Disturbance, succession and the coexistence of species in a lowland podocarp forest, South Westland, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Vegetation change in six forest stands affected by flood and wind disturbance was investigated using stand history reconstruction in a lowland podocarp forest, south Westland, New Zealand. A method is described for identifying even-aged patches of trees from tree location and age data. Identification of even-aged patches of trees, along with size and age structures, and stem spatial distributions, were used to reconstruct the spatial and temporal patterns of tree establishment in response to past disturbance. Periodic, major disturbance had affected the composition and structure of all six Stands. In four of the stands floods formed extensive, continuous canopy openings and initiated regeneration of relatively even-aged patches of trees. In the remaining two stands windthrow of canopy trees occurred over an extended period and resulted in abundant but patchy regeneration in smaller treefall openings. Species differed in their patterns of regeneration in response to disturbances of different type and intensity. Establishment site preferences and stem spatial distributions showed that the four species studied partitioned establishment sites along gradients related to canopy cover and forest floor microrelief. Partitioning of establishment sites may contribute to the maintenance of species diversity in these forests. Moreover, disturbance is critical for continued coexistence as it is the source of much of the environmental variation to which species differentially responded. The composition and structure of regenerating patches was determined by the interaction between species regeneration requirements and the availability of suitable establishment sites. The type and intensity of disturbance and underlying environmental gradients influenced the availability of establishment sites within canopy gaps and hence subsequent forest establishment. Catastrophic floods formed extensive openings and left relatively homogeneous deposits of silt. Regeneration on these sites was dominated by Dacrycarpus dacrydioides. Less intense flooding left logs and stumps that provided sites for establishment of Dacrydium cupressinum, Prumnopitys ferruginea, and Weinmannia racemosa. On older surfaces drainage patterns influenced establishment in openings formed by treefalls. Regeneration on drier sites was dominated by D. cupressinum, P. ferruginea, and W. racemosa reflecting the predominance of better drained level and elevated microsites. In contrast D. dacrydioides dominated regeneration on poorer drained surfaces that had abundant wet depressions. The importance of disturbance in influencing forest composition and patterns of species turnover suggests that a non-equilibrium model of vegetation change is appropriate for understanding the dynamics of these forests.