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

Abstract

Despite the importance of tropical forests to global carbon balance, our understanding of how tropical plant physiology will respond to climate warming is limited. In addition, the contribution of tropical forest understories to global carbon cycling is predicted to increase with rising temperatures, however, in situ warming studies of tropical forest plants to date focus only on upper canopies. We present results of an in situ field-scale +4 • C understory infrared warming experiment in Puerto Rico (Tropical Responses to Altered Climate Experiment; TRACE). We investigated gas exchange responses of two common understory shrubs, Psychotria brachiata and Piper glabrescens, after exposure to 4 and 8 months warming. We assessed physiological acclimation in two ways: (1) by comparing plot-level physiological responses in heated versus control treatments before and after warming, and (2) by examining physiological responses of individual plants to variation in environmental drivers across all plots, seasons, and treatments. P. brachiata has the capacity to up-regulate (i.e., acclimate) photosynthesis through broadened thermal niche and up-regulation of photosynthetic temperature optimum (T opt) with warmer temperatures. P. glabrescens, however, did not upregulate any photosynthetic parameter, but rather experienced declines in the rate of photosynthesis at the optimum temperature (A opt), corresponding with lower stomatal conductance under warmer daily temperatures. Contrary to expectation, neither species showed strong evidence for respiratory acclimation. P. brachiata down-regulated basal respiration with warmer daily temperatures during the drier winter months only. P. glabrescens showed no evidence of respiratory acclimation. Unexpectedly, soil moisture, was the strongest environmental driver of daily physiological temperature responses, not vegetation temperature. T opt increased, while photosynthesis and basal respiration declined as soils dried, suggesting that drier conditions negatively affected carbon uptake for both species. Overall, P. brachiata, an early successional shrub, showed higher acclimation potential to daily temperature variations, potentially mitigating negative effects of

Limited thermal acclimation of photosynthesis in tropical montane tree species

“…guineense (Figure 7). Although being in the direction predicted by optimality theory, our findings contrast to previous studies on tropical species which did not find any significant effect of warming on basal rates of photosynthetic capacity (Carter et al, 2020; Crous et al, 2018; Scafaro et al, 2017).…”

“…These large reductions in H. montana may partly reflect some pot limitation, with this species showing strongly increased growth at the two warmer sites, in contrast to S. guineense (Figure 7). Although being in the direction predicted by optimality theory, our findings contrast to previous studies on tropical species which did not find any significant effect of warming on basal rates of photosynthetic capacity (Carter et al, 2020;Crous et al, 2018;Scafaro et al, 2017).…”

“…However, this estimate of the thermal sensitivity of T optA is heavily based on studies on temperate and boreal species (Berry & Björkman, 1980; Kumarathunge et al, 2019; Sendall et al, 2015; Way & Yamori, 2014). Of the relatively few studies on tropical trees investigating responses of T optA to elevated growth temperatures, some report significant upward shifts in T optA with warming (Cunningham & Read, 2003; Kositsup et al, 2009; Read, 1990; Slot & Winter, 2017b), while others do not (Carter et al, 2020; Crous et al, 2018).…”

“…Meta-analyses, dominated by studies on boreal and temperate tree species, have showed that V cmax and J max rates measured at a common leaf temperature of 25℃ (V cmax25 and J max25 ) are largely unaltered by warming (Kattge et al, 2009;Kumarathunge et al, 2019;Way & Oren, 2010). Across individual studies, some did not find any effect of warming on either V cmax25 or J max25 (e.g., Bermudez et al, 2020;Carter et al, 2020;Fauset et al, 2019;Lamba et al, 2018;Stefanski et al, 2019), while others observed significant declines (e.g., Dusenge et al, 2015Dusenge et al, , 2020Way & Sage, 2008), or increases (e.g., Crous et al, 2013) in warm-grown plants. Therefore, there is no consistent acclimation response of photosynthetic capacity to warming.…”

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

“…1). 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).…”