Identifying suitable woodpecker nest trees using decay selection profiles in trembling aspen (Populus tremuloides)

Abstract

Woodpeckers are the primary tree-cavity producers in North America and, through their process of cavity excavation, they create nesting and roosting habitat for a complex web of cavity-using species. Managing for potential woodpecker nest trees requires an understanding of factors that influence decisions by which woodpecker select trees for nest cavity excavation, including tree decay availability associated with softened heartwood and sapwood in trees. We evaluated woodpecker nest cavity excavation in relation to the nature and availability of decay in trembling aspen (Populus tremuloides) using 12years of nest and tree decay class data from British Columbia. We used a cavity-nest web to visually depict decay selection profiles for six woodpecker species and compared these woodpecker decay selection profiles to the availability of decay in aspen (96.7% of all cavity nests found), identifying the most suitable nest tree stages used by woodpeckers. The suitable woodpecker nest tree stage consists primarily of live unhealthy trees and recently dead trees (93.9% of active nests found in aspen). Compared to non-nest trees (trees within which no cavity nests were found during the study period), live trees that were used for nest cavity excavation had higher annual probabilities of entering the suitable woodpecker nest tree stage than non-nest trees, and they also had a lower probability of progressing outside of this stage. This suggests excavation decisions involve a balance of competing tradeoffs between ease of excavation and tree security. Nest trees remained within the suitable woodpecker nest tree stage for an average of 11.8years, and almost half of this time involved live trees in an unhealthy state. We suggest that forest management guidelines that focus on retaining only dead trees to provide cavities for wildlife are missing a significant component of available and future tree cavity resources.