DOI: 10.5061/dryad.tn2d16t
title: Data from: Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

description: 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 (&gt;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.
description: Seedling_dataSurvival data of transplanted Acacia tortilis and A. robusta seedlings under different experimental treatmentsGermination_dataData of germination success of transplanted seeds of A. tortilis and A. robusta under various experimental treatmentsEnv_dataEnvironmental data (soil volumetric water content, and light) from experimental treatmentsPlot_Rainfall_Jan_2018Rainfall measurements of mean annual rainfall (from interpolated rain gauge data), and actual rainfall (from Arc satellite data) in seedling and germination plots.