Effects of Fire on Factors Controlling Plant Growth in Adenostoma Chaparral

Abstract

In the first few years following fire, burned chaparral areas are typified by a flush of seed germination and seedling growth not seen in unburned chaparral. The effects of fire on factors of potential importance to this plant response were examined in recently burned and long unburned chaparral, dominated by Adenostoma fasciculatum, in the Santa Ynez Mountains near Santa Barbara, California. Measurement of soil moisture, soil texture, and light indicated that changes in these factors following fire had little direct effect on postfire germination and growth. Bacteria and fungi were more abundant in burned than in unburned chaparral soil. Thus, the hypothesis that microbial depletion of soil oxygen in unburned chaparral inhibits seed germination appears untenable. Changes in organic matter content, pH, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium were examined in the upper soil layers of both burned and unburned chaparral during the first 18 mo following fire. Mineral addition as ash—fall was also evaluated. Burning increased levels of most mineral elements significantly. Furthermore, a large reservoir of readily available organic nutrients was added in the ash. Causes of low nutrient levels in unburned chaparral were also investigated. Additions of nutrients to unburned chaparral soil in the greenhouse and field resulted in increased growth of species common in burned chaparral areas. Bioassays of aqueous A. fasciculatum leaf washings were performed against 10 plant species common in burned chaparral areas. Significant depression of germination and growth was observed. Rain throughfall collected under the shrubs was also inhibitory. Efforts to isolate and identify the toxins are described. Numbers of small herbivorous mammals were greatly reduced in recently burned chaparral. This factor is shown to have a marked effect on seedling survival. Applications of various heat treatments to unburned chaparral soil and to fresh seeds of species common to burned chaparral indicated that the seed of several species are released from dormancy by heat. Insolational heating of soil in cleared, but unburned, chaparral is sufficient to stimulate germination.