Climate and disturbance regimes could change profoundly in the 21st century, and whether forests will respond by shifting to alternate ecosystem states remains poorly resolved. Rocky Mountain subalpine forests experience stand-replacing wildfires, and successful postfire tree regeneration is necessary to assure forest resilience. Postfire forest regeneration may be vulnerable to climate warming because tree seedlings are sensitive to drought, underscoring the need to quantify effects of warmer, drier conditions on postfire tree establishment. We conducted a field experiment in Yellowstone National Park to ask, how do climate conditions expected by mid-21st century alter postfire lodgepole-pine (Pinus contorta) seedling establishment, survival, and growth? We hypothesized lodgepole pine would establish following fire in their current distribution, but establishment would be reduced or fail under projected warmer, drier conditions. In 2014, we established three sites in burned lodgepole-pine forests and three sites at lower elevations where the warmer, drier current climate matches conditions projected for lodgepole-pine forests by 2050. We collected 168 soil cores (20-cm deep) from recent wildfires, transplanted cores to varied topographic positions at each experimental site, and planted locally collected lodgepole pine seeds in each core. Seedling establishment, survival and growth were recorded over subsequent years.
Results/Conclusions
Total first-year lodgepole pine seedling establishment was reduced by almost 90% at low-elevation experimental sites compared to sites within the current distribution of lodgepole pine. Seedlings established in 26% of low-elevation cores and 90% of cores in lodgepole-pine forest. When seedlings were present, establishment density was only 1.5 + 0.28 seedlings per core at low-elevation sites compared to 3.4 + 0.15 seedlings per core in lodgepole pine forest. Mortality during the first growing season was also more pronounced at low-elevation sites (81 + 13% of seedlings died) than at lodgepole pine forest sites (44 + 11% of seedlings), and all survivors at low-elevation sites were located on cooler north-facing aspects. However, height of surviving seedlings at the end of the first growing season was not influenced by climate. Climate in Yellowstone is expected to warm and dry substantially over coming decades, and projected mid-century climate does not appear conducive to successful postfire lodgepole pine regeneration. Understanding the changing abiotic context of postfire succession and identifying mechanisms that may underpin abrupt ecological change will help us to better anticipate 21st-century subalpine-forest dynamics.