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

SummaryVegetation change is expected with global climate change, potentially altering ecosystem function and climate feedbacks. However, causes of plant mortality, which are central to vegetation change, are understudied, and physiological mechanisms remain unclear, particularly the roles of carbon metabolism and xylem function.We report analysis of foliar nonstructural carbohydrates (NSCs) and associated physiology from a previous experiment where earlier drought-induced mortality of Pinus edulis at elevated temperatures was associated with greater cumulative respiration. Here, we predicted faster NSC decline for warmed trees than for ambient-temperature trees.Foliar NSC in droughted trees declined by 30% through mortality and was lower than in watered controls. NSC decline resulted primarily from decreased sugar concentrations. Starch initially declined, and then increased above pre-drought concentrations before mortality. Although temperature did not affect NSC and sugar, starch concentrations ceased declining and increased earlier with higher temperatures.Reduced foliar NSC during lethal drought indicates a carbon metabolism role in mortality mechanism. Although carbohydrates were not completely exhausted at mortality, temperature differences in starch accumulation timing suggest that carbon metabolism changes are associated with time to death. Drought mortality appears to be related to temperature-dependent carbon dynamics concurrent with increasing hydraulic stress in P. edulis and potentially other similar species.

The hydrological and geomorphological impacts of traditional swidden cultivation in Montane Mainland Southeast Asia are virtually inconsequential, whereas the impacts associated with intensified replacement agricultural systems are often much more substantial. Negative perceptions toward swiddening in general by governments in the region beginning half a decade ago have largely been based on cases of forest conversion and land degradation associated with (a) intensified swidden systems, characterized by shortened fallow and extended cropping periods and/or (b) the widespread cultivation of opium for cash after the Second World War. Neither of these practices should be viewed as traditional, subsistence-based swiddening. Other types of intensive agriculture systems are now replacing swiddening throughout the region, including semi-permanent and permanent cash cropping, monoculture plantations, and greenhouse complexes. The negative impacts associated with these systems include changes in streamflow response, increased surface erosion, a higher probability of landslides, and the declination in stream water quality. Unlike the case for traditional swiddening, these impacts result because of several factors: (1) large portions of upland catchments are cultivated simultaneously; (2) accelerated hydraulic and tillage erosion occurs on plots that are cultivated repetitively with limited or no fallowing to allow recovery of key soil properties, including infiltration; (3) concentrated overland flow and erosion sources are often directly connected with the stream network; (4) root strength is reduced on permanently converted hillslopes; (5) surface and ground water extraction is frequently used for irrigation; and (6) and pesticides and herbicides are used. Furthermore, the commercial success of these systems relies on the existence of dense networks of roads, which are linear landscape features renowned for disrupting hydrological and geomorphological systems. A new conservation focus is needed to reduce the impacts of these intensified upland agricultural practices.

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This study investigates basin-scale hydrologic implications of the replacement of forest-dominated land cover by rubber plantations in Montane Mainland Southeast Asia. The paper presents a new method for estimating the water demand of rubber and consequently water losses to the atmosphere through rubber evapotranspiration (ET). In this paper we argue that rubber ET is energy-limited during the wet season, but during the dry season water consumption is mostly governed by environmental variables that directly affect rubber phenology, namely, vapour pressure deficit, temperature and photoperiodicity. The proposed ET model is introduced into a hillslope-based hydrologic model to predict the basin-scale hydrologic consequences of rubber replacing native vegetation. Simulations suggest greater annual catchment water losses through ET from rubber dominated landscapes compared to traditional vegetation cover. This additional water use reduces discharge from the basin, or its storage.

This study investigates the hydrologic implications of land use conversion from native vegetation to rubber (Hevea brasiliensis) in Southeast Asia. The experimental catchment, Nam Ken (69 km 2 ), is located in Xishuangbanna Prefecture (22°N, 101°E), in the south of Yunnan province, in southwestern China. During 2005 and 2006, we collected hourly records of 2 m deep soil moisture profiles in rubber and three native land-covers (tea, secondary forest and grassland), and measured surface radiation above the tea and rubber canopies. Observations show that root water uptake of rubber during the dry season is controlled by day-length, whereas water demand of the native vegetation starts with the arrival of the first monsoon rainfall. The different dynamics of root water uptake in rubber result in distinct depletion of soil moisture in deeper layers. Traditional evapotranspiration and soil moisture models are unable to simulate this specific behaviour. Therefore, a different conceptual model, taking in account vegetation dynamics, is needed to predict hydrologic changes due to land use conversion in the area.

Aim Movement of water from the land surface to the atmosphere (evapotranspiration, ET) is the dominant output flux in the global terrestrial surface water budget. The partitioning of ET between soil evaporation (E) and plant transpiration (T) couples important ecological, hydrological and atmospheric processes. ET partitioning has been hypothesized to vary as a function of woody plant cover, yet a relationship between ET partitioning and woody cover has not been quantified empirically. Land surface models assume unit increase in T per unit increase in vegetation cover (woody cover), following a proportional linear relationship. Recent assessments have questioned the validity of this assumption for heterogeneous canopies, but we lack experimental data across an explicitly defined gradient of woody cover to characterize this relationship. Location North American monsoon region. Methods In a controlled dryland environment experimental facility, we manipulated woody cover and documented the response of ET and its component fluxes. We incorporated the resulting functions into a widely used coupled land–atmosphere model (WRF‐Noah) to document the implications of modifying specific model parameters that assume (1:1) proportionality. Results As total ET increased with woody cover, T/ET deviated below 1:1 proportionality. Using our experimentally determined relationship for ET partitioning and woody cover in the model, we observed reductions in ET of as much as 40% during the monsoon season and annual increases of almost 200% in regional E. Main conclusions Our results highlight a limitation of modelled ET that affects regional to global patterns of water flux, with implications for a number of earth surface processes. A better understanding of how changing woody cover influences patch‐scale ecohydrological processes is needed, particularly under current changes in woody cover associated with deforestation, afforestation and drought‐induced mortality. More specifically, improved representation of E and T fluxes will improve understanding and modelling of large‐scale ecological, hydrological and atmospheric processes.

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