The Effects of Mobile Tree Islands on Soil Phosphorus Concentrations and Distribution in an Alpine Tundra Ecosystem on Niwot Ridge, Colorado Front Range, U.S.A.

Niwot Ridge is populated by moving patches of trees that form islands of woody vegetation in a habitat dominated by graminoids. This study determined the amounts and distribution of soil phosphorus (P) in relation to tree islands. As tree islands retreat to leeward, we hypothesize that the colonized soils would become both depleted in organic P (Po) and would possess more geochemically bound P than in the adjacent tundra. A modified Hedley sequential fractionation indicated that the individual soil P fractions, along with the sums of these fractions, did not differ significantly between tundra, windward, and leeward sites. Overall, soil P appears to be distributed in almost equal portions between geochemical and biological forms. The lack of significant change of soil P concentrations and distribution in response to tree island passage on Niwot Ridge is likely due to the lack of recent glaciation as well as a long history of infrequent perturbances.