Abstract:
Four different plot types were established in shrub and tree comunitities in Little Elk Valley in southwest Oregon. Ten functional (F) plots were cleared of all vegetation in a radius of three times the height of the surrounding vegetation around a single Pacific madrone (Arbutus menziesii Pursh.) tree. Ten competition plots (C) were cleared as in the F plots, but a single Douglas-fir (Pseudotsuga menziesii Mirb. (Franco)) within 1 m of the central madrone stem in each plot was left intact. Ten baseline soil moisture plots (B) were cleared of all vegetation as in the F plots, including madrone and Douglas-fir. Twenty 2,4-D treated madrone plots (D) were not cleared, but had been sprayed 2 years prior to measurment. The F, C, and D plot madrone had stomatal conductance, net radiation, ambient air temperature, and vapor pressure deficit measured over the morning, midday, and afternoon periods of the same day during the early, mid and late summer seasons of 1981 to estimate evapotranspiration. Soil moisture was measured at the 0.5, 2, and 3 m distances from the plot center at the 30 and 60 cm soil depths in the F, C, and B soil moisture plots. In the D plots, soil moisture was measured at both the 30 and 60 cm depths but at only the 0.5 m distance from the madrone stem over the same periods during the day and seasons as in the F, C, and B plots. Madrone leaf area was measured directly on all study plants. Predawn plant moisture stress measurements were made on the Douglas-fir in the C plots. Results showed that madrone leaf area in the F and C plots increased over the previous year's leaf area by 0.48 and 0.38 m², respectively (equivalent to 9.6 and 9.7 percent increases, respectively). 2,4-D treated madrone had a leaf area increase of 1.0 m² (equivalent to a 30 % increase). Soil moisture in the cleared plot types was not significantly influenced by the presence of an individual madrone, Douglas-fir, or peripheral vegetation. Soil moisture at 60 cm had a greater influence on stomatal conductance of madrone than soil moisture at 30 cm depth. Stomatal conductance and evapotranspiration of madrone leaves were most strongly influenced by ambient air temperature. Evapotranspiration per unit surface area did not differ significantly between the 2,4-D treated and untreated madrone two years after treatment. 2,4-I) treatment reduces the leaf area index of Pacific madrone for a number of years, decreasing the total plant evapotranspiration. The rapid recovery of madrone leaf area after treatment indicates that leaf area index will also be increasing rapidly. Therefore, evapotranspiration will return to pretreatment levels as fast as the leaf area index and the total leaf area of the plant attain pretreatment levels. Thus the competition reduction benefits of 2,4-D) application appear to be short lived (on the order of 5 years), unless applied under conditions that kill madrone completely.
