Comparing estimates of forest site quality in old second-growth oak forests

Site index is a useful tool for evaluating forest site quality, but the application of site index is currently problematic within the central Appalachian hardwoods because several of the assumptions required for site index are violated, particularly within older stands. This study evaluated the applicability of site index to relatively old (age range=74–134 years) second-growth, mixed-oak stands in western Maryland by comparing site index as traditionally calculated from age and height with two other models of site index based on (1) height and diameter, and (2) topographic position. Reproducibility statistics indicated that while site index calculated from age and height appeared to adequately reflect forest site quality relative to other old stands, they underestimated the expected site quality predicted by topographic position. Site index curves provided by Olson [Site index curves for upland oaks in the southeast, Research Note 125, USDA Forest Service, Southeastern Experiment Station, Ashville, NC, 1959, 2 pp.] performed better than those provided by Schnur [Yield, stand, and volume tables for even-aged upland oak forests, USDA Technical Bulletin 560, 1937], probably because asymptotic height was predicted to be more rapid using Olsonʹs curves. However, both of these site index formulations are outdated, particularly because they are not based on stem analyses. We recommend further study of asymptotic height growth in oak to better approximate forest site quality for upland oaks at more advanced growth stages, and new stem analyses of oaks in the central Appalachians to improve site quality evaluation within the region.