Department of Geography, College of Geosciences, Texas A&M University, College Station, Texas 77843-3147, USA.
Ecology. 2011 Feb;92(2):496-506. doi: 10.1890/09-1725.1.
Throughout interior Alaska (U.S.A.), a gradual warming trend in mean monthly temperatures occurred over the last few decades (approximatlely 2-4 degrees C). The accompanying increases in woody vegetation at many alpine treeline (hereafter treeline) locations provided an opportunity to examine how biotic and abiotic local site conditions interact to control tree establishment patterns during warming. We devised a landscape ecological approach to investigate these relationships at an undisturbed treeline in the Alaska Range. We identified treeline changes between 1953 (aerial photography) and 2005 (satellite imagery) in a geographic information system (GIS) and linked them with corresponding local site conditions derived from digital terrain data, ancillary climate data, and distance to 1953 trees. Logistic regressions enabled us to rank the importance of local site conditions in controlling tree establishment. We discovered a spatial transition in the importance of tree establishment controls. The biotic variable (proximity to 1953 trees) was the most important tree establishment predictor below the upper tree limit, providing evidence of response lags with the abiotic setting and suggesting that tree establishment is rarely in equilibrium with the physical environment or responding directly to warming. Elevation and winter sun exposure were important predictors of tree establishment at the upper tree limit, but proximity to trees persisted as an important tertiary predictor, indicating that tree establishment may achieve equilibrium with the physical environment. However, even here, influences from the biotic variable may obscure unequivocal correlations with the abiotic setting (including temperature). Future treeline expansion will likely be patchy and challenging to predict without considering the spatial variability of influences from biotic and abiotic local site conditions.
在美国阿拉斯加内陆地区，过去几十年中月平均温度呈逐渐上升趋势（约 2-4 摄氏度）。许多高山林线（以下简称林线）地区木本植被的相应增加为研究生物和非生物局部地点条件如何相互作用以控制变暖期间树木建立模式提供了机会。我们设计了一种景观生态学方法，在阿拉斯加山脉的一个未受干扰的林线地区研究这些关系。我们在地理信息系统 (GIS) 中确定了 1953 年（航空摄影）和 2005 年（卫星图像）之间的林线变化，并将它们与从数字地形数据、辅助气候数据和到 1953 年树木的距离中得出的相应局部地点条件联系起来。逻辑回归使我们能够对控制树木建立的局部地点条件的重要性进行排序。我们发现，树木建立控制的重要性存在空间转变。在林线下限以下，生物变量（与 1953 年树木的接近程度）是树木建立的最重要预测因子，这提供了与非生物环境响应滞后的证据，并表明树木建立很少与物理环境达到平衡或直接对变暖做出反应。海拔和冬季阳光暴露是林线上限树木建立的重要预测因子，但树木接近度仍然是重要的三级预测因子，表明树木建立可能与物理环境达到平衡。然而，即使在这里，生物变量的影响也可能掩盖与非生物环境（包括温度）之间明确的相关性。如果不考虑生物和非生物局部地点条件影响的空间变异性，未来的林线扩张可能是零散的，难以预测。
