Shinneman Douglas J, Baker William L
Department of Botany, University of Wyoming, Laramie, Wyoming 82071, USA.
Ecol Appl. 2009 Jul;19(5):1231-45. doi: 10.1890/08-0846.1.
Fire is known to structure tree populations, but the role of broad-scale climate variability is less clear. For example, the influence of climatic "teleconnections" (the relationship between oceanic-atmospheric fluctuations and anomalous weather patterns across broad scales) on forest age structure is relatively unexplored. We sampled semiarid piñon-juniper (Pinus edulis-Juniperus osteosperma) woodlands in western Colorado, U.S.A., to test the hypothesis that woodland age structures are shaped by climate, including links to oceanic-atmospheric fluctuations, and by past fires and livestock grazing. Low-severity surface fire was lacking, as fire scars were absent, and did not influence woodland densities, but stand-replacing fires served as long-rotation (>400-600 years), stand-initiating events. Old-growth stands (>300 years old) were found in 75% of plots, consistent with a long fire rotation. Juniper and piñon age structures suggest contrasting responses during the past several centuries to dry and wet episodes linked to the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO). Juniper density increased slightly during periods of drought, positive (warm) AMO (after approximately 10-year lag), and negative (cool) PDO. In contrast, piñon populations may still be recovering from a long, drought-filled period (AD 1620-1820), with pulses of recovery favored during cool AMO, warm PDO, and above-average moisture periods. Analysis of 20th-century tree establishment and instrumental climate data corroborate the long-term relationships between age structure and climate. After Euro-American settlement (AD 1881), livestock grazing reduced understory grasses and forbs, reducing competition with tree seedlings and facilitating climate-induced increases in piñons. Thus tree populations in these woodlands are in flux, affected by drought and wet periods linked to oceanic-atmospheric variability, Euro-American livestock grazing, and long-rotation, high-severity fires. Reductions in livestock grazing levels may aid ecological restoration efforts. However, given long-term fluctuations in tree density and composition, and expected further drought, thinning or burning to reduce tree populations may be misdirected.
众所周知，火灾会影响树木种群的结构，但大尺度气候变化的作用尚不清楚。例如，气候“遥相关”（大洋 - 大气波动与大尺度异常天气模式之间的关系）对森林年龄结构的影响相对较少被研究。我们在美国科罗拉多州西部对半干旱的矮松 - 杜松（矮松 - 硬果杜松）林地进行了采样，以检验以下假设：林地年龄结构受气候影响，包括与大洋 - 大气波动的联系，以及过去的火灾和牲畜放牧的影响。由于没有火灾疤痕，所以缺乏低强度地表火，且其并未影响林地密度，但高强度林冠火是长周期（>400 - 600年）的、引发林分形成的事件。在75%的样地中发现了老龄林分（>300年），这与长火灾周期一致。杜松和矮松的年龄结构表明，在过去几个世纪中，它们对与大西洋多年代际振荡（AMO）和太平洋年代际振荡（PDO）相关的干湿事件有不同的响应。在干旱期、正（暖）AMO（约10年滞后）和负（冷）PDO期间，杜松密度略有增加。相比之下，矮松种群可能仍在从漫长的干旱期（公元1620 - 1820年）中恢复，在冷AMO、暖PDO和高于平均湿度时期，恢复的脉冲更为明显。对20世纪树木建立和仪器观测气候数据的分析证实了年龄结构与气候之间的长期关系。在欧美移民定居（公元1881年）后，牲畜放牧减少了林下草本植物和阔叶植物，减少了与树苗的竞争，并促进了气候导致的矮松数量增加。因此，这些林地中的树木种群处于变化之中，受到与大洋 - 大气变率相关的干旱和湿润时期、欧美牲畜放牧以及长周期、高强度火灾的影响。降低牲畜放牧水平可能有助于生态恢复工作。然而，考虑到树木密度和组成的长期波动，以及预期的进一步干旱，为减少树木数量而进行的疏伐或焚烧可能是错误的做法。
