Structural Changes in Chamise (Adenostoma fasciculatum) along a Fire-Induced Age Gradient

Abstract

Chamise (denostoma fasciculatum) undergoes major structural changes in above-ground biomass along a gradient of increasing stand age since fire. Shrub growth in volume and biomass is rapid through 16 years but levels off in older stands. In this early phase of linear biomass increase, net above-ground productivity has a mean of 430 g-yr-t for each shrub, or 60 g.m-2 yrover the period of 2-16 years stand age. Individual shrubs at 37 years in Sequoia in the southern Sierra Nevada have more than twice the biomass and four times the annual above-ground productivity of chamise of similar age in the San Gabriel Mountains of Los Angeles County. Between 16 and 37 years, shrub senescence increases with no increase in above-ground biomass and a sharp reduction of available photosynthetic surface area. Total stand biomass continues increasing with stand age up to 37 years as shrub biomass increases, with a maximum at approximately 15,000 kg-ha-'. Fine fuels less than 10 mm comprise more than 500 g.m-2 in all chamise canopy ages, providing an important structural element of flammability. The characteristic evergreen shrub or chaparral vegetation of central and southern California is a classic example of a fim-type community (Hanes 1977; Biswell 1974). Occurring in both pure and mixed stands throughout most of the state's foothill zone, chamise (Adenostomafasciculatum H. & A.) is the single most common chaparral species (Hanes 1965, 1977). Physical characteristics which are well adapted to rapid rates of energy release (Countryman and Philpot 1970) combine with typical hot and dry summers, volatile ether extractives, and low fiel moistures to make chamise an especially flammable fuel type. Its wide distribution and frequent proximity to developed and urban areas has made chamise the subject of growing management concern as well as scientific investigation (Parsons 1976; Hanes 1977). Following fire, chamise characteristically resprouts from a root crown as well as germinating from heat-treated seeds (Christensen and Muller 1975). In most cases the sprouts form the basis of a rejuvenated stand which rapidly matures to the point where within about 15 years the fuel loading (weight per unit area) is thought to be able to again support a moderately intense fire (Philpot 1973). During the past century fire Authors are associate professor, Department of Ecology and Evolutionary Biology, University of California, Irvine 92717; and research scientist, Sequoia and Kings Canyon National Parks, Three Rivers, California 93271. The authors greatly acknowledge the assistance of Gail Baker, Sue MacLeod, and Steve DeBenedetti in carrying out this study. Funding was provided by the National Park Service and by a National Science Foundation Grant. Manuscript received May 1, 1978. suppression policies have greatly reduced fire frequencies in most chaparral areas. As a result there now exist large areas of overmature chamise chaparral which constitute highly flammable, potentially catastrophic fuel accumulations as well as being of little value as browse of wildlife. This report provides a detailed analysis of structural changes in chamise following different periods since fire. Such information is of value in understanding the concept of senescence as it applies to shrub growth as well as adaptations of chamise to periodic burning. Analysis of growth patterns, SEQUOIA AND KINGS CANYON NATIONAL