Designing old forest for the future: Old trees as habitat for birds in forests of Mountain Ash Eucalyptus regnans

Abstract

Old trees are one of many habitat attributes associated with old forests. They are known to be required by many wildlife species including cavity-dependent birds. Forest managers need to select strategies for retaining and regrowing old trees to maintain suitable numbers over time at appropriate configurations in the forest landscape. This paper presents data from wet forests in south-eastern Australia to help elucidate the effects of different strategies on forest birds as one element of biodiversity that needs to be conserved in these forests. Bird abundance was assessed by standard area search at 117 sites in forests of Mountain Ash Eucalyptus regnans in the Victorian Central Highlands, over the course of two spring-summer breeding seasons from 1996 to 1998. The sites contained 60-year regrowth from extensive wildfires in 1939, along with varying numbers of older trees that had survived the fire. Some of the old trees were dead and at varying stages of decay. Numbers of old trees were counted at each site and a range of other habitat measurements taken. Birds were considered by species or guilds of species (feeding and nesting guilds). Bird abundance data were regressed against numbers of old trees and other relevant habitat data. Cavity-nesting birds were positively related to numbers of old trees, and especially to those that remained alive. Bird species that nest in longitudinal cavities in small branches (spouts) were rare at sites that lacked old trees, and were related strongly to numbers of live old trees. Large cavity-nesters were related to numbers of live old trees or all old trees. Treecreepers correlated less strongly, perhaps because they use cavities in decaying snags and stumps as well as large old trees. Honeyeaters and Mistletoe birds were also positively associated with numbers of live old trees, although they do not need cavities. Total bird abundance was positively related to live, dead or all old trees. Several other relationships were found. For example, shrub foragers were related positively to abundance of tall shrubs but not low shrubs; wattle foragers were related positively to abundance of wattles (especially Silver Wattles Acacia dealbata); and Eastern Spinebills Acanthorhynchus tenuirostris were positively associated with abundance of Mountain Correa Correa lawrenciana (the flowers of which provide a major source of nectar). Non-linear relationships between bird abundance and densities of old trees were found for at least two cavity-nesting bird species. The relationship was convex for Striated Pardalote Pardalotus striatus (optimum density 5–12 live old trees/ha) and mildly concave for Crimson Rosella Platycercus elegans. This suggests that the former would benefit from strategies to disperse old trees among regrowth at suitable scales whereas the latter would benefit from strategies to produce clumped distributions of old trees. However, Striated Pardalotes showed a strong preference for live old trees, which cannot easily be kept alive among regrowth after logging or fire, suggesting clumped retention may be more practical for that species too. Three other species showed weak evidence of non-linear responses, but most species showed no significant evidence of departure from linearity. No species or guild showed strong evidence of a threshold level of old tree density (above ∼1 ha −1) below which those birds would not occur. Hence for most of the diurnal birds considered in this study, old trees appear to be useful in proportion to the overall number of suitable old trees that can be retained and regrown (rather than their spatial pattern), and especially the number that can be kept alive. Clumped retention strategies give the best chance of keeping many retained trees alive in retained patches. Otherwise, spacing patterns should be a secondary consideration, except where they are known to be important for species such as owls and arboreal mammals.