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, abunda nce, and productivity of the alpine tundra is an ongoing area of ecological inquiry. At Niwot Ridge in the Front Range of Colorado, plant species rich ness and foliage production were studied with respect to N and P additions in three alpine communities varying in snowpack depth and duration. These e ffects were also measured in conjunction with a snowpack enhancement experi ment. Measurements of plant responses were made 4 yr following the initiati on of the manipulations. The addition of either N or P enhanced plant folia ge productivity (P = 0.05 and P = 0.03, respectively). Nitrogen additions h ad 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). Sno wpack did not affect foliage productivity (P = 0.20), but species richness was negatively affected (P < 0.001). Snowpack also appeared to mediate spec ies-specific responses to N and P additions. In the alpine, the relationshi p between species diversity and plant productivity is mediated by species-s pecific traits. After 4 yr, the increased production by plant species sensi tive to P additions did not reduce species richness. This suggests that pro duction-induced competitive exclusion is not a generalization that can be u sed to explain the decline in species richness. Moreover, the reduction in species richness due to N addition occurred across all of the tundra commun ities studied here. These communities differ with respect to the strength o f other potential limiting resources such as light (self-shading) or water. Thus, this negative response is best explained by changes in soil chemistr y that resulted directly or indirectly from N additions.