Fire and Population Dynamics of Woody Plants in a Neotropical Savanna: Matrix Model Projections

Abstract

Human activity has resulted in high fire frequency in many moist tropical savannas. To simulate the effects of increased fire frequency on woody plants of the cerrado savannas of Brazil, I constructed matrix population models for five species, including a subshrub, two shrubs, and two trees, using four years of demographic data. The models projected that four of the five species will decline under frequent burning but will increase in abundance under infrequent burning. For these four, fire intervals of 2–9 yr are required for long-term persistence, depending on the species. The fifth species, a shrub, was virtually unaffected by burning. Similar data for three herbaceous species were obtained from literature sources. Among the eight species, there was a negative relationship between population growth under annual burning and population growth under unburned conditions. Woody plant species performed best under low fire frequency, and herbaceous species performed best under high fire frequency. Three of the study species propagate vegetatively by producing root suckers. Fire stimulates vegetative propagation in all three species but reduces sexual reproduction. Elasticity analysis revealed that vegetative propagation is more important than sexual reproduction for population maintenance of these three species, and that vegetative propagation is relatively more important under frequent burning. However, stasis and progression were always much more important than either mode of reproduction, with stasis making the largest contribution to population growth. Analyses of life history response experiments were performed to determine what demographic variables contributed most to the population decline under frequent burning. The effect of fire on sexual reproduction contributed little to the overall effect of fire on population growth, particularly for clonal species. Fire-induced mortality also had little effect on population growth, except for Miconia albicans, which has extremely fire-sensitive seedlings. The negative effect of fire on individual size tended to make the greatest contribution to the overall negative effect of fire on population growth, and this effect was greater for the two trees than for the shrub species. The results indicate that growth form was the primary determinant of fire response, while ability to reproduce vegetatively is important to a lesser extent. Current fire regimes are predicted to reduce woody plant density and to favor smaller life forms and species capable of clonal propagation.