Diameter growth across an altitudinal gradient in the cloud forest tree genus Weinmannia

Background/Question/Methods

Tree growth typically decreases with decreasing temperature, for both species and communities. Lower absolute growth may help explain cold margin range limits, while lower growth relative to co-occurring species leading to increased competitive pressure could help explain warm margin range limits. For trees growing along an altitudinal gradient we predicted that (1) growth should decline with elevation within species, and (2) high elevation species should have lower growth rates than related low elevation species. We tested these predictions within a hierarchical Bayes modeling framework, combining data from repeated diameter censuses of all trees with data from dendrometers installed on a subset of the same trees in 10 one hectare tree plots spanning a 2000 meter altitudinal gradient. We use this model to estimate growth rates for all trees and years and test for differences in growth for species in the cloud forest tree genus Weinmannia across species, year, tree size, elevation, and basal area of neighbors (as a proxy for competitive pressure).

Results/Conclusions

Contrary to expectations, preliminary results show that low elevation species do not have higher growth rates than high elevation species, and growth often increases with elevation within species. However, growth rates of adult trees do follow expectations based on the hypothesized competitive hierarchy of the species. Shade tolerant species and those with large adult size have the highest growth rates (and are most abundant) within closed canopy forest. In contrast, light demanding species have the lowest growth rates in closed canopy forest, and appear to be declining in the plots surveyed. Lower growth rates at low elevation suggest that climatic effects on growth may help explain warm margin range limits in these species. Decreasing growth at lower elevations may be exacerbated by climate change, as temperatures in the region have increased by an average of 0.05°C per year in recent decades, and warming is expected to accelerate.