Breeding and post-breeding forest bird community dynamics in regenerating clearcuts and two-age harvests in the central Appalachians
Two-age harvesting has been used more frequently in the management of Central Appalachian hardwood stands as an alternative to clearcutting, but long-term responses of avifauna to these harvests have not been investigated during the breeding season. No studies have examined post-breeding bird communities in these harvests; recent research concerning post-breeding use of early-successional habitats has only examined clearcuts. Greater cover from predators and increased food resources in young seral forests are suspected factors behind the attraction of mature-forest birds post-breeding, and one goal was to test these hypotheses. In addition to microhabitat characteristics, avian habitat use of forest fragmented by timber harvesting may also be affected by stand attributes such as size, amount of edge, and retained basal area. My primary objectives were to (1) determine short- and long-term effects of two-age harvesting on breeding birds in comparison to clearcuts, (2) examine post-breeding bird responses to cover and food resource variables, and (3) relate post-breeding bird responses to residual basal area, stand size, and edge of young harvests.
In 1994-1996, breeding bird surveys were completed in two-age and clearcut stands as well as mature unharvested forest stands. In 2005 and 2006, I conducted point counts in the stands from the 1994-1996 study (now 19-26 years old) and in younger clearcut and two-age stands (6-10 years old). I determined differences in breeding bird metrics among these five treatments and temporal differences comparing time periods in the old harvests and unharvested stands.
I used mist-nets from late-June to mid-August 2006 to sample post-breeding bird communities in 9 regenerating stands with a gradient of residual basal areas. I measured vegetation characteristics, fruit, and arthropod resources at 10 nets within each stand. I analyzed capture data using Poisson regression and information-theoretic approaches to model selection. Vegetative cover and food variables were used to predict bird capture rates. Area and edge effects were tested in 13 stands sampled post-breeding in 2005-2006, which ranged from 4-21 ha in size, and bird metrics were contrasted among high-leave two-age (5.3-7.0 m2/ha retained basal area), low-leave two-age (2.0-3.7 m2/ha retained basal area) and clearcut treatments using mist-net and transect data during post-breeding.
Relative abundance of early-successional breeding species was similar in young two-age stands and young clearcuts. Many of these species, which are typically absent from group selection cuts, were present in two-age stands thus supporting their promise as an alternative to clearcutting. Although the older harvests had lower overall relative abundance, species richness, and diversity, they provided habitat for Neotropical migrant mature-forest songbirds that were absent or uncommon in the young harvests, and several late-successional species became more common in the older harvests over the 10 year period between studies. Consequently, two-age management provides habitat for a diverse group of species assemblages as these stands mature and may be an ecologically sustainable alternative to clearcutting in landscapes where Brown-headed Cowbirds (Molothrus ater) are uncommon.
For use of harvested stands by post-breeding birds, cover variables often were among the most important. Strong positive relations between vertical complexity of the vegetation and capture rates of mature-forest birds and molting adults support the predator-avoidance hypothesis. At the net level, basal area was a poor positive predictor of bird captures, except frugivores. Although captures in two-age stands were comparable to clearcuts, residual trees within a stand tended to depress proximate capture rates. Fruit variables inconsistently predicted frugivore captures, but appeared to be important for molting birds of both age classes. Arthropod variables best explained capture rates for some groups, but the difficulty of measuring food availability for birds and the lack of data concerning post-breeding food limitation precludes forming conclusions about the resource-selection hypothesis. Two-age and clearcut stands provided habitat for many early-successional and mature-forest birds post-breeding with cover appearing to be the primary factor for use by most species.
Clearcuts and two-age harvests were used similarly by generalists and late-successional individuals during post-breeding. Early-successional birds avoided high-leave two-age stands and were more common in clearcuts. Area and edge sensitivity were evident for both mature-forest and early-successional bird species. Mature-forest birds (both generalists and late-successional specialists) were found in fewer numbers in large stands, but edge effects were inconclusive, with more species associated with harvest edge. In contrast, early-successional species tended to use stand interiors more often and positively responded to stand size. Despite within-stand edge effects evident for several species, few birds in the forest periphery responded to harvest edge types. Mist-netting and transect surveys were important for helping to determine post-breeding habitat requirements for a variety of species. Understanding stand-specific bird survival is needed to determine the true quality of silvicultural harvests for post-breeding birds.