Identification of tree groups used by secondary cavity-nesting birds to simplify forest management in subtropical forests

Abstract

In tropical and subtropical forest ecosystems, cavities formed by decay processes are a key but scarce resource for birds that nest and roost in them, which makes them a highly sensitive group to logging. The piedmont forest of northwestern Argentina is a complex ecosystem with 113 tree and 120 bird species. It has high logging pressure on the few, well-conserved forest remnants, complicating the delineation of sustainable management guidelines for each tree or bird species in a short time. Our objective was to reduce the complexity of subtropical forests by grouping tree species according to the characteristics used by secondary cavity-nesting birds (i.e. non excavators). In the piedmont forest, 50 plots of 0.25 ha were sampled to record cavity trees and cavity characteristics. These were then used in a cluster analysis to form tree groups. Additionally, cavities were searched to identify the bird species using the decay-formed cavities. A total of 187 cavity trees, comprising 23 tree species, were recorded, and these formed four tree groups or clusters. We recorded 86 cavities that were used by secondary cavity-nesting bird species. The four tree groups were unequally used by secondary cavity nesters. The tree group that included valuable timber species (Myroxylon peruiferum, Anadenanthera colubrina and Calycophyllum multiflorum) and had the greatest cavity availability represented 71% of total cavity use. Another tree group with valuable timber species (Cedrela balansae and Amburana cearensis), measured > 73 cm DBH and > 21 m tall, had cavity entrances > 0.10 cm2, and contributed 14% of all cavity use by birds. A third group had no highly economically valuable tree species, and included the snag category (i.e. standing dead trees) as well as a 15% of cavity use. The fourth tree group had a DBH < 0.40 cm, only one highly economically valuable tree species (Cordia trichotoma), and supported no cavity use. The clustering of subtropical trees can reduce the complexity of these forests, hence easing their management by focusing on those groups with tree species showing similar characteristics and providing suitable nesting sites for secondary cavity-nesting birds.