Fire enhances the complexity of forest structure in alpine treeline ecotones

Alpine treelines are expected to move upward in a warming climate, but downward in response to increases in wildfire. We studied the effects of fire on vegetation structure and composition across four alpine treeline ecotones extending from Abies lasiocarpa/Picea engelmannii forests at lower elevations, through Pinus albicaulis/Larix lyallii parkland, to alpine tundra. We estimated the probabilities of burning and transitions between states following fire among four canopy-cover (structural) classes: non-forest (0% tree cover), sparse woodland (< 10% tree cover), open forest (10-40% tree cover), and closed forest (> 40% tree cover). We also evaluated changes in the size structure and composition of live overstory trees (>= 1.4 m height) due to mortality following fire. The severity and resulting effects of fire varied among structural classes: Non-forest was less likely to burn than the landscape as a whole; open forest was more likely to remain forest than to change to non-forest; and closed forest never changed to non-forest, irrespective of burn severity. Higher-severity fires caused greater mortality of larger-diameter trees than of smaller-diameter trees. Our results suggest that structural components of the alpine treeline will not respond unidirectionally to a warming climate nor to an increase in fire. Instead, the ecotone will expand bidirectionally and develop larger, more heterogeneous patches of vegetation.