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Factors affecting the physiognomy and species richness of plant communities of valleys of the Darling Plateau, Western Australia

Hancock, Christopher Nigel (2002) Factors affecting the physiognomy and species richness of plant communities of valleys of the Darling Plateau, Western Australia. PhD thesis, Murdoch University.

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Native vegetation along and near watercourses on the Darling Plateau, east of Perth, Western Australia was studied in order to identify the key environmental parameters affecting structure and species richness. Measurements were taken of vegetation height, cover and species richness, together with groundwater levels and a range of edaphic parameters.

Floristic data were used to sort releves into community types. One upland, three riparian margin and four riparian plant community types were identified. The greatest degree of floristic and environmental change across the valley floor occurred between the inner riparian margin and the riparian zone proper. At this point substantial increases in soil moisture and nutrients coincided with a change to a dense but species poor riparian community. Differences between riparian community types reflected differences in summer water availability along each of the watercourses, and therefore differences in the length of the growing season.

The crown size, girth and height of eucalyptus peaked in the riparian margin at the down-slope limit of Eucalyptus wandoo. The shrub stratum increased in height from the lower valley slopes and across the valley floor, peaking as Melaleuca thickets in the riparian zone where groundwater levels remained elevated into early summer. Vegetation cover as a whole increased from upland to riparian habitats, primarily in response to increasing soil moisture and penetrability or 'softness'. The perennial component increased with summer soil moisture and penetrability and decreased with pH, while the cover of annuals was inversely related to perennial cover. Tree and shrub cover increased with the duration of groundwater elevation, as did ground cover, though there was still an inverse relationship between tree/shrub cover and ground cover.

Species richness and life form diversity declined from upland, through riparian margin to riparian vegetation. Upland sites featured a mix of phanerophytes, chamaephytes, hemicryptophytes: sedges, geophytes and therophytes, while riparian sites were dominated by only phanerophytes and sedges. The decline in total species richness was correlated with increasing moisture, soil fertility and plant cover. Trends in species richness differed between plant life forms. The smaller life forms in particular were increasingly excluded as one moved from upland to riparian habitats, leaving sedges and phanerophytes to dominate. Although there were strong correlations between alpha species richness and some abiotic environmental parameters, these would have influenced species richness at the scale of species pools, rather than the releve (25m2) scale. Biotic factors may influence species richness at both the species pool and the releve scales. A conceptual model has been presented to better focus any further studies of species richness. In it, the types of factors liable to affect species richness are summarised, along with the level (regional species pool vs releve) at which each factor acts. The concept of a coexistence coefficient, i.e. a measure of what proportion of the species pool may coexist within a quadrat/releve, is discussed.

Seasonal comparisons of tissue moisture content and xylem pressure potentials indicated that riparian plants on perennial watercourses did not experience a reduction in tissue moisture or xylem pressure over summer, i.e. they 'avoided' summer drought. On the other hand, riparian plants on seasonal watercourses did experience drops in tissue moisture or xylem pressure over summer. Together with the hydrological data, these data indicate that the type of riparian community supported at any site depends on the amount of water available through summer, and the ability of species to tolerate summer drought.

The study underscored the overriding importance of hydrology in determining the structure and diversity of vegetation in valleys of the Darling Plateau. It showed that seasonal hydrology, as it affects vegetation, can be viewed in at least two ways - namely the degree of soil saturation/waterlogging in winter vs the amount of water available over summer. Hydrological regimes affect species distributions in different ways. These may be direct, as when species are precluded by waterlogging or summer drought or indirect, as when elevated soil moisture in summer produces a thick shrub cover, which in turn prevents the growth of minor species.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Ladd, Phil and Froend, Ray
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