Survival of Mount Eden Bush, an urban forest remnant in Auckland, New Zealand

Abstract

Mount Eden Bush is the only reserved fragment of primary broadleaved forest on basaltic lava on the Auckland isthmus in northern New Zealand. An edaphic variant of northern coastal short forest, the reserved forest of 0.7 ha is approximately 1-2% of the estimated original <50 ha tract and contains 84% of the vascular species recorded in it. The canopy is dominated by Griselinia lucida, Litsea calicaris, and Pseudopanax lessonii, the subcanopy by Melicytus ramiflorus, and the understorey by Coprosma macrocarpa and Macropiper excelsum. A depauperate vascular flora compared with other basaltic lava forests in the district may result from long isolation of the original tract in a deforested landscape remote from seed sources. Low tree density, low basal area, and a strongly rupestral/xerophytic ground layer reflect the drought-prone lava substrate (mean boulder cover 38%). It has probably been dominated by the adventive trailing herb Tradescantia fluminensis for >60 years (mean cover 38%), which reduces abundance of woody native seedlings and cover of native ground layer herbaceous species. Over two-fifths of the vascular flora is now alien native (all planted) or adventive, the latter almost all garden escapes from the surrounding suburban matrix and including many of the most threatening weeds of urban Auckland. Despite widespread T. fluminensis, currently important canopy/subcanopy and understorey dominants appear to be replacing themselves. In the absence of intervention, however, indigenous species are likely to become less important in the canopy and adventive Ligustrum lucidum and Prunus serrulata more important; the latter two species are respectively the second and third commonest canopyforming species regenerating in gaps. Future extinction of some indigenous species with critically small populations cannot be ruled out.