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Seasonality and facilitation drive tree establishment in a semi-arid floodplain savanna

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Abstract

A popular hypothesis for tree and grass coexistence in savannas is that tree seedlings are limited by competition from grasses. However, competition may be important in favourable climatic conditions when abiotic stress is low, whereas facilitation may be more important under stressful conditions. Seasonal and inter-annual fluctuations in abiotic conditions may alter the outcome of tree–grass interactions in savanna systems and contribute to coexistence. We investigated interactions between coolibah (Eucalyptus coolabah) tree seedlings and perennial C4 grasses in semi-arid savannas in eastern Australia in contrasting seasonal conditions. In glasshouse and field experiments, we measured survival and growth of tree seedlings with different densities of C4 grasses across seasons. In warm glasshouse conditions, where water was not limiting, competition from grasses reduced tree seedling growth but did not affect tree survival. In the field, all tree seedlings died in hot dry summer conditions irrespective of grass or shade cover, whereas in winter, facilitation from grasses significantly increased tree seedling survival by ameliorating heat stress and protecting seedlings from herbivory. We demonstrated that interactions between tree seedlings and perennial grasses vary seasonally, and timing of tree germination may determine the importance of facilitation or competition in structuring savanna vegetation because of fluctuations in abiotic stress. Our finding that trees can grow and survive in a dense C4 grass sward contrasts with the common perception that grass competition limits woody plant recruitment in savannas.

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Acknowledgments

This study was funded by the Cotton Catchment Communities Cooperative Research Centre in collaboration with the Namoi and Central West Catchment Management Authorities. We thank landholders for allowing access to sites and sharing local knowledge, Nick Schultz, Rhiannon Smith, Morag Stewart and Peter Berney for field assistance and expert advice. Feedback from Ian Lunt and Rod Fensham on an early version of this work and comments from Katherine Gross and two anonymous reviewers greatly improved the manuscript. Jodi Price was funded by the ARC Centre of Excellence for Environmental Decisions. The experiments comply with the current laws of Australia, in which the experiments were performed.

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  1. Megan K. Good

    Present address: Department of Botany, School of Life Sciences, La Trobe University, Bundoora, VIC, 3086, Australia

Authors and Affiliations

  1. Ecosystem Management, School of Environmental and Rural Science, University of New England, Armidale, NSW, 2351, Australia

    Megan K. Good & Nick Reid

  2. Botany, School of Environmental and Rural Science, University of New England, Armidale, NSW, 2351, Australia

    Peter J. Clarke

  3. School of Plant Biology, University of Western Australia, Crawley, Perth, 6009, WA, Australia

    Jodi N. Price

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  1. Megan K. Good
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Correspondence to Megan K. Good.

Additional information

Communicated by Truman Young.

Appendices

Appendix 1

See Fig. 6

Fig. 6
figure 6

Comparison of daily maximum temperatures (lines) and rainfall (bars) in the first 2 weeks after seedling transplant in summer and winter

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Appendix 2

See Fig. 7

Fig. 7
figure 7

Rainfall (bars) and daily maximum temperatures (line) throughout the winter field experiment (from August 2010 to April 2011). High rainfall in December 2010 resulted in flooding of the study sites

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Good, M.K., Clarke, P.J., Price, J.N. et al. Seasonality and facilitation drive tree establishment in a semi-arid floodplain savanna. Oecologia 175, 261–271 (2014). https://doi.org/10.1007/s00442-014-2886-x

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