Aim
The Caucasus is a global biodiversity hotspot that includes a wide diversity of temperate forests, from xeric to mesic and rain forest. Little is known about their vulnerability to climate ...change. We aimed to identify the major climate constraints on tree growth.
Location
Western Caucasus of Georgia, Russia and Turkey (40–43° N, 41–43° E).
Time period
Twentieth century.
Major taxa studied
Trees, angiosperms and gymnosperms.
Methods
We used a new network of 35 tree‐ring width chronologies from four angiosperm and four gymnosperm species across an elevational gradient of > 2,000 m. We used correlations to identify the major climate factors (temperature, precipitation and drought) at monthly and seasonal scales affecting tree growth and to assess whether their effects change over time. To explore common response patterns among species, we used self‐organizing maps, a type of artificial neural network.
Results
Spring or summer drought reduced radial growth of most tree species, despite large differences in elevation. As expected, drought was particularly detrimental at warm, low‐elevation sites. Besides drought, growth of conifers at high elevations was also limited by cold winters and summers. Important species‐specific climate–growth responses were also evident. In general, climate–growth relationships were stable over time, except at some cold‐limited sites, where positive responses to summer and winter temperatures have diminished over the last few decades.
Main conclusion
Growth responses to precipitation and drought among species were more similar than they were to temperature, even at humid sites, providing further evidence of drought vulnerability in mesic forests. The productivity of high‐elevation conifer forests, limited by summer drought and low temperatures, will depend on the balance between temperature and precipitation. Given that climate change is expected to induce larger climatic gradients in the region, the potential reduction of forest cover at a regional scale would make the conservation of these mesic forests more essential.
Climate and disturbance alter forest dynamics, from individual trees to biomes and from years to millennia, leaving legacies that vary with local, meso and macroscales. Motivated by recent insights ...in temperate forests, we argue that temporal and spatial extents equivalent to that of the underlying drivers are necessary to characterize forest dynamics across scales. We focus specifically on characterizing mesoscale forest dynamics because they bridge fine‐scale (local) processes and the continental scale (macrosystems) in ways that are highly relevant for climate change science and ecosystem management. We revisit ecological concepts related to spatial and temporal scales and discuss approaches to gain a better understanding of climate–forest dynamics across scales.
Forest decline under environmental stress is expressed by regeneration failure and accelerated mortality in all ontogenic stages at the population level. Characterizing functional traits and ...mechanisms that best capture species decline and mortality is essential to assess forest dynamics.
We analysed sensitivity to increasing water stress in two species with different water‐use strategies on a mixed Quercus pyrenaica–Pinus sylvestris forest where adult pines express vulnerability to climate change but oaks do not. We compared the dynamics of radial growth, wood δ13C and sapwood non‐structural carbohydrates (NSCs) in response to drought at different time‐scales in both species and two age cohorts in pine.
Both species were very sensitive to water stress, which influenced trait phenotypic plasticity at short‐ and long time‐scales. Water‐use strategy in pines of both ages was more conservative than in the more drought‐tolerant oak. Both species showed negative growth trends despite increasing intrinsic water‐use efficiency. Recent growth of pines is slower than it was in the past. Carbon isotope discrimination trends in young pines suggested increasing leaf gas exchange constraints. NSCs were far from depletion in both species and all pine ages. Intra‐ and inter‐annual NSC variability was higher in oaks than in pines and in soluble sugars (SS) than in starch. SS were lowest in young pines. Sensitivity of NSCs to contrasting climatic years was low in pines, and NSC levels mostly remained homeostatic for this species. The sensitivity to climate expressed suggests different C allocation strategies, with less coupling between radial growth and current‐year photosynthesis in young pines.
Synthesis. Pines expressed negative responses to increased water stress regardless of age, showing rising gas exchange constraints through tighter stomatal control of water losses than oaks. Young pines showed similar functional responses to water stress than old pines in decline, which suggests species‐level vulnerability and could be regarded as early warning signals anticipating mortality in pines. Yet, given the high sensitivity to drought also expressed by the non‐declining oak, it would have been difficult to unequivocally disentangle species decline based only on the functional traits analysed.
Resumen
La ausencia de regeneración y la mortalidad acelerada por estrés ambiental en todas las escalas ontogénicas y a nivel poblacional indican decaimiento forestal.
En este trabajo analizamos la sensibilidad al aumento del estrés hídrico en un bosque mixto de Quercus pyrenaica y Pinus sylvestris, dos especies con diferentes estrategias en el uso del agua. En esta masa los pinos adultos muestran síntomas de decaimiento pero los robles no. Comparamos las respuestas a la sequía del crecimiento radial, el δ13C de la madera y los carbohidratos no estructurales de la albura (NSC) a diferentes escalas temporales en las dos especies y en pinos de dos clases de edad.
Ambas especies mostraron una alta sensibilidad al estrés hídrico, el cual mostró una estrecha relación con la plasticidad fenotípica en escalas temporales cortas y largas. Los pinos de todas las edades estudiadas mostraron estrategias más conservadoras en el uso del agua que la especie más tolerante a la sequía, el roble. Las dos especies mostraron tendencias de crecimiento negativas, pese a mostrar tendencias positivas en iWUE. El crecimiento radial de los pinos es menor actualmente que a la misma edad en el pasado. Las tendencias de la discriminación isotópica del carbono en los pinos jóvenes sugieren un incremento de las limitaciones en el intercambio gaseoso foliar. Tanto la variabilidad intranual como la interanual en los NSC fueron mayores en los robles que en los pinos y en azúcares solubles (SS) que en almidón. Los pinos mostraron los valores más bajos de SS. La sensibilidad de los NSC a dos años climáticamente contrastados fue muy baja en pinos, cuyos niveles de NSC permanecieron homeostáticos en consecuencia. La sensibilidad al clima mostrada por los diferentes rasgos funcionales analizados sugiere diferencias en la redistribución del carbono dentro de la planta, con menor acoplamiento entre crecimiento radial y fotosíntesis en pinos jóvenes.
Síntesis. Los pinos respondieron negativamente al incremento del estrés hídrico independientemente de su edad, aumentando sus limitaciones en el intercambio gaseoso mediante un mayor control estomático que los robles. Los pinos jóvenes y viejos tuvieron una respuesta functional ante el estrés hídrico parecida. Esto sugiere vulnerabilidad a nivel de especie y podría considerarse como una señal de aviso temprano previa a futuros aumentos en la mortalidad en pinos. A pesar de ello, dada la alta sensibilidad a la sequía de los robles, que no están en decaimiento, hubiera sido difícil deducir solamente mediante el análisis de los caracteres funcionales estudiados si las especies están en decaimiento o no.
Pines expressed negative responses to increased water stress regardless of age, showing rising gas exchange constraints through tighter stomatal control of water losses than oaks. Young pines showed similar functional responses to water stress than old pines in decline, which suggests species‐level vulnerability and could be regarded as early‐warning signals anticipating mortality in pines.
This paper describes devices to extract α-cellulose from small whole wood samples developed at the Lamont-Doherty Earth Observatory Tree-Ring Lab and explains the procedures for chemical extractions ...and for the dual analysis of carbon (δ13C) and oxygen (δ18O) stable isotopes. Here, we provide the necessary steps and guidelines for constructing a cellulose extraction system for small amounts of wood and leaves. The system allows the simultaneous extraction of cellulose from 150 samples by means of in-house filter tubes, where chemicals used for the cellulose extraction are exchanged and eliminated in batches. This new implementation diminishes the processing time, minimizes physical sample manipulation and potential errors, increases sample throughput, and reduces the amount of chemicals and analytic costs. We also describe the dual measurement of δ13C and δ18O ratios in tree-ring cellulose using high-temperature pyrolysis in a High Temperature Conversion Elemental Analyzer (TC/EA) interfaced with a Thermo Delta V plus mass spectrometer.
•We estimate seasonal- and site-specific growth responses to drought for Scots pine.•A tree-ring forward model was used to capture the intra-annual growth pattern.•We implement seasonal responses of ...growth to drought in a forest succession model.•The new model better simulates tree growth and forest dynamics in dry areas.
Drought is a key factor affecting forest ecosystem processes at different spatio-temporal scales. For accurately modeling tree functioning – and thus for producing reliable simulations of forest dynamics – the consideration of the variability in the timing and extent of drought effects on tree growth is essential, particularly in strongly seasonal climates such as in the Mediterranean area. Yet, most dynamic vegetation models (DVMs) do not include this intra-annual variability of drought effects on tree growth. We present a novel approach for linking tree-ring data to drought simulations in DVMs. A modified forward model of tree-ring width (VS-Lite) was used to estimate seasonal- and site-specific growth responses to drought of Scots pine (Pinus sylvestris L.), which were subsequently implemented in the DVM ForClim. Ring-width data from sixteen sites along a moisture gradient from Central Spain to the Swiss Alps, including the dry inner Alpine valleys, were used to calibrate the forward ring-width model, and inventory data from managed Scots pine stands were used to evaluate ForClim performance. The modified VS-Lite accurately estimated the year-to-year variability in ring-width indices and produced realistic intra-annual growth responses to soil drought, showing a stronger relationship between growth and drought in spring than in the other seasons and thus capturing the strategy of Scots pine to cope with drought. The ForClim version including seasonal variability in growth responses to drought showed improved predictions of stand basal area and stem number, indicating the need to consider intra-annual differences in climate–growth relationships in DVMs when simulating forest dynamics. Forward modeling of ring-width growth may be a powerful tool to calibrate growth functions in DVMs that aim to simulate forest properties in across multiple environments at large spatial scales.
•Regional assessment of open oak woodland tree decline.•Open oak woodland decline was related to high annual temperatures.•Intensive management probably contributed to oak decline.•Alternative ...practices are urgent to mitigate tree decline in similar agroecosystems.
Evergreen oak woodlands in the Western Mediterranean exploited as agrosilvopastoral systems have often been considered as a sustainability paradigm. Yet, these ecosystems show profound symptoms of degradation with widespread tree decline and increased mortality, making them a paradigmatic example of overexploited ecosystems threatened by global change. Understanding the biotic and abiotic, environmental and management factors involved in the current decline of these open woodlands is key to derive sustainable management options. Our goal was to evaluate the potential role of climate and land-use legacies as drivers of tree decline in Quercus ilex open woodlands at the regional level in western Spain. We analysed tree recruitment and health as proxies to the key processes implied in ecosystem decline. Overall, tree health was poor. Levels of tree decline followed a latitudinal gradient, with cooler stands in the North exhibiting better health (i.e. fewer, less severe decline symptoms) and higher sapling and seedling abundance than warmer stands in the South. Warmer conditions and more intense human management, indirectly expressed by stands with lower canopy cover and larger trees, were directly related to both worse plot health and lower tree regeneration. The widespread tree decline and very low recruitment abundance observed in the open oak woodlands studied may be the consequence of negative synergistic effects of a more limiting climate and land-use legacies from human overexploitation of a fragile ecosystem. These results warn of the negative impacts that land-use practices can exert on similar agrosilvopastoral ecosystems with the added risks of ongoing climatic changes, threatening ecological and economical sustainability.
Tree-ring chronologies were examined to investigate the influence of climate on radial growth of Pinus nigra in southeastern Spain. We addressed whether drought differentially affected the ...ring-widths of dominant and suppressed trees and if our results supported the hypothesis that, in a Mediterranean climate, suppressed conifer trees suffer greater growth reductions than dominant trees. Climate-growth relationships were analyzed using response and correlation functions, whereas the effect of drought on trees growth was approached by superposed epoch analysis in 10 dry years. A cool, wet autumn and spring, and/or mild winter enhanced radial growth. Latewood was the most sensitive ring section in both kinds of trees and it was primarily influenced by current year precipitations. Earlywood was mostly influenced by climatic conditions previous to the growing season. In general, May was the most influential month. Pinus nigra was shown to be very drought sensitive tree in the study area. Tree-rings in suppressed trees showed lower growth reductions caused by drought than those of dominant trees. However, dominant trees recovered normal growth faster. Dominant trees showed a more plastic response, and suppression appeared to reduce the effect of climate on tree radial growth. Some possible causes for these effects are discussed. Our results support the essential role of the balance between light and moisture limitations for plant development during droughts and show that it is not appropriate to generalize about the way in which suppression affects climate-growth relationship in conifers.
•Radial-growth averaging and absolute-increase had fewer type I and II errors.•Radial-growth averaging require the least a priori data.•Time-series produces information on magnitude and duration of ...release events.•False positives were more common in forests with infrequent and low severity disturbance.•Yearly binning performs better to identify local maxima indicating disturbance.
The retrospective study of abrupt and sustained increases in the radial growth of trees (hereinafter ‘releases’) by tree-ring analysis is an approach widely used for reconstructing past forest disturbances. Despite the range of dendrochronological methods used for release-detection, a lack of in-depth comparison between them can lead researchers to question which method to use and, potentially, increases the uncertainties of disturbance histories derived with different methods.
Here, we investigate the efficacy and sensitivity of four widely used release detection methods using tree-ring width series and complete long-term inventories of forest stands with known disturbances. We used support vector machine (SVM) analysis trained on long-term forest census data to estimate the likelihood that Acer rubrum trees experiencing reductions in competition show releases in their tree-ring widths. We compare methods performance at the tree and stand level, followed by evaluation of method sensitivity to changes in their parameters and settings.
Disturbance detection methods agreed with 60–76% of the SVM-identified growth releases under high canopy disturbance and 80–94% in a forest with canopy disturbance of low severity and frequency. The median competition index change (CIC) of trees identified as being released differed more than two-fold between methods, from −0.33 (radial-growth averaging) to −0.68 (time-series). False positives (type I error) were more common in forests with low severity disturbance, whereas false negatives (type II error) occurred more often in forests with high severity disturbance. Sensitivity analysis indicated that reductions of the detection threshold and the length of the time window significantly increased detected stand-level disturbance severity across all methods.
Radial-growth averaging and absolute-increase methods had lower levels of type I and II error in detecting disturbance events with our datasets. Parameter settings play a key role in the accuracy of reconstructing disturbance history regardless of the method. Time-series and radial-growth averaging methods require the least amount of a priori information, but only the time-series method quantified the subsequent growth increment related to a reduction in competition. Finally, we recommend yearly binning of releases using a kernel density estimation function to identify local maxima indicating disturbance. Kernel density estimation improves reconstructions of forest history and, thus, will further our understanding of past forest dynamics.
In the southwestern USA, recent large‐scale die‐offs of conifers raise the question of their resilience and mortality under droughts. To date, little is known about the interannual structural ...response to droughts. We hypothesized that piñon pines (Pinus edulis) respond to drought by reducing the drop of leaf water potential in branches from year to year through needle morphological adjustments. We tested our hypothesis using a 7‐year experiment in central New Mexico with three watering treatments (irrigated, normal, and rain exclusion). We analyzed how variation in “evaporative structure” (needle length, stomatal diameter, stomatal density, stomatal conductance) responded to watering treatment and interannual climate variability. We further analyzed annual functional adjustments by comparing yearly addition of needle area (LA) with yearly addition of sapwood area (SA) and distance to tip (d), defining the yearly ratios SA:LA and SA:LA/d. Needle length (l) increased with increasing winter and monsoon water supply, and showed more interannual variability when the soil was drier. Stomatal density increased with dryness, while stomatal diameter was reduced. As a result, anatomical maximal stomatal conductance was relatively invariant across treatments. SA:LA and SA:LA/d showed significant differences across treatments and contrary to our expectation were lower with reduced water input. Within average precipitation ranges, the response of these ratios to soil moisture was similar across treatments. However, when extreme soil drought was combined with high VPD, needle length, SA:LA and SA:LA/d became highly nonlinear, emphasizing the existence of a response threshold of combined high VPD and dry soil conditions. In new branch tissues, the response of annual functional ratios to water stress was immediate (same year) and does not attempt to reduce the drop of water potential. We suggest that unfavorable evaporative structural response to drought is compensated by dynamic stomatal control to maximize photosynthesis rates.
The leaf and sapwood structures determine the design of the hydraulic system of a tree and affect the water exchanges between the plant and the atmosphere. We investigated the effect of drought on the yearly addition of sapwood area, leaf area, and elongation in branches, as well as their interannual variability. Using two functional ratios, we showed that during drought, new tissues added in branches do not support a reduction in the leaf water potential.
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