Fire and Biotic Factors Afecting Germination

Abstract

Most plant communities are so complex that a clear-cut analysis of the relative importance of these 7 mechanisms is either very difficult or impossible. By working with simplified systems or with natural conditions where already a number of these mechanisms are excluded or inoperative, the significance of the other mechanisms can be assessed. By working with communities of annual plants, or first-year establishment of perennials, particularly in their early stages of development, mechanisms 3 and 7 can be eliminated. This is the significance of the study of desert annuals (Went 1948). By taking the problem into the laboratory, mechanisms 4, 5 and 6 can be eliminated, using the same medium (sterile sand and vermiculite) on which small amounts of soil are spread, and placing the containers in greenhouses or laboratories with controlled climate in the absence of pests and diseases. This was also done with desert soils (Went 1948). In the latter case surface samples of desert soils were used, containing a natural mixture of seeds. In the present paper data are recorded on the germination of weeds and chaparral species in relation to fire, climate and competition. A few general conclusions had been drawn previously concerning the effect of fire on a chaparral vegetation (Sampson 1944). Shrubs were divided into species resprouting from the root crown after a fire (Adenostoma fasciculaturn, Ceanothus leucodermis, Quercus spp.) and nonsprouting species (other Ceanothus). Growth of annuals and particularly germination of shrub species was strongly increased as the result of a fire. According to Sampson, however, unusually dry or wet season preceding or following burning may unpredictably affect the plant population of burned areas. The following experiments were carried out to find out more about germination as influencing re-establishment of vegetation after a fire. Several areas in the neighborhood of Pasadena, where fires had destroyed the chaparral cover, were investigated. The Bryant fire occurred on August 5, 1947, burning an area of 3,500 acres with a range of elevations between 500 and 1,600 m. No appreciable rain occurred until December, when a total of 72 mm. (2.83) fell, followed in February, March and April by 69 mm. (2.71), 112 mm. (4.40) and 86 mm. (3.38) (see Table I). Observations the following season showed that on areas adjacent to the burn there was practically no germination of either annuals or perennials (1 Salvia mellifera) while on the burn a fairly typical area showed the extensive germination of Eriodictyon, 10 or more seedlings per square meter of Adenostoma and Salvia mellifera, 20-50 per square meter of Ceanothus, up to 200 of Dendromecon rigida, and 1 per square meter of Rhus laurina. There were thick stands of annuals.