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Survival of tree cavities : a critical resource for cavity-nesting communities Edworthy, Amanda


Tree cavities are a multi-annual resource used by cavity-nesting vertebrates for nesting and roosting, and the abundance of this resource is influenced by the rates at which cavities are created and destroyed. Tree cavities are vulnerable to forest harvesting and retention of long-lived cavity trees is an important strategy in maintaining the richness and abundance of forest species. My objectives were to: 1) investigate factors influencing longevity of cavities in nesting trees; and 2) assess the effect of harvesting on the risk of loss to cavities in retained trees. To examine factors influencing cavity persistence, I modeled survival of more than1300 nesting cavities over 16 years (1995-2010) for temperate forests of interior British Columbia, Canada. Sources of loss were tree stem blowdown and breakage (90%), chamber decay (7%), and cavity entrance healing over (3%). Cavities in live trees lasted 2.7 times longer than those in dead trees with advanced decay. There were also differences in cavity longevity across forest types and formation agents, which will affect cavity availability for some populations. Comparison of hazard of loss for cavities in two partial-harvest treatments and unharvested stands revealed that hazard of loss increased by 70% for cavities in the partial harvest treatment (44–95% removal of basal area) compared with those in uncut forest, while cavities in wildlife reserves (retention patches; ~ 1 ha) had a 48% increase in hazard of loss relative to uncut forest. Rates of loss were highest in the five years following harvest, and after 8 years declined to pre-harvest levels. My results showed that cavities in live trees lasted longer than cavities in dead trees, cavities in continuous forest stands lasted longer than those in aspen groves within grassland areas, and cavities created by strong excavators lasted longer than those created by weak excavators, or the keystone excavator at my study sites, the northern flicker. Additionally, cavities in trees retained after harvesting had an increased risk of loss across all decay classes, but this effect was somewhat mitigated by retaining cavities embedded in wildlife reserves.

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