Disentangling drivers of tree population size distributions

• We evaluated the effects of major drivers of changes in tree population structure.
• We used data for a national network of forest plots and exclosures.
• Widespread invasive animals had distinct impacts on tree populations.
• This provides indicators for management of invasive mammal impacts in forests.

There are pressing needs to evaluate and manage the effects of biological invaders in forest ecosystems in the face of long-term changes and anthropogenic influences on these systems. Hence, disentangling the effects of different drivers is critical for understanding the relative importance of biological invaders on shaping future forest structure and composition, and how management of these invaders changes forest ecosystem functioning. We evaluated the effects of major drivers of changes in tree population structure in New Zealand forests using a national grid of permanent vegetation plots and long-term (>15 year) ungulate exclosures. The long-term effects of disturbance and other drivers 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 quantified variation in tree size class distributions for common tree species, and plant functional groups defined by mammal diet selection, in 986 permanent forest plots and in 88 paired ungulate exclosure and control plots. Our analyses revealed that: (i) strong changes in 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 overwhelmed by temporal changes and effects of forest stand successional stage. These findings demonstrate that the effects of invasive herbivores can be distinguishable from other drivers such as disturbance or successional stage, and suggests that these changes will ultimately be manifested in altered forest community composition and ecosystem processes.