Inter and intra-specific plant competition between four Cumberland Plain grassland species at Penrith Lakes and the implications for local land rehabilitation.
Revegetation and reconstruction of simplified plant communities on degraded land in Western Sydney is likely to play a role in the conservation of the Cumberland Plain Woodland, and in the conservation of ecological processes within the landscape. Significant challenges face the endeavour of broad-scale reconstruction, including the supply of seed for direct seeding and the control of weeds during the establishment phase of under-story revegetation.
Reconstruction sites are by nature radically disturbed and degraded, and present particular weed control challenges. Physical disruption of the soil increases light availability at the soil surface, and light exposure combined with greater temperature fluctuations can increase nitrogen mineralisation (D’Antonio, et al, 2002). Such increases in short-term resource availability favours many exotic weed species. This combined with the observation that, weeds that germinate and establish early cause greater yield-loss than weeds that emerge later in the life of the crop (Grime, 1977), make the establishment phase of grassland rehabilitation particularly critical. In the slightly longer-term native grasses are very likely to succeed in a rehabilitated environment if the short-term risks of mortality due to weed competition can be managed. For example, Tillman (1988) has shown that, at moderate to low mortality rates, transient dominance is predicted whereby species may actually be replaced by slower growing species with lower resource requirements (Grace, 1990).
The management of weeds in an integrated way, such as low disturbance ground preparation, timing of sowing, chemical applications and species selection, combine to enable successful grassland rehabilitation. A component of such an integrated approach is to better understand the relative competitive abilities of native species and to select species and mixtures most likely to compete successfully with and resist weed invasion.
Four Cumberland Plain provenance grassland species, Microlaena stipoides, Chloris ventricosa, Austrodanthonia racemosa and Glycine microphylla, have been planted in an addition style (Firbank, et al, 1985) competition trial on post-mining land at Penrith Lakes in March 1996. Fifty-one separate treatments in four replicates were planted-out in a series of monocultures, and binary and trinary mixtures at four starting densities to examine inter and intra-specific competition between the species and to identify the most productive and weed resilient treatments for use in local land rehabilitation.
Measures of basal circumference, plant height and inflorescence numbers were undertaken on four specific plants of each species present in each trial plot in the Autumn, Winter, Spring and Summer of 1996, and in March 1998. Cover and density measures were collected in June and September 1997 and March 1998, and destructive sampling of dry weight herbage mass was collected in June 1998 and March 1999. In addition a measurement of cover was undertaken in November 2004. A brief overview of some results and possible implications, from herbage mass measures of 1998 and 1999 and cover measurements of 2004, in relation to the three grass species, in monoculture and binary mixtures, are presented herein.
Chloris ventricosa displayed early dominance, having higher productivity during the early sampling occasions than in the later ones, and suppressing the productivity of both Microlaena and Austodanthonia during this time. In this undisturbed environment (since 1996) Chloris lost its initial dominance and ultimately provided a more productive cover when in mix with the other two grass species than when in monoculture. Chloris may act as a useful native cover crop exhibiting stoloniferous habit and relatively rapid growth and seed set.
Chloris suppressed Austodanthonia racemosa during early establishment. Austodanthonia suppressed Microlaena during early establishment and performed better in mixtures on all sampling occasions except during early sampling in competition with Chloris. Austrodanthonia monocultures had amongst the highest levels of productivity in early monitoring and the least productivity and cover scores during the 2004 sampling, eight years after planting.
Microlaena stipoides is a slower growing and longer-lived species that performed best over time when in monoculture. The species was suppressed by Chloris and Austodanthonia in the early sampling occasions and became the superior competitor over time despite its early suppression by the other two species.
Overall, Microlaena is a slower growing species that increased its dominance over time. This species performed best and produced the highest levels of native cover when grown in monoculture. This observation suggests that “shotgun” mixtures of seed for direct seeding may not always produce the best results and that some species may do better sown as mosaics of monocultures rather than in mixtures. It also suggests that in an undisturbed environment, slower growing species may dominate in the longer-term. The early dominance of faster-growing native species such as Chloris and Austodanthonia suggests that in a disturbed environment these species may act as a useful cover or nurse crop for slower growing species and may provide better short-term protection to erosive soils. Further, it may suggest that slower growing species should be established into an uncultivated environment, if a sufficient seedbed can be provided, thereby reducing the presence of quick-growing exotic species that may inhibit their survival. Low disturbance ground preparation and seedling establishment may also enable successful sowing of monoculture mosaics of slower growing species where considered desirable.
D’Antonio, C., and Meyerson, L.A. (2002). Exotic Plant Species as Problems and Solutions in Ecological Restoration: A Synthesis. Restoration Ecology. Volume 10, No. 4, 703-713.
Firbank, L.G., and Watkinson, A.R. (1985b). On the analysis of competition within two-species mixtures of plants. J. Appl. Ecol. 22, 503-517.
Grace, J.B. (1990). On the Relationship between Plant Traits and Competitive Ability. In, Perspectives on Plant Competition 1990 (eds) James B. Grace and David Tilman. Academic Press, San Diego.
Grime, J.P. (1979). Plant strategies and vegetation processes. Wiley, London.
Tilman, G.D. (1988). Plant Strategies and the Dynamics and Structure of Plant Communities. Princeton Monographs, Princeton, New Jersey.
This research is part of an ongoing PhD by Tim Berryman at the University of Western Sydney (Hawkesbury). Tim’s academic supervisor is Dr. Frank Kelleher. Funding for the research has been received from the NSW Roads and Traffic Authority, the Penrith Lakes Development Corporation, the Horticultural Research and Development Corporation, and Lowlands Lawn Turf.