Reference conditions and historical changes in an unharvested ponderosa pine stand: Implications for forest health

Abstract

Reference conditions are a critical component of ecological restoration and provide a point of comparison for the current degree of deviation from an ecosystem's evolutionary trajectory. In Arizona, the body of knowledge on reference conditions for stand density, age distribution, and spatial patterns is heavily weighted towards moderately productive basalt derived soils on the Coconino National Forest. These conditions may represent the average conditions for frequent fire ponderosa pine forests, but do not adequately capture the full range of variation. With landscape scale restoration treatments proposed across the Southwestern ponderosa pine type, establishing reference conditions that include spatial patterns for a variety of site conditions is a necessary step to assure that treatments are tailored to each site's reference conditions to avoid artificially homogenizing the landscape with a one-size-fits-all approach. Spatial pattern analyses have allowed researchers to link spatially-dependent process such as regeneration, competition, and mortality to structural characteristics; however, the mechanism(s) that control these processes are complex and may never be fully understood. The study of spatial patterns may also allow us to predict stand development following restoration treatments. Using an unharvested, old-growth ponderosa pine stand on a highly productive site with sedimentary derived soils; a forest reconstruction model, univariate and bivariate Ripley's K functions, and regression analysis we determined structural and spatial reference conditions that can be used to guide restoration treatments. We also compared contemporary (2010) and reconstructed (1883) stand structure and spatial patterns to evaluate historical changes. We found a reconstructed (1883) tree density of 88.0 trees ha−1, basal area of 7.97 m2 ha−1, and a quadratic mean diameter of 33.95 cm at breast height. The reference spatial pattern of ponderosa pine was significantly aggregated from 1–2 m and randomly distributed at distances greater than 2 m. Crown radius and interconnectivity projections indicated a few significant "patches" containing 2–3 trees within 1–2 m. A comparison of different sampling scales, 0.5, 1.0, 2.0, and 3.0 ha, indicated that a 0.5 ha plot was adequate to capture stand level spatial patterns. Compared to the reconstructed stand conditions, the contemporary stand had substantially greater tree densities (614 trees ha−1 ) than prior to 1883 and is aggregated up to 50 m. Bivariate spatial analysis indicated early recruitment patterns following fire exclusion were attracted to live presettlement trees from 1–4 m and independent at distances greater than 4 m. We conclude that these results are a product of a variety of factors including soil parent material, climatic variables and decreased dominance of mechanisms that drive aggregation. Using forest health evaluation criteria, we also determined that current stand conditions at Barney Spring are unhealthy and require intervention. To restore a healthy, sustainable, self-regulating system we recommend a complete restoration to include structural manipulations for fuels reduction and to mimic the random reference spatial pattern aided by the reintroduction of frequent, low intensity fire