干旱区生态环境知识资源中心

Changes in vegetation composition after fire in a Sarcopoterium spinosum dwarf-shrub community on phosphorus-deficient terra rossa soil overlying limestone were studied over a 10-yr period. Herbicide application and phosphate fertilizer were combined to provide a range of initial conditions for recovery. The site was undisturbed during the growing season in winter but was grazed by cattle in summer. Encroachment rate of S. spinosum was mainly a function of the density of viable buds on plants that survived the fire and/or the shrub control treatments. This, together with the very low rate of shrub establishment from seed set a short-to-medium term limit to the attainable cover of S. spinosum. The leguminous shrub Calycotome villosa appeared only in the third year after the fire, apparently from seed. After shrub control, the encroachment rate of C. villosa was low, even after 9 yr. On the other hand, where no herbicide was applied, and with the cessation of goat grazing, C. villosa became increasingly prominent, to the extent that by the 8th year after the fire, it inhibited the continued expansion of S. spinosum. Phosphorus nutrition tended to reduce the cover of S. spinosum because of its stimulating effect on the predominantly leguminous herbaceous vegetation. C. villosa also appeared to benefit from greater phosphorus availability, despite greater competition from the annual vegetation. These findings show that, depending on the context, secondary succession after fire can take different courses. Whereas the combined effect of herbicide control and P-enrichment cannot prevent shrub encroachment, it can slow down the rate of succession for more than a decade, despite the presence of ample shrub seed sources in the area.