Competition drives the decline of a dominant midstorey tree species. Habitat implications for an endangered marsupial

Abstract

ContextMulti-species native forests are an important source of habitat for many plant and animal species. The dynamics of these forests are shaped by species-specific differences in growth, shade tolerance, life span, and other life-history traits, which lead to complex development patterns. Whether a particular forest stand is suitable habitat for a specific animal species may depend on various stand structural elements, which may vary in abundance due to stand dynamics, thereby generating temporal variability in habitat suitability. In the Central Highlands of Victoria, Australia, the critically endangered Leadbeater’s possum (LBP) requires both a dense lower storey of Acacia spp.for foraging and suitable hollow-bearing trees (HBTs; primarily large eucalypts) for nesting. While most LBP habitat research has focused on the dynamics of HBTs, there are uncertainties about the extent to which the occurrence and abundance of Acacia spp.are affected by competition with eucalypts. Material and methodsWe used 230 forest inventory plots to quantify how the abundance of Acacia dealbata, the most abundant of the Acacia species in these forests, varies with stand age and the abundance of overstorey eucalypts. We also took tree cores from 92 A. dealbata growing in either pure stands or stands mixed with eucalypts to test for the effects of tree age and eucalypt suppression on Acacia growth and decay patterns. ResultsThe frequency and abundance of A. dealbata decreased as stand age increased, but was strongly influenced by eucalypt basal area. When eucalypt basal area was >50m2/ha, Acacia was almost never present. In multi-species stands, Acacia grew more slowly (by a factor of 0.66, 95% credible interval 0.51–0.86) and was more likely to have internal decay (probability increased by 0.25; 0.07–0.48) than when grown in pure stands. DiscussionAcacia dealbata is a native pioneer species regenerating following fires. Eucalyptus overtop the Acacia after 8–10 years. The ensuing stand stratification leads to the decline and loss of the Acacia. We propose silvicultural approaches to slow Acacia decline and prolong the suitability of regrowth Eucalyptus stands as potential LBP habitat.