Document: SYN-3-63-33

Species and functional types affect ecosystem response to fertilization in arctic tundra.

BRET-HARTE, M.S.*, F.S.CHAPINIII, J.R.WHORLEY, E.A.GARCIA and V.SACRE

University of Alaska, Fairbanks, AK 99775 USA ¹

Abstract:
Human activites increasingly affect earth's environment and ecosystems not only through direct impacts on climate and element cycling, but also by mediating introductions and extinctions of species. Species differ in traits that affect the cycling of carbon and nitrogen, but the extent to which species affect ecosystem functioning and the relationship between species diversity and ecosystem response to perturbation is still largely unknown. We conducted an experiment in arctic tundra to measure the effects of removing plant species and functional groups of plant species, defined in two different ways, on ecosystem carbon and nutrient cycling in the presence and absence of fertilization. Our objective was to understand to what extent species can substitute for each other in ecosystem functioning and response to perturbation. We measured biomass non-destructively using a point-intercept method initially in 1997 and again in 1999. We found that in 1999, 1) fertilization stimulated the growth of most vascular species and led to declines in most non-vascular species in the absence of species removals, 2) fertilization when key species were removed led to increases in some species in the same functional type (e.g., Rubus chamaemorus increased by 200% when fertilized in the absence of Betula nana, as compared to a slight decline when Betula was present), 3) removal of a single functional type, mosses, led to an increase in growth of all vascular plants without additional fertilization, and 4) removal of mosses plus the two most common evergreen and deciduous shrubs without fertilization led to an increase in graminoid biomass similar that seen with fertilization with no species removals. These results suggest that nitrogen is being redistributed among the remaining species and that compensatory growth of the remaining species is occurring. It remains to be seen whether carbon storage will be fully compensated in the removal treatments, as short-term and long-term responses to fertilization in tundra are quite different.

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This abstract is being presented at: 10:15 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.