Tuart (Eucalyptus gomphocephala) decline is not associated with other vegetation structure and composition changes

Abstract

Tree decline affects forests and woodlands on most continents. The loss of canopy species from these landscapes has marked impacts on the surrounding environment. Understanding the consequences of tree mortality on vegetation structure and composition will contribute to developing management strategies for these ecosystems. The tuart (Eucalyptus gomphocephala), once the dominant tree species on the coastal plain in southwest Western Australia, has been cleared extensively and today occurs over less than a third of its former range. Compounding the effects of this clearing, dramatic declines in tree health over the past two decades has been recorded. We examined whether E. gomphocephala decline is reflected in changes to overall vegetation structure and composition. Using Landsat Thematic Mapper satellite data, we identified 12 sites with stable or improving vegetation vigour over the previous 15 years (‘healthy’ sites) and 12 sites with declining vegetation vigour over this time (‘declining’ sites). Health of E. gomphocephala trees, as well as litter, vegetation structure and vegetation composition were compared across these sites. Four health measures indicated that E. gomphocephala trees from healthy and declining sites were statistically distinguishable (p < 0.001). Trees at declining sites showed marked epicormic growth (up to 96% of canopy), evidence of crown dieback, reduced crown density and increased foliage transparency. The canopy changes were reflected in marked reduction in leaf litter for declining sites (p = 0.024). Despite the changes in E. gomphocephala canopy and leaf litter, however, there were no significant differences in vegetation community composition, relative vegetation cover for five height strata, vegetation structural and compositional diversity, presence of coarse woody debris, or overall canopy cover. There were also no differences in soil chemistry between the healthy and declining sites. The agent of tuart decline therefore appears to be specific to Eucalyptus gomphocephala at these sites, or alternatively the changes to other plant species have been too subtle to detect at this stage.