions, showing the highest complexity in recruitment history, canopy accession, and growth trajectories. The best tree-ring metrics, condensed in a Naturalness Score to provide a synthetic functional ranking of forests, varied coherently with structural complexity, which represented stand dynamics more closely than biomass-related metrics. Synthesis and applications. We propose a synthetic ranking of forest functional naturalness based on the ecological processes experienced by trees. This ranking helps to overcome the limitations associated with the use of arbitrary size- or age-related thresholds of old-growth status and provides a functional approach to establish chronosequences in ecological studies. The quantitative description of complex processes underpinning the unique biological and ecological features (e.g. extreme tree longevity) found in primary old-growth forests enhances their irreplaceable value in nature conservation. The proposed framework of tree-ring indicators describes functional traits tightly related to forest naturalness, and may thus become a tool to identify and protect old-growth forests, benchmark the impact of silvicultural practices, prescribe targets or evaluate the effectiveness of restoration programs. This article is protected by copyright. All rights reserved.

Anatomical features of Pinus flexilis under warmer and drier conditions along an altitudinal transect revealed a shorter growing season and shifts in the timing of wood formation. Future climate change driven by greenhouse warming is expected to increase both frequency and severity of drought events and heat waves. Possible consequences for forest ecosystems include changes in foundation species and extended die-off phenomena. We investigated tree growth under the set of biotic and abiotic conditions, and their interactions, that are expected in a drier and warmer world using mountain observatories designed to capture elevation gradients in the Great Basin of North America. Stem cambial activity, wood anatomy, and radial growth of limber pine (Pinus flexilis) were examined at two different elevations using automated dendrometers and repeated histological microcores in 2013–2014. Mean annual temperature was 3.7° cooler at the higher site, which received 170 mm year−1 of precipitation more than the lower site. Mean air temperature thresholds for xylogenesis computed using logistic regression were 7.7 and 12.0 °C at the higher and lower site, respectively. No differences in the onset date of cambial activity were found under such naturally contrasted conditions, with the global change analog provided by the lower site. Growing season was shortened by increasing drought stress at the lower site, thereby reducing xylem production. Stem expansion was only detectable by automated dendrometers at the higher site. Using elevation to simulate climatic changes and their realized ecosystem feedbacks, it was possible to express tree responses in terms of xylem phenology and anatomical adaptations.

The annual cycle of precipitation in the southwestern United States is characterized by a bimodal distribution. El Nino-Southern Oscillation (ENSO) episodes influence cool-season precipitation, while the North American Monsoon (NAM) brings warm-season rainfall to the region. Because the covariability of the two seasons may result in both wet and dry water-year anomalies, we evaluated the long-term contribution of cool and warm-season precipitation to water-year totals using newly collected and/or updated tree-ring chronologies. Precipitation reconstructions back to the late 17th century, from 1663 to 2015, were produced using a total of 183 stem increment cores collected from seven ponderosa pine (Pinus ponderosa) stands located along the southern edge of the Colorado Plateau in Arizona and New Mexico. Latewood chronologies were most correlated with July–August rainfall brought by the North American Monsoon, while earlywood chronologies were related to November–February precipitation, also matching the time-series variability of the Southern Oscillation Index. Total ring-width chronologies reflected water-year precipitation, and were used to account for precipitation falling outside of either the cool or the warm season. While seasonal variability of precipitation was the main driver of regional climatic extremes, the amount of precipitation falling outside the cool and warm seasons did not play a consistent role in determining frequency and severity of droughts and pluvials. This study highlights how intra-annual paleoclimatic reconstructions help with understanding the contribution of seasonal climate to wet and dry episodes influenced by large-scale climatic modes.

Xylem structure and cambial phenology (i.e. onset and cessation of cambial cell division) of conifers growing under severe water-limitations can change dramatically in relation to moisture availability. In hyperarid environments, analytical tools commonly used to investigate intra-annual variability of xylem anatomy (i.e. tracheidograms), may fail to capture the complexity of tree phenological responses to environmental conditions. This greatly limits our ability to accurately date the onset of intra-annual density variations, including the transition between earlywood and latewood. I present a new approach for developing phenological tracheidograms (“pheno-tracheidograms”) calibrated to account for the seasonal variations in cell division rates. Pheno-tracheidograms were developed for a population of Pinus ponderosa in the Mojave Desert (Nevada, USA) during the period 2015–2016 in order 1) to determine the onset date of latewood formation and 2) to investigate relationships between environmental conditions, lumen diameter, and cell wall thickness targeting specific climatic windows for each tracheid. Pheno-tracheidograms were standardized at the tree-level, showing more flexibility compared to tracheidograms standardized according to a pre-determined number of cells. By displaying cellular parameters with respect to the date of formation of the tracheid to which each measurement is associated, pheno-tracheidograms allowed to determine the beginning of latewood formation with daily resolution. Lumen diameter was significantly correlated with the onset date of cellular enlargement, while cell wall thickness showed a weaker relationship with the beginning of secondary wall deposition. Soil moisture positively affected the duration of cell enlargement and tracheid lumen diameter, particularly in the earlywood, while cell wall thickness was not significantly influenced by environmental conditions. Pheno-tracheidograms represent an empirical, yet effective way to date intra-annual xylem structures and to investigate high-resolution climate-anatomy relationships in conifer species from arid environments characterized by high phenological plasticity.

Future seasonal dynamics of wood formation in hyperarid environments are still unclear. Although temperature-driven extension of the growing season and increased forest productivity are expected for boreal and temperate biomes under global warming, a similar trend remains questionable in water-limited regions. We monitored cambial activity in a montane stand of ponderosa pine (Pinus ponderosa) from the Mojave Desert for 2 consecutive years (2015-2016) showing opposite-sign anomalies between warm- and cold-season precipitation. After the wet winter/spring of 2016, xylogenesis started 2 months earlier compared to 2015, characterized by abundant monsoonal (July-August) rainfall and hyperarid spring. Tree size did not influence the onset and ending of wood formation, highlighting a predominant climatic control over xylem phenological processes. Moisture conditions in the previous month, in particular soil water content and dew point, were the main drivers of cambial phenology. Latewood formation started roughly at the same time in both years; however, monsoonal precipitation triggered the formation of more false rings and density fluctuations in 2015. Because of uncertainties in future precipitation patterns simulated by global change models for the Southwestern United States, the dependency of P. ponderosa on seasonal moisture implies a greater conservation challenge than for species that respond mostly to temperature conditions.

Mountain conifers in the Great Basin of North America have provided some of the longest, continuous, and annually resolved paleoclimate records. Climate-growth relationships at the cellular level, which help understand wood formation processes that underlie dendroclimatic reconstructions, are at present largely unexplored in the Great Basin. We analyzed 42 trees located in the Snake Range (eastern Nevada, USA) at three sites along an elevation gradient. Sampled species included white fir (Abies concolor), Douglas fir (Pseudotsuga menziesii), limber pine (Pinus flexilis), bristlecone pine (Pinus longaeva), and Engelmann spruce (Picea engelmannii). Wood anatomical features were quantified for two consecutive years, 2011 and 2012. Lumen area, cell wall thickness, lumen diameter, and wall-to-cell ratio were measured for the total ring as well as for earlywood and latewood. Mean standardized tracheidograms highlighted differences between 2011 and 2012, in particular concerning lumen area and wall-to-cell ratio. Most annual variation was due to earlywood, rather than latewood. Anatomical parameters of limber pine, the only species that could be tested at both the montane and subalpine sites, varied with elevation. Principal component analysis showed that the main axis of variability was related to dimensional parameters (e.g. lumen area), which reflected differences in water availability.

Identifying tree growth releases is crucial to understand forest stand dynamics, their drivers (including climate change), and their interactions with other long-term ecosystem processes. We tested how bioclimate and geologic substrate affect the application of boundary line release criteria, which are based on scaling percent growth change to prior growth rate on a large number of samples, and then using a nonlinear function (e.g., negative exponential) to draw the maximum release potential (“boundary line”). Tree-ring series from 36 European beech ( Fagus sylvatica L.) forests, comprising more than 130 thousand observations and spanning a range of elevations in the Italian Alps (200–1500 m) and Apennines (400–1900 m), were used to develop boundary lines for two geographical regions ( Alps and Apennines ) and three bioclimatic zones ( high-mountain , mountain , low-elevation ) within each region. In the Apennines, two distinct boundary lines were also developed for beech stands growing on sedimentary and volcanic soils. We uncovered different amounts of maximal growth releases for equal levels of prior growth when comparing the northern (central Europe) and southern (Italy) portion of the species’ geographic distribution. High elevation sites in the Apennines and the Alps had similar boundary lines, which in both cases remained well below the regional ones because of distinctly lower growth releases. A limiting effect of site fertility on growth releases was uncovered in the Alps for old-growth stands growing on poor soils (e.g. dolomite). To quantify the influence of bioclimate on growth releases we produced disturbance chronologies for the two oldest stands pertaining to the same bioclimatic zone according to regional and bioclimatic boundary lines. Moderate and major growth releases were severely underestimated when the regional boundary line was used rather than the one specifically computed for the bioclimatic zone. Since boundary lines were affected by biogeoclimate, reconstructing disturbance events from growth releases requires detailed analysis of tree growth records according to site conditions with special attention to extreme environments.

Tree size distributions of old-growth forests are a fundamental tool for both scientific analysis and conservation management. In old-growth forests the diameter distribution shape may depend on spatial scale, but theoretical models often show fundamental similarities that suggest general underlying mechanisms controlling regeneration, mortality, and growth under different disturbance regimes. In this study we investigated the horizontal structure of two old-growth beech (Fagus sylvatica L.) forests in central Italy using a series of spatial scales and analytical approaches. Individual plots (each one 0.13 ha in size) were progressively aggregated into larger groups to simulate an increase in sampled area. A third-order polynomial was fit to the diameter distributions of the aggregated areas, and the curves were ranked by the AIC method. The rotated sigmoid (RS) shape was the most common descriptor of tree size distributions, and became dominant once aggregated spatial scales exceeded 0.6–0.8 ha. The same result was obtained when a single plot was made progressively larger. Given the relatively small scale (starting from 0.1 ha) at which the RS model emerged, old-growth beech forests of the Italian Apennines show a complex, yet highly resilient structure, recognizable even in small (

This paper describes the experience of developing an educational trail inside an old-growth beech forest in central Italy. In the last 150 years local people exploited this stand almost exclusively for recreational purposes and now the beechwood has acquired new ecological value after having reached old-growth status. Despite the cultural-historical importance of the site, there is a substantial lack of knowledge about this magnificent stand among members of the public, who have no tools to understand its dynamics nor to accept those structural changes related to its natural evolution. Tourist facilities were restored at the site, according to non-restricting criteria, and an educational program was developed in order to promote a shared comprehension of old-growth forests, their ecological processes and value, the services they provide, and to increase tourist awareness toward this fragile ecosystem. We discuss the educational role ecotourism might potentially hold in western developed countries to link the public with the scientific world, educating people about nature, while preventing negative impacts on natural environments. Even if the importance of local communities in conservation of natural resources has been widely recognized by a number of conservationists, public involvement, particularly in forest conservation and management, still represents a difficult challenge, since it requires the implementation of a common understanding about forest ecosystems, forest goods and forest services. The educational trail inside the beech forest of Monte Cimino may represent a low impact strategy for preserving forest aesthetic functionality, limiting potential damages to the forest while increasing its importance for nature conservation.

The timing of wood formation is crucial to determine how environmental factors affect tree growth. The long-lived bristlecone pine (Pinus longaeva D. K. Bailey) is a foundation treeline species in the Great Basin of North America reaching stem ages of about 5000 years. We investigated stem cambial phenology and radial size variability to quantify the relative influence of environmental variables on bristlecone pine growth. Repeated cellular measurements and half-hourly dendrometer records were obtained during 2013 and 2014 for two high-elevation stands included in the Nevada Climate-ecohydrological Assessment Network. Daily time series of stem radial variations showed rehydration and expansion starting in late April-early May, prior to the onset of wood formation at breast height. Formation of new xylem started in June and lasted until mid-September. There were no differences in phenological timing between the two stands, or in the air and soil temperature thresholds for the onset of xylogenesis. A multiple logistic regression model highlighted a separate effect of air and soil temperature on xylogenesis, the relevance of which was modulated by the interaction with vapor pressure and soil water content. While air temperature plays a key role in cambial resumption after winter dormancy, soil thermal conditions coupled with snowpack dynamics also influence the onset of wood formation by regulating plant-soil water exchanges. Our results help build a physiological understanding of climate-growth relationships in P. longaeva, the importance of which for dendroclimatic reconstructions can hardly be overstated. In addition, environmental drivers of xylogenesis at the treeline ecotone, by controlling the growth of dominant species, ultimately determine ecosystem responses to climatic change.

The impacts of climate extremes on terrestrial ecosystems are poorly understood but important for predicting carbon cycle feedbacks to climate change. Coupled climate-carbon cycle models typically assume that vegetation recovery from extreme drought is immediate and complete, which conflicts with the understanding of basic plant physiology. We examined the recovery of stem growth in trees after severe drought at 1338 forest sites across the globe, comprising 49,339 site-years, and compared the results with simulated recovery in climate-vegetation models.We found pervasive and substantial >legacy effects> of reduced growth and incomplete recovery for 1 to 4 years after severe drought. Legacy effects were most prevalent in dry ecosystems, among Pinaceae, and among species with low hydraulic safety margins. In contrast, limited or no legacy effects after drought were simulated by current climate-vegetation models. Our results highlight hysteresis in ecosystem-level carbon cycling and delayed recovery from climate extremes.

Premise of the study: A network of mountain observing stations has been installed in the Great Basin of North America. NevCAN (Nevada Climate-ecohydrological Assessment Network), which spans a latitudinal range of 2.5° and two elevation ranges of about 2000 m each, enabled us to investigate tree growth in relation to climate. Methods: We analyzed wood anatomy and tree-ring characteristics of four conifer species in response to different levels of water availability by comparing a low- and a high-elevation population. Chronologies of earlywood and latewood widths, as well as cellular parameters, were developed from the year 2000 to 2012. Results: At the southern (drier and warmer) sites, Pinus monophylla had smaller cell lumen, tracheid diameter, and cell wall thickness. Pinus monophylla and P. flexilis showed bigger cellular elements at the higher elevations, whereas the opposite pattern was found in Picea engelmannii and Pinus longaeva. When all species and sites were pooled together, stem diameter was positively related with earlywood anatomical parameters. Discussion: We have provided a glimpse of the applications that NevCAN, as a new scientific tool, could allow in the general field of botany. In particular, we were able to investigate how differences in water stress related to elevation lead to changes in xylem anatomy.

In the framework of the Project of National Interest (PRIN) “Climate change and forests - Dendroecological and ecophysiological responses, productivity and carbon balance on the Italian network of old-growth beech forests”, 9 old beech (Fagus sylvatica L.) forests were sampled in the eastern Alps and in the central Apennines to assess: i) the degree of ‘old-growthness’ on a structural and dendroecological basis; ii) the carbon (C) stocks in the different ecosystem compartments; iii) some ecophysiological traits using stable isotopes. Live and dead tree structure, soil features and C stocks were examined in some among the oldest and less disturbed beech forests in Italy. Furthermore, leaves, litter and wood cores were sampled for concurrent dendroecological, nutrient and stable isotope analysis, to study age structure, disturbance history and medium- and long-term response to climate and ecophysiological traits. In all sampled stands, values of basal area, volume and large trees density reached or exceeded control values reported for European and North American old-growth forests, while total amount of deadwood was generally low, except in two sites. Diameter distribution showed a remarkable differentiation from bimodal to ‘rotated-sigmoid’ curve. In accordance with structural analyses, disturbance chronologies revealed the importance of frequent low-to-moderate events in generating a fine-scale structure, confirming previous results on beech forests. Different degrees of ‘old-growthness’ were identified for Alpine and Apennine beech stands through the ‘structural-based approach’, which proved to be a valid tool for old-growth forests detection. Preliminary results on carbon stocks on a subset of sites indicate that these old-growth forests are relevant carbon reservoirs, with 192-268 MgC ha-1 of total biomass (67-73% aboveground; 27-33% belowground, 4 stands) and 7-21 MgC ha-1 of deadwood. In these stands, also forest floor (excluding deadwood) and soils are stores of relevant amount of carbon (5-9 MgC ha-1 litter layer, 4 stands; 168-420 MgC ha-1 mineral soil, 3 stands). Finally, carbon isotope discrimination (Δ) analysis, a proxy of wateruse efficiency, was applied on tree-ring cores from a subset of sites. The perspectives of this technique for a retrospective ecophysiological interpretation of climate-change impact on old-growth forests were described. Preliminary results obtained within this PRIN project indicate that the reported ecological indicators can be used to produce a description of forest structures and processes driving stand dynamics (‘structural-based approach’). Furthermore, the use of multiple sampling and research techniques and the integration of research groups with complementary expertise can foster deeper understanding of the ecology and dynamics of old-growth forests. L'articolo è disponibile sul sito dell'editore http://www.aisf.it/