Disturbance is required for CO2-dependent promotion of woody plant growth in grasslands
Beth R. Loveys A B D , John J. G. Egerton A , Dan Bruhn C and Marilyn C. Ball AA Plant Science Division, Research School of Biology, Australian National University, Canberra, ACT 0200, Australia.
B Present address: School of Agriculture, Food and Wine, The University of Adelaide, Waite Campus, PMB 1 Glen Osmond, SA 5064, Australia.
C Present address: Biosystems Department Risø National Laboratory, Technical University of Denmark – DTU, Building 330, PO Box 49 DK-4000 Roskilde, Denmark.
D Corresponding author. Email: beth.loveys@adelaide.edu.au
Functional Plant Biology 37(6) 555-565 https://doi.org/10.1071/FP09257
Submitted: 27 October 2009 Accepted: 6 March 2010 Published: 20 May 2010
Abstract
The relative effects of disturbance (here defined as bare soil), competition for edaphic resources, thermal interference and elevated [CO2] on growth of tree seedlings in grasslands were studied under field conditions. Snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings were grown in open-top chambers flushed with either ambient or elevated [CO2] from March 2004 to January 2005 (autumn to summer). These seedlings were planted into three treatments (i.e. bare soil, soil covered with straw or soil supporting a sward of live pasture grass) to separate effects of grass on seedling growth into those due to competition with grass for soil resources or to alteration of the thermal environment caused by a grassy surface (Ball et al. 2002). After the first major autumn frost, seedlings growing in competition with grass lost 59% of their canopy area, whereas those growing in bare soil or straw suffered negligible damage. These results reveal the complexity of competitive inhibition of plant growth in which ineffective competition for resources such as soil water enhances the vulnerability of the plant to abiotic stress, in this case frost. Tree seedlings growing in bare soil and straw commenced growth earlier in spring than those growing in competition with grass, where soil moisture was consistently lowest. Under ambient [CO2], growth was greater in bare soil than in straw, consistent with thermal interference, but these differences disappeared under elevated [CO2]. Elevated [CO2] significantly increased biomass accumulation for seedlings growing in bare soil and straw treatments, but not in grass. Thus, elevated [CO2] alleviated apparent thermal interference of seedling growth in spring but did not overcome adverse effects on seedling growth of either competitive reduction in soil resources or competitive enhancement of environmental stress. Nevertheless, elevated [CO2] could promote invasion of grasslands due to enhancement of woody plant growth in bare soil created by disturbances.
Additional keywords: climate change, elevated [CO2], Eucalyptus pauciflora, freeze damage, grass–tree interactions, microclimate, vegetation thickening.
Acknowledgements
The authors thank Mr Paul Sillis for permission to conduct experiments on his property, Mr Wayne Pippen for technical assistance, Drs Stephen Roxburgh and John Evans for useful comments on earlier versions of this manuscript and Dr Stephen Clayton for assistance with nitrogen analysis.
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