Differential regeneration responses to lack of disturbance in a fire-dependent community

Background/Question/Methods

Recruitment is influenced by disturbance regimes, and many communities are dominated by species dependent on large-scale disturbances.  Chaparral is a shrub vegetation dominated by species dependent on fire to stimulate germination of persistent soil seed banks.  At the same time, these communities also contain a large number of species with completely or partially transient seed banks.  Their regeneration niche differs considerably as do many morphological and physiological characteristics.  Only a few studies have investigated recruitment in the long-term absence of disturbance in these types of communities, and those studies focused on individual species.  Here I use a large number of large plots (932 -15 m2 plots) to assess the composition and richness of adults and seedling recruitment in diverse stands of undisturbed chaparral. Stands were located in three types of chaparral, a maritime chaparral strongly influenced by summer fog, a serpentine community, and a common interior chaparral community.

Results/Conclusions

The chaparral was composed of 43 species with many restricted to one chaparral type.  Of those, 13 of 36 shrub species were found to have recruits in the vegetation as were 7 of 8 tree species.   The average number of seedlings for all species was 0.23 m-2.  These were not evenly distributed and some plots exhibited 7 m-2; at the community scale, serpentine chaparral averaged the fewest seedlings.  Among shrubs, seedlings of Quercus wislizenii var. frutescens dominated the seedling community, representing over 87% of the total.  Trees were responsible for 3.5% of all seedlings, and Umbellularia californica established more than all the other tree species combined.  No significant differences were found by dispersal vectors, which were distributed roughly evenly by any categorization. Transient seed bank species dominated recruitment in undisturbed chaparral, accounting for over 99%, but species exhibited differential rates compared to their representation in the community, and many were absent in the seedling community. Species with fire-stimulated seed banks were absent or poorly represented.  The results suggest chaparral and mixed evergreen forest have soft boundaries and that observed patterns of dominance depend on the variation in the disturbance regime.