Vegetation Trends and Carbon Balance in a Ponderosa Pine Plantation: Long-Term Effects of Different Shrub Densities

摘要

Long-term studies designed for one purpose often can serve many purposes with time. This is illustrated by a study established in 1961 to test the efficacy of herbicides on a poor site in Northern California. There, a treated brushfield community in ayoung ponderosa pine plantation left initial densities of no shrubs, light, medium, or heavy shrubs. In 1966 a second study was established to see how these initial densities affected the growth and composition of the subsequent plantation plant community. Greenleaf manzanita prospered during the study. Snowbrush initially developed well, but then declined. Sierra plum endured, but was relegated to the understory. Needlegrass invaded rapidly, peaked early, and was mostly gone by the end of the study. Needlegrass developed well in shrub-free areas, but was nearly absent in heavy shrubs. Ponderosa pine grew well in no-shrub plots, fair in light-shrub plots, and poor in medium- and heavy-shrub plots. Even a light cover of shrubs decreased pine height, diameter, and volume growth substantially. In 2001, or four decades after the original treatment, a third study examined the long-t erm effects of vegetation control on ecosystem carbon storage. Preliminary measurements showed that no-shrub plots containedone-third more carbon in the standing vegetation than those in the heavy-shrub plots. Soil carbon levels were slightly greater where shrubs were present—a finding also found on a poorer site in the eastside Cascades and on a better site in the westside Sierras. Findings suggest that vegetation control had little influence on total ecosystem carbon storage (carbon mass was only 6 percent higher where shrubs had been controlled).