Conspecific Negative Distance-Dependence and New Zealand Podocarp Forest Dynamics

Abstract

The Janzen-Connell hypothesis proposed that conspecific negative density or distance dependence (CNDDD) helps maintain the species richness of tropical rainforests through the action of specialist natural enemies that suppress conspecific recruitment beneath canopy trees. Empirical studies have since shown that CNDDD occurs not only in tropical rainforests but in a wide range of other vegetation types. Studies of CNDDD have invariably focused on its implications for species coexistence and diversity; rarely has its potential relevance to successional replacement patterns been explored. Here we present evidence for CNDDD that may shed new light on a long-standing problem in New Zealand forest dynamics—the failure of relatively shade-tolerant conifers (in particular Dacrydium cupressinum, Podocarpaceae) to regenerate beneath their own canopies in dense podocarp stands. We mapped tree positions on a 3-ha plot in a central North Island podocarp/broadleaf forest in which diameter distributions indicated two discrete cohorts of D. cupressinum trees and used pair-correlation functions to measure hetero- and conspecific spatial associations between small and large trees. Small trees (5–60 cm diameter) of D. cupressinum were significantly negatively associated with large conspecific trees of the older cohort at radii of up to 7 m. This pattern cannot readily be explained by light requirements, as small trees of a less shade-tolerant podocarp (Phyllocladus trichomanoides) were distributed independently of large D. cupressinum. Small trees of the only other species well-represented by large diameters (the broadleaved species Elaeocarpus dentatus, Elaeocarpaceae) were also negatively associated with large conspecific trees, albeit across a narrower range of radii. Our results suggest CNDDD might contribute to the characteristic failure of D. cupressinum to regenerate in central North Island dense podocarp stands, regeneration resuming only when attrition or exogenous disturbance reduces the D. cupressinum overstorey to scattered emergent trees.