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

Morrison, T. A., Holdo, R. M., Rugemalila, D. M., Nzunda, M. and Anderson, T. M. (2019) Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti. Journal of Ecology, 107(1), pp. 216-228. (doi:10.1111/1365-2745.13010)

Morrison, T. A., Holdo, R. M., Rugemalila, D. M., Nzunda, M. and Anderson, T. M. (2019) Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti. Journal of Ecology, 107(1), pp. 216-228. (doi:10.1111/1365-2745.13010)

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Abstract

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.

Item Type:Articles
Additional Information:National Science Foundation grants (BCS-1461728, DEB‐1145787 and DEB-1145861) supported this work
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Morrison, Dr Thomas
Authors: Morrison, T. A., Holdo, R. M., Rugemalila, D. M., Nzunda, M., and Anderson, T. M.
College/School:College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
Journal Name:Journal of Ecology
Publisher:Wiley
ISSN:0022-0477
ISSN (Online):1365-2745
Published Online:24 May 2018
Copyright Holders:Copyright © 2018 The Authors
First Published:First published in Journal of Ecology 107(1):216-228
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher
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