Fire enhances the complexity of forest structure in alpine treeline ecotones


Cansler, C.A., D. McKenzie, and C.B. Halpern. 2018. Fire enhances the complexity of forest structure in alpine treeline ecotones. Ecosphere (2):e02091.


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.