FLOOD DISTURBANCE AND THE COEXISTENCE OF SPECIES IN A LOWLAND PODOCARP FOREST, SOUTH WESTLAND, NEW-ZEALAND

1. The structure and composition of four forest stands affected by catastrophic floods on a lowland floodplain, south Westland, New Zealand, was investigated using stand history reconstruction. Age and size structures, and tree locations were used to identify cohorts of trees that were initiated by flood events. This, along with information on species microsite preferences and spatial patterning, was used to reconstruct the patterns of establishment of the four dominant canopy trees (Dacrycarpus dacrydioides, Dacrydium cupressinum, Prumnopitys ferruginea and Weinmannia racemosa) in response to flood disturbance. 2. The four species coexisted by partitioning establishment sites with respect to the amount of overhead cover and the type of forest floor microsite. D. dacrydioides and D. cupressinum established in large gaps but colonized contrasting microsites; D. dacrydioides established mostly on mineral soil while D. cupressinum established on elevated microsites. W. racemosa and P. ferruginea both established on elevated microsites beneath largely intact forest canopies. However, W. racemosa colonized canopy gaps while P. ferruginea established mostly under closed cover. Disturbance was critical to the maintenance of species diversity because it was the source of much of the environmental variation to which species responded differentially. 3. Among plots, differences in forest structure and composition reflected -the availability of sites suitable for the recruitment of each species. Site availability was in turn largely a consequence of disturbance history. Intense floods formed large gaps and left homogenous surface deposits of silt that became dominated by D. dacrydioides. Less-intense floods left exposed debris that provided additional establishment sites for D. cupressinum and for occasional P. ferruginea, while floods that left surviving canopy intact provided establishment opportunities for all four species. Recruitment into small canopy openings in developing stands was dominated by W. racemosa. 4. The importance of disturbance in influencing patterns of forest regeneration suggests that a model emphasizing the response of individuals to a fluctuating physical and biotic environment is appropriate for understanding the dynamics of these forests. Forest dynamics were a consequence of the differential response of species to changes in the nature of canopy gaps through time.