A field experiment was used to evaluate the effects of fire and cattle grazing on the initiation oi Lantana camara invasions in dry rainforest-open forest ecotones in the gorges of the Macleay River, NSW. A factorial combination of four factors (burning, biomass removal, soil scarification, and fertilization) at two levels (presence and absence) was established to assess the suitability of disturbed patches for germination, survival, and growth in association with changes in microclimate and resource availability. Burning, biomass removal and soil scarification, either singularly or in any combination, significantly increased germination, on average, by 19%, from 50.2% to 59.7%. Survival increased on average 26% across all treatments while mortality decreased by 26% when compared with the control. The differences between treatment combinations were not significant. Seedling growth was significantly enhanced by all disturbances, except by soil scarification alone. Treatment combinations that reduced vegetation cover (burning and/or biomass removal) and, therefore reduced shading, significantly increased L. camara biomass production by an average of 140% for all treatments. The control yielded 14.0 g m"', while fertilizer alone and biomass removal alone yielded 27.6 g m" -and 40.5 g m~', respectively. Other treatment combinations averaged 35.2 gm~" and were not significantly different from each other. Consequendy, successful invasions are likely to occur whenever canopy disturbances create patches of greatly decreased competition and/or increased resource availability. Shading plays a greater role as a limiting factor than any other, while surface soil macronutrient levels are also important, particularly when combined with canopy disturbances that increase light availability. The effects of biomass reduction and soil disturbance associated with fire and cattle grazing are significant in the successful invasion of L. camara. Management strategies to reduce weed encroachment and community degradation must identify and maintain ecological barriers to L. camara invasion in order to promote rainforest conservation and biodiversity.
No abstract
Suppression of Choricarpia leptopetala (F. Muell.) Domin., an early native coloniser of mesic forests, by Lantana camara L. was evaluated using field-based variable density and substitution experiments. Sites were located in disturbed areas within wet sclerophyll forest and warm temperate rainforest near Lake Macquarie on the Central Coast of NSW. In the variable density experiment, C. leptopetala growth was significantly suppressed by 36.4% from 3.3 to 2.1 g plant −1 in the presence of equal proportions of L. camara at a total density of 20 seedlings m −2 , only when nutrients (60 g m −2 of a slow release NPKS fertiliser) were added to the soil surface. Lantana camara growth significantly increased by 14.6% from 4.1 to 4.7 g plant −1 when nutrients were added. At a density of 40 seedlings m −2 , growth of C. leptopetata was suppressed by 37.5% from 3.2 to 2.0 g plant −1 in the zero nutrient treatment with equal proportions of L. camara, and was further suppressed by 40.0% from 2.0 to 1.2 g plant −1 with the addition of nutrients. Lantana camara growth significantly increased by 43.7% from 2.3 to 3.3 g plant −1 when nutrients were added. Pure stands of C. leptopetala did not respond significantly to nutrient addition at either density. In the substitution experiment (with a constant density of 20 seedlings m −2), C. leptopetala growth was significantly suppressed by 44.1% from 3.4 to 1.9 g plant −1 when L. camara reached 75% of stand composition in the zero nutrient treatment and by 43.2% from 3.7 to 2.1 g plant −1 when it reached 50% of stand composition in the nutrient addition treatment. The results link interference and suppression of native colonisers by exotic invaders with demonstrated increases in resource availability following ecosystem disturbance. Lantana camara is able to take better advantage of increased resource availability than C. leptopetala, thereby accumulating more biomass and suppressing the growth of C. leptopetala. Suppression of C. leptopetala also increased when the density of L. camara was increased. Consequently, the negative effect of the invading species on the indigenous species is both nutrient-addition and density dependent. Models describing interference in ecosystem recovery following disturbance need to include interference processes associated with weed invasion that disadvantage indigenous species. Nomenclature: Harden (1990).
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