Experimental warming across a tropical forest canopy height gradient reveals minimal photosynthetic and respiratory acclimation

Abstract

Tropical forest canopies cycle vast amounts of carbon, yet we still have a limited understanding of how these critical ecosystems will respond to climate warming. We implemented in situ leaf-level + 3 C experimental warming from the understory to the upper canopy of two Puerto Rican tropical tree species, Guarea guidonia and Ocotea sintenisii. After approximately 1 month of continuous warming, we assessed adjustments in photosynthesis, chlorophyll fluorescence, stomatal conductance, leaf traits and foliar respiration. Warming did not alter net photosynthetic temperature response for either species; however, the optimum temperature of Ocotea understory leaf photosynthetic electron transport shifted upward. There was no Ocotea respiratory treatment effect, while Guarea respiratory temperature sensitivity (Q 10 ) was down-regulated in heated leaves. The optimum temperatures for photosynthesis (T opt ) decreased 3-5 C from understory to the highest canopy position, perhaps due to upper canopy stomatal conductance limitations. Guarea upper canopy T opt was similar to the mean daytime temperatures, while Ocotea canopy leaves often operated above T opt . With minimal acclimation to warmer temperatures in the upper canopy, further warming could put these forests at risk of reduced CO 2 uptake, which could weaken the overall carbon sink strength of this tropical forest.

Tropical rainforest species have larger increases in temperature optima with warming than warm‐temperate rainforest trees

“…The decrease of g s with increasing temperatures is not uncommon and has been observed in several studies (e.g. Slot et al ., 2016; Carter et al ., 2021) suggesting that photosynthesis rates could additionally be constrained by stomatal conductance at higher growth temperatures.…”

Temperature responses of photosynthesis and respiration in evergreen trees from boreal to tropical latitudes

“…These lower rates combined with the smaller RR observed in J : V 25 and A net25 in the tropics can contribute to seemingly small or no thermal acclimation response of photosynthesis or photosynthetic capacity in tropical species (Scafaro et al ., 2017; Crous et al ., 2018; Fauset et al ., 2019; Carter et al ., 2020; Dusenge et al ., 2021). The mechanisms behind this limited thermal acclimation in the tropics need to be further explored, including limits to acclimation capacity and the potentially stronger role of stomatal conductance limitations to photosynthesis at higher temperatures (Carter et al ., 2021; Slot et al ., 2021). Slot & Winter (2016) indicated a possible VPD effect at higher temperatures, which may influence photosynthesis by stomatal conductance.…”

“…3a; R 2 = 0.28; P < 0.0001; Table 2). Further support for the sensitivity of T optA to a change in surrounding growth temperatures, is provided by temperature gradients, even within tree canopies where the T optA is often higher at the top of the canopy than in more shaded parts lower in the canopy (Carter et al ., 2021).…”

Improved representation of plant physiology in the JULES-vn5.6 land surface model: Photosynthesis, stomatal conductance and thermal acclimation

“…In contrast to temperate and boreal forests, tropical forests are thought to be more susceptible to climate change, having evolved under relatively narrow temperature regimes, and experiencing less seasonal and day-to-day variation in temperature changes (Cunningham and Read, 2003). As a consequence, an increasing number of studies show that tropical trees have less capacity to physiologically acclimate photosynthesis to increasing temperatures (Carter et al, 2021;Dusenge et al, 2021;Mau et al, 2018;Miller et al, 2021;Vårhammar et al, 2015). Other studies have determined high temperature threshold responses of photosynthesis, indicating an ability of tropical trees to acclimate to moderate warming, but more severe warming decreases carbon gain (Doughty and Goulden, 2008;Pau et al, 2018;Slot and Winter, 2017;Sullivan et al, 2020).…”