Effect of Quercus douglasii (Fagaceae) on herbaceous understory along a rainfall gradient

Abstract

Variation in effect of approximately 50% Quercus douglasii (blue oak) cover on herbaceous understory biomass and composition was studied along a rainfall gradient between five sites. Biomass and composition were compared between understory and adjacent open grassland at each site to evaluate changes in canopy effect along the gradient. Biomass was measured at the time of greatest standing biomass (PEAK) in 1986 and 1987. Composition was measured at PEAK 1986. Annual rainfall was above average in 1985-1986, and below average in 1986-1987. In both years PEAK biomass was greater in grassland than understory at sites with >50 cm yr_1 average rainfall, and no difference was apparent at sites with <50 cm yr_1 rainfall. Variation in species composition between grassland and understory was independent of rainfall gradient. Differences in individual species presence and abundance between grassland and understory were found at all sites. We conclude that variation in canopy effect on biomass resulted from changes in relative production between understory and open grassland, not from differences in relative composition. Winter deciduous Quercus douglasii Hook. & Arn. (blue oak) forms a patchy canopy over and within a continuous annual herbaceous layer on more than a million hectares in California (McClaran and Bartolome 1989). Average rainfall in blue oak woodland varies geographically from 30 to 100 cm yr_1 (Griffin 1977). In high rainfall areas understory biomass was lower than open grassland (Bartolome 1 986; Jansen 1987), and understory biomass increased after thinning or removal of oak canopy (Johnson et al. 1959; Heady and Pitt 1 979; Kay 1 987). In low rainfall areas understory biomass was greater than in open grassland (Duncan and Reppert 1960; Holland 1980). Composition differences between grassland and understory were reported for several sites (Heady and Pitt 1979; Holland 1980; Bartolome 1986; Jansen 1987). Because Q. douglasii cover was not constant among these study sites, relative influence of rainfall and tree cover on herbaceous biomass and composition cannot be interpreted. Madrono, Vol. 36, No. 3, pp. 141-153, 1989 This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 05:32:13 UTC All use subject to http://about.jstor.org/terms 142 MADRONO [Vol. 36 In general, differences between understory and grassland composition have been attributed to negative competition effects or positive site modification effects (e.g., Parker and Muller 1982; Yavitt and Smith 1983; Schott and Pieper 1985). Biomass differences between understory and grassland also have been attributed to these positive and negative effects. Examples of positive effects described for relatively xeric areas (e.g., Tiedemann and Klemmedson 1977; Patten 1978), contrast with negative effects in mesic regions (e.g., Halls and Schuster 1965; Jameson 1967; Ford and Newbould 1977). Negative effects of light and water competition were suggested in tree-grass models (Walker et al. 1981; McMurtie and Wolf 1983) and the interpretation of empirical biomass data (Knoop and Walker 1985). The suggestion that species interactions vary along environmental gradients (Begon et al. 1986) provides a framework to organize the diversity of canopy effects by Quercus douglasii. We used this framework to identify our objective and generate a null hypothesis. To evaluate effects of rainfall and Q. douglasii canopy on herbaceous understory, we used the null hypothesis that the effect of 50% Q. douglasii canopy on the similarity of grassland and understory biomass and composition does not vary along a rainfall gradient.