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), and particularly by reduced root hydraulic conductance (k), which has been widely hypothesized. We tracked stomatal conductance at the leaf level (g) and canopy scale (G) 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, G, k and k. Path analysis showed strong support for direct effects of k on g and for flood duration on k, but not k on k. A process-based model that accounted for the k reduction predicted the timeseries of G in flood and control treatment trees reasonably well (predicted versus observed G R = 0.80 and 0.51 for M. grandiflora and Q. virginiana, respectively). However, accounting only for k reduction in flooded trees was insufficient for predicting observed G reduction. Together, these results suggest that hydraulic constraints were not limited to roots and highlight the need to account for flooding effects on k when projecting forest ecosystem function using process-based models.