Temperate tree seedlings show extensive plastic response to water deficit and heat stress, but it is insufficient to fully mitigate decline in growth

Lina Marcela Aragón Baquero, Christian Messier, Julie Messier

Heat, cold, drought, salinity, oxidative stress, and nutrient deficiency are among the abiotic stressors that plants face because of climate change. However, these stressors do not occur in isolation, which increases plant vulnerability to pathogens and herbivory. While plants cannot quickly escape changes in air temperature and soil water availability, they still can acclimate to the new abiotic and biotic conditions in their current environments. Since temperate forests in Canada cover 9% of the world’s total forest area (approximately 346 million hectares) and boreal forests store one-third of the world’s terrestrial carbon, it is crucial to evaluate how heat and drought together impact the growth and functioning of native trees in these ecosystems. To support this effort, we conducted a greenhouse experiment exposing five native tree species to six different environmental treatments involving heat and drought, to assess specifically: 1) effects of water deficit and heat, alone and together, on seedling performance, 2) the plastic response of seedlings to water deficit and heat; and 3) the role of phenotypic plasticity to maintain plant performance under stress. We found, first, that growth is significantly reduced under water deficit, while warmer temperatures have a neutral to positive effect on growth, depending on the species (broadleaves versus conifers). Second, the effects of water deficit and heat on the phenotype are idiosyncratic among species, given that a) no single trait show a consistent response to the stressors imposed across all species; b) some species only showed a multivariate trait response to water deficit (i.e., Acer saccharum, and Picea glauca), others only to heat (i.e., Betula alleghaniensis, and Pinus resinosa) and only Q. rubra responded to both stressors; and c) a unique set of traits per species was associated with their responses to water deficit and/or heat. Our results illustrate the various and unique ways in which plant species are affected by and respond to environmental stress, highlighting the vulnerability of natural ecosystems to global warming.