Large-scale biogeographical shifts in vegetation are predicted in response to the altered precipitation and temperature regimes associated with global climate change. Vegetation shifts have profound ecological impacts and are an important climate-ecosystem feedback through their alteration of carbon, water, and energy exchanges of the land surface. Of particular concern is the potential for warmer temperatures to compound the effects of increasingly severe droughts by triggering widespread vegetation shifts via woody plant mortality. The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates-temperature-sensitive carbon starvation in response to a period of protracted water stress or temperature-insensitive sudden hydraulic failure under extreme water stress (cavitation). Here we show that experimentally induced warmer temperatures (Ϸ4°C) shortened the time to droughtinduced mortality in Pinus edulis (piñ on shortened pine) trees by nearly a third, with temperature-dependent differences in cumulative respiration costs implicating carbon starvation as the primary mechanism of mortality. Extrapolating this temperature effect to the historic frequency of water deficit in the southwestern United States predicts a 5-fold increase in the frequency of regional-scale tree die-off events for this species due to temperature alone. Projected increases in drought frequency due to changes in precipitation and increases in stress from biotic agents (e.g., bark beetles) would further exacerbate mortality. Our results demonstrate the mechanism by which warmer temperatures have exacerbated recent regional die-off events and background mortality rates. Because of pervasive projected increases in temperature, our results portend widespread increases in the extent and frequency of vegetation die-off.biosphere-atmosphere feedbacks ͉ drought impacts ͉ global-change ecology ͉ Pinus edulis ͉ carbon starvation

[1] In water-limited ecosystems, partitioning ecosystemscale evapotranspiration fluxes between plant transpiration and soil/canopy evaporation remains a theoretical and technical challenge. We used the Biosphere 2 glasshouse to assess partitioning of evapotranspiration across an experimentally manipulated gradient of woody plant cover using continuous measurements of near-surface variations in the stable isotopic composition of water vapor (d 2 H). Our technique employs a newly-developed laser-based isotope analyzer and the Keeling plot approach for surface flux partitioning. The applicability of the technique was verified by comparison to separate, simultaneous lysimeter and sap flow estimates of ET partitioning. The results showed an expected increase in fractional contribution of transpiration to evapotranspiration as woody cover increased-from T/ET = 0.61 at 25% woody cover to T/ET = 0.83 at 100% cover. Further development of this technique may enable field characterization of evapotranspiration partitioning across diverse woody cover gradients, a central issue in addressing dryland ecohydrological responses to land use and climate change. Citation:

Summary1. Climate extremes such as drought can trigger large-scale tree die-off, reducing overstorey canopy and thereby increasing near-ground solar radiation. This directly affects biotic and abiotic processes, including plant physiology, reproduction, phenology, soil evaporation and nutrient cycling, which themselves affect understory facilitation, productivity and diversity, and land surface-atmosphere fluxes of energy, carbon and water. 2. Although important, assessing extreme-event solar radiation responses regionally following dieoff is complex compared with characterizing patch-scale inputs. Estimating regional-scale changes requires integration of broad-scale downward-looking shading patterns due to canopy and topography with fine-scale upward-looking canopy details (e.g. live vs. dead trees, height, diameter, spatial pattern and foliar diffusivity). 3. We quantified increases in near-ground solar radiation following overstorey loss of pin˜on pine cover in response to a recent extreme drought event (2002)(2003). We evaluated 211 km 2 in southwestern USA seasonally and annually using high-spatial resolution satellite imagery, hemispherical ground photography, GIS (Geographic Information System)-based solar radiation modelling tools, in situ meteorological data and tree measurements. , an increase of 9.1%, in summer -while simultaneously decreasing spatial variation. Annually the increase was c. 17 W m . Larger increases occurred where initial canopy cover was greater or at higher elevations, by as much as c. 80 W m )2 (a 40% increase).5. Synthesis. Our results are notable in that they quantify increases regionally in near-ground solar radiation in response to a climate extreme triggering widespread tree die-off. The substantial increases quantified are expected to have primary direct effects on processes such as plant physiology, reproduction, phenology, soil evaporation and nutrient cycling, and secondary effects on understory facilitation, productivity and diversity, and land surface-atmosphere fluxes of energy, carbon and water. Consequently, extreme event-induced changes in near-ground solar radiation need to be considered by both ecologists and physical scientists in assessing global change impacts. More generally, our results highlight an important but sometimes overlooked aspect of plant Journal of Ecology 2011Ecology , 99, 714-723 doi: 10.1111Ecology /j.1365Ecology -2745Ecology .2011 ecology -that plants not only respond to their physical environment and other plants, but also directly modify their physical environment from individual plant to regional scales.

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Much of the terrestrial biosphere can be viewed as part of a gradient, with varying amounts of woody plant cover ranging from grassland to forest—the grassland–forest continuum. Woody plant cover directly impacts the soil microclimate through modifications of near‐ground solar radiation and soil temperature, and these interactive effects are relevant for key ecohydrological processes such as soil evaporation. Trends in how increasing woody plant cover affect soil surface microclimate have recently been evaluated for gradients of evergreen woody plants, but analogous trends for deciduous plants, where phenology should be influential, are lacking. We evaluated season‐dependent changes in soil microclimate along a deciduous grassland–forest continuum of velvet mesquite (Prosopis velutina Wooton) using repeated hemispherical photography and continuous soil temperature measurements at the 5‐cm depth. Both near‐ground solar radiation and soil temperature decreased with increasing canopy cover, even during the leafless season. The trends varied substantially among seasons, however, with differences between canopy and intercanopy patches readily evident only during the period of full leaf‐out, during which the correlation between near‐ground solar radiation and soil temperature was strongest. Our results provide a more comprehensive understanding about the interactions of canopy cover, canopy structure attributes, and plant phenology that produce seasonally pulsed heterogeneity in the soil surface microclimate. Notably, our results add a new dimension to the moisture “pulse dynamics” perspective commonly applied to dryland ecohydrology, highlighting seasonally pulsed heterogeneity in soil microclimate that could influence soil moisture dynamics in drylands.

Water limited ecohydrological systems (WLES), with their broad extent, large stores of global terrestrial carbon, potential for large instantaneous fluxes of carbon and water, sensitivity to environmental changes, and likely global expansion, are particularly important ecohydrological systems. Strong nonlinear responses to environmental variability characterize WLES, and the resulting complexity of system dynamics has challenged research focussed on general understanding and site specific predictions. To address this challenge our synthesis brings together current views of complexity from ecological and hydrological sciences to look towards a framework for understanding ecohydrological systems (in particular WLES) as complex adaptive systems (CAS). This synthesis suggests that WLES have many properties similar to CAS. In addition to exhibiting feedbacks, thresholds, and hysteresis, the functioning of WLES is strongly affected by self-organization of both vertical and horizontal structure across multiple scales. As a CAS, key variables for understanding WLES dynamics are related to their potential for adaptation, resistance to variability, and resilience to state changes. Several essential components of CAS, including potential for adaptation and rapid changes between states, pose challenges for modelling and generating predictions of WLES. Model evaluation and predictable quantities may need to focus more directly on temporal or spatial variance in contrast to mean state values for success at understanding system-level characteristics. How coupled climate and vegetation changes will alter available soil, surface and groundwater supplies, and overall biogeochemistry will reflect how self-organizational ecohydrological processes differentially partition precipitation and overall net metabolic functioning.

In tropical regions, particularly in Central and South America (CSA), the projections of climate seasonality under climate change are still uncertain. This is especially true for ecologically-relevant variables such as precipitation and temperature. However, assessments of model-based projections of seasonal climate for this region are scarce. We analyzed the simulation of seasonal precipitation and air surface temperature in CSA and six sub-regions within from 49 models included in the Coupled Intercomparison Project Phase 5 (CMIP5) and 33 models from CMIP6. In general, continental patterns and seasonality of both variables are moderately well resembled, while most models show systematic biases over the oceans, producing unrealistic spatial patterns. To quantify how well CMIP5/CMIP6 models simulate these variables, we used Taylor diagrams with respect to TRMM for precipitation and ERA5 for temperature.Precipitation shows the largest spread among models. Conversely, temperature shows a better simulation. CMIP5/CMIP6 models exhibit a better performance simulating both variables during December-January-February and March-April-May than during the other seasons. This is partly due to the reduced model biases in representing the Intertropical Convergence Zone during these two seasons. In general, biases are reduced in the CMIP6 models with respect to CMIP5. Regarding regional evaluations, precipitation patterns for Mesoamerica, Cerrado and Chaco regions are better reproduced compared to TRMM, while the annual cycles for the Andes hotspot, Central Chile and Guianas are not well simulated, mainly during their wet seasons. However, these biases are reduced in CMIP6 models. In regard to precipitation projections, models only agree over most of the regions with decreasing precipitation. Conversely, temperature exhibits a general consensus on persistent warming even during the historical period, with an average increase of 6 C by the end of the century, according to the CMIP6 models.

[1] The North American monsoon is a key feature affecting summer climate over Southwestern North America. During the monsoon, evapotranspiration from the Southwest promotes transference of water to the atmosphere which is subsequently distributed across the continent -linking the SW to other regions via atmospheric hydrologic connectivity. However, the degree to which atmospheric connectivity redistributes monsoonal terrestrial moisture throughout the continent and its sensitivity to climate disturbances such as drought is uncertain. We tracked the trajectory of moisture evapotranspired within the semiarid Southwest during the monsoon season using a Lagrangian analytical model. Southwest moisture was advected north-east accounting for $15% of precipitation in adjacent Great Plains regions. During recent drought (2000 -2003), this amount decreased by 45%. Our results illustrate that the spatial extent of the North American monsoon is larger than normally considered when accounting for hydrologic connectivity via soil moisture redistribution through atmospheric pathways.