Disturbance and climatic influences on age structure of ponderosa pine at the pine/grassland ecotone, Colorado Front Range

In the Colorado Front Range, disturbances and climatic Variation influence stand structure of ponderosa pine (Pinus ponderosa) along the lower timberl ine ecotone. Over the past 100 years there has been a shift to a greater de nsity and extent of ponderosa pine at the forest-grassland boundaries. Pond erosa pine regeneration at lower timberline appears to be influenced by fir es in the 1860s and decreased grazing pressure in the 1970s-1980s. Climatic variation may also have influenced age structure, even though analyses of age structure at a 10-year class scale prevented the detection of climatic influences occurring at a finer scale. These changes in disturbance regimes , possibly together with moister springs/early summers, created favourable conditions for the increase in density and extent of ponderosa pine at lowe r timberline ecotone.