Long‐term changes in composition, diversity and distribution patterns in four herbaceous plant communities along an elevational gradient

Abstract Questions Have species expanded their ranges upward in elevation? Have α‐ and β‐diversity changed over time? Have species richness changes along the gradient shown distributional patterns? Have community compositions changed over time, and if so, which plant families are responsible for variation? Do herbaceous changes correspond to increased warming, overstorey shifts and disturbance events at each site? Location Long‐term Ecological Research ( LTER ) sites near Niwot Ridge, CO , US . Methods We surveyed four herbaceous plant communities along an elevational gradient for species composition, diversity and frequency. Our results were compared to those found in 1981 and 1996 in the same plots to determine long‐term community change at each site and along the elevational gradient. We evaluated changes in elevational species range expansion, community diversity, and composition. We related herbaceous change to tree data and site‐specific literature on climate and disturbance to analyze change patterns and drivers. Results Our investigation showed the upward range expansion of eight species. Alpha‐diversity changes were site‐specific, while species richness changes along the gradient did not exhibit distinctive patterns. Each community changed significantly ( P < 0.05) in composition and in β‐diversity. Shade‐tolerant families increased in dominance at the three lower sites, which reflected succession‐driven canopy closure. Changes correlated to increased warming, overstorey shifts and disturbance. Conclusions Herb aceous communities have changed in dynamic ways, as evidenced by shifts in composition, diversity and distribution. We confirm the need for more long‐term studies that can offer robust insights into the interplay between change drivers, such as climate, biotic interactions and disturbance, which work in concert to determine community assemblage.