Density and characteristics of tree cavities inside and outside Volcanoes National Park, Rwanda

Abstract

Tree cavities, formed by animal excavation or processes of fungal decay and mechanical damage, may provide nesting, roosting, or resting opportunities to many invertebrate and vertebrate species. Although cavity availability has been linked to patterns of biodiversity and ecosystem functioning elsewhere, there have been few such studies in the Afrotropics. Here, we present a baseline survey of cavity availability inside the high elevation (2200-3714 m) Afromontane forest ecosystems of Volcanoes National Park (VNP), Rwanda. We aimed to provide such reference data in the form of summary statistics on cavity density and characteristics in a collection of 400 m² plots that together cover 8.8 ha inside and 0.68 ha outside VNP. We also explored the relative importance of fungal decay vs. excavators in the formation of cavities, tested for the relative role of standing dead trees and living trees as cavity substrates, considered differences in diameter and height between cavity-bearing trees and trees without cavities, tested whether cavity density varies across elevation, and determined the orientation of cavity entrances. We found 109 cavities in 52 cavity-bearing trees (dominated by Hagenia abyssinica) inside VNP, for a density of 12.4 cavities and 5.9 cavity-bearing trees per hectare, and none outside the park. More cavities were decay-formed (n = 90) than excavated (n = 19), and though most cavities were found in living trees (n = 44), the number of cavities in dead trees (n = 8) was high relative to dead tree substrate availability. We also found that cavity-bearing trees were larger than those without cavities, that excavated cavities were predominantly oriented toward the southeast and decay-formed cavities to the northeast, and that cavity density declined with increases in elevation. Our results show that large and dead trees of particular species are important cavity substrates that need to be given attention in conservation and management, as is clearly illustrated by the lack of cavities in the highly managed Eucalyptus stands outside VNP.

Keywords: Albertine Rift; excavators; fungal decay; microclimate; nest web; snag density; tree cavities.