New tree‐level temperature response curves document sensitivity of tree growth to high temperatures across a US‐wide climatic gradient

Abstract

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. I find that tree radial growth responds non-linearly to temperature across many ecoregions of the United States: 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 temperatehumid South-East or among temperature-sensitive trees. I find limited evidence that trees acclimatize to high temperatures within their lifetime: local variation in early exposure to high temperatures, which stems from local variation in the timing of tree birth, does not significantly impact the response to high temperatures, although temperature-sensitive trees acquire some heightened sensitivity from early exposure.I also find some evidence that trees adapt to high temperatures in the long run: across humid ecoregions of the United States, 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, applied to representative tree ring data, could contribute to predicting forest carbon uptake potential and composition under global change.

The energy–water limitation threshold explains divergent drought responses in tree growth, needle length, and stable isotope ratios

“…Though temperature mediated the ecohydrological transition in this system, different climatic factors likely drive transitions among other ecohydrological classes that emerge across larger spatial scales (Babst et al, 2019;Gantois, 2022;Roebroek et al, 2020).…”

“…In California's White Mountains, for example, which are drier than the Sierra Nevada, only trees growing within 60–80 m of treeline were positively correlated with temperature (Salzer et al, 2014). In addition, a recent study suggested that tree growth‐temperature inflection points vary dramatically within the United States (i.e., ranging between 10°C and 30°C; Gantois, 2022). Species traits, as well as other ecohydrological classes, may be leading to highly variable temperature inflection points across large geographic gradients (Roebroek et al, 2020).…”

“…In California's White Mountains, for example, which are drier than the Sierra Nevada, only trees growing within 60-80 m of treeline were positively correlated with temperature (Salzer et al, 2014). In addition, a recent study suggested that tree growth-temperature inflection points vary dramatically within the United States (i.e., ranging between 10°C and 30°C;Gantois, 2022).…”

“…Our sampling design enabled us to disentangle climate from elevation and suggested that temperature, not precipitation, was the strongest determinant of whether this system was more energy or water limited (Roebroek et al, 2020). Though temperature mediated the ecohydrological transition in this system, different climatic factors likely drive transitions among other ecohydrological classes that emerge across larger spatial scales (Babst et al, 2019; Gantois, 2022; Roebroek et al, 2020). Interestingly, water limitation still occurred in energy‐limited regions—precipitation was positively correlated with whitebark pine growth until reaching the precipitation threshold (Figure S6).…”

Drought sensitivity in mesic forests heightens their vulnerability to climate change

“…In theory, drought adaptations could occur at the scale of individuals (e.g., phenotypic plasticity) or local communities (e.g., tree density) or through evolutionary processes that occur across generations. Recent research, however, suggests that most climate adaptation develops over longer timescales than an individual tree’s life span ( 54 ). This suggests that genetic adaptation is an important mechanism that protects trees from climate change, but it also highlights the risk that rapid warming may outpace the rate of adaptation.…”