Characterizing structural conditions in mature managed red pine: Spatial dependency of metrics and adequacy of plot size

Restoration of the range of forest types and stages that once composed the landscape mosaic of the Upper Great Lakes region of North America will involve manipulating managed red pine stands to recreate now rare structural conditions. Because many of the attributes used to characterize structural condition depend upon spatial arrangement, sampling schemes to assess condition must match sampling extent to attribute variability. To evaluate the metrics and scales appropriate for characterizing structural complexity in managed red pine, we applied metrics that incorporated one-, two-, and three-dimensional structural attributes to eight 1.0 ha mature stem-mapped stands. Most metrics were also calculated within simulated moving windows of 0.05–1.0 ha in sampling extent (“plot size”) within each stand. Two standards were used to evaluate the adequacy of plot sizes for these metrics: (1) estimates were precise, varying less than 10% among moving windows for a given scale, and (2) estimates were accurate to within 10% of the 1.0 ha value at both the 5th and 95th percentiles of the moving window distributions. Necessary sample sizes to achieve precise and accurate estimates were also calculated for each scale.Stands were relatively homogeneous in metrics commonly used in traditional timber management (basal area, mean diameter, total density), which were precisely and accurately estimated in 0.50 ha or larger plots. Metrics capturing attributes associated with old stands (number of large trees, number of snags) were more variable and required larger plots sizes (0.75–1.0 ha). A single 0.50 ha plot would be adequate to estimate mean tree size, the variation in tree size, basal area, and the density of all trees, but the minimum plot size for characterizing structural attributes commonly associated with old-growth appears to be 0.75–1.0 ha in managed red pine. The two-dimensional spatial arrangement (Ripley’s K) and the structural complexity index (SCI) were satisfactorily estimated in plots of 0.50 ha in extent. These early transition phase stands were characterized by smaller trees, greater red pine dominance, density-dependent dispersion, and lower SCI than old-growth. This suggests that relatively large plot sizes may be necessary to evaluate structural condition of stands with increasingly “old-growth” structural conditions.