Temperate and Tropical Forest Canopies are Already Functioning beyond Their Thermal Thresholds for Photosynthesis

Abstract

Tropical tree species have evolved under very narrow temperature ranges compared to temperate forest species. Studies suggest that tropical trees may be more vulnerable to continued warming compared to temperate species, as tropical trees have shown declines in growth and photosynthesis at elevated temperatures. However, regional and global vegetation models lack the data needed to accurately represent such physiological responses to increased temperatures, especially for tropical forests. To address this need, we compared instantaneous photosynthetic temperature responses of mature canopy foliage, leaf temperatures, and air temperatures across vertical canopy gradients in three forest types: tropical wet, tropical moist, and temperate deciduous. Temperatures at which maximum photosynthesis occurred were greater in the tropical forests canopies than the temperate canopy (30 ± 0.3 • C vs. 27 ± 0.4 • C). However, contrary to expectations that tropical species would be functioning closer to threshold temperatures, photosynthetic temperature optima was exceeded by maximum daily leaf temperatures, resulting in sub-optimal rates of carbon assimilation for much of the day, especially in upper canopy foliage (>10 m). If trees are unable to thermally acclimate to projected elevated temperatures, these forests may shift from net carbon sinks to sources, with potentially dire implications to climate feedbacks and forest community composition.

Significant Diel Variation of Soil Respiration Suggests Aboveground and Belowground Controls in a Tropical Moist Forest in Puerto Rico

“…For example, the morning rise in PAR also corresponds with the morning peak in soil respiration, which could be an indication of photosynthetic activity influencing soil respiration. Further, midday decreases in RH roughly correspond to the midday depression of soil respiration, which could be a reflection of stomatal closure and reduced photosynthetic rates, especially when canopy temperatures are above optimum (>30 °C; Mau et al, 2018). During evening hours, when photosynthesis is not occurring, we observed a linear decline of soil respiration rates as temperatures cooled, which could reflect either a slowing of microbial activity in response to temperature, and/or a slowing of carbon supply to roots as photosynthesis stops.…”

Photosynthetic heat tolerances and extreme leaf temperatures

“…Consistent with this hypothesis, previous studies found that experimental warming of tropical tree seedlings had no effect on their PHTs (Krause et al., 2010). Indeed, many tropical species are already operating above their temperature optima and maxima for carbon assimilation (Doughty & Goulden, 2009; Mau, Reed, Wood, & Cavaleri, 2018), which is partially influenced by PSII function and provides additional indirect evidence of their limited capacity to acclimate to higher temperatures. Nevertheless, we did measure PHTs during the hottest part of the year which should have minimized any potential effect of cool‐to‐warm season acclimation on our heat tolerances.…”

Photosynthetic and Respiratory Acclimation of Understory Shrubs in Response to in situ Experimental Warming of a Wet Tropical Forest

“…Based on previous in situ studies showing a lack of upregulation of net photosynthesis with warming (Doughty, 2011), evidence that tropical plants are operating near physiological thresholds (Doughty and Goulden, 2008;Mau et al, 2018), and the fact that tropical forests have evolved within a very narrow climatic envelope (Janzen, 1967), we expected these tropical shrubs to have limited photosynthetic acclimation potential. We further hypothesized that respiration would acclimate to experimental warming, as respiratory acclimation has been shown in tropical plants developed in situ (Slot et al, 2014).…”

“…In fact, due to their high leaf area index, the shaded canopy and understory leaves can contribute more than 50% of gross primary productivity of tropical forests (Chen et al, 2012;He et al, 2018). The role that shaded leaves play in tropical forest carbon cycling may further increase as temperatures rise if upper canopies surpass their thermal thresholds (He et al, 2018;Mau et al, 2018;Pau et al, 2018). Shaded leaves could continue to play an even larger role in forest carbon cycling if elevated CO 2 lowers transpiration (Kirschbaum and McMillan, 2018) and, therefore, evaporative cooling in the upper canopy leaves, as this could potentially further increase upper canopy leaf temperatures and reduce photosynthesis (Doughty and Goulden, 2008;Fauset et al, 2019).…”