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 germi- nation, 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.

北美西部亚高山针叶林的林分更替型野火在数千年间以100至300年的间隔周期性发生，但随着气候变暖，林火发生频率显著升高。若幼龄林尚未从此前的火烧中恢复便再次遭遇火烧，或火后数年发生干旱，都会降低火后林木更新效果。不过，短间隔（<30年）火烧是否会加剧幼苗对恶劣微气候的脆弱性，目前尚不明确。
本研究开展野外试验以解答三个核心问题：（1）在高烈度短间隔火烧后，扭叶松（Pinus contorta var. latifolia）与灰绿花旗松（Pseudotsuga menziesii var. glauca）的种子萌发、幼苗存活及定植情况如何随坡向发生变化？（2）哪些环境因子调控这两个物种的种子萌发、幼苗存活与定植过程？（3）基于本试验证据，在经历此类短间隔火烧的景观中，预计有多大比例的有效种子能够成功完成定植？
火后一年，我们在美国怀俄明州大黄石生态系统的4个研究样地中，于北坡、南坡与平坦地块分别播种两个物种的种子。我们持续监测土壤微气候，并在后续生长季每两周测定一次种子萌发与幼苗存活情况，同时在第二个生长季的初始与末期开展相关测量。
研究结果显示，种子萌发率未随坡向产生显著差异，但随生长季早期土壤湿度与温度升高而增加。南坡的幼苗存活率与定植率均较低（两个物种的种子定植率均<1%），且随土壤温度升高、土壤湿度降低而进一步下降。针对扭叶松，我们预测在复燃景观中，有超过25%的区域其有效种子定植率低于1%。短间隔火烧区域的土壤温度较长间隔火烧的相似区域高出2℃，最高温度常超过40℃。
地形异质性会调控短间隔火烧对幼苗定植的影响，随着气候变暖提升短间隔火烧的发生概率，林木更新将呈现更强的斑块化特征。