Integrating effects of species composition and soil properties to predict shifts in montane forest carbon-water relations

Isotope-Based Techniques to Investigate Factors Influencing Water Use Efficiency in Pinus koraiensis Leaves during Plant Growth

Plant water use efficiency (WUE) is a comprehensive physiological indicator of plant growth and ability to adapt to drought. However, research on the mechanisms controlling WUE during plant growth and development remains weak. Here, we studied Pinus koraiensis as a typical evergreen conifer species in Northeast China. After collecting 80 tree samples with varying diameters at breast height (DBH), we measured δ13C and δ18O as an indicator of WUE, leaf morphology (volume, dry weight, and total epidermal area), ecological stoichiometry (carbon, nitrogen, and phosphorus content), and abiotic factors (light environment, soil pH, soil water content, and soil nutrient content). Correlational analysis of these variables revealed distinct differences between smaller/younger and larger/older plants: (1) In plants with DBH less than 52 cm, δ13C was positively related to DBH, and δ18O was negatively related to DBH. Plants with DBH greater than 52 cm showed no relationship between δ13C and DBH, and δ18O was positively related to DBH. (2) In plants with DBH less than 52 cm, there was a negative correlation between δ13C and δ18O and between δ13C and leaf phosphorus content (LP), but a positive correlation between δ13C and DBH, leaf mass per area (LMA), and leaf density (LD). The slopes of DBH-δ13C, δ18O-δ13C, leaf nitrogen content (LN)-δ13C, and LMA-δ13C correlations were greater in smaller plants than large plants. (3) Structural equation modelling showed that in smaller plants, DBH had a direct positive effect on δ13C content and a direct negative effect on δ18O, and there was a direct positive effect of light environment on δ18O. In larger plants, there was a direct negative effect of light environment on δ13C and a direct positive effect of DBH on light environment, as well as a negative effect of soil nitrogen content on leaf nitrogen. In smaller plants, DBH was the most important factor influencing δ13C, followed by δ18O and soil moisture, with light and soil pH showing minimal influence. In larger plants, light environment influenced δ13C the most, followed by soil nitrogen content and soil moisture content, with leaf nitrogen and DBH contributing little. The results suggest that water use efficiency strategies of P. koraiensis vary according to growth stage, and the effects of abiotic factors and functional traits vary at different growth stages.

Reverse diversity-biomass patterns in grasslands are constrained by climates and stoichiometry along an elevational gradient

Diversity and biomass play an important role in grassland ecosystem functions. However, diversity and biomass are variable because of their high sensitivity to environmental change in natural ecosystems. How plant diversity, biomass, and driving factors (climates, soils, and plants) in grasslands vary with environmental change remains unclear. We conducted intensive fieldwork (≈1000 km transect) on plant diversity, biomass, and associated drivers (i.e., climates, soils, and plants) to identify the patterns of diversity and biomass along an elevational gradient (50-4000 m) in grasslands of southwest China. Grassland biomass decreased significantly, but grassland diversity increased with increasing elevation. Consequently, a significant reverse pattern between biomass and diversity was detected along an elevational gradient. We also observed that the reverse pattern was primarily driven by the shifts in climates (i.e., temperature and precipitation), leaf stoichiometric traits (i.e., leaf N:P ratio), and soil properties (i.e., soil N content) along the elevational gradient. Our results contradicted previous studies on the positive diversity-biomass relationships, suggesting that previous studies might weaken the effects of climatic factors and plant stoichiometry under environmental change. These findings revealed that the reverse pattern between diversity and biomass in grasslands was shaped by the combined effects (climates, plants, soils) in grasslands, thus providing new insights into the debates and predictions on the diversity and biomass in grasslands under climate change.

Woody species with higher hydraulic efficiency or lower photosynthetic capacity discriminate more against 13C at the global scale

Leaf carbon isotope composition (δ13C) provides an integrative record on the carbon and water balance of plants over long periods. Photosynthetic ability and hydraulic traits which are highly associated with stomatal behavior could affect leaf δ13C. Association between photosynthetic ability and leaf δ13C has been examined, however, how hydraulic traits influence leaf δ13C has not been fully understood. To fill this gap, we investigated the variations in leaf δ13C among 2591 woody species (547 shrub and 2044 tree species), and analyzed the link of leaf δ13C with leaf photosynthetic and xylem hydraulic traits. Our result showed that leaf δ13C was positively correlated to leaf photosynthetic ability and capacity. For hydraulic traits, leaf δ13C was negatively related to hydraulic conductivity (Ks), xylem pressure inducing 50 % loss of hydraulic conductivity (P50) and vessel diameter (Vdia). Associations of leaf δ13C with xylem hydraulic traits indicate woody species with stronger hydraulic safety discriminated less against 13C, while woody species with higher hydraulic efficiency had more negative leaf δ13C. Shrub species, which showed a lower Vdia and P50, had a significant less negative leaf δ13C than tree species. Furthermore, woody species inhabiting in dry regions discriminated less against 13C than those growing in humid regions. Moreover, leaf δ13C displayed a low phylogenetic signal based on Blomberg's K statistic. Overall, woody species with a higher leaf photosynthetic ability or stronger hydraulic safety system discriminated less against 13C and adopt the provident water use strategy.

A comprehensive resilience assessment of Mexican tree species and their relationship with drought events over the last century

The resilience of forests to drought events has become a major natural resource sustainability concern, especially in response to climate change. Yet, little is known about the legacy effects of repeated droughts, and tree species ability to respond across environmental gradients. In this study, we used a tree-ring database (121 sites) to evaluate the overall resilience of tree species to drought events in the last century. We investigated how climate and geography affected the response at the species level. We evaluated temporal trends of resilience using a predictive mixed linear modeling approach. We found that pointer years (e.g., tree growth reduction) occurred during 11.3% of the 20th century, with an average decrease in tree growth of 66% compared to the previous period. The occurrence of pointer years was associated with negative values of the Standardized Precipitation Index (SPI, 81.6%) and Palmer Drought Severity Index (PDSI, 77.3%). Tree species differed in their resilience capacity, however, species inhabiting xeric conditions were less resistant but with higher recovery rates (e.g., Abies concolor, Pinus lambertiana, and Pinus jeffreyi). On average, tree species needed 2.7 years to recover from drought events, with extreme cases requiring more than a decade to reach pre-drought tree growth rates. The main abiotic factor related to resilience was precipitation, confirming that some tree species are better adapted to resist the effects of droughts. We found a temporal variation for all tree resilience indices (scaled to 100), with a decreasing resistance (-0.56 by decade) and resilience (-0.22 by decade), but with a higher recovery (+1.72 by decade) and relative resilience rate (+0.33 by decade). Our results emphasize the importance of time series of forest resilience, particularly by distinguishing the species-level response in the context of legacy of droughts, which are likely to become more frequent and intense under a changing climate.

Evaluation of the effects of long-term natural and artificial restoration on vegetation characteristics, soil properties and their coupling coordinations

Vegetation restoration is the most important factor to restrain soil and water loss in the Chinese Loess Plateau, and its effect is long-term. Among them, the coupling and coordination relationship between vegetation and soil is the key to the smooth implementation of ecological restoration and the project of returning farmland to forest and grassland. However, people have neglected whether the choice of vegetation restoration method is suitable for the development of ecological environment in this region, and whether vegetation and soil coexist harmoniously. In this paper, the typical watersheds with similar terrain environment but different vegetation restoration methods were selected as the research objects, which were Dongzhuanggou (natural restoration, NR) and Yangjiagou (artificial restoration, AR). Through vegetation investigation and soil physical property experiment, the comprehensive evaluation function was used to quantify the impact of restoration methods on vegetation characteristics and soil properties, and the vegetation-soil coupling model was used to explore the coordinated development of vegetation and soil under different restoration methods. The results showed that there were significant differences between the two restoration methods in terms of vegetation characteristics (P < 0.05). The vegetation diversity indices of NR were 1.59-4.81 times that of AR. For root characteristic indices, NR was 1.05-2.25 times that of AR. For soil physical properties, there was no significant difference between the two restoration methods (P > 0.05). The comprehensive evaluation function of vegetation (VCE) and soil (SCE) under NR were 0.74 and 0.42, respectively, while those under AR were 0.55 and 0.63, respectively. The comprehensive function showed that the vegetation population performance under NR was slightly better than that under AR, while the soil restoration effect was opposite. Under the two restoration methods, the vegetation-soil coupling relationship was barely coordinated (NR: 0.53; AR: 0.54), and both were the intermediate coordinated development mode. The vegetation diversity, tending level and soil management level should be improved simultaneously during the process of vegetation restoration on the Chinese Loess Plateau.

Seasonal variations in water uptake and transpiration for plants in a karst critical zone in China

Despite substantial drought conditions in the karst critical zone (KCZ), the KCZ landscapes are often covered with forest woody plants. However, it is not well understood how these plants balance water supply and demand to survive in such a water-limited environment. This study investigated the water uptake and transpiration relationships of four coexisting woody species in a subtropical karst forest ecosystem using measurements of microclimate, soil moisture, stable isotopes (δ¹⁸O, δ²H, and δ¹³C), intrinsic water-use efficiency (WUE_i), sap flow, and rooting depth. The focus was on identifying differences within- and between-species across soil- and rock-dominated habitats (SDH and RDH) during the rainy growing season (September 2017) and dry season (February 2018). Species across both habitats tended to have higher transpiration with lower WUE_i during the rainy season and lower transpiration with higher WUE_i during the dry season. Compared to those in the SDH, species in the RDH showed lower transpiration with higher WUE_i in both seasons. The dominant water sources were soil water and rainwater for supporting rainy-season transpiration in the SDH and RDH, respectively, and groundwater was the main water source for supporting dry-season transpiration in both habitats. A clear ecohydrological niche differentiation was also revealed among species. Across both habitats, shallower-rooted species with higher soil-water uptake, compared to deeper-rooted species with higher groundwater uptake, showed higher transpiration and lower WUE_i during the rainy season and vice versa during the dry season. This study provides integrated insights into how forest woody plants in the KCZ regulate transpiration and WUE_i in response to drought stress through interactions with seasonal water sources in the environment.

Limiting resource and leaf functional traits jointly determine distribution patterns of leaf intrinsic water use efficiency along aridity gradients

Intrinsic water use efficiency (iWUE) is a critical eco-physiological function allowing plants to adapt to water- and nutrient-limited habitats in arid and semi-arid regions. However, the distribution of iWUE in coexisting species along aridity gradients and its controlling factors are unknown. We established two transects along an aridity gradient in the grasslands of Losses Plateau (LP) and Inner Mongolia Plateau (MP) to elucidate the patterns and underlying mechanisms of iWUE distribution in coexisting species along aridity gradient. We determined leaf carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotopes, functional traits related to carbon fixation, and limiting resources. Bulk leaf δ¹³C and δ¹⁸O were used as proxies for time-integrated iWUE and stomatal conductance (gs) during the growing season. Our results showed that variability in iWUE within transect was primarily controlled by species, sampling sites and an interactive effect between species and sampling sites. Mean values of iWUE (iWUE_Mean) increased and coefficient of variation (CV) in iWUE (iWUE_CV) decreased with an increase in aridity, demonstrating that increases in aridity lead to conservative and convergent water use strategies. Patterns of iWUE_Mean and iWUE_CV were controlled primarily by the ratio of soil organic carbon to total nitrogen in LP and soil moisture in MP. This revealed that the most limited resource drove the distribution patterns of iWUE along aridity gradients. Interspecific variation in iWUE within transect was positively correlated with Δ¹⁸O, indicating that interspecific variation in iWUE was primarily regulated by gs. Furthermore, relationship between iWUE and multi-dimensional functional trait spectrum indicated that species evolved species-specific strategies to adapt to a harsh habitat by partitioning limiting resources. Overall, these findings highlighted the interactive effects of limiting resources and leaf functional traits on plant adaptation strategies for iWUE, and emphasized the importance of considering biological processes in dissecting the underlying mechanisms of plant adaptation strategies at large regional scales.

Contrasting plant responses to multivariate environmental variations among species with divergent elevation shifts

The general predictions of climate impacts on species shifts (e.g., upward shift) cannot directly inform local species conservation, because local-scale studies find divergent patterns instead of a general one. For example, our previous study found three shift patterns with elevation (strong down-, moderate down-, and up-slope shifts) in temperate mountain forests. The divergent shifts are hypothesized to arise from both multivariate environmental variations with elevation and corresponding species-specific responses. To test this hypothesis, we sampled soils and leaves to measure elevation variations in soil conditions and determined plant responses using discriminations against heavier isotopes, carbon (¹³ C) and nitrogen (¹⁵ N). Functional traits of the species studied were also extracted from a public trait dataset. We found that: (1) With low soil water contents at low elevations, only the leaves of up-shifters had lower ¹³ C discriminations at low vs. high elevations; (2) With low soil P contents at high elevations, only the leaves of moderate down-shifters had higher ¹⁵ N discriminations at high vs. low elevations; (3) The leaves of strong down-shifters did not show significant elevation patterns of the discriminations; (4) The contrasting responses among the three types of shifters agree with their functional dissimilarity, suggested by their separate locations in a multitrait space. Taken together, the divergent shifts are associated with the elevation variations in environmental conditions and contrasting plant responses. The contrasting responses could result from the functional dissimilarity among species. Therefore, a detailed understanding of both local environmental variations and species-specific responses can facilitate accurate predictions of species shifts to inform local species conservation.

Soil properties mediate ecosystem intrinsic water use efficiency and stomatal conductance via taxonomic diversity and leaf economic spectrum

The interactions between plants and soils lead to complex feedbacks that regulate intrinsic water use efficiency (iWUE) and stomatal conductance (gs) at ecosystem level and reflect water constraints on plant productivity. However, the relationships among soil properties, biodiversity, and leaf functional traits contributing to the variability in ecosystem iWUE and gs remain largely unknown. To elucidate these relationships, we used principal component analysis to reduce soil properties to a fertility spectrum and a limiting-resource spectrum across grassland, and early-, mid- and late-successional forests in a karst catchment. Leaf functional traits at community level were calculated based on leaf biomass, and were reduced to an economic spectrum and a limiting-resource spectrum. Leaf carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotopes at community levels were used as proxies for ecosystem iWUE and g_s. The effects of soil properties, biodiversity (taxonomic, functional and phylogenetic diversity) and leaf traits on δ¹³C and δ¹⁸O were evaluated using structural equation models. Our results showed that variability in ecosystem iWUE and gs was determined overwhelmingly by indirect effects of soil properties via two different pathways: the soil fertility spectrum, determining the number of coexisting species (taxonomic diversity) and turnover of species (leaf economic spectrum), and the soil limiting-resource spectrum, shaping the specific phylogenetic lineages (phylogenic diversity). In addition, δ¹³C and δ¹⁸O were constrained by the interactive effects of leaf economic spectrum, and taxonomic and phylogenic diversity; total effects of biodiversity on δ¹³C and δ¹⁸O were larger than those of leaf economic spectrum. Our study highlighted the critical role of the evaluating interaction relationships between leaf functional traits, biodiversity metrics and soil properties in understanding the mechanisms of ecosystem function responding to environmental change.

Increase in leaf organic acids to enhance adaptability of dominant plant species in karst habitats

Estimation of leaf nutrient composition of dominant plant species from contrasting habitats (i.e., karst and nonkarst forests) provides an opportunity to understand how plants are adapted to karst habitats from the perspective of leaf traits. Here, we measured leaf traits-specific leaf area (SLA), concentrations of total carbon ([TC]), nitrogen ([TN]), phosphorus ([TP]), calcium ([Ca]), magnesium ([Mg]), manganese ([Mn]), minerals ([Min]), soluble sugars, soluble phenolics, lipids, and organic acids ([OA])-and calculated water-use efficiency (WUE), construction costs (CC), and N/P ratios, and searched for correlations between these traits of 18 abundant plant species in karst and nonkarst forests in southwestern China. Variation in leaf traits within and across the abundant species was both divergent and convergent. Leaf [TC], [Ca], [Min], [OA], and CC were habitat-dependent, while the others were not habitat- but species-specific. The correlations among [TN], [TP], SLA, [TC], CC, [Min], WUE, [OA], and CC were habitat-independent, and inherently associated with plant growth and carbon allocation; those between [CC] and [Lip], between [Ca] and [Mg], and between [Mg] and [WUE] were habitat-dependent. Habitat significantly affected leaf [Ca] and thus indirectly affected leaf [OA], [Min], and CC. Our results indicate that plants may regulate leaf [Ca] to moderate levels via adjusting leaf [OA] under both high and low soil Ca availability, and offer new insights into the abundance of common plant species in contrasting habitats.

Linking Remotely Sensed Carbon and Water Use Efficiencies with In Situ Soil Properties

The capacity of terrestrial ecosystems to sequester carbon dioxide (CO2) from the atmosphere is expected to be altered by climate change and CO2 fertilization, but this projection is limited by our understanding of how the soil system interacts with plants. Understanding the soil–vegetation interactions is essential to assess the magnitude and response of terrestrial ecosystems to the changing climate. Here, we used soil profile and satellite data to explore the role that soil properties play in regulating water and carbon use by plants. Data obtained for 19 terrestrial ecosystem sites in a warm temperate and humid climate were used to investigate the relationship between remotely sensed data and soil physical and chemical properties. Classification and regression tree results showed that in situ soil carbon isotope (δ13C), and soil order were significant predictors (r2 = 0.39, mean absolute error (MAE) = 0 of 0.175 gC/KgH2O) of remotely sensed water use efficiency (WUE) based on the Moderate Resolution Imaging Spectroradiometer (MODIS). Soil extractable calcium (Ca), and land cover type were significant predictors of remotely sensed carbon use efficiency (CUE) based on MODIS and Landsat data-(r2 = 0.64–0.78, MAE = 0.04–0.06). We used gross primary productivity (GPP) derived from solar-induced fluorescence (SIF) data, based on the Orbiting Carbon Observatory-2 (OCO-2), to calculate WUE and CUE (referred to as WUESIF and CUESIF, respectively) for our study sites. The regression tree analysis revealed that soil organic matter and soil extractable magnesium (Mg), δ13C, and soil silt content were the important predictors of both WUESIF (r2 = 0.19, MAE = 0.64 gC/KgH2O) and CUESIF (r2 = 0.45, MAE = 0.1), respectively. Our results revealed the importance of soil extractable Ca, soil carbon (S13C is a facet of soil carbon content), and soil organic matter predicting CUE and WUE. Insights gained from this study highlighted the importance of biotic and abiotic factors regulating plant and soil interactions. These types of data are timely and critical for accurate predictions of how terrestrial ecosystems respond to climate change.

Smoke from regional wildfires alters lake ecology

Wildfire smoke often covers areas larger than the burned area, yet the impacts of smoke on nearby aquatic ecosystems are understudied. In the summer of 2018, wildfire smoke covered Castle Lake (California, USA) for 55 days. We quantified the influence of smoke on the lake by comparing the physics, chemistry, productivity, and animal ecology in the prior four years (2014-2017) to the smoke year (2018). Smoke reduced incident ultraviolet-B (UV-B) radiation by 31% and photosynthetically active radiation (PAR) by 11%. Similarly, underwater UV-B and PAR decreased by 65 and 44%, respectively, and lake heat content decreased by 7%. While the nutrient limitation of primary production did not change, shallow production in the offshore habitat increased by 109%, likely due to a release from photoinhibition. In contrast, deep-water, primary production decreased and the deep-water peak in chlorophyll a did not develop, likely due to reduced PAR. Despite the structural changes in primary production, light, and temperature, we observed little significant change in zooplankton biomass, community composition, or migration pattern. Trout were absent from the littoral-benthic habitat during the smoke period. The duration and intensity of smoke influences light regimes, heat content, and productivity, with differing responses to consumers.

Climate-induced reversal of tree growth patterns at a tropical treeline

Globally, cold-limited trees and forests are expected to experience growth acceleration as a direct response to warming temperatures. However, thresholds of temperature limitation may vary substantially with local environmental conditions, leading to heterogeneous responses in tree ecophysiology. We used dendroecological and isotopic methods to quantify shifting tree growth and resource use over the past 143 years across topographic aspects in a high-elevation forest of central Mexico. Trees on south-facing slopes (SFS) grew faster than those on north-facing slopes (NFS) until the mid-20th century, when this pattern reversed notably with marked growth rate declines on SFS and increases on NFS. Stable isotopes of carbon, oxygen, and carbon-to-nitrogen ratios suggest that this reversal is linked to interactions between CO₂ stimulation of photosynthesis and water or nitrogen limitation. Our findings highlight the importance of incorporating landscape processes and habitat heterogeneity in predictions of tree growth responses to global environmental change.

Tree-ring δ18O climate signals vary among tree functional types in South Asian tropical moist forests

We present the first annually resolved and statistically reliable tree-ring δ¹⁸O (δ¹⁸O_T) chronologies for the three South Asian tropical moist forest tree species (Chukrasia tabularis A. Juss., Toona ciliata M. Roem., and Lagerstroemia speciosa Roxb.) which differ in their shade tolerance and resistance to water stress. We found significantly higher mean δ¹⁸O_T values in light-demanding T. ciliata than in intermediate shade tolerant C. tabularis and shade tolerant L. speciosa (p < 0.001). δ¹⁸O_T in C. tabularis was mainly influenced by pre-monsoon vapor pressure deficit (VPD; r = -0.54, p < 0.01) and post monsoon maximum temperature (T_max) (r = 0.52, p < 0.01). δ¹⁸O_T in T. ciliata was strongly negatively correlated with a dry season drought index PDSI (r = -0.65, p < 0.001) and VPD (r = -0.58, p < 0.001). Pre-monsoon T_max was strongly positively linked with δ¹⁸O_T in L. speciosa (r = 0.65, p < 0.001), indicating that climatic influences on δ¹⁸O_T are species-specific and vary among tree functional types. Although there was a week correlation between local precipitation and δ¹⁸O_T in our studied species, we found a strong correlation between δ¹⁸O_T and precipitation at a larger spatial scale. Linear mixed effect models revealed that multiple factors improved model performance only in C. tabularis, yielding the best model, which combined VPD and T_max. The top models in T. ciliata and L. speciosa included only the single factors PDSI and T_max, highlighting that the way C. tabularis interacts with climate is more complex when compared with other two species. Our analyses suggest that stable oxygen isotope composition in tree rings of South Asian tropical moist forest trees are a suitable proxy of local and regional climate variability and are an important tool for understanding the physiological mechanisms associated with the global hydrological cycle.

A record of vapour pressure deficit preserved in wood and soil across biomes

The drying power of air, or vapour pressure deficit (VPD), is an important measurement of potential plant stress and productivity. Estimates of VPD values of the past are integral for understanding the link between rising modern atmospheric carbon dioxide (pCO2) and global water balance. A geological record of VPD is needed for paleoclimate studies of past greenhouse spikes which attempt to constrain future climate, but at present there are few quantitative atmospheric moisture proxies that can be applied to fossil material. Here we show that VPD leaves a permanent record in the slope (S) of least-squares regressions between stable isotope ratios of carbon and oxygen (13C and 18O) found in cellulose and pedogenic carbonate. Using previously published data collected across four continents we show that S can be used to reconstruct VPD within and across biomes. As one application, we used S to estimate VPD of 0.46 kPa ± 0.26 kPa for cellulose preserved tens of millions of years ago-in the Eocene (45 Ma) Metasequoia from Axel Heiberg Island, Canada-and 0.82 kPa ± 0.52 kPa-in the Oligocene (26 Ma) for pedogenic carbonate from Oregon, USA-both of which are consistent with existing records at those locations. Finally, we discuss mechanisms that contribute to the positive correlation observed between VPD and S, which could help reconstruct past climatic conditions and constrain future alterations of global carbon and water cycles resulting from modern climate change.

Strong overestimation of water-use efficiency responses to rising CO2 in tree-ring studies

The carbon isotope ratio (δ¹³ C) in tree rings is commonly used to derive estimates of the assimilation-to-stomatal conductance rate of trees, that is, intrinsic water-use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO₂ concentrations (C_a ), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree-ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising C_a . We found that increases in tree size and stand age greatly influenced iWUE. The effect of C_a on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to C_a , mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO₂ effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO₂ effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.

A State Factor Model for Ecosystem Carbon-Water Relations

With increasing calls for improving terrestrial carbon sequestration and sustainable water use, scientists are faced with the challenge of predicting changes in carbon-water relations from organisms to landscapes. We propose an integrative framework to help in answering basic and applied questions pertaining to coupled carbon-water functions in a variety of ecosystems. The conceptual framework is based on data from a globally representative set of ecosystems that hold vast amounts of carbon and provide water for rural and urban land uses. We focus on examples that demonstrate the value of an integrated approach that combines fast- and slow-changing state factors (i.e., variables that define structural properties and functional processes at the soil-plant-atmosphere interface) to improve predictions of carbon-water relations across scales.

Similar potential of foliar δ13C and silicon levels for inferring local climate information in the Tibetan Plateau region

Stable carbon isotope ratios (δ¹³C) are widely used as climate proxies for assessing and predicting climatic information at an annual resolution. However, the detailed information in the isotopes that results from intra-annual climate scenarios and is associated with mineral accumulation remains unclear. Combined with investigations of elements and ash contents, variations in foliar δ¹³C in relation to annual, winter and summer climate scenarios were investigated in a dendroclimatologically important tree species Sabina przewalskii Kom. Foliar δ¹³C exhibited a significant negative correlation with mean annual temperature, mean annual precipitation and mean annual relative humidity as well as significant positive correlations with elevation. Climatic factors in winter and summer have opposite effects on the variation of δ¹³C. The beneficial mineral element Si had a significant positive correlation with foliar δ¹³C, whereas the essential mineral elements K, Ca, and Mg did not. Specifically, Si and δ¹³C have similar correlations with climate factors and elevation. These results suggest that measurement of Si content has a similar potential to δ¹³C for use as an alternative climate indicator when detailed climatic information may otherwise be limited and provide a basis for understanding the integration of δ¹³C in plant responses to climate.

Land carbon models underestimate the severity and duration of drought's impact on plant productivity

The ability to accurately predict ecosystem drought response and recovery is necessary to produce reliable forecasts of land carbon uptake and future climate. Using a suite of models from the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP), we assessed modeled net primary productivity (NPP) response to, and recovery from, drought events against a benchmark derived from tree ring observations between 1948 and 2008 across forested regions of the US and Europe. We find short lag times (0-6 months) between climate anomalies and modeled NPP response. Although models accurately simulate the direction of drought legacy effects (i.e. NPP decreases), projected effects are approximately four times shorter and four times weaker than observations suggest. This discrepancy between observed and simulated vegetation recovery from drought reveals a potential critical model deficiency. Since productivity is a crucial component of the land carbon balance, models that underestimate drought recovery time could overestimate predictions of future land carbon sink strength and, consequently, underestimate forecasts of atmospheric CO₂.

North America's oldest boreal trees are more efficient water users due to increased [CO2], but do not grow faster

Due to anthropogenic emissions and changes in land use, trees are now exposed to atmospheric levels of [[Formula: see text]] that are unprecedented for 650,000 y [Lüthi et al. (2008) Nature 453:379-382] (thousands of tree generations). Trees are expected to acclimate by modulating leaf-gas exchanges and alter water use efficiency which may result in forest productivity changes. Here, we present evidence of one of the strongest, nonlinear, and unequivocal postindustrial increases in intrinsic water use efficiency ([Formula: see text]) ever documented (+59%). A dual-isotope tree-ring analysis ([Formula: see text] and [Formula: see text]) covering 715 y of growth of North America's oldest boreal trees (Thuja occidentalis L.) revealed an unprecedented increase in [Formula: see text] that was directly linked to elevated assimilation rates of [Formula: see text] (A). However, limited nutrient availability, changes in carbon allocation strategies, and changes in stomatal density may have offset stem growth benefits awarded by the increased [Formula: see text] Our results demonstrate that even in scenarios where a positive [Formula: see text] fertilization effect is observed, other mechanisms may prevent trees from assimilating and storing supplementary anthropogenic emissions as above-ground biomass. In such cases, the sink capacity of forests in response to changing atmospheric conditions might be overestimated.