Does Geijera parviflora Lindl. (Rutaceae) facilitate understorey species in semi-arid Australia?
A. D. Warnock A C , M. E. Westbrooke A , S. K. Florentine A and C. P. Hurst BA Centre for Environmental Management, School of Science and Engineering, University of Ballarat, PO Box 663, Victoria 3350, Australia.
B Institute of Health and Biomedical Innovation, Queensland University of Technology, 60 Musk Avenue, Kelvin Grove, Queensland 4059, Australia.
C Corresponding author. Email: a.warnock@ballarat.edu.au
The Rangeland Journal 29(2) 207-216 https://doi.org/10.1071/RJ07032
Submitted: 20 June 2007 Accepted: 31 August 2007 Published: 14 November 2007
Abstract
Species composition under tree canopies often differs from that of surrounding micro-environments. In arid and semi-arid zones, trees can be beneficial to understorey vegetation. This study examined zones of vegetation composition and soil physiochemical parameters associated with Geijera parviflora Lindl. The importance of shade, rainfall redistribution, seed bank and soil moisture were examined. Species abundance, soil moisture, seed bank composition, rainfall redistribution and soil nutrient concentration were measured under five randomly selected mature G. parviflora trees in south-western New South Wales, Australia. To complement the findings from this study, artificial shade plots were constructed in a canopy-free area and species abundance measured seven months after shade construction. The study demonstrated that G. parviflora was associated with zonation of understorey vegetation. Two zones of understorey vegetation were found in relation to G. parviflora: (i) under the tree canopy with high species diversity, and (ii) beyond the canopy, this community being dominated by Dissocarpus paradoxus throughout the year with Crassula colorata appearing after rainfall. The zone beyond the canopy also had lower soil nutrient concentrations. Soil moisture, nutrient concentration and the seed bank density were significantly higher under the canopy. However, the canopy reduced precipitation reaching the soil surface. The effects of the canopy on understorey species composition and soil moisture were enhanced after winter rainfall. Artificial shade increased species abundance and richness under a 90%-shading treatment. The results indicated that G. parviflora generated spatial heterogeneity over the broader plant community increasing species richness, abundance and diversity under the canopy. This emphasises the importance of arid zone trees in conserving understorey plant diversity. Shading, soil nutrient concentration and increased seed bank density and soil moisture appeared to be key influences on the plant communities under the canopy.
Additional keywords: canopy, microhabitat, nurse plant, overstorey, shade, spatial heterogeneity.
Acknowledgements
The authors thank the University of Ballarat for financial support and the provision of laboratory facilities, and Jo Gorman and John Warren of New South Wales National Parks and Wildlife Service for access to sites within Tarawi Nature Reserve. We are also grateful to students from the University of Ballarat who assisted with field work. Kate Gosney provided additional help with statistical analysis. John Pickard and an anonymous reviewer provided useful comments on an earlier draft of the manuscript.
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