SummaryRecent regional tree die-off events appear to have been triggered by a combination of drought and heat -referred to as 'global-change-type drought'. To complement experiments focused on resolving mechanisms of drought-induced tree mortality, an evaluation of how patterns of tree die-off relate to highly spatially variable precipitation is needed.Here, we explore precipitation relationships with a die-off event of pinyon pine (Pinus edulis Engelm.) in southwestern North America during the 2002-2003 global-change-type drought. Pinyon die-off and its relationship with precipitation was quantified spatially along a precipitation gradient in north-central New Mexico with standard field plot measurements of die-off combined with canopy cover derived from normalized burn ratio (NBR) from Landsat imagery.Pinyon die-off patterns revealed threshold responses to precipitation (cumulative 2002-2003) and vapor pressure deficit (VPD), with little to no mortality (< 10%) above 600 mm and below warm season VPD of c. 1.7 kPa. [Correction added after online publication 17 June 2013; in the preceding sentence, the word 'below' has been inserted.]Our results refine how precipitation patterns within a region influence pinyon die-off, revealing a precipitation and VPD threshold for tree mortality and its uncertainty band where other factors probably come into play -a response type that influences stand demography and landscape heterogeneity and is of general interest, yet has not been documented.

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.

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.

Hydrologic exchange flows (HEFs) across the river‐aquifer interface have important implications for biogeochemical processes and contaminant plume migration in the river corridor, yet little is known about the hydrogeomorphic factors that control HEFs dynamics under dynamic flow conditions. Here, we developed a 3‐D numerical model for a large regulated river corridor along the Columbia River to study how HEFs are controlled by the interplays between dam‐regulated flow conditions and hydrogeomorphic features of such river corridor system. Our results revealed highly variable intra‐annual spatiotemporal patterns in HEFs along the 75‐km river reach, as well as strong interannual variability with larger exchange volumes in wet years than dry years. In general, the river was losing during late spring to early summer when the river stage was high, and river was gaining in fall and winter when river stage was low. The magnitude and timing of river stage fluctuations controlled the timing of high exchange rates. Both river channel geomorphology and the thickness of a highly permeable river bank geologic layer controlled the locations of exchange hot spots, while the latter played a dominant role. Dam‐induced, subdaily to daily river stage fluctuations drove high‐frequency variations in HEFs across the river‐aquifer interfaces, resulting in greater overall exchange volumes as compared to the case without high‐frequency flows. Our results demonstrated that upstream dam operations enhanced the exchange between river water and groundwater with strong potential influence on the associated biogeochemical processes and on the fate and transport of groundwater contaminant plumes in such river corridors.

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