Plant species richness, productivity, and nitrogen and phosphorus limitations across a snowpack gradient in Alpine Tundra, Colorado, USA

The extent to which nutrient limitation affects species composition, abundance, and productivity of the alpine tundra is an ongoing area of ecological inquiry. At Niwot Ridge in the Front Range of Colorado, plant species richness and foliage production were studied with respect to N and P additions in three alpine communities varying in snowpack depth and duration. These effects were also measured in conjunction with a snowpack enhancement experiment. Measurements of plant responses were made 4 yr following the initiation of the manipulations. The addition of either N or P enhanced plant foliage productivity (P = 0.05 and P = 0.03, respectively). Nitrogen additions had a negative effect on the species richness censused in 1-m(2) plots (P < 0.001), while P additions had no effect on species richness (P > 0.60). Snowpack did not affect foliage productivity (P = 0.20), but species richness was negatively affected (P < 0.001). Snowpack also appeared to mediate species-specific responses to N and P additions. In the alpine, the relationship between species diversity and plant productivity is mediated by species-specific traits. After 4 yr, the increased production by plant species sensitive to P additions did not reduce species richness. This suggests that production-induced competitive exclusion is not a generalization that can be used to explain the decline in species richness. Moreover, the reduction in species richness due to N addition occurred across all of the tundra communities studied here. These communities differ with respect to the strength of other potential limiting resources such as light (self-shading) or water. Thus, this negative response is best explained by changes in soil chemistry that resulted directly or indirectly from N additions.