Hydraulic constraints to stomatal conductance in flooded trees.

Stomatal closure is a pervasive response among trees exposed to flooded soil. We tested whether this response is caused by reduced hydraulic conductance in the soil-to-leaf hydraulic continuum (k_total), and particularly by reduced root hydraulic conductance (k_root), which has been widely hypothesized. We tracked stomatal conductance at the leaf level (g_s) and canopy scale (G_s) along with physiological conditions in two temperate tree species, Magnolia grandiflora and Quercus virginiana, that were subjected to flood and control conditions in a greenhouse experiment. Flooding reduced g_s, G_s, k_root and k_total. Path analysis showed strong support for direct effects of k_total on g_s and for flood duration on k_total, but not k_root on k_total. A process-based model that accounted for the k_total reduction predicted the timeseries of G_s in flood and control treatment trees reasonably well (predicted versus observed G_s R² = 0.80 and 0.51 for M. grandiflora and Q. virginiana, respectively). However, accounting only for k_root reduction in flooded trees was insufficient for predicting observed G_s reduction. Together, these results suggest that hydraulic constraints were not limited to roots and highlight the need to account for flooding effects on k_total when projecting forest ecosystem function using process-based models.