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Interactions between Annuals and Woody Perennials in a Western Australian Nature Reserve

R. J. Hobbs and L. Atkins
Journal of Vegetation Science
Vol. 2, No. 5 (Dec., 1991), pp. 643-654
Published by: Wiley
Stable URL: http://www.jstor.org/stable/3236175
Page Count: 12
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Interactions between Annuals and Woody Perennials in a Western Australian Nature Reserve
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Abstract

We studied the interactions between woody perennial species and native and non-native annual species in a number of vegetation types within a nature reserve in the Western Australian wheatbelt. In particular, we examined the responses of annuals to perennial canopy removal, fire, soil disturbance and nutrient additions, and the effects of removal of annuals on perennial seedling regeneration. Experimental shrub removal significantly increased the abundance of annuals in a dense shrubland dominated by Allocasuarina campestris, but had no effect in a more open species-rich sandplain heath. Soil disturbance and nutrient addition in the heath area had no significant influence on annual abundance until three years after treatment. Fire had no clear effect on annual abundance in the heath within the reserve, but promoted a large increase in non-native species within an adjacent roadverge. A pattern of increased soil nutrient levels was accompanied by greatly increased nonnative annual abundance beneath individual trees of Santalum spicatum. Exploratory laboratory bioassay experiments indicated that several woody perennials produced leachates that were capable of reducing the germination or growth of the introduced grass Avenafatua, indicating that allelopathy may be an important component of the interaction between the annual and perennial components. Within a woodland community, fire temporarily reduced the abundance of annual species and increased the establishment of perennial seedlings. Field experiments showed that annuals significantly reduced the survival of seedlings of the shrub Allocasuarina campestris. Our results indicate that intact native vegetation canopies effectively prevent invasion by non-native annuals, and that regeneration by native perennials is likely to be inhibited by the presence of an abundant annual cover.

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