Convergence During Secondary Forest Succession

(1) Successional convergence in community composition was examined in terms of three questions: (i) for a given site is there a continuous shift in composition toward that characteristic of climax? (ii) does variation in community composition along an environmental gradient increase or decrease with succession? (iii) to what extent is the species composition along a successional gradient determined by site characteristics rather than by chance factors? Hypotheses regarding the nature of successional convergence are reviewed in terms of these questions. (2) These hypotheses were tested using data for tree (>0.5 cm dbh) and herb (all foliage 80-yr-old pines, and uneven-aged hardwoods). For each age-group first axis ordination scores (detrended correspondence analysis) were highly correlated most consistently with soil pH as compared with other soil and site variables. This correlation was greatest in the intermediate-age (40-60and 60-80-yr-old) pines and in the hardwood stands, and lowest in old pine stands. The distribution of species in the ordination space was most similar between the intermediate-age pine stands and the hardwoods; in contrast, young and old pine age classes were dissimilar to hardwoods. Species distributions in relation to the soil pH gradient were also most similar between intermediate-age pines and hardwoods. Thus, there is a shift in species composition toward that characteristic of climax, but it is probably not monotonic. (3) Beta-diversity was highest in the hardwoods and lowest among the pines. Community differentiation along gradients increased in this chronosequence. (4) Predictability of soil pH, based on stand species composition, was highest in the intermediate-age pines and in the hardwoods. Habitat breadth in relation to soil pH decreased steadily with successional age. These results are consistent with the hypothesis that the role of chance factors (e.g. site history and seed rain) decreases with successional age.