Abstract
Due to global warming, many species will face greater risks of thermal stress, which can lead to changes to performance, abundance and/or geographic distributions. In plants, high temperatures above a species-specific critical thermal maximum will permanently damage photosystem II, leading to decreased electron transport rates, photosynthetic failure, and eventual leaf and plant death. Previous studies have shown that plant thermal tolerances vary with latitude, but little is known about how they change across smaller-scale thermal gradients (i.e., with elevation) or about how these thermal tolerances relate to species’ local performances and geographic distributions. In this study, we assess the maximum photosynthetic thermal tolerances (T50) of nearly 200 tropical tree species growing in 10 forest plots distributed across a >2500 meter elevation gradient (corresponding to a 17oC temperature gradient) in the northern Andes Mountains of Colombia. Using these data, we test the relationships between species’ thermal tolerances and 1) plot elevations and temperatures, 2) species’ large-scale geographic distributions, and 3) changes in species’ abundances through time within the plots. We found that species’ T50 do in fact decrease with plot elevation but significantly slower than the corresponding adiabatic lapse rate (-0.4 vs. -5.7oC km-1) and that there remains a large amount of unexplained variation in the thermal tolerances of co-occurring tree species. There was only a very weak association between species’ thermal tolerances and their large-scale geographic distributions and no significant relationships between species’ thermal tolerances and their changes in relative abundance through time. A potential explanation for these results is that thermal tolerances are adaptations to extreme leaf temperatures that can be decoupled from regional air temperatures due to microclimatic variations and differences in the species’ leaf thermoregulatory properties.
Highlights
With anthropogenic climate change driving rapid increases in global temperatures, many plant species will face greater risks of thermal stress (Saxe et al, 2001; Parmesan and Hanley, 2015)
In accord with a priori predictions, the photosynthetic thermal tolerances (T50) of tree species were positively related to plot temperature and elevation, such that the species sampled from hot lowland plots tended to have higher T50 than species sampled from cold highland plots (T50 = 49.72◦-0.0004 × Elevation, R2 = 0.07, P = 0.00101; T50 = 47.39◦ + 0.0827 × MAT, R2 = 0.07, P = 0.00115; T50 = 48.16◦ + 0.0372 × MTWM, R2 = 0.02, P = 0.0701; Figure 2)
There was a tendency for the average T50 to vary with temperature and elevation, but the relationships were not significant
Summary
With anthropogenic climate change driving rapid increases in global temperatures, many plant species will face greater risks of thermal stress (Saxe et al, 2001; Parmesan and Hanley, 2015). Thermal stress can limit the performance of individuals and populations, eventually leading to the retraction of some species’ ranges (or shifts in species’ ranges if local extinctions are offset by simultaneous invasions into cooler areas) and local extinctions (Chen et al, 2011; Feeley, 2012; Lenoir and Svenning, 2015) These local extinctions can in turn lead to decreased diversity through biotic attrition (Colwell et al, 2008; Wiens, 2016), changes in community composition (Feeley et al, 2011, 2013; Duque et al, 2015; Fadrique et al, 2018) and potential changes in important ecosystem services such as carbon sequestration/storage (Clark et al, 2003; Brienen et al, 2015), regional climate regulation (Cox et al, 2000; Luo, 2007), and food production (Tito et al, 2018). This suggests that many tropical species may have reduced thermal safety margins (difference between temperatures and tolerances) compared to temperate species and will be at elevated risk of thermal stress due to global warming (Perez et al, 2016; O’sullivan et al, 2017)
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