Plot Size Recommendations for Biomass Estimation in a Midwestern Old-Growth Forest

We examine the relationship between disturbance regime and plot size for woody biomass estimation in a midwestern old-growth deciduousforestfrom 1926 to 1992. Analysis was done on the core 19.6 ac of a 50. I ac forest in which every tree 4 in. dbh and greater has been tagged and mapped since 1926. Five windows of time are compared-1926, 1976, 1981, 1986 and 1992. The most ejficient plot sizes requiring measurement of the least total area rangefrom 0.02 to 1.6 ac. A small plot size was recommended after grazing disturbance, anda large size was recommended when mortality was high andacceptable errorwas low. North. J. Appl.For. 15(4):165-168. O ld growth forests currently represent small forests and small numbers of forests throughout the region (Spetich 1995). However, these sites will continue to grow in area and numbers over the next century due to management strategies now in place (Shifley et al. 1995). For instance, Spetich et al. (1997) found that, over the next century in Indiana alone, oldgrowth forests will increase in area by a factor of 2,292 and in number by a factor of 7 based on management plans of public land holding agencies. In Missouri, old-growth forests will increase in area by a factor of 25 over the same time period (Shifley et al. 1995). The currently existing oldgrowth sites are important for the information they can provide us to develop as guidelines for monitoring these future old-growth sites. They will also provide us with data to better understand successional pathways and how those pathways change when sites are disturbed, forming the basis of management tools for other forests. Little is known about the spatial variation of aboveground biomass and related sampling issues. Papers recommending plot size for Midwestern old-growth forests of the United States have not considered the size necessary to inventory biomass (Cain 1934, Shifley and Schlessinger 1994). Nor have these papers cqnsidered the type or level of disturbance NOTE: M.A. Spetich’s current address is USDA Forest Service, Southern Research Station, P.O. Box 1270, 100 Reserve St., Hot Springs, AR 71902. Phone: (501) 623-1180; Fax: (501) 623.0186; E-mail: mspetich@[email protected]. The authors thank Stephen R. Shifley, Carl Mize, and two anonymous reviewers for valuable comments and suggestions; Betsy L. Spetich for numerous editorial comments; and Mark Hansen for providing a spreadsheet containing biomass formulas and tables. Purdue Agricultural Station Publication No. 14972. that might affect variation and plot size decisions. The objective of this study was to recommend plot size for estimating biomass under two typical disturbance regimes that have occurred in Midwestern old-growth forests (exogenous and endogenous). The exogenous disturbance at our study site was grazing and the endogenous disturbances were two levels of mortality (high and low). These recommendations were based on the most efficient plot size and sample size combination that required the least total area. Early in United States forest inventory history the 0.25 ac plot size was used extensively (Avery 1975). However, plots < 0.1 ac are now more commonly used (Avery 1975). This change in size is partly due to the fact that second-growth forests are more homogenous than old-growth forests (Avery 1975). Today fixed-radius plots rarely exceed 0.25 ac in size due to the increasing number of borderline trees with increasing plot size (Loetsch et al. 1973). In a comparison of 144 sampling designs, Kulow (1966) found that sampling precision and accuracy were proportional to the size of sampling unit. He also found that 0.2 and 0.1 ac plots resulted in significantly better results than smaller sampling units by significantly reducing the difference between the true mean and the estimated mean. Livestock grazing has influenced the structure of forests throughout most of the midwestern region (Barnes 1989, Parker 1989, Shifley et al. 1995). Many old-growth remnant forests have had grazing disturbance through the beginning of this century (Lindsey et al. 1969, McCune et al. 1988, Parker and Ward 1988, Barnes 1989, Parker 1989, Richards et al. 1995). A study of 20 yr of grazing in an old-growth forest in Pennsylvania found that, for the young trees (trees Reprinted from the Northern Journal ofApplied Forestry, Vol. 15, No. 4, December 1998. Not for further reproduction. 16’ < 2.5 ft tall and > 1 in. in dbh), the grazed side of the forest had 59% less basal area than the ungrazed side (Lutz 1930). Steinbrenner (195 1) found similar results for six woodlots in Wisconsin. Grazing by livestock can result in a more patchy distribution of small size class trees (Ward et al. 1996) and should be taken into account when sampling. High rates of tree mortality have been noted in several midwestern old-growth forests (Barton and Schmelz 1987, McCune et al. 1988). The Barton and Schmelz study of a central Indiana old-growth forest from I954 to 1984 indicated that mortality increased in almost every size class.