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Vegetation plays an important role in regulating global carbon cycles and is a key component of the Earth system models (ESMs) that aim to project Earth's future climate. In the last decade, the vegetation component within ESMs has witnessed great progress from simple "big-leaf" approaches to demographically structured approaches, which have a better representation of plant size, canopy structure, and disturbances. These demographically structured vegetation models typically have a large number of input parameters, and sensitivity analysis is needed to quantify the impact of each parameter on the model outputs for a better understanding of model behavior. In this study, we conducted a comprehensive sensitivity analysis to diagnose the Community Land Model coupled to the Functionally Assembled Terrestrial Simulator, or CLM4.5(FATES). Specifically, we quantified the first- and second-order sensitivities of the model parameters to outputs that represent simulated growth and mortality as well as carbon fluxes and stocks for a tropical site with an extent of 1×1 ∘. While the photosynthetic capacity parameter (Vc,max25) is found to be important for simulated carbon stocks and fluxes, we also show the importance of carbon storage and allometry parameters, which determine survival and growth strategies within the model. The parameter sensitivity changes with different sizes of trees and climate conditions. The results of this study highlight the importance of understanding the dynamics of the next generation of demographically enabled vegetation models within ESMs to improve model parameterization and structure for better model fidelity. [ABSTRACT FROM AUTHOR]

Soil respiration (Rs), the flow of CO2 from the soil surface to the atmosphere, is one of the largest carbon fluxes in the terrestrial biosphere. The spatial variability of Rs is both large and poorly understood, limiting our ability to robustly scale it in time and space. One factor in Rs spatial variability is the autotrophic contribution from plant roots, but it is uncertain how the proximity of plants affects the magnitude and temperature sensitivity of Rs. This study examined the effect of tree proximity on Rs in the growing and dormant seasons, as well as during moisture-limited times, in a temperate, coastal, deciduous forest in eastern Maryland, USA. In a linear mixed-effects model, tree basal area within 5 m (BA5) exerted a significant positive effect on the temperature sensitivity of soil respiration. Soil moisture was the dominant control on Rs during the dry portions of the year while soil moisture, temperature, and BA5 all exerted significant effects on Rs in wetter periods. Our results suggest that autotrophic respiration is more sensitive to temperature than heterotrophic respiration at these sites, although we did not measure these source fluxes directly, and that soil respiration is highly moisture-sensitive, even in a record-rainfall year. The Rs flux magnitudes (0.3-16.6 µmol m-2 s-1) and variability (coefficient of variability 10 %-22 % across plots) observed in this study were comparable to values observed over decades in similar forests. We estimate that four Rs observations were required to be within 50 % of the stand-level mean, and 311 to be within 5 %, at 90 % confidence. A better understanding of the spatial interactions between plants and microbes that results in measured Rs is necessary to link these processes with large scale soil-to-atmosphere C fluxes. [ABSTRACT FROM AUTHOR]

Nonstructural carbohydrates (NSCs) are essential for maintenance of plant metabolism and may be sensitive to short‐ and long‐term climatic variation. NSC variation in moist tropical forests has rarely been studied, so regulation of NSCs in these systems is poorly understood. We measured foliar and branch NSC content in 23 tree species at three sites located across a large precipitation gradient in Panama during the 2015–2016 El Niño to examine how short‐ and long‐term climatic variation impact carbohydrate dynamics. There was no significant difference in total NSCs as the drought progressed (leaf P = 0.32, branch P = 0.30) nor across the rainfall gradient (leaf P = 0.91, branch P = 0.96). Foliar soluble sugars decreased while starch increased over the duration of the dry period, suggesting greater partitioning of NSCs to storage than metabolism or transport as drought progressed. There was a large variation across species at all sites, but total foliar NSCs were positively correlated with leaf mass per area, whereas branch sugars were positively related to leaf temperature and negatively correlated with daily photosynthesis and wood density. The NSC homoeostasis across a wide range of conditions suggests that NSCs are an allocation priority in moist tropical forests. Nonstructural carbohydrates (NSCs), the organic compounds that drive plant metabolism, have rarely been studied in moist tropical forests, so their regulation in these systems is poorly understood. We measured foliar and branch NSCs in canopy trees across a large precipitation gradient in Panama during the 2015–2016 El Niño to examine how short‐ and long‐term climatic variation impact carbohydrate dynamics. There was a large variation in NSCs across species; however, there was no change in total NSCs as the drought progressed nor across the rainfall gradient, suggesting that NSCs are an allocation priority in moist tropical forests. [ABSTRACT FROM AUTHOR]

Understanding historical hydroclimate change during the last millennium is of fundamental importance for forecasting and evaluating the regional hydrologic cycle and water security under global warming. Here we compared observations of tree‐ring stable isotope ratios (δ13C and δ18O), tree‐ring widths, lake sediments estimates of regional hydroclimate, and glacier advance and retreat for northeastern Qinghai‐Tibetan Plateau, China. The hydroclimate dynamics in our composite isotope record is similar to that derived from the juniper tree‐ring widths throughout the last millennium until the latter half of the twentieth century, after which they diverged with the tree‐ring widths indicating an unprecedented wet period while the isotopic results, sediment proxies, and glaciers demonstrated a drying pattern. Rising atmospheric CO2 promoted increases in canopy‐scale intrinsic water‐use efficiency and growth, causing tree‐ring widths to overestimate the regional hydroclimate signal in the growth record. Thus, our study suggests that CO2 fertilization over the last century may confound hydroclimate reconstructions inferred from juniper tree‐ring width at northeastern Qinghai‐Tibetan Plateau, which may cause overestimates of water resources. Correction for CO2‐induced changes in tree‐ring widths may be necessary to accurately utilize tree rings for paleoclimate reconstructions. Plain Language Summary: During the past 50 years, the northeastern Qinghai‐Tibetan Plateau region experienced alpine glacial retreat, increased desertification, and a reduction of river and lake levels. Understanding the region's background hydroclimate variation is critically important to assess current and future water‐management policies in this water‐limited region. Hydroclimate reconstructions exceeding a millennium from tree‐ring width revealed that the late twentieth century experienced an unprecedented wet climate, while lake sediments demonstrated a drying pattern. Here we reconstructed a new hydroclimate chronology over the last millennium using independent tree‐ring stable isotope series. We found a drying trend in the late twentieth century from the new hydroclimate chronology, which is different from the tree‐ring width reconstruction, but compared well with lake‐sediment results. Except for the late twentieth century, our new hydroclimate chronology compared well with tree‐ring width reconstruction throughout the last millennium. We found hydroclimate overestimation from tree‐ring width was caused by the CO2 fertilization during the late twentieth century. Rising atmospheric CO2 improves tree photosynthesis, increases canopy‐scale water‐use efficiency, and accelerates tree radial growth. Therefore, the hydroclimate in the northeastern Qinghai‐Tibetan Plateau is drying in the late twentieth century, and paleo‐moisture inferred only from tree‐ring width may be overestimated due to atmospheric CO2 fertilization. Key Points: Hydroclimate variations over last 1,020 years were reconstructed from tree‐ring stable isotope ratios and tree‐ring widthHydroclimate dynamics of the two series were similar throughout the last millennium until the latter half of the twentieth centuryTree growth resulting from CO2 fertilization confounds the regional hydroclimate signal in the tree‐ring width record [ABSTRACT FROM AUTHOR]

Vegetation plays a key role in regulating global carbon cycles and is a key component of the Earth System Models (ESMs) aimed to project Earth's future climates. In the last decade, the vegetation component within ESMs has witnessed great progresses from simple 'big-leaf' approaches to demographically-structured approaches, which has a better representation of plant size, canopy structure, and disturbances. The demographically-structured vegetation models are typically controlled by a large number of parameters, and sensitivity analysis is generally needed to quantify the impact of each parameter on the model outputs for a better understanding of model behaviors. In this study, we use the Fourier Amplitude Sensitivity Test (FAST) to diagnose the Community Land Model coupled to the Ecosystem Demography Model, or CLM4.5(ED). We investigate the first and second order sensitivities of the model parameters to outputs that represent simulated growth and mortality as well as carbon fluxes and stocks. While the photosynthetic capacity parameter Vcmax25 is found to be important for simulated carbon stocks and fluxes, we also show the importance of carbon storage and allometry parameters, which are shown here to determine vegetation demography and carbon stocks through their impacts on survival and growth strategies. The results of this study highlights the importance of understanding the dynamics of the next generation of demographically-enabled vegetation models within ESMs toward improved model parameterization and model structure for better model fidelity. [ABSTRACT FROM AUTHOR]

Understanding how climate alters plant–soil water dynamics, and its impact on physiological functions, is critical to improved predictions of vegetation responses to climate change. Here we analyzed how belowground interactions for water shift under warming and drought, and associated impacts on plant functions. In a semi-arid woodland, adult trees (piñon and juniper) and perennial grasses (blue grama) were exposed to warming and precipitation reduction. After 6 years of continuous treatment exposure, soil and plant water isotopic composition was measured to assess plant water uptake depths and community-level water source partitioning. Warming and drought modified plant water uptake depths. Under warming, contrasting changes in water sources between grasses and trees reduced belowground water source partitioning, resulting in higher interspecific competition for water. Under drought, shifts in trees and grass water sources to deeper soil layers resulted in the maintenance of the naturally occurring water source partitioning among species. Trees showed higher water stress, and reduced water use and photosynthesis in response to warming and drought. This case study demonstrates that neighboring plants shift their competitive interactions for water under prolonged warming and drought, but regardless of whether changes in moisture sources will result in increased competition among species or maintained partitioning of water resources, these competitive adaptations may easily be overridden by climate extremes. [ABSTRACT FROM AUTHOR]

The leaf area to sapwood area ratios of trees (Al:AS) can shift to maintain homeostatic gas exchange per unit leaf area in response to climate variability. We tested the hypothesis that trees alter their Al:AS ratios in response to long-term warming and reduced precipitation in order to maintain leaf-specific gas exchange rates under more stressful conditions. Whole-tree Al:AS was measured on mature piñon pine (Pinus edulis Engelm.) and one-seed juniper (Juniperus monosperma (Engelm.) Sarg.) trees after 5 years (2012–16) of chronic exposure to increased temperature (+4.8 °C), precipitation reduction (−45%), or both simultaneously. No difference was found in Al:As among treatments for either species. Associated with this lack of shift in Al:As were large changes in pre-dawn leaf water potential and stomatal conductance, consistent with theoretical expectations of interactions between leaf and whole-tree hydraulic supply. Our results suggest that a lack of whole-tree acclimation in Al:As results in the reductions in plant gas exchange and water status associated with long-term warming and reduced precipitation in semi-arid woodlands. [ABSTRACT FROM AUTHOR]

How plants respond physiologically to leaf warming and low water availability may determine how they will perform under future climate change. In 2015-2016, an unprecedented drought occurred across Amazonia with record-breaking high temperatures and low soil moisture, offering a unique opportunity to evaluate the performances of Amazonian trees to a severe climatic event. We quantified the responses of leaf water potential, sap velocity, whole-tree hydraulic conductance (Kwt), turgor loss and xylem embolism, during and after the 2015-2016 El Niño for five canopy-tree species. Leaf/xylem safety margins (SMs), sap velocity and Kwt showed a sharp drop during warm periods. SMs were negatively correlated with vapour pressure deficit, but had no significant relationship with soil water storage. Based on our calculations of canopy stomatal and xylem resistances, the decrease in sap velocity and Kwtwas due to a combination of xylem cavitation and stomatal closure. Our results suggest that warm droughts greatly amplify the degree of trees' physiological stress and can lead to mortality. Given the extreme nature of the 2015-2016 El Niño and that temperatures are predicted to increase, this work can serve as a case study of the possible impact climate warming can have on tropical trees. This article is part of a discussion meeting issue 'The impact of the 2015/ 2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'. [ABSTRACT FROM AUTHOR]

• Desert plants had high hydraulic safety and low xylem hydraulic conductivity. • Shrubs and trees diverged in hydraulics but not for native and introduced species. • Hydraulic safety and growth rather than climate of origin driven plant survival. The species-poor woodlands in arid regions are particularly vulnerable to climate change induced plant die-off. Searching for species suitable to replant in these areas is an effective way to prevent further expansion of land desertification. Good knowledge of drought resistance of species is critical for current replanting efforts, however, the related plant survival mechanisms in the process of restoration remain limited. Here, we tested the dominant drivers of plant survival in relation to climate factors, growth characteristics and functional traits including hydraulics and gas exchange across 35 multiple shrub and tree woody species in a common garden at the Tengger Desert, northwest China. We found a wide range of hydraulic thresholds among these desert species, with relatively high hydraulic safety and low xylem hydraulic conductivity. Pant survival was largely determined by hydraulic safety and growth status, but was not affected by the climate of the species' site of origin. Shrubs and trees diverged in hydraulic and growth characteristics, while no difference between native and introduced species was found. Our results highlight the critical roles of hydraulic safety and growth on determining the survival and drought adaptation during desert vegetation development, which provide a new insight into ecological restoration in arid regions under changing climate conditions. [ABSTRACT FROM AUTHOR]

We measured carbon (respiration, photosynthesis, and production) and nitrogen allocation to male and female cones of Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca) to quantify gender-specific: (1) resource allocation to reproduction, and (2) contribution to carbon costs of reproduction via photosynthesis. We also measured foliar photosynthesis and nitrogen concentration ([N]) near and far from female cones to examine the relationship between reproduction and foliar physiology. Over one growing season, male cones required only 8% of all carbon allocated to reproduction, with females consuming the remaining 92%. Female cones, however, had maximum instantaneous refixation rates of 54%, which, integrated over the season, offset 6% of their total carbon requirements, while male cones were completely dependent on vegetative tissues for carbon. Male cones received 22% of all nitrogen allocated to reproduction and female cones received the remaining 78%. Foliage near female cones had elevated photosynthesis during the early stages of cone development anti consistently lower [N] than foliage far from cones. Although female cones may photosynthesize, the annual sum of carbon fixed by reproductive structures is minor in comparison to the total carbon allocated to production and respiration. Key words: carbon; gender; nitrogen; photosynthesis; Pinaceae; Pseudotsuga menziesii; resource allocation; respiration.