Self-thinning in early postfire chaparral succession: mechanisms, implications, and a combined approach

For two consecutive years (1995 and 1996), the self-thinning processes in nearly pure, even-aged stands of four dominant postfire chaparral species on Santa Monica Mountains, Southern California were quantified through the use of the "upper thinning boundary" and regression lines. During thinning, these species became less dense and more evenly distributed over space, but the total biomass significantly increased; the upper thinning boundaries (of total biomass) of all four species were better described by a slope of -1/2. In contrast, regression slopes of either total biomass or mean biomass per individual against density tended to be shallower than the upper thinning lines. The regression slopes were significantly shallower in 1995 than in 1996, indicating that self-thinning became more evident as biomass accumulated. The differences in regression slopes among species suggested that the stands of these four species were developing under different physical or biological regimes. Self-thinning was closely related to successional species replacement, and changes in canopy structure might be the main cause of variations in the biomass-density relationships. Use of both upper thinning line and regressions could greatly improve our understanding of self-thinning processes and their implications for chaparral succession.