The Effect of Blue Oak Removal on Herbaceous Production on a Foothill Site in the Northern Sierra Nevada1

The effect on herbaceous production of blue oak canopies of 25, 50, and 75 percent, and the effect of complete removal of such canopies were determined over a 7-year period for a single site in the foothills of the northern Sierra Nevada. Tree covered plots produced 12 percent less than open grassland plots, while cleared plots produced 46 percent more than tree covered plots. Percentage differences in production were about the same for the middle and end of the grow­ ing season, but were considerably less at the beginning of the growing season. Large, climatical­ ly induced, annual variation in production mask the long term trend of the effect of tree removal on annual production. Tree canopy, whether present or removed, has no discernable correlation with production due to tree size, stocking and other plot effects. Johnson et al. and by Murphy and Crampton provided support for state-wide recommendations for the removal of blue oak from rangeland as a means of increasing forage production, livestock grazing, and ranch income (Bell 1963; Murphy and Berry 1973; Holland 1976). In contrast to these findings and recommenda­ tions, Holland (1973; 1976; and 1980) reported that forage production was 40 to 100 percent greater under blue oaks than in open grassland and recommended that blue oak clearing be stopped (Steinhart 1978). Holland's findings were based on studies in the south-central Sierra Nevada foothills and southern Coast Ranges and are supported by observations made by Duncan and Reppert (1960), Duncan (1967), and Duncan and Clawson (1980). Possible explanations for these contra­ dictory findings are given by Duncan and Clawson (1980) and Holland and Morton (1980) and include: (1) regional climatic differences; (2) different levels of blue oak canopy; and (3) edaphic differ­ ences. This study was initiated in the fall of 1979 to determine the relationship between the level of blue oak canopy and herbaceous production throughout the season of, rapid plant growth (approxima­ tely March 1 to May 31) for two contrasting situ­ ations: (1) tree canopy left intact; and (2) tree canopy removed. A second purpose of the study was to determine the long term trend in herbaceous production following tree removal. To provide for replication of the study and ac­ count for regional variation in climatic and eda­ phic conditions, 3 study sites were proposed. However, lack of available resources and funding prevented the inclusion of 2 sites in the study. Results obtained from the one remaining study site should therefore be considered as descriptive and no inferences should be drawn beyond the narrow confines and limitations of this study.