422 Ford et al.: Use of Water by Eastern Hemlock described by Ford and Vose (2007). All lead wires were connected to double-shielded cable wires and differentially connected to a data logger with a multiplexer peripheral (models CR10X and AM416; Campbell Scientific, Inc., Lo- gan, UT). The temperature difference between the upper and lower probes was converted to sap flux density using the equation of Granier (1985). For all trees, readings for the two replicate sets of sensors were averaged. Occasionally, sensors were replaced in newly drilled holes if null, out-of-range, erratic, or negative readings were recorded or if probes were physically damaged. We also used three or more variable length sap flow probes to measure the radial distribution of sap flux within the sapwood in two of the 16 trees in the SA and two of the eight trees in NE. Further descriptions of methods used to measure sap flux and climatic variables can be found in Ford and Vose (2007) and Daley et al. (in press). Trees were considered healthy with minimal (SA) to absent (NE) infestation at the onset of these studies. We use the data we obtained to develop relationships between: 1) water flux and depth into the xylem; 2) tree water use and the seasonal variation in water flux; and 3) tree water use and tree size. In addition, we use those data to develop simple mathematical and graphical functions for predicting daily water use by eastern hemlock on the basis of climatic variables and tree diameter. A multiple variable linear regression approach was used to predict daily water use by eastern hemlock. We treated days as replicate observations; however, one underlying assump- tion of regression is that replicate observations are indepen- dent. Water use observations for an individual tree on suc- cessive days are not independent, attributable partially to lag effects associated with capacitance. Seasonally, successive observations often have a high autocorrelation resulting from the influence of factors such as leaf area, sapwood area, cli- mate, and individual genetic effects. To avoid capacitance effects, we subsampled the master data set and only used 7 days of measured water use for each tree. Subsamples had to occur on rain-free days, have measurements from all trees on both sites, and had to represent all seasons (the seven sub- sampled days were roughly 60 days apart). We assumed that any remaining autocorrelation in the subsampled data set was attributable to seasonal effects and that replicate observations were independent. In other words, we assumed that daily water use measured at one point in time did not directly affect daily water use measured approximately 60 days later and that any trend in the daily water use over time was the result of external factors affecting the tree such as climate and cli- mate effects on seasonal leaf and sapwood area growth. An additional assumption of regression analysis is that observations are normally distributed. To meet this assump- tion, daily water use data were natural log-transformed after which data were distributed normally (Shapiro-Wilk test, P0.21). Because this transformation was used, it was nec- ©2007 International Society of Arboriculture essary to eliminate from consideration 2 days of subsampled observations in the NE data set. These observations were made when transpiration was zero, during January and March. Our final subsampled data set contained 152 obser- vations. Thus, our dependent variable was the natural log of daily water use in each of the 24 trees on 7 days that were sampled (n 152). The independent variables used in the analysis were re- gional location, day of year, year, diameter at breast height, daytime average air temperature, and daytime average vapor pressure deficit. We used a forward selection regression ap- proach in which variables were entered into the model each in turn until no remaining variable produced a significant F statistic (SAS 2003). We present two models: one model is more complex, but the independent variables explain more variation in eastern hemlock water use; the second model is less complex and the independent variables explain slight- ly less variation in water use. We used the less complex model to develop an easy-to-use graphical relationship among tree diameter, average daytime temperature, and total daily water use. RESULTS AND DISCUSSION Distribution of Water Flux Within the Stem The distribution of water flux within the stem was consistent among the four trees monitored. Although diameter of the four trees ranged from 34.6 to 77.7 cm (13.8 to 31.1 in), the maximum water flux consistently occurred 1 to 2 cm (0.4 to 0.8 in) beneath the cambium in the xylem (Figure 1). This depth represented 12% to 30% of the hydroactive xylem ra- dius in the trees measured. The integrated water flow in the 0 to 2 cm (0 to 0.8 in) sapwood region represented 54% (34.6 cm [13.8 in] diameter at breast height [dbh]), 74% (44.6 cm [17.8 in] dbh), 23% (55.3 cm [22.1 in] dbh), and 97% (77.7 cm [31.1 in] dbh) of the total flow. Most of the variability in the distribution of water flux was found in the deeper sap- wood areas and was largely attributable to the 55.3 cm (22.1 in) dbh tree. This skewed distribution of water flux in the stem is typical in trees with tracheid xylem elements (Ford et al. 2004) and it can be related to the development and age of the xylem elements. For example, at the cambium interface, newly de- veloped living xylem cells are not yet functional in mass water transport. Xylem cells further away from the cambium are dead and hollow and are thus functional in mass water transport. Over time, these xylem cells become increasingly displaced from the cambium, and they can become progres- sively more dysfunctional as a result of chemical, biological, and physical disruptions (see discussion and references in Ford et al. 2004). Our data suggest that the greatest move- ment of stem water generally occurs in zones 1 to 2 cm (0.4 to 0.8 in) past the inner bark.
November 2007
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