Relationship of stand characteristics to drought‐induced mortality in three Southwestern piñon–juniper woodlands

Abstract

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.

Methodology and performance of a rainfall manipulation experiment in a piñon–juniper woodland

“…Extended drought in the southwestern United States has resulted in extensive forest and woodland tree mortality (Breshears et al 2005, Floyd et al 2009, Allen et al 2010. During a regional drought that occurred from [2000][2001][2002][2003], widespread mortality occurred in populations of piñon pine (Pinus edulis); a species which dominates piñon-juniper woodlands-one of the most extensive vegetation types in western North America (Gottfried et al 1995, Mueller et al 2005, Breshears et al 2009, Floyd et al 2009, Clifford et al 2011. Global climate change associated with continued, high fossil fuel emissions and subsequent increase in atmospheric [CO 2 ] is expected to exacerbate the frequency and severity of future droughts in many areas (Houghton et al 1996, Easterling et al 2000, IPCC 2001, IPCC 2007, Seager et al 2007, Allison et al 2009).…”

Effects of climatic variability and change on forest ecosystems: a comprehensive science synthesis for the U.S

“…In addition, projected changes in forest distribution, structure, and function need to be synthesized using recent GCM projections, including quantification of uncertainties about the effects of climate on forest processes. North America during the past decade, an indicator that a physiological threshold was exceeded because of the effects of low soil moisture (Floyd et al 2009). Although this is not direct evidence of the effects of climate change, it demonstrates the effects of severe drought, a phenomenon expected more frequently in the future, on large-scale forest structure and function in arid environments.…”

Refined forest land use classification with implications for United States national carbon accounting

“…For example, Melillo et al, 2014 suggests increased drought, fire, and other disturbances across these areas. Given that moisture is a limiting factor in most of these systems (Coulston et al, 2010;Floyd et al, 2009;Marlon et al, 2012) future evaluations will need to be made as dominant tree species may change and disturbance impacts become apparent. Further, a re-evaluation of results under current and potential future climate will elucidate potential rural land use implications of plausible climate change impacts.…”