Department of Geography, University of Guelph, Guelph, ON, Canada.
Glob Chang Biol. 2018 Oct;24(10):4489-4504. doi: 10.1111/gcb.14338. Epub 2018 Jul 1.
Treeline advance has occurred throughout the twentieth century in mountainous regions around the world; however, local variation and temporal lags in responses to climate warming indicate that the upper limits of some treelines are not necessarily in climatic equilibrium. These observations suggest that factors other than climate are constraining tree establishment beyond existing treelines. Using a seed addition experiment, we tested the effects of seed availability, predation and microsite limitation on the establishment of two subalpine tree species (Picea engelmannii and Abies lasiocarpa) across four treelines in the Canadian Rocky Mountains. The effect of vegetation removal on seedling growth was also determined, and microclimate conditions were monitored. Establishment limitations observed in the field were placed in context with the effects of soil properties observed in a parallel experiment. The seed addition experiment revealed reduced establishment with increasing elevation, suggesting that although establishment within the treeline ecotone is at least partially seed limited, other constraints are more important beyond the current treeline. The effects of herbivory and microsite availability significantly reduced seedling establishment but were less influential beyond the treeline. Microclimate monitoring revealed that establishment was negatively related to growing season temperatures and positively related to the duration of winter snow cover, counter to the conventional expectation that establishment is limited by low temperatures. Overall, it appears that seedling establishment beyond treeline is predominantly constrained by a combination of high soil surface temperatures during the growing season, reduced winter snowpack and unfavourable soil properties. Our study supports the assertion that seedling establishment in alpine treeline ecotones is simultaneously limited by various climatic and nonclimatic drivers. Together, these factors may limit future treeline advance in the Canadian Rocky Mountains and should be considered when assessing the potential for treeline advance in alpine systems elsewhere.
整个 20 世纪，世界各地的山区都出现了树线推进现象；然而，对气候变暖的响应存在局部变化和时间滞后，这表明并非所有树线的上限都处于气候平衡状态。这些观察结果表明，除了气候之外，还有其他因素限制了树木在现有树线以外的地方生长。本研究使用种子添加实验，测试了种子可获得性、捕食和微生境限制对两种亚高山树种（云杉和落叶松）在加拿大落基山脉四个树线位置上的建立的影响。还确定了植被去除对幼苗生长的影响，并监测了微气候条件。在野外观察到的建立限制与平行实验中观察到的土壤特性的影响放在一起进行了考虑。该种子添加实验表明，随着海拔的升高，建立的可能性降低，这表明尽管在树线生态交错带内的建立至少部分受到种子限制，但在现有树线以外，其他限制更为重要。食草作用和微生境可获得性的影响显著降低了幼苗的建立，但在树线以外的影响较小。微气候监测表明，建立与生长季节温度呈负相关，与冬季积雪持续时间呈正相关，这与传统的低温限制建立的预期相反。总的来说，看来树线以外的幼苗建立主要受到生长季节土壤表面温度高、冬季积雪减少和不利的土壤特性的限制。本研究支持了这样一种观点，即高山树线生态交错带中的幼苗建立同时受到各种气候和非气候驱动因素的限制。这些因素共同作用可能限制了加拿大落基山脉未来的树线推进，并在评估其他高山地区树线推进的潜力时应加以考虑。
