Crown dieback events as key processes creating cavity habitat for magellanic woodpecker

OJEDA, VALERIA; SUAREZ, MARIA LAURA; KITZBERGER, THOMAS

AUSTRAL ECOLOGY

Ecological Society of Australia

Año: 2007 vol. 32 p. 436 - 445

1442-9985

Woodpeckers are considered keystone species for webs of cavity nesters and habitat and resourcespecialists that strongly depend on availability of trees suitable for cavity excavation. Most studies carried out innorthern hemisphere temperate coniferous forests emphasize the importance of old growth stages of forests or largedead trees as habitat for cavity builders.We present a study of Nothofagus pumilio tree selection by the magellanicwoodpecker (Campephilus magellanicus) that incorporates dendroecological data on long-term growth trends oftrees that provides new insights into the processes that create suitable habitat for cavity excavating species.Weanalysed 351 cavity and neighbouring control trees in terms of age and radial growth patterns, as well as externaltree characteristics. In addition, from a subsample of these trees we developed tree-ring chronologies for each groupusing standard methods in order to analyse potential differences in radial growth patterns between cavity andnon-cavity trees. Multivariate models that account for differences between paired cavities versus control treesindicated that growth decline and the degree of crown dieback were the primary variables explaining magellanicwoodpecker tree selection for cavity building. In contrast to previous work, neither diameter (above a certainthreshold) nor age, were important determinants of selection. Furthermore, trees that became present cavity arethose that had synchronously declined in radial growth during the 1943–44 and 1956–57 droughts and the1985–86 massive caterpillar defoliation. Insect outbreaks and extreme climatic events may episodically reducevigour, induce partial crown mortality, trigger increased fungal attack and heart rot formation at different treeheights on the bole in a group of trees and thus increase availability of soft substrate and their likelihood of cavityexcavation by the magellanic woodpecker. These results underscore the importance of drought/biotically-inducedcanopy dieback events in creating habitat for woodpeckers and their dependent cavity users.