Changes in stand composition and structure between 1981 and 1996 in four Front Range plant communities in Colorado

We conducted a study analyzing the vegetation dynamics in four communities along an elevational gradient in the Front Range of the Colorado Rocky Mountains and compared our results with previous research conducted in 1981. The objective of this study was to determine whether the successional plant community trajectories hypothesized by the original investigator (Marr) in 1953 were consistent with plant community parameters measured in subsequent years. The ponderosa pine and Douglas-fir/ponderosa pine communities' herbaceous composition remained relatively constant between 1981 and 1996; however, a few individual species in both communities experienced significant changes in frequency over time. These individual species changes, along with dominant shifts in the forest canopy, were inconsistent with Marr's original successional hypotheses for these stands that stated the ponderosa pine stand would remain dominated by ponderosa pine and grassy openings and the Douglas-fir/ponderosa pine stand would eventually have equal dominance between the two tree species. The aspen herbaceous community experienced the most significant change between 1981 and 1996. Significant decreases in species richness and diversity were recorded along with large changes in species composition and frequencies of individual species. These changes were associated with successional changes in the forest canopy, which was congruent with Marr's successional hypothesis for this stand. The kobresia meadow herbaceous community showed the least change between 1981 and 1996 among the four communities; however, there was variation in frequency percentages for a few individual species over time and a small turnover in species composition. Marr's hypothesis that this stand represented a climax community is consistent with our results.