Ecological responses to altered fire frequency in treeline forests of the north Yukon, Canada.

摘要

Climate change is expected to alter patterns of disturbance, which in turn may drive future ecosystem change. This interaction may be particularly important in sub-arctic regions due to rapid climate change and frequent fire. With increased temperatures in northern ecosystems, there is an assumption that typically southern species will shift their distributions northwards. In the northwestern boreal forest, however, the bottleneck to forest establishment is recruitment, which is strongly mediated by reproductive output and disturbance effects. This research focused on the indirect pathway of disturbance-mediated recruitment. The interaction between fire and forest successional processes in sub-arctic stands undergoing climatic warming was investigated by using field observations and experimental studies at the northern limit of the boreal forest, where the recent fire history created a mosaic of black spruce ( Picea mariana [Mill.] BSP) stands burned with varying fire return intervals. This research had two main objectives: 1) Identify how fire influences tree recruitment in northern treeline forests, and; 2) Investigate the responses of northern forests to a change in fire return interval. Fire may either accelerate or hinder treeline movement, depending upon regional conditions and fire severity. Recruitment of black spruce was drastically reduced following two closely timed fires (∼15 year interval), compared to stands burned under a typical fire return interval, and carbon storage across disturbance intervals was dramatically reduced following short-interval burns. Recruitment was also limited in mature forest stands. On-site germination experiments demonstrated that recruitment was limited by seed availability after a short fire return interval, and by seedbed quality in unburned stands. Once established as seedlings, all native tree species studied benefited from the more favourable environmental conditions for growth in short-interval burned stands. It is predicted that a positive effect of warming, in the form of direct climate effects or indirect fire effects, on post-fire recruitment potential in the absence of seed limitation. However, this pattern could reverse, becoming a negative effect, if conditions become too dry. Increases in fire activity may thus lead to a degradation of treeline forests in some areas, rather than the increased tree density and forest extent predicted with climate warming.