Multi-stemmed trees in montane rain forests: their frequency and demography in relation to elevation, soil nutrients and disturbance

Multi-stemmed trees are a common component of tropical and temperate montane rain forests, where they co-occur with single-stemmed trees. We hypothesized that multi-stemmed architecture should predominate in less productive sites (such as at high elevation or with low soil nutrient availability) and that, because it frequently results from sprouting, it should prevail in sites that are frequently disturbed. We also hypothesized that where multi-stemmed architecture predominates, there should be lower rates of mortality and recruitment of individuals. We tested these hypotheses using data from permanent plots in tropical montane rain forests in Jamaica (14 years) and in temperate montane rain forests in New Zealand (19 years). The frequency of multi-stemmed trees varied across plots in both forests (4-34% in Jamaica; 0-21% in New Zealand) along gradients of elevation and soil nutrients. Analyses examined the direct and indirect effects of the various environmental variables that are likely to drive site productivity and disturbance. We showed contrasting predictors of the frequency of multi-stemmed trees in the tropical and temperate forests. Multi-stemmed trees in Jamaican forests were most frequent on sites of low soil nutrient status (especially available soil phosphorus, P) whereas in New Zealand they were most frequent at high elevation sites, especially towards tree line. In both forests there was no relationship between multi-stemmed tree frequency and slope. Turnover (the mean of mortality and recruitment rates) of multi-stemmed trees in Jamaican forests was lowest on sites of low soil nutrient status (especially available P) but was unrelated to environmental predictors in New Zealand. In both forests, turnover rates of multi-stemmed trees overall were 60% lower than those for co-occurring single-stemmed trees, offering support for the hypothesis that multi-stemmed architecture favours persistence. Synthesis. Our study demonstrates that multi-stemmed trees can predominate in low productivity sites in montane rain forests. Their low turnover compared with co-occurring single-stemmed trees confirms the importance of evaluating the persistence niche as a mechanism promoting woody species coexistence in forests.