Life history differences and tree species coexistence in an old-growth New Zealand rain forest

Abstract

We examined stand disturbance history, population structures, spatial relationships of age classes and size classes, and tree growth histories in an old-growth temperate rain forest in southern New Zealand. We aimed to determine the role of juvenile and adult life history differences in promoting coexistence of the four main overstory species (Nothofagus menzies, Weinmannia racemosa, Dacrydium cupressinum, and Prumnopitys ferruginea). There was no evidence that major compositional shifts were occurring: N. menzies, D. cupressinum, and P. ferruginea were represented by all-aged populations, indicating continual recruitment of all three species within the 5-ha study area during recent centuries. No age data were obtained for W. racemosa, but the diameter distribution of this species was consistent with an all-aged population structure. The temporal distribution of releases in tree ring sequences suggested a history of chronic patchy disturbance during at least the last 400 years. Ring width sequences were consistent with species differences in growth histories. About half (53%) of N. menzies >20 cm dbh appeared to have reached the canopy in one growth spurt, compared to only 11% of P. ferruginea. Many trees of the latter species showed multiple episodes of release and suppression, indicating successive responses to several gap events before reaching the canopy. D. cupressinum growth histories were intermediate between these two extremes. Nearest neighbor age relationships also indicated species differences in regeneration patterns. Neighboring individuals of N. menzies -10 cm dbh were, on average, significantly more similar in age than random pairs drawn from the age data pool under a bootstrap null model. This pattern is consistent with regeneration of N. menzies in small, even-aged patches, in response to treefall gaps. In contrast, age differences between nearest neighbor pairs of P. ferruginea and D. cupressinum trees did not deviate significantly from the null model of randomly selected ages, suggesting that establishment and survival were not closely linked to gap formation. Maximum radial growth rates of N. menzies saplings were nearly 2.5 times those of D. cupressinum and P. ferruginea. Fast growth of N. menzies beneath treefall gaps may compensate for the greater shade tolerance of P. ferruginea, in particular. D. cupressinum, although apparently outperformed by one or another of its competitors in both shade and treefall gaps, has a longer life-span than any of the other species. Thus, relatively low recruitment rates may be sufficient to maintain its present abundance. Fewer data were available for W. racemosa, but its ability to reproduce vegetatively may be an important factor in its persistence in competition with the other dominants. Under a disturbance regime dominated by treefall gaps, coexistence of the four dominants appears to be associated with complementary differences in growth rate, shade tolerance, canopy residence time, and facility for vegetative reproduction. Our results serve to emphasize that understanding plant species coexistence may require attention to interspecific differences in both juvenile and adult life history characteristics.