Topographic position amplifies consequences of short-interval stand-replacing fires on postfire tree establishment in subalpine conifer forests

Stand-replacing fires burned at 100 to 300-year intervals for millennia in subalpine conifer forests of western North America, but forests are burning more frequently as climate warms. Postfire tree regeneration is reduced when young forests reburn before recovering from previous fires or when drought occurs during postfire years. However, whether seedling vulnerabilities to harsh microclimate conditions may be amplified in short-interval (<30 years) fires is unclear. We conducted a field experiment to answer three questions: (1) How do germination, survival, and establishment of lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii var. glauca) vary by aspect following high-severity, short-interval fires? (2) What environmental factors control germination, survival, and establishment of both species? (3) Based on our experimental evidence, what proportion of available seed would be expected to establish across landscapes that burned in these short-interval fires? One year postfire, we planted seeds of both species in north-facing, south-facing and flat plots at four sites across the Greater Yellowstone Ecosystem (Wyoming, USA). Soil microclimate was monitored continuously. Seed germination and seedling survival were measured every two weeks during the following growing season and at the beginning and end of the second growing season. Germination did not vary with aspect but increased with early-season soil moisture and temperature. Survival and establishment were low on south-facing aspects (<1% of seeds established for both species) and declined with warmer soil temperatures and drier soils. For lodgepole pine, we predicted establishment rates of <1% of available seed over 25% of the reburned landscape. Soil temperatures in short-interval fires were 2˚C warmer than similar areas of long-interval fire, with maximum temperatures frequently exceeding 40 °C. Topographic variation will mediate the consequences of short-interval fire for seedling establishment, leading to patchier tree regeneration as climate warming raises the likelihood of short-interval fires.