Assistant Professor Austin Peay State University, United States
Abstract: Climate change will result in an increase in the duration, frequency, and severity of heat waves. When temperatures cross certain thresholds, leaves can stop photosynthesis altogether or be damaged beyond repair. Yet the upper heat tolerances of temperate trees are severely understudied. We determined heat tolerances for forest species common to the eastern United States. Additionally, we sought to determine any shifts in the tolerance in populations along a latitudinal gradient.
We chose 11 species of hardwood trees to test heat tolerance across the community in Clarksville, TN in summer 2022. These species are common and span a range of successional traits such as shade tolerance. Leaf samples were placed in controlled water baths between 38 and 60°C, with one sample kept at 23°C. After leaving the water bath leaves sat for 24 h and we then measured chlorophyll fluorescence to determine the temperature where heat stress begins (LT15). To evaluate heat tolerance shifts along a latitudinal gradient, we repeated this methodology at sites in Alabama, Tennessee, Indiana, and Michigan but for just three species that are common at all four sites: Liriodendron tulipifera, Acer saccharum, and Fagus grandifolia.
At the community scale, 3 of the 11 species exhibited LT15 temperatures of ~44°C, which is consistent with observed canopy high air temperatures of 43.97°C for the site. Conversely, three species showed no stress until 48°C providing these species with a significant buffer against extreme heat events. This means that some species are likely experiencing significant heat stress under current heat events while other species are not.
Shifts in heat tolerance along the latitudinal gradient were inconsistent, with heat tolerance of L. tulipifera increasing from the cooler-northern site (mean heat tolerance [95% CI] 41.8°C [40.8-43.1]) to warmer-southern sites (46.4°C [44.7-48.9]). However, for A. saccharum and F. grandifolia, we found no consistent trend in heat tolerance across latitude. Thermal safety margins decreased from Michigan to Alabama, indicating those in southern sites are experiencing temperatures closer to their LT15 than those at northern sites.
As the effects of climate change continue manifest on forests, trees will be pushed closer to or beyond their thermal heat limits. We may see thermally-sensitive species shift their ranges (especially at the southern edge of current ranges) or compositional changes in forests favoring those with higher heat tolerances.
