Mortality, leaf drop, and growth: Tree seedling drought stress and warming responses in the Boston Area Climate Experiment

Background/Question/Methods

Global climate models predict increases in average annual temperatures and changes in the amount and pattern of precipitation worldwide. Plant responses to global warming are mediated both by the timing and the amount of precipitation. In the Boston Area Climate Experiment (BACE) in Waltham, Massachusetts, we address the question of how warming and precipitation interact by exposing an old-field community to a factorial combination of heating and precipitation treatments. The BACE uses infrared heaters to achieve four levels of warming ranging from no warming to ~+4°C. Rainout shelters remove half of the incoming precipitation from drought treatments and sprinkle the removed water immediately onto rain addition treatments. The gradient factorial approach of the BACE enables tests of hypotheses whether species- and ecosystem-level responses to warming are linear or non-linear, and to test treatment interactions. We planted tree seedlings (Acer rubrum, Betula lenta, Betula nigra, Pinus strobus and Quercus rubra) into each treatment plot and monitored their survival, growth, and phenology during one year with a cold, wet summer (2009) and one year with a hot, dry summer (2010). Here, we report on tree seedling mortality, drought responses, and phenology in response to our experimental manipulations and to inter-annual weather variations.

Results/Conclusions

Precipitation and warming treatments both affected tree seedling phenology and mortality. Tree seedling mortality was highest in warmed drought treatments. Of the five species examined, A. rubrum survived best under drought stress. The dry, hot summer in 2010 caused most tree seedlings to drop their leaves early (Aug., Sept.) and increased tree mortality. Warming advanced spring phenological events in the deciduous tree seedlings, and decreased the time from bud burst to full leaf expansion. The formation of new needles in P. strobus did not respond to the experimental treatments. Warming delayed the peak of fall coloration in all deciduous species, while precipitation effects were species-specific. In A. rubrum, warming also lengthened the time period from the onset of fall coloration to full fall coloration. Our results indicate that interactive climate effects have the potential to alter tree seedling survival and to affect growing season length, which may impact future forest composition and carbon sequestration, respectively.