Limited thermal acclimation of photosynthesis in tropical montane tree species

Abstract

The temperature sensitivity of physiological processes and growth of tropical trees remains a key uncertainty in predicting how tropical forests will adjust to future climates. In particular, our knowledge regarding warming responses of photosynthesis, and its underlying biochemical mechanisms, is very limited. We grew seedlings of two tropical montane rainforest tree species, the early‐successional species Harungana montana and the late‐successional species Syzygium guineense, at three different sites along an elevation gradient, differing by 6.8℃ in daytime ambient air temperature. Their physiological and growth performance was investigated at each site. The optimum temperature of net photosynthesis (ToptA) did not significantly increase in warm‐grown trees in either species. Similarly, the thermal optima (ToptV and ToptJ) and activation energies (EaV and EaJ) of maximum Rubisco carboxylation capacity (Vcmax) and maximum electron transport rate (Jmax) were largely unaffected by warming. However, Vcmax, Jmax and foliar dark respiration (Rd) at 25℃ were significantly reduced by warming in both species, and this decline was partly associated with concomitant reduction in total leaf nitrogen content. The ratio of Jmax/Vcmax decreased with increasing leaf temperature for both species, but the ratio at 25℃ was constant across sites. Furthermore, in H. montana, stomatal conductance at 25℃ remained constant across the different temperature treatments, while in S. guineense it increased with warming. Total dry biomass increased with warming in H. montana but remained constant in S. guineense. The biomass allocated to roots, stem and leaves was not affected by warming in H. montana, whereas the biomass allocated to roots significantly increased in S. guineense. Overall, our findings show that in these two tropical montane rainforest tree species, the capacity to acclimate the thermal optimum of photosynthesis is limited while warming‐induced reductions in respiration and photosynthetic capacity rates are tightly coupled and linked to responses of leaf nitrogen.

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

“…Moreover, recent field studies on tropical tree species have observed limited thermal acclimation (Carter et al, 2021;Dusenge et al, 2021). Our findings extends this work by investigating the mechanisms controlling the photosynthesis temperature response across a range of growth temperatures to determine differences in acclimation capacity in rainforest trees from different climates.…”

“…T optA may be related to species' climate of origin (Slatyer, 1978; Robakowski et al ., 2012), but other studies reported a lack of T optA response to a species' climate of origin (Gunderson et al ., 2000; Kumarathunge et al ., 2019; Dusenge et al ., 2021). A change in growth temperatures was found to be the most common driver of T optA (Kumarathunge et al ., 2019).…”

“…The limited studies investigating thermal acclimation of photosynthesis in rainforest species have generally reported reduced, not increased, photosynthetic rates with warming (Cunningham & Read, 2003; Slot & Winter, 2016; Scafaro et al ., 2017; Dusenge et al ., 2021). Slot & Winter (2017) found that tropical seedlings can acclimate to moderate warming via an increase in T optA , but photosynthesis declined under a high level of warming.…”

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

“…Moreover, stomatal conductance typically constrains the CO 2 supply to carboxylation even further at high temperatures (Lloyd & Farquhar, 2008; Grossiord et al ., 2020). As a result, declines in A net at high temperatures are usually the consequence of Rubisco limitation rather than electron transport limitation (Vårhammar et al ., 2015; Busch & Sage, 2017; Dusenge et al ., 2021). This is also the case at 25°C in most tree species globally (De Kauwe et al ., 2016).…”

“…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).…”