Competition and the coexistence of species in a mixed podocarp stand

Examined the importance of intra- and interspecific competition in affecting the growth and survival of Dacrycarpus dacrydioides and Dacrydium cupressinum, the two dominant trees in a mixed podocarp stand in Ohinemaka Forest, Westland, New Zealand. Two lines of evidence (spatial patterning of trees and correlates of tree performance with neighbourhood density) were used to infer the importance of competition. Intraspecific competition among D. dacrydioides trees appeared to be intense. Mean radial growth rate (MGR) was significantly negatively correlated with the number of conspecific neighbours, and patterns of non-random mortality resulted in a shift in spatial pattern from a significantly clumped to a significantly regular distribution. Intraspecific competition among D. cupressinum was less intense. D. cupressinum had a low rate of mortality, predominantly random spatial patterns of mortality, and a weak correlation between MGR and the number of conspecific neighbours. Differences in the relative importance of intraspecific competition in affecting survival and growth of the two species was attributed to differences in the distribution and availability of suitable establishment sites. D. dacrydioides preferentially established on abundant level or slightly elevated sites, and in patches reached densities where resources were limiting. The number and distribution of D. cupressinum were determined primarily by the availability of suitable elevated establishment sites that were scattered throughout the stand. The restricted number of suitable sites limited D. cupressinum to densities where resources were not severely limiting. The two species avoided interspecific competition by partitioning the variability in forest-floor micro-relief. -from Author