Light environments occupied by conifer and angiosperm seedlings in a New Zealand podocarp–broadleaved forest

Abstract

Interaction between conifers and angiosperms in New Zealand’s podocarp–broadleaved forests is a topic of enduring interest. We aimed to determine if the often discontinuous regeneration of the podocarps Dacrydium cupressinum and Prumnopitys ferruginea can be attributed to their seedlings’ tolerating less shade than those of angiosperm canopy trees and/or to occupying a narrower range of light environments. We quantified the light environments (% diffuse light availability) naturally occupied by large seedlings (50–200 cm tall) of these two conifers and five co-occurring angiosperms, in an old-growth podocarp–broadleaved forest in the central North Island of New Zealand. Randomisation was used to compare the mean and variance of the light environments occupied by each species with those of the distribution of light environments in the forest understorey. The 10th percentiles of distributions were also calculated as an indicator of the deepest shade tolerated by each species. These parameters showed D. cupressinum to be essentially randomly distributed in relation to light availability, like the angiosperm Beilschmiedia tawa. Although this was also true of the mean light environment of the other conifer, P. ferruginea, there was marginally significant evidence that this species was underrepresented at the shadiest microsites. In contrast, the angiosperms Elaeocarpus dentatus and Weinmannia racemosa showed strongly non-random patterns, occupying significantly brighter minimum and mean light environments than would be expected by chance. It therefore seems unlikely that the discontinuous population structures of podocarps in many forests result from an intolerance of shade at the large seedling stage. Furthermore, the similarity of the ranges of light environments occupied by D. cupressinum and P. ferruginea suggests that reported differences in population structure and successional position of these species are not attributable to differences in seedling shade tolerance.