Individualistic Growth Response of Tundra Plant Species to Environmental Manipulations in the Field

In undisturbed arctic tussock and wet meadow tundras we increased air temperature with a plastic greenhouse, increased nutrient availability by NPK fertilization, and decreased light intensity with shade cloth to determine the factors limiting growth of tundra plants. After 2 yr of these manipulations we measured growth of each major vascular species and one moss species. Each species showed a different pattern of growth response to alteration of light, air temperature, and nutrient regimes, indicating that no single factor limits growth of all species in these communities. Growth of canopy species (Betula nana, Ledum palustre, Carex bigelowii, and Eriophorum vaginatum) was reduced by experimental shading more than was growth of understory species (e.g., Vaccinium vitisidaea and Rubus chamaemorus). Species typical of nutrient—rich sites (Betula nana, Rubus chamaemorus, and Polygonum bistorta) generally responded more to nutrient addition than did species typical of nutrient—poor sites (e.g., Empetrum nigrum), although there were species characteristic of fertile sites (Salix pulchra) and infertile sites (Ledum palustre) which did not show this pattern of nutrient response. Species that grow in warm hollows between tussocks showed less growth in response to increased air temperature than did canopy species. We suggest that lack of a single common factor limiting growth of all species in tussock and wet meadow tundras implies that (1) each species is individualistically distributed, as described by the continuum model of community organization, (2) as a result of competition and/or distinct evolutionary histories, the growth of each species is limited by a different combination of environmental factors, and (3) production by individual species varies greatly from year to year, but production by the whole vegetation is more stable, because years that are favorable for growth of some species cause a compensatory decrease in growth of other species.