Xylem cavitation as a mechanism for water storage in Douglas-fir boles.
Kavanagh, Kathleen *,1, Bond, Barbara2, 1 University of Idaho, Mocsow, ID2 Oregon State University, Corvallis, OR
ABSTRACT- Water storage in trees allows for increased carbon fixation. It has been hypothesized that most of the stored water from the bole is released as a result of xylem cavitation, but this hypothesis is controversial and has not been tested rigorously. To determine if cavitation in sapwood tracheids is the source of water from the bole, we monitored cavitation events (AEs) in two severed Douglas-fir bole segments as they dehydrated over a 2-month period along with concurrent estimates of transpiration (F), stomatal conductance (gs), leaf water potential (l), and predawn bole water potential (pdB), ). In addition, we monitored AEs during periods of drought stress in intact Douglas-fir trees along with (l), (pd), and sapflux. We noted three distinct phases of water loss, AEs and physiology during the desiccation process. In the initial phase, approximately 2.4% of total bole water was lost before we observed a decline in daily maximum gs; during this time AEs were very low. After several hours, as the stored water in the severed boles was depleted, gs, and F declined and AEs increased. Finally, pdB in the severed bole declined to -2.5 MPa, stomata remained closed and cuticular water loss from the needles became the primary water flux from the severed boles; throughout this phase, diurnal bole AEs increased significantly and closely tracked VPD. Following an initial rapid drop in l, and bole mass (phase 1), minimum l and bole mass declined very slowly. The results of this experiment are consistent with bole sapwood being a significant source of stored water to maintain stomatal conductance, although the mechanism of release does not appear to be related to tracheid cavitation. However, cavitation in the bole sapwood does appear to be a source of water to maintain l during periods of high rates of cuticular water loss and low pd.
Key words: cuticular water loss, transpiration, capacitance, sap flow
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