- Campbell, Steven P.
University of Maine
Bird populations have been monitored at the Holt Research Forest in Arrowsic, Maine, since 1983 as part of a long-term forest ecosystem study. In the winter of 1987-1988, 10 ha of the 40-ha study area were subjected to a group-selection timber harvest (i.e., a harvest that creates canopy gaps by removing small groups of trees). I analyzed the first 20 years of these data (5 years of pre-harvest data and 15 years of post-harvest data) for changes in abundance and spatial distribution of birds in response to the harvest. Although species’ responses to the group-selection harvest were idiosyncratic, two general patterns emerged. Bird species dependent on early successional habitat exhibited temporary (≤ 8 years) increases in abundance and a positive spatial response to the gaps (i.e., use of gaps increased, distance from gaps decreased, and use of edges [0-25 m from gaps] increased). In contrast, mature-forest bird species showed little change in abundance but relatively strong distributional shifts away from disturbed areas and the surrounding forest. The duration of the responses was generally short-lived; by 15 years after the harvest, abundance levels of nearly all species and their use of the disturbed areas had approached pre-harvest levels.
Using the five years of pre-harvest data, I also assessed the roles of stochastic and deterministic processes in year-to-year changes in habitat use by comparing observed patterns in habitat use with patterns generated from null models. Although some species exhibited near random habitat use, observed patterns of variability for most species were matched by those generated from null models when random variability was constrained, which suggests that the variability in most species has both deterministic and stochastic components. In particular, null models that incorporated habitat preference or site fidelity each reproduced the observed patterns of variability for nearly half of the species examined. Support for these two models suggest that any factor that causes birds to return to the same site repeatedly and limits the spatial extent of the variability can generate spatial distribution patterns similar to those that were observed.