Variation in mesophyll conductance across local and regional aridity gradients

Background/Question/Methods

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 (g_m), which can have a pronounced effect on photosynthesis under water stressed conditions.  This study assessed the importance of g_m 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-C_i) 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-C_i, A-Q, and fluorescence data and semi-informative priors for biochemical parameters that are not directly informed by such data.  Estimated g_m did not vary as expected between environments or species; g_m (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 g_m of Juniperous monosperma (one-seed juniper) growing near a riverbed was not significantly different from the g_m of juniper growing on a drier, nearby ridge.  However, the juniper on the ridge had significantly higher g_m (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 g_m compared to the less drought-tolerance species (Gambel oak).  Though the estimated g_m differed from expectations, such variation in g_m may be partly governed by leaf-level water status, suggesting that such data may help refine our understanding of g_m.