BARK CHARACTERISTICS AND SOIL TYPE ARE RELATED TO WOODPECKER USE OF LIVE FORAGING TREES

Little is known about the relationship between anatomical and physiological characteristics of live trees and woodpecker foraging behavior, despite some woodpecker species foraging on live trees more often than dead trees. We assessed whether excavation of prey by insectivorous bark-foraging woodpeckers was associated with five characteristics (phloem thickness, oleoresin exudation, oleoresin viscosity, and bark thickness and hardness) of ponderosa pine (Pinus ponderosa) trees, and whether those characteristics, and the abundance of woodpecker foraging excavations, differed between trees growing on geologically older fine soils vs. coarser, volcanically derived, younger cinder soils in northern Arizona. We hypothesized that trees experienced greater water and nutrient stress on cinder soils and that stress would lead to differences in tree traits that affect woodpecker foraging. We found that trees growing on fine soils were >10 times less likely to be used by woodpeckers than were trees growing on cinder soils. Tree use by both woodpeckers and their wood-borer prey was negatively associated with bark thickness and hardness, but was not associated with other tree characteristics. The negative relationship with bark characteristics was stronger for woodpeckers than wood-borers, and woodpecker predation rates on wood-borers were negatively associated with bark hardness, suggesting that woodpeckers were responding to bark hardness independent of wood-borer presence. Tree traits we examined did not explain the 10-fold difference in tree use by woodpeckers across soil types, suggesting there were other variables we did not measure affecting this pattern. Wood-borers like those feeding on live trees we studied typically do not kill trees and remain part of the landscape without destroying forests, suggesting forests on cinder soils could act as important woodpecker foraging habitat without experiencing expansive forest losses typical of more irruptive beetle species.