Disentangling drivers of tree population size class distributions
There are pressing needs to evaluate and manage the effects of biological invaders in ecosystems in the face of long-term changes and anthropogenic influences on these systems. We evaluated the effects of several likely drivers of changes in tree population structure in New Zealand forests using a national grid of permanent vegetation plots and long-term ungulate exclosures. The effects of disturbance and other drivers can leave imprints in tree population dynamics and size class structure. For example, the effects of natural disturbance are expected to have different effects from those of browsing or folivorous animals. We evaluate the effects of ten potential drivers of variation in tree size class distributions, and test these drivers as complimentary hypotheses. Changes in size class distributions and leaf functional traits were assessed through analyses of variation in size class distributions of plant species and palatability groups in 986 permanent forest plots, and by analyses of changes in 88 paired ungulate exclosure and control plots.
Our findings demonstrate that: i) strong changes in tree size class distributions have occurred in recent decades, ii) tree species selected or preferred by invasive ungulates (primarily red deer) demonstrate particularly strong shifts, iii) inclusion of climate or soil variables into statistical models of exclosure effects on size class distributions were rarely warranted, and these effects were typically overwhelmed by temporal changes and forest stand successional stage, iv) long-term ungulate exclosures were not representative of national-scale size class distributions for taxa preferred by deer. These findings also demonstrate that the effects of browsers is distinguishable from other drivers such as disturbance and successional stage, but that these effects have been overstated in results using ungulate exclosures because they are not representative of effects at a national scale. Changes in tree size class distributions are ultimately linked to forest community composition and ecosystem properties or processes. Hence, disentangling the effects of different drivers on tree size class distributions is critical for understanding how biological invaders alter forest ecosystem functioning, and how management of these invaders shapes future forest structure and composition.