Integrated demographic and physiological measure of warming impacts on seedlings

Background/Question/Methods

Record temperatures and droughts are impacting forests across North America. We have a rough understanding of how physiological and demographic rates change with warming temperatures. Predictions of enhanced photosynthesis and increased growth rates are common, but these rates do not act independently. Instead these and other rates all simultaneously react to the environment. By measuring a suite of traits at a variety of scales we develop an integrated measure of tree performance due to underlying environmental and physiological conditions. In 2008, a manipulative warming experiment was started in North Carolina and Massachusetts to determine seedling responses to changing climatic conditions. The experiment applied a factorial design for warming (control, ambient, +3-5°C), two light levels (gap and understory) for a total of 48 plots, and focused on twelve species with well over 1000 total seedlings. Seedling level measurements included: sapflux, photosynthesis, respiration, biomass, growth, mortality, soil moisture relative humidity, and light levels.

Results/Conclusions

Overall, even though higher temperatures enhanced photosynthesis and increase transpiration rates they did not consistently yield higher growth rates or reduced mortality. Individual seedlings are the result of layers of idiosyncratic responses to the environment. In our experiment, the primary drivers of seedling responses were light levels and soil moisture. It is within these constraints that temperatures affected growth. With high light and high soil moisture species almost universally responded positively to elevated temperatures. However, when there were limitations (low light or drought), the effects of elevated temperature could be negative. Enhanced respiration in heated plots at low light led to smaller net carbon gain and reduced plant vigor. This pattern was similar during drought where not only high temperatures drive the increased loss of carbon to respiration, but elevated evapotranspiration created drier conditions for longer periods. It is critical to consider how these responses interact with species' life histories as we attempt to predict how forests change over the next 50 to 100 years.