Tree-line Dynamics: Adding Fire to Climate Change Prediction

Abstract

Fire is the dominant disturbance that drives ecosystem structure and function in the boreal forest of North America (Weber and Flannigan, 1997). Fire frequency varies widely across different regions of the boreal forest, but the interval between fires is generally long enough for trees to re-establish and recover to mature stands before the stand burns again (Heinselman, 1981; Payette, 1992). In the western boreal forest of North America, historic fire-return intervals have been estimated as typically 80–120 years between fires (Johnson, 1992). Tree establishment occurs most rapidly in the first years following fire, when the availability of viable seed and nutrients is at its highest (Johnson and Fryer, 1989; Johnstone et al., 2004). In a typical fire cycle of one to two centuries, the individuals that germinate in the first few years following a fire are the same individuals that reach reproductive maturity and are burned in the next fire, continuing the cycle of post-fire self-replacement. Boreal tree species demonstrate a variety of adaptations to fire, including serotinous cones (that require heat for seed release) maintained in aerial seed banks (Pinus spp., Picea mariana) and vegetative re-sprouting from roots and stumps following fire (Betula spp., Populus spp.; Li, 2000). In the latitudinal tree line (or forest-tundra ecotone) of the boreal forest, the growing conditions for trees are suboptimal. Because of the underlying permafrost, the soils of this region are relatively cold and wet and thus have slow nutrient turnover rates. Black spruce (Picea mariana), a dominant boreal conifer, exhibits slow growth under these conditions, taking several decades to reach reproductive maturity. However, historical fire cycles allow for black spruce to accumulate enough viable seed for self-replacement after fire. Many regions of the circumpolar North have undergone an increase in temperature, particularly in the past four decades (Chapin et al., 2005). The northern regions of Yukon and Alaska have experienced a 2 C increase in summer temperatures since the 1960s (Chapin et al., 2005). In the boreal forest, fires are expected to occur more often as the climate warms (Stocks et al., 1998; Flannigan et al., 2005; Soja et al., 2007), and increased fire activity is already being noted in some parts of the region (Gillett et al., 2004; Kasischke and Turetsky, 2006). Because of the long period black spruce require to become reproductively mature, an increase in fire activity may interrupt the cycle of post-fire self-replacement for this dominant boreal conifer. This interruption could initiate a change in the structure and function of these northern ecosystems that will have important implications for the global carbon cycle because it alters patterns of carbon accumulation and storage (Grace et al., 2002; Bond-Lamberty et al., 2004). The long-term, landscape-level consequences of altering the fire return interval in the tree-line forest are unknown. On the decadal scale, shifts from coniferous to deciduous dominance after high-severity or frequent fire have been documented in tree-line and interior northern boreal forests (Landhausser and Wein, 1993; Johnstone and Chapin, 2006). The failure of coniferous species following fire is thought to occur because of reduced availability of viable seed (Landhausser and Wein, 1993; Lloyd et al., 2002; Johnstone and Chapin, 2006). Tree-line forests, by definition, occupy areas on the extremes of species tolerance limits, and thus are likely to be sensitive to direct climate effects. With increased temperatures in northern ecosystems, it is assumed 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. Through my PhD research, I am specifically focusing on the indirect pathway of disturbance-mediated recruitment by investigating whether an altered fire return interval will initiate ecosystem change in the tree-line forest of northern Yukon.