Data from: Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

1. Savanna ecosystems span a diverse range of climates, edaphic conditions
and disturbance regimes, the complexity of which has stimulated
long-standing interest in the mechanisms that maintain tree-grass
coexistence. One hypothesis suggests that tree establishment is strongly
limited by one or several demographic bottlenecks at early stages of the
tree life cycle. A major impediment to testing this hypothesis is the lack
of data on the relative strengths of different bottlenecks across key
environmental gradients. 2. To identify demographic bottlenecks that limit
early tree establishment (0-18 months), we conducted a series of
transplant experiments with two savanna trees species (Acacia robusta and
A. tortilis) across a natural rainfall and soil fertility gradient in the
Serengeti ecosystem, Tanzania. We tested the interactive effects of
precipitation, herbivory, seed scarification, grass competition, water
limitation and tree species identity on two key life stages: germination
and early seedling survival (0-2 months) and juvenile seedling survival
(2-18 months). 3. Germination and early seedling survival increased as a
function of rainfall, in the absence of herbivores and when seeds were
scarified. Juvenile seedling survival, in contrast, decreased with
rainfall but increased in the absence of herbivores. Grass removal had the
single strongest (positive) effect on juvenile seedling survival of any
treatment. Soil moisture monitoring and grass-addition treatments revealed
that grasses negatively affected seedlings in ways that were not
necessarily linked to soil moisture. 4. A demographic model combining all
effects across early life stages showed that the strength of grass
competition on juvenile seedling survival was the key factor limiting
early tree establishment. While rainfall had an unexpected opposing effect
on the two life stages, the net effect of mean annual precipitation on
early tree establishment was positive. 5. Synthesis: Successful tree
establishment in Serengeti is maximized by a seemingly unlikely sequence
of events: (1) scarification of seeds by browsers, (2) heavy rainfall to
promote germination, (3) intensive grazing (but absence of browsers) and
(4) dry conditions during juvenile seedling growth (>2 months) to
reduce competition with grasses. By considering a wide suite of conditions
and their interactions, our experimental results are relevant to ongoing
debates about savanna vegetation dynamics and structural shifts in
tree:grass ratios.