Print PageGiant sequoia (Sequoiadendron giganteum) are the largest and among the oldest living organisms on Earth, with many individuals possessing massive and complex crowns. Spatial and temporal heterogeneity in climatic factors interact with changes in xylem water potential and tree structure throughout individual crowns to determine water and carbon exchange rates at leaf, branch, and whole-tree scales. We measured sap flow rates throughout the crowns of 3 large Sequoiadendron trees growing in the southern Sierra Nevada to (1) characterize within-crown and whole-tree water-use patterns, and (2) examine structural and environmental factors driving these patterns. The crown structure of each tree was mapped using rope-based arborist techniques. Sap flow was measured in 8 branches in lower, middle, and treetop crown locations in each tree in 2009, and in 6 branches in lower, middle, and treetop crown locations in 2 trees in 2010. Sap flow was also measured at the tree base, live crown base, and treetop in the main trunk of 2 trees in 2010. Temperature and humidity were measured and hemispherical photographs were obtained to characterize environmental conditions of each branch. Diurnal xylem water potential was also measured on all branches in 2009 and on a subsample of branches in 2010.
Results/Conclusions
Diurnal patterns of xylem water potential varied considerably among branches and reflected the influence of both height and solar radiation exposure. In contrast, atmospheric vapor pressure deficit was generally low (< 2.0 kPa) and did not significantly vary among sample branches during the two measurement periods. Diurnal patterns of average hourly branch transpiration and stomatal conductance were significantly correlated with solar radiation with time lags ranging from 0-4 hours, but were not correlated with vapor pressure deficit. The strength of the correlation between sap flow and solar radiation was determined by the amount of direct solar radiation received by each branch throughout the day. We observed large azimuthal variation in trunk sap flow rates that closely followed the daily trajectory of solar radiation exposure within the canopy. Radial profiles of trunk sap velocity showed the highest flow rates several centimeters inside the cambium and declining thereafter with increasing sapwood depth. We estimate that sap flow through the main trunk of large individual Sequoiadendron trees can exceed 2,000 liters of water per day during the summer, more than any other tree species reported in the literature, with important implications for forest hydrology and ecosystem water balance.
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