Browsing is a strong filter for savanna tree seedlings in their first growing season

Newly germinated seedlings are vulnerable to biomass removal but usually have at least 6 months to grow before they are exposed to dry-season fires, a major disturbance in savannas. In contrast, plants are exposed to browsers from the time they germinate, making browsing potentially a very powerful bottleneck for establishing seedlings. Here we assess the resilience of seedlings of 10 savanna tree species to topkill during the first 6 months of growth. Newly germinated seeds from four dominant African genera from across the rainfall gradient were planted in a common garden experiment at the Wits Rural Facility and clipped at 1 cm when they were ~2, 3, 4 and 5 months old. Survival, growth and key plant traits were monitored for the following 2.5 years. Seedlings from environments with high herbivory pressure survived topkill at a younger age than those from low-herbivore environments, and more palatable genera had higher herbivore tolerance. Most individuals that survived were able to recover lost biomass within 12 months, but the clipping treatment affected root mass fraction and branching patterns. Synthesis. The impact of early browsing as a demographic bottleneck can be predicted by integrating information on the probability of being browsed and the probability of surviving a browse event. Establishment limitation through early browsing is an under-recognized constraint on savanna tree species distributions.

Automated summary

This study assessed the resilience of seedlings of 10 savanna tree species to topkill during the first 6 months of growth. Seedlings from environments with high herbivory pressure survived topkill at a younger age, while more palatable genera had higher herbivore tolerance. The impact of early browsing as a demographic bottleneck is an under-recognized constraint on savanna tree species distributions.