Abrupt changes in subalpine forest landscapes in a warmer world with more fire

Abstract:

Subalpine forests in the northern US Rocky Mountains have been resilient to stand-replacing fires that historically burned at 100–300-yr intervals. During the 21^st century, warming climate and novel fire regimes could lead to drastic forest change or even collapse (abrupt loss). Anticipating kinds, rates and magnitude of change is challenging; fires are stochastic, and many interacting factors drive forest dynamics. We studied 5 representative landscapes of Greater Yellowstone (Wyoming, USA) and asked: (1) How are forests likely to change with 21^st-century warming and fire activity? (2) Within a landscape, are forest changes gradual or abrupt, and do different forest attributes change synchronously or sequentially? (3) Can mid-century stabilization of anthropogenic carbon emissions avert forest collapse? Using the spatially explicit, individual-based forest model, iLand, we simulated each landscape with 3 GCMs (CanESM2, HadGEM2-CC, HadGEM2-ES) × 2 RCPs (4.5, 8.5) × 20 probabilistic sequences of fire timing, location and maximum size (n=600 simulations). Fire spread and severity varied with available fuels, weather and species traits. Tree regeneration varied with propagule pressure, re-sprouting capacity, seed dispersal and climate controls on establishment and growth. With CanESM2 (precipitation increases with warming), future fire activity was similar to historical fire. Forest extent, structure, composition and aboveground carbon stocks were sustained in all landscapes, with little difference between RCPs. With HadGEM2-CC and HadGEM2-ES (precipitation does not increase with warming), burned area was 2-4x greater than CanESM2, and fire severity peaked ~2050. Compared to RCP 4.5, burned area doubled in RCP 8.5. Forest extent, structure and distributions of subalpine tree species experienced a series of abrupt, synchronous declines starting in mid-century. Dense forests became very sparse, and 50-75% of forested area was lost by 2080. Landscapes dominated by Picea-Abies or Pinus contorta were much more vulnerable to collapse than those dominated by Pseudotsuga menziesii. Differences in projected future precipitation produced divergent fire projections and forest-change trajectories in Yellowstone, but stabilizing carbon emissions would lower the risk of forest collapse.