Thursday, August 9, 2007 - 3:20 PMOOS 42-2: Dieback of forests and woodlands across elevational gradients in response to global-change-type drought in the southwestern US, North America
Neil Cobb1, Kirsten Ironside1, John D. Shaw2, Kiona Ogle3, Craig D. Allen4, David D. Breshears5, and Michael Clifford1. (1) Northern Arizona University, (2) USDA Forest Service, (3) University of Wyoming, (4) Jemez Mountains Field Station, (5) University of Arizona
Since 1996 the southwestern US has been subject to unusually warm conditions and the most severe drought since the 1950s, triggering significant climate-induced dieback in many tree species across montane elevational gradients. The spatial pattern of climate-induced dieback often reveals preferential mortality initially along the drier, lower fringes of tree species distributions, with dieback subsequently extending upslope. Since 2002 substantial dieback has been observed in Engelmann spruce (Picea engelmannii), aspen (Populus tremuloides), Douglas-fir (Pseudotsuga menziesii), white fir (Abies concolor), ponderosa pine (Pinus ponderosa), piñon (Pinus edulis), junipers (Juniperus spp.), Quercus spp., Cercocarpus spp., Arctostaphylos spp., Cercidium sp., and other woody (and non-woody) species in various portions of the Southwest, from Sonoran Desert woodlands to subalpine forests. In some cases, insect outbreaks have markedly amplified the magnitude of tree mortality. For example, climate-stressed piñon was subject to a massive outbreak of a bark beetle (Ips confusus), with significant piñon mortality affecting more than 1,000,000 ha in 2002-2004 and exceeding 90% of mature individuals across extensive areas, thereby shifting woodland compositions strongly toward Juniperus dominance. In this paper we will specifically examine patterns in mortality of individual species across elevation gradients that span their distribution from lower to upper ecotones, and we will compare community level mortality patterns from lower-elevation to upper-elevation life zones. Finally, we explore the role of biotic agents in magnifying the degree of mortality across elevation gradients. The general patterns of tree die-off provide important insights relevant for assessing the impacts and implications of climate change.