Endemic Forest Disturbances and Stand Structure of Ponderosa Pine (Pinus ponderosa) in the Upper Pine Creek Research Natural Area, South Dakota, USA

Disturbances are natural and essential components of healthy ecosystems, but their ecological roles in the maintenance of endemic conditions for an area (that is, long-established levels of activity that are of low magnitude and relatively static intensity and cause unnoticed or relatively low amounts of tree killing, defoliation, or deformation) are poorly understood. The purpose of this study was to develop a conceptual model of stand development that links stand structure with underlying tree-killing disturbances. Transect surveys were used to identify and assess stand structure of a 60-ha study site in a ponderosa pine (Pinus ponderosa Douglas ex P. Laws. & C. Laws.) stand with no harvest or management history. The site was composed of a mosaic of four different stages of stand development. The conceptual model hypothesized that different disturbance agents were associated with different stand types, and that these agents played two basic ecological roles: (1) fire, wind, and epidemic populations of mountain pine beetle (Dendroctonus ponderosae Hopkins) killed trees over large enough areas to allow new stands to develop, and (2) suppression, competition, ice/snow buildup, western gall rust, endemic mountain pine beetle populations, wildfire, shrub competition, poor site quality, low light intensity, limb rust, wind, lightning, and armillaria root disease created smallscale canopy gaps that changed the growth environment for established trees and thereby influenced stand development and structure. The importance of single agents may be difficult to estimate because disturbances interact concurrently and sequentially in time and space.