Interval squeeze: altered fire regimes and demographic responses interact to threaten woody species persistence as climate changes

© The Ecological Society of America www.frontiersinecology.org D this century, climate change is projected to cause profound plant species losses and migrations – and shifts in ecosystem structure and function – in many parts of the world (including several biodiversity hotspots; Malcolm et al. 2006). However, such assessments are based almost entirely on empirical niche (or climate envelope) models, which measure only one component of climate vulnerability (Dawson et al. 2011); published evidence points to more complex interaction effects, suggesting that proximate causes such as altered disturbance frequencies are more important than direct climate impacts (Cahill et al. 2013; Grimm et al. 2013). Despite accumulating evidence of indirect effects, conceptual models offering broader, mechanistic views that facilitate prediction are lacking. Fire is the dominant terrestrial, landscape-scale disturbance factor globally (Bowman et al. 2009), and recent studies using a range of global climate models project a more fire-prone future for most terrestrial ecosystems, especially in the middle and high latitudes (Moritz et al. 2012). Particularly vulnerable to the interactive effects of climate and fire-regime change are biomes where climate is projected to become both warmer and drier, a combination that will increase droughtrelated impacts and fire frequency (eg Mediterraneantype ecosystems; Figure 1). Current conceptions of plant species responses to climate change in fire-prone ecosystems highlight the potential impacts either of climatic envelope shifts (where climate change leaves species stranded at sites no longer suitable for their growth; eg Fitzpatrick et al. 2008) or of shortened fire intervals, which increase “immaturity risk” (ie insufficient growing time before fire recurs; eg Westerling et al. 2011). Nevertheless, these impacts ignore – or at least conflate – multiple threatening processes acting together (Brook et al. 2008), which could produce markedly different outcomes for species and ecosystems, including notable shifts in composition and structure in response to comparatively small shifts in species climate envelopes (Scheffer et al. 2001; Bowman et al. 2014a). Furthermore, in light of the ongoing global trend of habitat fragmentation, plant species conservaCONCEPTS AND QUESTIONS