Density-Yield Relationship of an Invasive Annual Grass inCalifornia Chaparral and Its Restoration Implications
The density-yield relationship of wild plant populations, which in agricultural systems is typically consistent with the constant final yield hypothesis, has remained largely unexplored in a restoration context despite its potential importance. In degraded California shrublands, non-native annual grasses are associated with reduced native shrub recruitment. By studying their density-yield…
In degraded California shrublands, non-native annual grasses hinder native shrub growth. By studying their density-yield relationship, restoration efforts may be optimized by determining the necessary level of grass control to alleviate competitive pressure on woody plants. This research examined Bromus diandrus, a prominent invasive annual grass, at a chaparral site in Santa Barbara County, California.
The study manipulated grass density through 0-100% removal treatments while monitoring the performance of Ceanothus megacarpus seedlings, used as phytometers. The results confirmed that Bromus diandrus follows the constant final yield hypothesis, exhibiting strong intraspecific competition at moderate to high densities. This competition results in constant population biomass and an allometric exponent similar to those found in agricultural studies.
Removing 75% of the grass biomass reduced competition, yet only at 100% removal did resource availability (light and soil moisture) increase, as evidenced by direct measurements and improved phytometer performance. In California Chaparral and other Mediterranean ecosystems, where weed biomass reduction is a common management goal, these findings suggest that complete eradication of non-native annual grasses may be necessary to facilitate the establishment of native woody seedlings.
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