Date of Award

2000

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr. Ray Froend

Second Advisor

Dr. Peter Davies

Abstract

This thesis provides baseline information on the ecological processes involved in the recruitment and regeneration of riparian vegetation. As there has been a paucity of basic ecological studies on riparian vegetation in Australia, the project is broad in scope, and gives a general picture of the factors influencing, the regeneration of riparian vegetation and provides a starting point for more detailed work. The project focuses on factors determining recruitment events and the life history traits of particular species in a river in the cool temperate zone of south western Australia (Blackwood River) and on a river in the dry tropics of the Kimberley region of north-western Australia (Ord River). By studying two contrasting river systems in different climatic zones, the influence of the physical environment and biotic factors in vegetation persistence and recruitment in the riparian zone can be distinguished. This will contribute to providing an ecological basis for the rehabilitation and management of riparian vegetation in these types of systems. The structure of the vegetation on the Blackwood River consists of an overstorey dominated by Eucalyptus rudis, with a shrub understory at ungrazed sites and with annual species dominant in areas grazed by livestock. On the Ord River there is a much more diverse overstsorey and a species-poor understorey dominated by perennial grasses. Fencing to exclude stock, or to at least manage grazing, is a fundamental step towards achieving rehabilitation of degraded riparian sites where recruitment may be severely limited. Exclosure experiments in the Blackwood River show little improvement in recruitment after three years, with only minor increases in the occurrence and cover of native species. Establishment of these species may be difficult with the increase in abundance of exotic grasses and annual herbs which has occurred as a result of the absence of grazing. These results are however for the short-term and a much longer period is required to look at the vegetation dynamics and successional processes of these sites. In the riparian zone, regeneration of the vegetation from soil seedbanks is important for annual species of herbs and grasses but of only minor significance for perennial species. For perennial species, particularly the overstorey, direct seedfall from existing vegetation occurs, and enhanced dispersal by floating downstream with flood debris is a consequential recruitment mechanism. Hence, reproductive phenology of the four species monitored in this study appears to be well adapted to the hydrological regime on the respective rivers. Position in the riparian landscape where seedlings manage to establish is strongly related to environmental conditions that provide adequate moisture but protect seedlings from flooding. Historical flow records can be used to develop an understanding of the natural flow regime for a particular river which can then be related to patterns of vegetation development in terms of reproductive phenology, seedling establishment and population structure, as well as plant community patterns in the riparian zone. Variability in natural flow regimes, as a disturbance, can therefore be used in conjunction with other abiotic and biotic factors in developing a model of vegetation dynamics for the riparian zone. For example, the regime of intermittent high frequency large flood disturbances on the Ord River prevents the establishment of stable states of the vegetation and the ecosystem is characterised by long periods of transition between short-lived stable states. This riparian ecosystem is thus driven by physical (allogenic) processes rather than by vegetation successional (autogenic) processes. In contrast, lower energy seasonal flooding on the Blackwood River allows mature stands of trees to develop throughout the river profile. Recruitment is continual, although species can also respond to large flood events. This disturbance regime results in long periods of stable status with short periods of transition. The vegetation is thus subject to longer periods of autogenic processes and, because of lower frequency flooding disturbance, shorter periods of allogenic processes. These results highlight the effect of the different fluvial regimes of the two rivers on the respective vegetation dynamics. Management of the riparian vegetation should therefore take into account the frequency and rate of change in the vegetation and that disturbed states and long periods of transition between states, particularly on the Ord River, are part of the natural process. This would suggest that altering the natural flow regimes, such as through river regulation, would have significant effects on riparian vegetation dynamics.
This work has relevance to all aspects of riparian zone vegetation, including management of natural systems unaffected by man-made disturbances, for systems affected by stock grazing, for areas requiring rehabilitation and on regulated rivers. It highlights the importance of fluvial processes to riparian vegetation and indicates that understanding the natural flow regime of a target river is a critical first step in the management of riparian vegetation and in the planning of riparian vegetation zone rehabilitation. Where the riparian zone is highly modified, through, for example, livestock grazing and/or weed invasion, natural regeneration of the riparian vegetation may be a long term process. If intervention, such as replanting, is appropriate, care should be taken that species selected are adapted to particular site conditions, such as flooding regime, landscape position and river geomorphology.

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