Structure and Change in Herbaceous Communities of Four Ecosystems in the Front Range, Colorado, U.S.A.^*

* We dedicate this paper to John W. Marr.

Abstract

The structure of four herbaceous communities located in lower montane (2200 m), upper montane (2600 m), subalpine (3050 m), and alpine (3750 m) zones in the Front Range, Colorado, in 1981 was compared with that in 1953 when these same areas had been studied previously. Plant communities were compared primarily on the basis of number of species, species composition, species frequency, and community diversity. The herbaceous community at 2200 m had changed significantly since 1953 in frequencies and composition of species. The change is associated with a considerable disturbance of the tree canopy. The herbaceous community at 2600 m had remained relatively stable despite some moderate changes in the tree canopy. The herbaceous community at 3050 m was less diverse in 1981 than in 1953. The differences are associated with (1) successional changes in the overstory at this location and (2) the likelihood that somewhat different areas were studied in the 2 yr. The herbaceous community at 3750 m had shown remarkably little change in composition or relative abundances of species between 1953 and 1981.

The diversity of the herbaceous community at 2200 m in 1981 was significantly lower than the diversities of the communities at the other three elevations, possibly because this community is in an earlier successional stage due to recent disturbances. The herbaceous communities at 2200 and 2600 m had many species in common, but there were differences in frequencies of individual species. There was little overlap in species composition among the other communities.

At all elevations, the distribution of the species over frequency classes was quite similar for the herbaceous communities in 1953 and 1981, even when there were large differences in species composition between these dates. At the lower three elevations many species were present in low frequencies, and few species were common, while at 3750 m many species were common or abundant. Patterns of species abundance and estimates of species diversity did not coincide with each other across the four habitats.