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 lin es. During thinning, these species became less dense and more evenly d istributed over space, but the total biomass significantly increased; the upper thinning boundaries (of total biomass) of all four species w ere better described by a slope of -1/2. In contrast, regression slope s of either total biomass or mean biomass per individual against densi ty tended to be shallower than the upper thinning lines. The regressio n slopes were significantly shallower in 1995 than in 1996, indicating that self-thinning became more evident as biomass accumulated. The di fferences in regression slopes among species suggested that the stands of these four species were developing under different physical or bio logical regimes. Self-thinning was closely related to successional spe cies replacement, and changes in canopy structure might be the main ca use of variations in the biomass-density relationships. Use of both up per thinning line and regressions could greatly improve our understand ing of self-thinning processes and their implications for chaparral su ccession.