Abundance of tree cavities in forests of the Upper Midwest, USA

Background/Question/Methods

Tree cavities provide important habitat features for a variety of wildlife species. Cavity formation and presence is related to tree species, size (height, diameter at breast height - d.b.h.), status (live, dead), and other conditions. Fan et al. (2003) estimated tree cavity abundance for mature second-growth timberland in Missouri, and Denton et al. (2012) developed models for cavity-nesting waterfowl in hardwood forests of the north-central United States. However, estimates of tree cavities are lacking for the majority of cavity-dependent wildlife species, across the majority of forests in the midwestern USA. Attributes of tree cavity diameter (minimum entrance diameter >= 2.54 cm) and cavity height above ground were recorded for sampled live and dead trees >= 12.7 cm d.b.h. by the U.S. Forest Service, Forest Inventory and Analysis (FIA) program in upper Midwestern states during the 1990s. (Tree cavity data no longer are collected by FIA in this region.) We analyzed tree cavity data from field inventory plots from Illinois and Indiana (1998), Iowa and Minnesota (1990), Michigan (1993), Missouri (1989), and Wisconsin (1996) to produce estimates of historical frequency and total numbers of cavity trees, by tree species group, tree diameter, cavity size, and cavity position.

Results/Conclusions

Six percent of all sampled trees contained one or more cavities, with higher percentages for hard hardwood species group, dead trees, and larger d.b.h. Seventy four percent of all trees were hardwoods, but hardwoods accounted for over 90 percent of all cavity trees; 63 percent of all trees were <28 cm d.b.h., but only 26 percent of cavity-trees were in this size class.  Seventy-four percent of all cavity trees were >28 cm in. d.b.h., a size class containing 37 percent of all trees. Iowa and Missouri had the highest percentages of cavity-containing trees; Michigan and Minnesota had the lowest. Sixty five, 29, and 6 percent of cavities were located <3.1, 3.1-9.0, and 9.0+ m above the ground, respectively. Across the seven states, about 64, 22, and 14 percent of cavities were in cavity entrance diameter classes of 2.54-10, 10.1-17.5, and 17.5+ cm, respectively. We estimated a total of 280 million cavity-containing trees in forests of these seven states. Our results suggest that cavity availability for cavity-nesting birds may be more common than previously recognized and that cavity abundance likely will increase with forest maturation.  Ongoing studies are developing models of cavity occurrence based on historical and current FIA data.