Overstory and Understory Dynamics in a Ponderosa Pine Plantation Vary with Stand Density in the Sierra Nevada: 40-Year Results

Abstract

We periodically measured overstory ponderosa pine (Pinus ponderosa) growth and understory cover and abundance in a long-term study on the west slope of the Sierra Nevada, California, USA. The study was established in 1969 in a 20-year-old plantation, thinned to basal areas of 9, 16, 23, 30, and 37 m2 ha−1 and rethinned three times. The objective was to determine the effect of stand density regimes on productivity, understory vegetation, aboveground carbon storage, and mortality caused mainly by Dendroctonus infestations. Results showed that without mortality, basal area and volume increments were not affected by thinning the plantation to 16 m2 ha−1 from 37 m2 ha−1. With Dendroctonus actively attacking trees, the plantation could be thinned to 9 m2 ha−1 without sacrificing volume increment, because the level of mortality increased with stand density index. The thinning intensity did not affect the abundance of understory vegetation but did affect the cover of understory trees and graminoids. Intensively managed plantations could sequester and store the same amount of carbon as less intensively managed plantations. However, because fire is a major component in a pine-dominant ecosystem, carbon in larger diameter trees would be more resilient to wildfire than carbon in small diameter trees or understory vegetation.