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.