Life history differences and tree species coexistence in an old-growth New Zealand rain forest.
A study was conducted to investigate 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 Jackson River Valley, southern New Zealand, during 1995-96. The role of juvenile and adult life history differences in promoting coexistence of the 4 main overstorey species (Nothofagus menziesii, Weinmannia racemosa, Dacrydium cupressinum, and Prumnopitys ferruginea) was studied. No evidence that major compositional shifts were occurring was observed. N. menziesii, D. cupressinum and P. ferruginea were represented by all-age populations, indicating continual recruitment of all 3 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-age population structure. The temporal distribution of releases in tree ring sequences indicated 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. menziesii >20 cm dbh appeared to have reached the canopy in one growth spurt, compared to only 11% of P. ferruginea. Many P. ferruginea trees 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 extremes. Nearest neighbour age relationships also indicated species differences in regeneration patterns. Neighbouring individuals of N. menziesii ≥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 was consistent with regeneration of N. menziesii in small, even-aged patches, in response to tree fall gaps. In contrast, age differences between nearest neighbour 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. menziesii saplings were nearly 2.5 times those of D. cupressinum and P. ferruginea, and it is suggested that the fast growth of N. menziesii beneath tree fall 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 tree fall gaps, had the longest life-span, and it is suggested that 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 tree fall gaps, coexistence of the 4 dominants was associated with complementary differences in growth rate, shade tolerance, canopy residence time and facility for vegetative reproduction.