Carbon uptake via photosynthesis is an important controller of tree growth. Given predictions of increasing temperature and decreasing precipitation for the southwestern US, it is important to develop a mechanistic understanding of the constraints on photosynthesis. One important limitation to CO2 uptake is mesophyll conductance (gm), which can have a pronounced effect on photosynthesis under water stressed conditions. This study assessed the importance of gm to photosynthesis, which was expected to be lower (increased limitation): 1) as water stress increased along local gradients, 2) as aridity increased across regional gradients, and 3) for species adapted to low water availability (e.g., desert upland vs. riparian species). We made measurements of photosynthesis and chlorophyll florescence across local water stress and regional aridity gradients spanning three locations in Arizona differing in mean annual precipitation (325, 408, and 508 mm) and mean maximum temperature (16.4, 29.0, and 30.6°C). Species at each site were chosen to represent different water-use strategies (e.g., upland vs. riparian). Assimilation versus leaf-internal CO2 (A-Ci) and versus light (A-Q) response curves were measured for each species. A biochemical-based model of photosynthesis was fit to these data via a hierarchical Bayesian framework.
Results/Conclusions
The Bayesian framework allowed for the simultaneous integration of A-Ci, A-Q, and fluorescence data and semi-informative priors for biochemical parameters that are not directly informed by such data. Estimated gm did not vary as expected between environments or species; gm (µmol m-2s-1Pa-1) was significantly lower in Prosopis velutina (mesquite) at the driest site (mean = 1.23, 95% CI = [0.32, 2.86]) compared to the intermediate site (mean = 10.50, 95% CI = [1.45, 26.40]). At the wettest site, the predicted gm of Juniperous monosperma (one-seed juniper) growing near a riverbed was not significantly different from the gm of juniper growing on a drier, nearby ridge. However, the juniper on the ridge had significantly higher gm (mean = 8.63, 95% CI = [2.21, 20.40]) than Quercus gambelii (Gambel oak, mean = 1.57, 95% CI = [0.52, 3.38]) growing near the river; this result contradicts our initial expectation that the more drought-tolerant species (juniper) should exhibit a lower gm compared to the less drought-tolerance species (Gambel oak). Though the estimated gm differed from expectations, such variation in gm may be partly governed by leaf-level water status, suggesting that such data may help refine our understanding of gm.