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, bu t their ecological roles in the maintenance of endemic conditions for an ar ea (that is, long-established levels of activity that are of low magnitude and relatively static intensity and cause unnoticed or relatively low amoun ts of tree killing, defoliation, or deformation) are poorly understood. The purpose of this study was to develop a conceptual model of stand developme nt that links stand structure with underlying tree-killing disturbances. Tr ansect 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. La ws.) stand with no harvest or management history. The site was composed of a mosaic of four different stages of stand development. The conceptual mode l hypothesized that different disturbance agents were associated with diffe rent stand types, and that these agents played two basic ecological roles: (1) fire, wind, and epidemic populations of mountain pine beetle (Dendrocto nus ponderosae Hopkins) killed trees over large enough areas to allow new s tands to develop, and (2) suppression, competition, ice/snow buildup, weste rn gall rust, endemic mountain pine beetle populations, wildfire, shrub com petition, poor site quality, low light intensity, limb rust, wind, lightnin g, and armillaria root disease created small-scale 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.