Reversal of human-induced vegetation changes in Sequoia National Park, California

Abstract

We resampled 76 permanent plots that had been established in the woodlands and forests of Sequoia National Park in 1969. Our objectives were to describe vegetation changes in the tree and shrub layers and determine the effects of prescribed burning that began in the 1960s. We compared changes in species importance and tree size class distributions between sample dates and between burned and unburned plots. Species composition had remained similar in all nine vegetation types sampled except in the ponderosa pine forest where Pinus ponderosa Dougl. ex P. & C. Laws. decreased in importance from 28 to 15% and Abies concolor (Gord. & Glend.) Lindl. increased from 18 to 31%. Structural changes were more common, as tree density decreased in the blue oak woodland (19%), and live oak woodlands (15%), as well as in ponderosa pine forest (41%), white fir forest (5%), giant sequoia groves (39%), and red fir forest (24%). Decreases in density were greater in burned plots but occurred in unburned plots as well, indicating that prescribed fire and self-thinning contributed to decreases in density. Tree density was unchanged in the lodgepole and subalpine forests, but increased in the Jeffrey pine forest (58%). The decreases in tree density represent a reversal of earlier trends.

Proceedings of the fifth symposium on oak woodlands: oaks in California’s changing landscape. 2001 October 22-25; San Diego, CA.

“…Associations between estimated fire history and blue oak regeneration drawn from dendrochronological studies (McClaren and ) result because fire temporally concentrates shoot ages of sprouts through topkill. Although other studies that report on direct effects of fire on blue oak have involved either larger (Haggerty 1994, Roy andVankat 1999) or smaller (Allen-Diaz andBartolome 1992, Schwan andothers 1997) oaks than those in our study, none of these studies provide evidence that fire favors blue oak regeneration.…”

“…In a previous study on blue oak sapling recruitment in 15 locations, we found that sapling recruitment was not dependent upon fire and that repeated fires reduced sapling populations (Swiecki and others 1997). Similarly, Roy and Vankat (1999) found no blue oak regeneration when they resurveyed plots in Sequoia National Park, some of which had burned. Mortality of mature trees was greater in burned plots, but the paper does not report whether the differences were statistically significant.…”

Apparent climatically induced increase of tree mortality rates in a temperate forest

“…If understory trees were undergoing self‐thinning, we would expect both increasing stand biomass and gradually decreasing mortality rates (Peet & Christensen 1987) instead of unchanging stand biomass and increasing mortality rates. Additionally, many of our forest plots are in higher elevation forests that are currently close to their historic fire return intervals (Pitcher 1987), while lower elevation forests have had 100–140 years since the cessation of burning to attain quasi‐equilibrium (Swetnam 1993; Roy & Vankat 1999). Finally, mortality rates are increasing for both the shade‐tolerant Abies and generally shade‐intolerant Pinus , and canopy closure – as reflected in cumulative tree size (i.e.…”

More than climate? Predictors of tree canopy height vary with scale in complex terrain, Sierra Nevada, CA (USA)

“…The Southern Sierra Nevada mountains are also home to the largest trees in the world (Giant Sequoias Sequoiadendron giganteum); although these trees did not occur within the footprint of the available LiDAR imagery, the climate is very similar to the mid-elevation transition sites 31 (Soaproot Saddle/ Providence Creek) and these isolated pockets of Sequoias will also experience Southern Sierran climatic changes in the next century. Extensive human management and fire in these forests has affected species composition and structure, highlighting the importance of anthropogenic influences on the forests of the Southern Sierra Nevada (Roy and Vankat, 1999).…”