Lack of seasonal change in hydraulic conductivity points to short-term embolism repair in redwood treetop branches

The coastal California climate is getting warmer and drier, leading to decreased soil and atmospheric moisture, and increased water stress on redwoods. We have a poor basis, however, for predicting the influence of such stress on branch hydraulic conductivity, a key physiological characteristic that, in turn, can limit growth and lead to dieback at the treetop. Treetop branches are of particular interest both because they are subject to the greatest stress (due to the influence gravity and position in the hydraulic pathway) and because branches in general support the foliage responsible for virtually all photosynthesis. This study investigates the questions: 1) How does redwood treetop branch Ks change throughout the year as periods of low water potential repeatedly occur? and 2) Does new xylem production in redwood treetop branches restore Ks by replacing dysfunctional conduits? We combine time-series measurements of water potential, conductivity, and xylem growth in treetop branches of stand-dominating redwoods at Landels-Hill Big Creek Reserve on the central California coast. On numerous occasions during the study, water potential surpassed the previously-determined threshold for decreasing conductivity, but no seasonal or accumulating loss of conductivity was observed. Branch diameter measurements and cell-level inspection of branch samples showed little to no growth occurring in treetop branches even during the expected growing season, eliminating the possibility that loss of conductivity was simply masked by the production of new xylem cells. Water potential on the day of branch harvest was a much stronger predictor of hydraulic conductivity than longer-term water potential history of a branch, suggesting that changes in hydraulic conductivity can be short term, perhaps the result of day-to-day embolism formation and repair. Overall, we have found redwood treetop branches to be surprisingly resistant to hydraulic dysfunction. Despite previous studies’ finding that redwood treetop branches experience decreasing hydraulic conductivity at stress levels that we observed, there was no accumulating loss of hydraulic conductivity occurring over time. These results emphasize the importance of conducting field studies to supplement vulnerability data obtained through traditional laboratory methods