Projecting changes in subalpine tree populations with warming across an elevation gradient
The elevational range of subalpine trees is known to be climate sensitive. Shifts in the distribution of trees with climate change are dependent on the demographic processes of recruitment, growth and mortality, which occur over decades to centuries for these long-lived species. We established the Alpine Treeline Warming Experiment at Niwot Ridge, CO, to examine effects of climate warming on recruitment 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, and watered some plots to distinguish heating from soil-trying effects. We used long-term demography plot data to quantify the sensitivity of tree growth and mortality to climate. We integrated this experimental and observational data into spatially explicit demography models to assess impacts of warming on tree population sizes and distributions over time.
Our preliminary results indicate century-long lags between the onset of climate changes and tree population establishment in the alpine due to a combination of seed limitation and low recruitment rates. We also find strong sensitivity of population growth to summer soil moisture, including in the alpine, which suggests that temperature increases may not always result in upslope range shifts in the absence of additional summer precipitation. Finally, we find rapid decline in the Engelmann spruce populations at low elevations due to recruitment failure. Climate driven changes in growth and mortality exacerbate this species’ decline. Linking field experiments and observations 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.