Light red meranti (Shorea parvifolia) is a canopy tree species native to tropical forests in Southeast Asia that can grow to over 65 m. In the two decades to 2005, the abundance of light red meranti decreased by 60% in a ForestGEO research plot in Pasoh, Malaysia. In a ForestGEO plot in Lambir, Malaysia, over 1000 km away, the same species decreased by 25% over a similar period. This example prompts the question of whether forest tree population dynamics are in general synchronised across space. In a paper just published in Proceedings of the Royal Society B, we found that the answer is yes.
We collated forest inventory data at three different scales—local, regional and global—and found signals of synchrony up to scales well beyond 100 km. For the Pasoh and Lambir forests, in particular, there was clear evidence of synchrony for larger trees (greater than 10 cm diameter) although not for smaller trees. A technical challenge in our analyses was developing statistical methods to produce an aggregate estimate of synchrony for each pair of forest sites while accounting for the noise inherent in datasets with large numbers of rare species. Although existing methods are available to estimate synchrony for individual species, these were not appropriate for our forest data because of the short duration of our datasets (a few decades at most) relative to the typical tree generation time. Our novel methods make use of copulas, which are used to model multivariate statistical phenomena in quantitative finance.
We attributed the observed synchrony in tree population dynamics to synchronised environmental drivers, such as climate, although we lacked the statistical power to identify particular drivers. The observed synchrony suggests greater risk of extinction for tree species that are more spatially constrained, especially those with range dimensions less than ~100 km. On the other hand, species with larger ranges may be buffered from extinction by the lack of synchrony in their population dynamics across space.
Chisholm Lab 