Extreme climatic event-triggered overstorey vegetation loss increases understorey solar input regionally: Primary and secondary ecological implications

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. Although important, assessing extreme-event solar radiation responses regionally following die-off 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). We quantified increases in near-ground solar radiation following overstorey loss of piñon pine cover in response to a recent extreme drought event (2002-2003). We evaluated 211km² in south-western 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. Overstorey loss due to die-off produced increases in near-ground solar radiation regionally each season - up to 28Wm^-2, an increase of 9.1%, in summer - while simultaneously decreasing spatial variation. Annually the increase was c. 17Wm^-2. Larger increases occurred where initial canopy cover was greater or at higher elevations, by as much as c. 80Wm^-2 (a 40% increase). 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 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.