Small-scale disturbance and regeneration dynamics in a neotropical mangrove forest

1 Mangrove forests are affected by a variety of natural disturbances that d iffer in scale, intensity and frequency. Small canopy gaps, although common , have not been well studied. We examined the role of lightning-created can opy gaps in the dynamics of a 47-km(2) intertidal mangrove community in the Dominican Republic, by quantifying the spatial patterns of overstorey tree distributions, spatial and temporal patterns of gap formation, and tree re generation in gaps and beneath the closed forest. We hypothesized that rege neration in these gaps would maintain and reinforce species' distribution p atterns across the intertidal gradient in this mangrove ecosystem. 2 All 52 gaps surveyed in the field were nearly circular canopy openings cr eated by lightning. Expanded gap size indicated an average diameter of 30.4 m. The total area of forest in the gap-phase is currently 1.9%, with an an nual formation rate of 0.23% year(-1.) Canopy turnover was calculated to ra nge from 421 to 842 years; however, gaps were not randomly distributed acro ss the mangrove forest and canopy turnover in the most gap-prone areas rang ed from 194 to 386 years. 3 Seedling density was not different under gaps and under the forest canopy , but sapling density and sapling growth rates of three species were signif icantly greater in canopy gaps. The annual mortality rate of Rhizophora man gle (9%) in canopy gaps was significantly lower than either Laguncularia ra cemosa (32%) or Avicennia germinans (56%). Gap regeneration was dominated b y R. mangle throughout the forest. 4 The results of this study do not support our initial hypothesis but sugge st that the distribution and abundance of R. mangle will increase throughou t the forest over time, given the present disturbance regime. Field observa tions indicated that the peat mat collapses in lightning-created gaps follo wing their formation, resulting in increased levels of standing water. We s uggest the successful regeneration of R. mangle is favoured in these gaps b ecause of its greater tolerance to flooding conditions.