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.

No abstract

No abstract

The invasion of Mesembryanthemum crystallinum into coastal grassland was observed at Surf on the central coast of California, USA. The species became established in areas unoccupied or sparsely occupied by other plants, and then grew into surrounding areas. The following season, few grassland seedlings established beneath dried Mesembryanthemum as compared to adjacent grassland. The reduction in numbers of grassland seedlings did not appear to be the result of limiting levels of moisture, light or macronutrients which were found in lower levels in the grassland, nor to be due to grazing by small mammals, since the pattern persisted in the absence of grazing. The differential pattern of seedling establishment was correlated with high levels of salt found in the soil beneath dried Mesembryanthemum.Mesembryanthemum crystallinum is an annual plant which accumulates salt throughout its lifespan. After the plant dies, this salt is released with leaching by fog and rain. The salt produces a detrimental osmotic environment preventing growth of nontolerant species. The salt does not appear to have a direct toxic influence on grassland species. Osmotic interference resulting from accumulation and release of salt appears to be the means by which Mesembryanthemum crystallinum dominates areas previously occupied by grassland.

No abstract