Patterns in bird community structure related to restoration of Minnesota dry oak savannas and across a prairie to oak woodland ecological gradient
Leakhena Au
David E. Andersen
Mark Davis
There is limited understanding of the influence of fire and vegetation structure on bird communities in dry oak (Quercus spp.) savannas of the Upper Midwest and whether bird communities in restored savanna habitats are similar to those in remnant savannas. During the 2001 and 2002 breeding seasons, we examined the relationship between bird communities and environmental variables, including vegetation characteristics and site prescribed-burn frequencies, across a habitat gradient in dry oak savannas in central Minnesota. The habitat gradient we studied went from: (1) prairie to (2) remnant oak savanna to (3) oak woodland undergoing savanna restoration via fire or mechanical removal of woody vegetation to (4) oak woodland. We conducted fixed-radius point counts (n = 120) within habitats with either prairie groundcover or predominately oak canopy. We described canopy and groundcover characteristics at a sub-sample (n = 28) of non-prairie points, and collected canopy and woody species richness data and prescribed-burn frequencies over the past 20 years for all points. Observed bird communities were most strongly correlated with canopy cover and burn frequency and, to a lesser extent, attributes of the shrub component. Most savanna points had bird communities that were distinct from those found at oak woodland or oak woodland points undergoing restoration via burning. Savanna points similar to oak woodland points were in areas managed by periodic cutting rather than burning. Remnant savanna bird communities were more strongly associated with prescribed burning than those in other habitat types, but it appeared that most oak woodlands that had undergone ≥ 20 years of prescribed burning remained ecologically distinct from remnant savannas. This suggests that some savannas that have converted to oak woodlands may exist in an alternative, or stable, ecological state even following extended restoration efforts.
Coarse Woody Debris in a Southern Appalachian Spruce-fir Forest of the Great Smoky Mountains National Park
Anita K. Rose
N. S. Nicholas
Spruce-fir forests in the southern Appalachian Mountains receive high atmospheric nitrogen inputs and have high nitrate levels in soil solution and streamwater. High levels of excess nitrogen have been associated with reduced tree vigor. Additionally, the balsam woolly adelgid (Adelges piceae Ratz.) has killed the majority of endemic Fraser fir [Abies fraseri (Pursh) Poir.] trees, resulting in large amounts of coarse woody debris. As part of a biogeochemical study in the Great Smoky MountainsNational Park, coarse woody debris was sampled to determine volume, mass, change in density, and change in concentration and content of carbon and nitrogen over the decomposition process. Dead wood volume was highly variable across the watershed, ranging from 4.5 m3 ha-1 to 306.8 m3 ha-1 for standing boles and from 21.2 m3 ha-1 to 402.7 m3 ha-1 for down boles. Wood density decreased significantly for all three major overstory species [red spruce (Picea rubens Sarg.), yellow birch (Betula alleghaniensis Britt.), and Fraser fir] by approximately 60%, from slightly decayed boles to boles in advanced decay. Standing and down dead biomass averaged 39.4 Mg ha-1 and 33.8 Mg ha-1, respectively. Carbon concentrations remained relatively constant and were approximately 47% for all decay classes and all species. Nitrogen concentrations increased sharply between live wood and highly decayed wood. The nitrogen content in live wood, compared to wood in advanced decay, increased by 40% to 118% for the species tested. At the watershed level, live bole wood contained 108.4 kg ha-1 of nitrogen, and dead bole wood contained 101.5 kg ha-1. Total carbon in live and dead bole wood averaged 93.8 Mg ha-1 and 34.9 Mg ha-1, respectively. The magnitude of coarse woody debris in this system is among the highest reported in the literature for the eastern United States, emphasizing the high degree of disturbance that has taken place in this ecosystem.