Does forest structure affect reproduction of northern goshawks in ponderosa pine forests?
Many management prescriptions are based on ecological hypotheses; evaluating empirical support for these hypotheses can improve management. There has been considerable dispute about the potential response of the northern goshawk to three management-driven forest structures in ponderosa pine forests of the south-western United States: (i) the structure recommended by US Forest Service's goshawk guidelines, designed to increase the abundance of 14 goshawk prey species and thus benefit goshawks; (ii) preferred foraging habitat as suggested by empirical evidence that goshawks forage selectively in areas with abundant large trees and dense canopy closure, rather than areas of highest prey abundance; and (iii) presettlement (i.e. prior to Euro-American settlement) structure characterized by clumps of large trees, canopy closure <40% and dense herbaceous understorey, which could have negative effects on goshawks. To evaluate empirical support for hypotheses that goshawk reproduction is affected by each of these three forest structures, we measured forest structure in a 1215-ha nest-centred circular area in each of 13 goshawk breeding areas on the Apache-Sitgreaves National Forest, Arizona. The breeding areas were selected to span the full range of productivity (fledglings per year monitored) over the previous 9-year period. Forest structure had a moderate effect on goshawk productivity (r2≤0.46). Contrary to expectation, goshawk productivity decreased with increasing similarity to the goshawk guidelines. Goshawk reproduction was not correlated with resemblance of the breeding area to preferred foraging habitat or resemblance to presettlement forest conditions. Synthesis and applications. Because the goshawk guidelines may not improve goshawk reproduction, the Forest Service should reconsider its decision to apply the guidelines to most forested lands in Arizona and New Mexico. Managers should evaluate empirical support routinely for the major ecological hypotheses that underlie forest prescriptions.