4.2 | Stomatal conductance
Previous studies in Panama suggested an important role for stomatal conductance in controlling the short-term temperature response of photosynthesis in lowland tropical trees (Slot & Winter 2017b,c; Hernández et al. 2020). The relatively small role of stomatal limitation in the control over the temperature response of photosynthesis in the current study on well-watered saplings suggests that stomata in tall trees are more sensitive to changes in VPD (as shown e.g. for Mediterranean oaks by Mediavilla & Escudero 2004), presumably due to the challenges of water delivery to the canopy. Stomatal limitations in response to VPD may therefore be underestimated when scaling results from studies such as the current one to tropical forests at large. Indeed, stomatal response to increased VPD has been shown to underpin the high temperature decrease of net ecosystem exchange across tropical forest sites (Tan et al. 2017; Smith et al. 2020).
Stomatal conductance was not strongly affected by the elevated CO2 and warming treatment in the current study and there was no obvious effect of the moderately lower relative humidity in the treatment dome. Stomatal limitation had only a small effect on TOpt of photosynthesis at 400 ppm CO2, as illustrated by moderately higher TOpt values of photosynthesis at a fixed Ci—consistent with global observations by Kumarathunge et al. (2019). Previous work found reductions (Cernusak et al. 2011) or no effect (Fauset et al. 2019) on stomatal conductance when tropical species were grown elevated CO2. Berryman, Eamus & Duff, (1994) showed that tropical tree species Maranthes corymbosa Blume exhibited both a short-term, reversible reduction in stomatal conductance in response to elevated CO2—regulated by stomatal aperture—and a long-term, non-reversible reduction associated with decreased stomatal density.
The response of stomatal conductance of tropical trees exposed to elevated temperature is also variable. Significant reductions were reported following ~2°C warming of leaves in the field (Doughty 2011) and 1.5°C warming of greenhouse-grown A. glandulosa , while Slot & Winter (2017a) did not find clear patterns for potted seedlings of three tropical tree species across a 10°C growth temperature range. Given the variation in observed patterns, the response of stomatal conductance to a combination of elevated CO2 and temperature is highly uncertain. Fauset et al. (2019) found a stronger reduction in stomatal conductance in A. glandulosa due to 1.5°C warming than due to doubling of CO2, resulting in reduced stomatal conductance in warmed plants at elevated CO2. In the current study on well-watered plants under +4°C warming and double ambient CO2, stomatal conductance did not acclimate. Stomatal conductance was slightly reduced in treatment plants relative to control plants, but only at temperatures (Fig. 5). Furthermore, there was no acclimation of the relationship of stomatal conductance with VPD, which is consistent with a meta-analysis on stomatal responses to elevated CO2 in which Medlyn et al. (2001) also observed that the relationship between conductance and VPD was not affected by growth at elevated CO2. In our study, control plants transferred to treatment conditions did have lower stomatal conductance than treatment plants. This suggests that the decrease in conductance is of transient nature, and that leaves developed under treatment conditions have more similar properties to control leaves than pre-existing leaves exposed to treatment conditions. Nonetheless, treatment and control plants had similar stomatal densities, possibly the result of opposing effects of warming and elevated CO2.