[1] Insights into how terrestrial ecosystems affect the Earth's response to changes in climate and rising atmospheric CO 2 levels rely heavily on the predictions of terrestrial biosphere models (TBMs). These models contain detailed mechanistic representations of biological processes affecting terrestrial ecosystems; however, their ability to simultaneously predict field-based measurements of terrestrial vegetation dynamics and carbon fluxes has remained largely untested. In this study, we address this issue by developing a constrained implementation of a new structured TBM, the Ecosystem Demography model version 2 (ED2), which explicitly tracks the dynamics of fine-scale ecosystem structure and function. Carbon and water flux measurements from an eddy-flux tower are used in conjunction with forest inventory measurements of tree growth and mortality at Harvard Forest (42.5°N, 72.1°W) to estimate a number of important but weakly constrained model parameters. Evaluation against a decade of tower flux and forest dynamics measurements shows that the constrained ED2 model yields greatly improved predictions of annual net ecosystem productivity, carbon partitioning, and growth and mortality dynamics of both hardwood and conifer trees. The generality of the model formulation is then evaluated by comparing the model's predictions against measurements from two other eddy-flux towers and forest inventories of the northeastern United States and Quebec. Despite the markedly different composition throughout this region, the optimized model realistically predicts observed patterns of carbon fluxes and tree growth. These results demonstrate how TBMs parameterized with field-based measurements can provide quantitative insight into the underlying biological processes governing ecosystem composition, structure, and function at larger scales.

Numerous current efforts seek to improve the representation of ecosystem ecology and vegetation demographic processes within Earth System Models (ESMs). These developments are widely viewed as an important step in developing greater realism in predictions of future ecosystem states and fluxes. Increased realism, however, leads to increased model complexity, with new features raising a suite of ecological questions that require empirical constraints. Here, we review the developments that

We assessed whether diversity in plant hydraulic traits can explain the observed diversity in plant responses to water stress in seasonally dry tropical forests (SDTFs). The Ecosystem Demography model 2 (ED2) was updated with a trait-driven mechanistic plant hydraulic module, as well as novel drought-phenology and plant water stress schemes. Four plant functional types were parameterized on the basis of meta-analysis of plant hydraulic traits. Simulations from both the original and the updated ED2 were evaluated against 5 yr of field data from a Costa Rican SDTF site and remote-sensing data over Central America. The updated model generated realistic plant hydraulic dynamics, such as leaf water potential and stem sap flow. Compared with the original ED2, predictions from our novel trait-driven model matched better with observed growth, phenology and their variations among functional groups. Most notably, the original ED2 produced unrealistically small leaf area index (LAI) and underestimated cumulative leaf litter. Both of these biases were corrected by the updated model. The updated model was also better able to simulate spatial patterns of LAI dynamics in Central America. Plant hydraulic traits are intercorrelated in SDTFs. Mechanistic incorporation of plant hydraulic traits is necessary for the simulation of spatiotemporal patterns of vegetation dynamics in SDTFs in vegetation models.

The response of tropical forests to droughts is highly uncertain 1 . During the dry season, canopy photosynthesis of some tropical forests can decline, whereas in others it can be maintained at the same or a higher level than during the wet season 2 . However, it remains uncertain to what extent water availability is responsible for productivity declines of tropical forests during the dry season 2,3 . Here we use global satellite observations of two independent measures of vegetation photosynthetic properties (enhanced vegetation index from 2002 to 2012 and solar-induced chlorophyll fluorescence from 2007 to 2012) to investigate links between hydroclimate and tropical forest productivity. We find that above an annual rainfall threshold of approximately 2,000 mm yr −1 , the evergreen state is sustained during the dry season in tropical rainforests worldwide, whereas below that threshold, this is not the case. Through a water-budget analysis of precipitation, potential evapotranspiration and satellite measurements of water storage change, we demonstrate that this threshold determines whether the supply of seasonally redistributed subsurface water storage from the wet season can satisfy plant water demands in the subsequent dry season. We conclude that water availability exerts a first-order control on vegetation seasonality in tropical forests globally. Our framework can also help identify where tropical forests may be vulnerable or resilient to future hydroclimatic changes.Photosynthetic metabolism in tropical forests controls ecosystem carbon uptake from the atmosphere, and it also influences critical ecosystem services, including carbon storage 4 , freshwater delivery 5 , maintenance of biodiversity 5 , and regulation of regional and global climate 6 . The photosynthetic metabolism of many tropical forests exhibits a recurring seasonality 2,7 . Understanding how climate influences these seasonal dynamics is an essential prerequisite for realistically predicting tropical forest responses to inter-annual and longer-term climate variation and change 3 . In particular, with a wide spectrum of varying total annual precipitation and dry-season length in the tropics ( Supplementary Fig. 7), the extent to which seasonality of vegetation productivity in tropical forests responds to water limitation remains unclear 2,3 . Although tropical forest seasonal dynamics have been studied at site and regional scales using eddy flux-tower networks and/or satellite remote sensing in Amazonia 2,8,9 , Insular Southeast (SE) Asia 7 and Africa 10 , a globally consistent functional inter-comparison of tropical forests is lacking. Thus, in this paper we address the following questions: What is the extent to which the seasonality of vegetation photosynthesis is limited by water availability in global tropical forests? Are there critical environmental thresholds that explain these seasonal variations? If so, what are the underlying physical mechanisms? What are the implications of such mechanisms on the future of tropical forests under cli...

Wind power is a rapidly growing alternative energy source to achieve the goal of the Paris Agreement under the United Nations Framework Convention on Climate Change, to keep warming well below 2 •C by the end of the 21 st century. Widely reported reductions in global average surface wind speed since the 1980s, known as terrestrial stilling, however, have gone unexplained and have been considered a threat to global wind power production. Our new analysis of wind data from in-situ stations worldwide now shows that terrestrial stilling reversed around 2010 and global wind speeds over land have recovered most of the losses since the 1980s. Concomitant increased surface roughness from forest growth and urbanization cannot explain prior stilling. Instead we show decadal-scale variations of nearsurface wind are very / quite likely caused by the natural, internal decadal ocean/atmosphere oscillations of the Earth's climate system. The wind strengthening has increased the amount of wind energy entering turbines by 17 ±2% for 2010-2017, likely increasing U.S. wind power capacity by 2.5%. The increase in global terrestrial wind bodes well for the immediate future of wind energy production in these regions as an alternative to fossil fuel consumption. Projecting future wind speeds using ocean/atmosphere oscillations show wind turbines could be optimized for expected wind speeds, including small and large speeds, during the productive life spans of the turbines. Reports of a 8% global decline in land surface wind speed (~1980 to 2010) have raised concerns about output from future wind power 1-5. Wind power varies with the cube of wind speed (u) 6. The decline in wind speed is evident in the northern mid-latitude countries where the majority of wind turbines are installed including China, the U.S. and Europe 1. If the observed 1980-2010 decline in wind speed continued until the end of the century, global u would reduce by 21%, halving the amount of power available in the wind. Understanding the drivers of this long-term decline in wind speed is critical not merely to maximize wind energy production 9-11 but also to address other globally significant environmental problems related to stilling, including reduced aerosol dispersal, reduced evapotranspiration rates, and adverse effects on animal behavior and ecosystem functioning 1,3,4,12. The potential causes for the global terrestrial stilling are complex and remain contested (e.g.,

The mechanisms governing tree drought mortality and recovery remain a subject of inquiry and active debate given their role in the terrestrial carbon cycle and their concomitant impact on climate change. Counter-intuitively, many trees do not die during the drought itself. Indeed, observations globally have documented that trees often grow for several years after drought before mortality. A combination of meta-analysis and tree physiological models demonstrate that optimal carbon allocation after drought explains observed patterns of delayed tree mortality and provides a predictive recovery framework. Specifically, post-drought, trees attempt to repair water transport tissue and achieve positive carbon balance through regrowing drought-damaged xylem. Furthermore, the number of years of xylem regrowth required to recover function increases with tree size, explaining why drought mortality increases with size. These results indicate that tree resilience to drought-kill may increase in the future, provided that CO fertilisation facilitates more rapid xylem regrowth.

Atmospheric measurements and land-based inventories imply that terrestrial ecosystems in the northern hemisphere are taking up significant amounts of anthropogenic carbon dioxide (CO 2 ) emissions; however, there is considerable disagreement about the causes of this uptake, and its expected future trajectory. In this paper, we use the ecosystem demography (ED) model to quantify the contributions of disturbance history, CO 2 fertilization and climate variability to the past, current, and future terrestrial carbon fluxes in the Eastern United States. The simulations indicate that forest regrowth following agricultural abandonment accounts for uptake of 0.11 Pg C yr À1 in the 1980s and 0.15 Pg C yr À1 in the 1990s, and regrowth following forest harvesting accounts for an additional 0.1 Pg C yr À1 of uptake during both these decades. The addition of CO 2 fertilization into the model simulations increases carbon uptake rates to 0.38 Pg C yr À1 in the 1980s and 0.47 Pg C yr À1 in the 1990s. Comparisons of predicted aboveground carbon uptake to regional-scale forest inventory measurements indicate that the model's predictions in the absence of CO 2 fertilization are 14% lower than observed, while in the presence of CO 2 fertilization, predicted uptake rates are 28% larger than observed. Comparable results are obtained from comparisons of predicted total Net Ecosystem Productivity to the carbon fluxes observed at the Harvard Forest flux tower site and in model simulations free-air CO 2 enrichment (FACE) experiments. These results imply that disturbance history is the principal mechanism responsible for current carbon uptake in the Eastern United States, and that conventional biogeochemical formulations of plant growth overestimate the response of plants to rising CO 2 levels. Model projections out to 2100 imply that the carbon uptake arising from forest regrowth will increasingly be dominated by forest regrowth following harvesting. Consequently, actual carbon storage declines to near zero by the end of the 21st century as the forest regrowth that has occurred since agricultural abandonment comes into equilibrium with the landscape's new disturbance regime. Incorporating interannual climate variability into the model simulations gives rise to large interannual variation in regional carbon fluxes, indicating that long-term measurements are necessary to detect the signature of processes that give rise to long-term uptake and storage.

Seasonally dry tropical forests (SDTF) are located in regions with alternating wet and dry seasons, with dry seasons that last several months or more. By the end of the 21st century, climate models predict substantial changes in rainfall regimes across these regions, but little is known about how individuals, species, and communities in SDTF will cope with the hotter, drier conditions predicted by climate models. In this review, we explore different rainfall scenarios that may result in ecological drought in SDTF through the lens of two alternative hypotheses: 1) these forests will be sensitive to drought because they are already limited by water and close to climatic thresholds, or 2) they will be resistant/resilient to intra-and inter-annual changes in rainfall because they are adapted to predictable, seasonal drought. In our review of literature that spans microbial to ecosystem processes, a majority of the available studies suggests that increasing frequency and intensity of droughts in SDTF will likely alter species distributions and ecosystem processes. Though we conclude that SDTF will be sensitive to altered rainfall regimes, many gaps in the literature remain. Future research should focus on geographically comparative studies and well-replicated drought experiments that can provide empirical evidence to improve simulation models used to forecast SDTF responses to future climate change at coarser spatial and temporal scales.