Recruitment of subalpine tree populations sensitive to warming within and above current altitudinal range

Abstract:

Forests are globally important contributors to carbon storage and release, and have strong regional leverage on surface energy balance and water fluxes. Highly uncertain feedbacks to climate change from dynamic shifts in forest distributions are increasingly incorporated into Earth system models, but lack data for parameterizations and quantitative benchmarks. We established the Alpine Treeline Warming Experiment at Niwot Ridge, CO, to examine effects of climate warming on tree seedling establishment near the lower limit of subalpine forest, at upper treeline, and in the alpine. We use infrared heaters to increase surface temperatures, and to lengthen the growing season. We also watered some plots to distinguish effects due to higher temperatures from those due to drier soil. We used meta-population models parameterized with data from the experiment and from the literature to assess impacts of warming on tree population sizes over time.

Averaged over 4 years, heating elevated soil temperatures, slightly reduced soil moisture, and advanced snowmelt. Heated and watered plots were not systematically wetter or drier than controls, as intended. Preliminary analyses show that in the forest, heating reduced germination for two species and, in most years, had no or negative effects on survival to 2 years, while watering increased demographic rates, consistent with expectations of reduced recruitment at lower elevations with warming and drying. At treeline, heating also reduced germination, but had year-dependent effects on survival to 2 years. In the alpine, contrary to expectations, experimental heating did not increase germination or survival, while watering increased survival to two years. Changing the seedling survival consistent with our experiment, and keeping all other parameters constant, we found strong effects of heating and watering on modeled population sizes. These effects vary across the three elevations. Due to timescales involved in tree growth, we find large lags between onset of climate changes and tree population responses. Linking field experiments directly with models of population change across the landscape provides a novel approach to projecting changes in altitudinal distributions of forest with climate change over time.