Acclimation of photosynthetic capacity and foliar respiration in Andean tree species to temperature change

Abstract

Summary Climate warming is causing compositional changes in Andean tropical montane forests (TMFs). These shifts are hypothesised to result from differential responses to warming of cold‐ and warm‐affiliated species, with the former experiencing mortality and the latter migrating upslope. The thermal acclimation potential of Andean TMFs remains unknown. Along a 2000 m Andean altitudinal gradient, we planted individuals of cold‐ and warm‐affiliated species (under common soil and irrigation), exposing them to the hot and cold extremes of their thermal niches, respectively. We measured the response of net photosynthesis (Anet), photosynthetic capacity and leaf dark respiration (Rdark) to warming/cooling, 5 months after planting. In all species, Anet and photosynthetic capacity at 25°C were highest when growing at growth temperatures (Tg) closest to their thermal means, declining with warming and cooling in cold‐affiliated and warm‐affiliated species, respectively. When expressed at Tg, photosynthetic capacity and Rdark remained unchanged in cold‐affiliated species, but the latter decreased in warm‐affiliated counterparts. Rdark at 25°C increased with temperature in all species, but remained unchanged when expressed at Tg. Both species groups acclimated to temperature, but only warm‐affiliated species decreased Rdark to photosynthetic capacity ratio at Tg as temperature increased. This could confer them a competitive advantage under future warming.

Variable thermal plasticity of leaf functional traits in Andean tropical montane forests

“…Cold-affiliated species did not change absolute δ 13 C or g 1 values with temperature (Figure 3 and Supporting Information: Figure S4), although STVs do suggest a slight increase in δ 13 C with warming (Figure 6). This is consistent with the approximately constant absolute values of LA and LW values and decreasing photosynthetic capacity (Cox et al, 2023) of these species under warmer conditions, as conservative water-use will limit the risk of desiccation (Cowan and Farquhar, 1977;Medlyn et al, 2017) and mortality from hydraulic failure (Rowland et al, 2015). δ 13 C can, however, be sensitive to altitudinal variation in CO 2 partial pressure.…”

“…Additionally, this would enable such species to shift from conservative strategies at suboptimal temperatures to more acquisitive strategies as temperatures rise, taking advantage of warmer conditions to maximise photosynthesis and growth, with larger leaves to intercept more sunlight and with a higher number of stomata to increase their capacity for C fixation (Kundu and Tigerstedt, 1999;Wright et al, 2017). The findings of Cox et al (2023) support this, as they report the highest photosynthetic capacities in warm-affiliated species when growing at temperatures closest to their thermal means.…”

“…Figure S4); combined with smaller leaves (Figure 4d,e) and lower photosynthetic capacities (Cox et al, 2023), this indicates a shift to more conservative growth strategies. Cold-affiliated species did not change absolute δ 13 C or g 1 values with temperature (Figure 3 and Supporting Information: Figure S4), although STVs do suggest a slight increase in δ 13 C with warming (Figure 6).…”

“…Absolute values of all nutrient traits, except N a , were significantly higher at 14°C than 22°C in both thermal affiliation groups (Supporting Information: Figure S3), while STVs showed significant shifts in P m and P a when species were transplanted away from their thermal means (Figure 5). For cold-affiliated species, higher leaf nutrients coincide with higher photosynthetic capacities at their cooler, home temperatures (Cox et al, 2023); as temperatures rise, faster enzymatic reactions allow carbon assimilation to be maintained with fewer photosynthetic enzymes (Arcus et al, 2016;van de Weg et al, 2012;Wang et al, 2020). N and P are both essential in carbon fixation, with N heavily invested in Rubisco and P modulating RuBP regeneration (Bahar et al, 2017;Bar-On & Milo, 2019;Bloomfield et al, 2014;Dusenge et al, 2021), but lower concentrations of both nutrients are required at lower photosynthetic capacities.…”

How a boiling river is helping to highlight the risks of warming for tropical forests

“…More recent global analyses of plant species have shown that high‐temperature thresholds of leaf physiology do generally rise with mean annual temperature (O'Sullivan et al ., 2017; Lancaster & Humphreys, 2020). Additionally, it is now well established that the optimum temperature for photosynthesis will often increase in response to warming (Yamori et al ., 2014), including in tropical tree species (Slot & Winter, 2017; Cox et al ., 2023). However, evidence has remained limited as to whether tropical trees are any more or less constrained in their capacity for thermal acclimation of leaf heat tolerance.…”