Bird populations and vegetation characteristics in managed and old-growth forests, northeastern Oregon

Populations of breeding birds and structure and composition of vegetation were examined in managed and old-growth mixed-coniferous forests in northeastern Oregon. Forest stands were about 85 and over 200 years of age and were dominated by Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa). Components of vegetation that distinguished old-growth forests from managed forests included the numbers of large trees (51 + cm dbh) and snags (31 + cm dbh), small understory grand fir (Abies grandis) trees (2.5-10 cm dbh), and tree height diversity; mean values of all of these components were greater in old-growth forests. Three of these variables could be associated, either directly or indirectly, with major differences in bird populations between managed and old-growth forest. The abundance of large snags in old-growth forests was probably responsible, in part, for the relatively high numbers of redbreasted nuthatches (Sitta canadensis), and most other hole-nesting birds, observed in this habitat. Large trees were indirectly important to hole-nesting birds because they provided a source of large snags. Grand fir trees were used by Townsend's warblers (Dendroica townsendi) and golden-crowned kinglets (Regulus satrapa) when foraging and nesting, and we attributed the abundance of these two bird species in old-growth forests to the presence of this understory tree component. Species of birds that were more abundant in managed forests than in old-growth forests appeared to be attracted to the open structure of the managed stands. We discuss the effects of replacting old-growth forests with managed forests on bird species in northeastern Oregon. Methods of maintaining habitat for those species that will decline in density following removal of old-growth forests are suggested.