, Volume 13, Issue 9, pp 2009-2022
Date: 22 May 2011

Resource heterogeneity and persistence of exotic annuals in long-ungrazed Mediterranean-climate woodlands

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Abstract

Processes that promote weed invasion are often well-demonstrated, but mechanisms that facilitate ecological resistance to weed invasion in non-invaded communities, or promote weed persistence in invaded communities, are poorly understood. Yet it is these processes that must be addressed to achieve sustainable ecological restoration. We surveyed soil heterogeneity in 25 long-ungrazed, unfertilized York gum (Eucalyptus loxophleba Benth. subsp. loxophleba)—jam (Acacia acuminata Benth.) woodlands of the Western Australian wheatbelt to investigate differences in soil characteristics between patches locally-invaded or non-invaded by widespread exotic annuals. Based on studies in other ecosystems, we hypothesized that (1) weed persistence is associated with elevated soil resource levels, and (2) of these soil resources, phosphorus is the key contributor to weed persistence in Western Australian woodlands, that typically occur on phosphorus-impoverished soils. Our first hypothesis was partly supported, with soil nutrients associated with up to 40% of the variation in cover of exotic annuals. In particular, low concentrations of total nitrogen, nitrate and available phosphorus are likely to contribute to resistance to invasion in many non-invaded woodland patches, especially in gaps between trees. However, other non-invaded patches had comparable nutrient concentrations to invaded patches, suggesting this resistance may be weak at more productive sites or that patches have not reached a stable equilibrium. Inconsistent with our second hypothesis, exotic annuals were as strongly correlated with elevated total nitrogen and associated variables as they were with available phosphorus, probably reflecting a history of grazing without fertilization. We conclude that effectiveness of ‘bottom-up’ approaches to weed control is likely to differ among ecosystems according to interactions with disturbance history and attributes of the non-invaded community, even where the weed species or functional types are the same.