Characteristics of snags containing excavated cavities in northern Arizona mixed-conifer and ponderosa pine forests

Abstract

Snags provide an important resource for a rich assemblage of cavity-nesting birds in the southwestern United States. To expand our knowledge of snag use by cavity-nesting birds in this region, we documented characteristics of snags with and without excavated cavities in mixed-conifer and ponderosa pine ( Pinus ponderosa Dougl. ex Laws) forest in north-central Arizona. Snags were sampled in 113 square plots (1 ha each) randomly located within a study area covering approximately 73,000 ha across two National Forests. Density of snags was three times greater in mixed-conifer forest ( n = 53 plots) than in ponderosa pine forest ( n = 60 plots), but density of snags containing cavities and overall cavity density did not differ between forest types. In both forest types, snags containing cavities were larger in diameter and retained less bark cover than snags without cavities. Most cavities were in ponderosa pine and Gambel oak ( Quercus gambelii Nutt.) snags, and most were in snags in advanced decay classes with broken tops. Our results are largely consistent with previous results from ponderosa pine forest, but differ from previous studies that documented heavy use of quaking aspen ( Populus tremuloides Michaux) by cavity nesters in mixed-conifer forest. These results support management to protect and recruit large snags well distributed across the landscape. The relatively high use of ponderosa pine and Gambel oak snags in both forest types suggests that recruitment of large pine and oak snags should be emphasized, and previous studies suggest emphasizing aspen recruitment as well. This may require special management efforts in mixed-conifer forest. These species are relatively shade-intolerant seral species in this forest type, and are apparently declining in this forest type due to fire-suppression efforts and resultant patterns of ecological succession.