Effect of Fires on Woody Vegetation in the Pine-grassland Ecotone of the Southern Black Hills

Abstract

This paper presents a 3-year study (1979-1981) on the effects of prescribed burning of ponderosa pine forests at Wind Cave National Park, in the southern Black Hills, South Dakota. The fires were largely restricted to surface fuels and forest understory vegetation. Effects upon understory shrubs. and deciduous trees were modest. In eight study plots, two cool-season (autumn and spring) fires consistently reduced densities of Ribes spp. and stimulated Amorpha canescens Pursh., while other shrubs were unaffected. These burns significantly reduced the density of immature and smaller mature ponderosa pines. The reductions were consistent across all eight study plots. By contrast, an autumn (1974) crown fire, in ponderosa pine, killed pines of all sizes and most shrub species increased dramatically. We attribute these differences in postfire vegetation response to variable fire intensities. INTRODUCTION In the southern Black Hills of South Dakota, ponderosa pines (Pinus ponderosa Laws.) tend to spread into surrounding grasslands along a pine-grassland ecotone (Gartner and Thompson, 1972). This invasion has been documented by a comparative study of photos from General Custer's 1874 Expedition of Exploration to the Black Hills with photos taken a century later (Progulske and Sowell, 1974). Participants in the 1874 Expedition observed that wildfires killed pine trees and encouraged shrub growth. Professor Aris B. Donaldson, the Expedition botanist, wrote of a postfire ponderosa pine forest where all large trees had been killed (Krause and Olsen, 1974, p. 64): But of raspberries there was no end. Acres and acres of the mountains' sides were covered with them. They were large and sweet and could be picked in clusters of from three to six. A recent paper by Dietz et al. (1980) further documents shrub increases on burned sites in this region. Ponderosa pine forests under natural conditions burn every 10-25 years throughout its range in the western United States (Weaver, 1967; Wright, 1978). At Wind Cave National Park, South Dakota, Shilts et al. (1980), using dendrochronological techniques, found the fire cycle to be between 13 and 21 years from 1820-1910. These fires tended not to kill mature trees, but they burned off the litter layer while killing young pines and other small-stemmed woody species (Weaver, 1967). When wildfires are absent, due either to successful efforts at fire suppression or unusual regional climatic patterns, dense dog-haired pine stands result. Such stands occurred within our study area, and pine had invaded the grassland in many places. The purpose of this study was to evaluate the effects of autumn and spring (cool season) prescribed burns on woody vegetation of the pine-grassland ecotone in Wind Cave National Park, of the southern Black Hills. The study covered one preburn and two postburn growing seasons. Our objectives were: (1) to determine if prescribed fires increased shrub and deciduous tree species in the forest understory; (2) to quantify mortality of immature pines in the forest-grassland ecotone, including both dense, spindly pine in the forest understory and seedlings and saplings invading adjacent grasslands, and (3) to measure the degree to which prescribed fires had consistent and predictable effects throughout the study area. METHODS The study area ranged in elevation from 1250-1425 m. Higher elevations, except