Postdoctoral Fellow University of British Columbia, IRES, Canada
Background/Question/Methods
Temperature is a key climate indicator, whose distribution is expected to shift right in a warming world. However, the high temperature tolerance of trees is less widely understood than their drought tolerance, especially when it comes to sub-lethal impacts of temperature on tree growth. I use a large data set of annual tree ring widths, combined with a flexible degree-day model, to estimate the relationship between temperature and tree radial growth across different ecoregions of the US. I then document acclimatization to high temperatures, by testing whether temperatures experienced early in a tree’s life, impact that tree’s response to high temperatures later in life. In addition, I document long-run adaptation to high temperatures, by comparing the tolerance of trees to high temperatures across a climatic gradient.
Results/Conclusions
I find that tree radial growth responds non-linearly to temperature across many ecoregions of the US: across temperate and/or dry ecoregions, spring-summer temperature increases are beneficial or mostly neutral for tree growth up to around 25-30°C in humid climates and 10-15°C in dry climates, beyond which temperature increases suppress growth. Thirty additional degree-days above the optimal temperature breakpoint lead to an average decrease in tree ring width of around 1-5%, depending on ecoregions, seasons, and inclusion or exclusion of temperature-mediated drought impacts. High temperatures have legacy effects across a 5-year horizon in dry ecoregions, but none in the temperate-humid South-East or among temperature-sensitive trees. I find limited evidence of acclimatization: local variation in early exposure to high temperatures, which stems from local variation in the timing of tree birth, does not significantly impact later response to high temperatures. Conversely, I find some evidence that trees adapt to high temperatures in the long-run: across humid ecoregions of the US, high temperatures are 40% less harmful to tree growth, where their average incidence is one standard deviation above average. Overall, these results highlight the strength of a new methodology, which could help predict forest carbon uptake potential and composition under global change.
