Assistant Professor University of Waterloo Waterloo, Ontario, Canada
Species response to climate change is not trivial to predict because warming involves the interaction of multiple stressors whose effects are simultaneous and therefore difficult to disentangle. To address this gap, we studied the individual effect of heat and drought on plant performance. We evaluated the multivariate phenotypic responses of five Canadian tree species to drought and heat to answer: 1) Do drought and heat have similar plastic effects on functional traits? 2) Do the same functional traits mediate the performance response to drought and heat?
In a greenhouse experiment, we measured the response of three broadleaves (Acer saccharum, Betula alleghaniensis, and Quercus rubra) and two conifers (Picea glauca and Pinus resinosa) to warming. We use a fully crossed factorial design of three water deficit levels and two temperatures to decouple the effect of drought and heat. We used capillarity irrigation to create the water deficit and open-top chambers to passively warm the air and soil. We measured 18 anatomical, morphological, and physiological traits from all organs to quantify five physiological functions: carbon-, water- and light-use, biomass allocation, and hydraulic safety. We measured plant performance as biomass relative growth rate (RGR).
The treatments reduced soil moisture content by 16% and 27% in the medium and high water-deficit levels relative to low. RGR responded differently to drought and heat: it was not affected by heat (except for P. glauca) but decreased by 40.4% (in Q. rubra) to 80.6% (in P. glauca) in high water deficit relative to low. There were no differences in RGR between medium and high-water deficits for most species, indicating that even a 16% reduction in water availability has large effects on plant growth. Based on preliminary results in the deciduous species, we did not find significant SLA and LDMC decrease in the most stressful treatment relative to the least stressful, although there is a trend in that direction. A. saccharum showed less trait variability under high stress (CV=3.6%) than low stress (CV=10.8%). In contrast to our expectations, heat and drought treatments did not affect predawn nor midday water potential; however, midday values are lower in conifers than in broadleaves. These preliminary results answered question one and part of question two. By August, we will have all the results to assess the individual effects of drought and heat on plant performance and how functional traits mediate these effects.