Spruce grouse decline in maturing lowland boreal forests of New York

Abstract

Forest succession affects species that rely on specific successional stages of forest development as core habitat. The spruce grouse (Falcipennis canadensis) is a mid-successional boreal forest obligate declining in population size and range extent in the northeastern United States. We evaluated habitat occupancy from 2002 to 2006 and we contrasted spruce grouse habitat use at multiple spatial scales to determine if advancing succession of boreal forest influenced the species’ distribution in New York from 2002 to 2012. We detected grouse at less than half (n=13) of forest patches occupied from 1976 to 1987 and at one previously undocumented patch suggesting in aggregate a 71% reduction in extent of area occupied. Spruce grouse kernel home ranges reflected a 13× greater selection for short (<6m) lowland conifer over tall (>6m) lowland conifer, a 4.5× greater selection of short lowland conifer over upland conifer, and an 8× greater selection for ericaceous vegetation over tall lowland conifer. Within home ranges, bird radiolocations revealed an 8.5× greater selection for short lowland conifer over upland conifer, a 6.6× greater selection of short lowland conifer over ericaceous vegetation, a 5× greater selection of tall lowland conifer over upland conifer, and a 4× greater selection of tall lowland conifer over ericaceous vegetation. Occupied forest patches were 75% closer to other occupied patches than were formerly occupied patches. Moreover occupied patches were 10% larger, they were characterized by 90% less Sphagnum spp., 5× greater ericaceous shrub cover, 34% greater tree density, 71% less live vegetation >6m tall, 8% less coniferous shrub cover, and contained about half of the live vegetation from 4 to 6m above the ground. Trees were 20% younger in occupied versus formerly occupied patches (x¯=43.37 [SE=41.82–44.95] versus 51.87 [SE=50.95–52.81], respectively). We also observed 17% greater importance value (i.e., IV=100×[relative basal area×relative density×relative frequency]/3) of tamarack (Larix laricina) and a 43% less balsam fir (Abies balsamea) importance value suggesting a decreased component of earlier successional species in occupied versus formerly occupied patches. We conclude that reduced size and greater isolation of remaining habitat patches as well as forest succession have contributed to spruce grouse declines in New York, processes that could be reversed through establishment of mid-successional forest patches near remaining grouse-occupied habitats.