Response to fertilization by various plant growth forms in an Alaskan tundra: nutrient accumulation and growth

The fertilization responses of six tundra species belonging to three plant growth forms were compared to test the hypothesis that species of the same plant growth form are more similar to one another than to other growth forms in their response to a controlled perturbation. The controlled perturbation was a complete factorial NPK fertilization experiment in tussock tundra at Eagle Creek, Alaska, USA. We compared deciduous shrubs, evergreen shrubs, and functionally deciduous gra- minoids in terms of mineral and total nonstructural carbohydrate (TNC) concentrations, and annual production per stem or tiller. Species differed in the extent to which nutrient and TNC concentrations were altered by fertil- ization, although concentrations were usually changed in the same direction in all species in response to a given nutrient addition. Growth forms were not consistently different from each other in the responses of nutrient and TNC concentration, and frequently the two species from the same growth form responded differently. Growth per stem or tiller was stimulated most strongly by N and/or N + P fertilization in five of six species, with small and usually insignificant differences in magnitude but not direction of the response among species. Nutrient concentrations generally responded without interaction between fertilization treatments but the growth response was usually characterized by the N*P interaction, indicating that individual nutrient concentrations may vary widely and independently but that growth at Eagle Creek requires a balanced plant nutrition with first N and then P as the principal limiting factors. Decreases in TNC with fertilization suggested that carbon supply was not strongly limiting to plant growth. We conclude that species respond individually to fertilization in terms of nutrient and TNC ac- cumulation, but that the species or growth forms studied are not distinctive from each other on the basis of limiting nutritional factors for growth.

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