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Extreme drought conditions accompanied by rising temperatures have characterized the American Southwest during the past decade, causing widespread tree mortality in piñion-juniper woodlands. Piñon pine (Pinus edulis Engelm.) mortality is linked primarily to outbreaks of the pinyon ips (Ips confusus (Leconte)) precipitated by drought conditions. Although we searched extensively, no biotic agent was identified as responsible for death in Juniperus L. spp. in this study; hence this mortality was due to direct drought stress. Here we examine the relationship between tree abundance and patterns of mortality in three size classes (seedling/sapling, pre-reproductive, reproductive) during the recent extended drought in three regions: southwest Colorado, northern New Mexico, and northern Arizona. Piñon mortality varied from 32% to 65%, and juniper mortality from 3% to 10% across the three sites. In all sites, the greatest piñon mortality was in the larger, presumably older, trees. Using logistic regression models, we examined the influence of tree density and basal area on bark beetle infestations (piñon) and direct drought impacts (juniper). In contrast to research carried out early in the drought cycle by other researchers in Arizona, we did not find evidence for greater mortality of piñon and juniper trees in increasingly high density or basal area conditions. We conclude that the severity of this regional drought has masked density-dependent patterns visible in less severe drought conditions. With climate projections for the American Southwest suggesting increases in aridity and rising temperatures, it is critical that we expand our understanding of stress responses expected in widespread piñon-juniper woodlands.

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.

Abstract. Climate variability, particularly the frequency of extreme events, is likely to increase in the coming decades, with poorly understood consequences for terrestrial ecosystems. Hydroclimatic variations of the Medieval Climate Anomaly (MCA) provide a setting for studying ecological responses to recent climate variability at magnitudes and timescales comparable to expectations of coming centuries. We examined forest response to the MCA in the humid western Great Lakes region of North America, using proxy records of vegetation, fire, and hydroclimate. Multi-decadal moisture variability during the MCA was associated with a widespread, episodic decline in Fagus grandifolia (beech) populations. Spatial patterns of drought and forest changes were coherent, with beech declining only in areas where proxyclimate records indicate that severe MCA droughts occurred. The occurrence of widespread, drought-induced ecological changes in the Great Lakes region indicates that ecosystems in humid regions are vulnerable to rapid changes in drought magnitude and frequency.