Fire, grazers and browsers interact with grass competition to determine tree establishment in an African savanna

In savanna ecosystems, fire and herbivory alter the competitive
relationship between trees and grasses. Mechanistically, grazing
herbivores favor trees by removing grass, which reduces tree-grass
competition and limits fire. Conversely, browsing herbivores consume trees
and limit their recovery from fire. Herbivore feeding decisions are in
turn shaped by risk-resource trade-offs that potentially determine the
spatial patterns of herbivory. Identifying the dominant mechanistic
pathways by which fire and herbivores control tree cover remains
challenging, but is essential for understanding savanna dynamics. We used
an experiment in the Serengeti ecosystem and a simple simulation driven by
experimental results to address two main aims: (1) determine the
importance of direct and indirect effects of grass, fire and herbivory on
seedling establishment; and (2) establish whether predators determine the
spatial pattern of successful seedling establishment via effects on
mesoherbivore distribution. We transplanted tree seedlings into plots with
a factorial combination of grass and herbivores (present/absent) across a
lion kill-risk gradient in the Serengeti, burning half of the plots near
the end of the experiment. Ungrazed grass limited tree seedling survival
directly via competition, indirectly via fire, and by slowing seedling
growth, which drove higher seedling mortality during fires. These effects
restricted seedling establishment to below 18% and, in conjunction with
browsing, resulted in seedling establishment dropping below 5%. In the
absence of browsing and fire, grazing drove a 7.5-fold increase in
seedling establishment. Lion predation risk had no observable impact on
herbivore effects on seedling establishment. The severe negative effects
of grass on seedling mortality suggests that regional patterns of tree
cover and fire may overestimate the role of fire in limiting tree cover,
with regular fires representing a proxy for the competitive effects of
grass.