Floyd M Lisa, Clifford Michael, Cobb Neil S, Hanna Dustin, Delph Robert, Ford Paulette, Turner Dave
Environmental Studies Program, Prescott College, Prescott, Arizona 81301, USA.
Ecol Appl. 2009 Jul;19(5):1223-30. doi: 10.1890/08-1265.1.
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.
在过去十年中，极端干旱条件伴随着气温上升一直是美国西南部的特征，导致矮松-杜松林地出现大面积树木死亡。矮松（Pinus edulis Engelm.）死亡主要与干旱条件引发的矮松大小蠹（Ips confusus (Leconte)）爆发有关。尽管我们进行了广泛搜索，但在本研究中未发现任何生物因子导致杜松属植物死亡；因此，这种死亡是由于直接干旱胁迫所致。在这里，我们研究了在科罗拉多州西南部、新墨西哥州北部和亚利桑那州北部这三个地区近期持续干旱期间，三个大小等级（幼苗/幼树、未成熟、成熟）的树木丰度与死亡模式之间的关系。在这三个地点，矮松死亡率在32%至65%之间，杜松死亡率在3%至10%之间。在所有地点，最大的矮松死亡率出现在较大、可能较老的树木中。我们使用逻辑回归模型，研究了树木密度和基部面积对树皮甲虫侵害（矮松）和直接干旱影响（杜松）的影响。与亚利桑那州其他研究人员在干旱周期早期进行的研究不同，我们没有发现在密度或基部面积不断增加的情况下，矮松和杜松树死亡率更高的证据。我们得出结论，该地区干旱的严重程度掩盖了在不太严重干旱条件下可见的密度依赖模式。鉴于对美国西南部气候的预测表明干旱加剧和气温上升，至关重要的是，我们要扩大对广泛的矮松-杜松林地预期应激反应的理解。
