Plant and soil responses to neighbour removal and fertilization in Alaskan tussock tundra

1 Species interactions will probably be important in determining plant community structure as availability of soil nutrients changes due to climate warming or anthropogenic N deposition. We removed dominant species, combinations of species, and entire plant functional types, in fertilized and unfertilized plots in tussock tundra. 2 After 2 years, graminoids responded more strongly to fertilizer than other growth forms, and the responses of graminoids and deciduous shrubs to fertilizer were greater under neighbour removal than in the intact community. Deciduous shrubs, evergreen shrubs and graminoids increased their biomass with fertilization, whereas non-vascular plants decreased. 3 Dominant species from each growth form usually responded strongly to fertilization, but half of all subdominant species responded weakly or not at all. Few species responded to neighbour removal. 4 Soil nutrient availability, however, was elevated significantly by both fertilization and neighbour removal. Neighbour removal increased nutrient availability in fertilized plots by up to two orders of magnitude, and availability of NH(4)(+) and NO(3)(-) in some unfertilized removal treatments was greater than in the fertilized intact community. 5 The failure of many plant species to respond with enhanced growth to soil nutrients made available by neighbour removal, despite their response to fertilization, could be due to (i) tundra plants having such rigid niche complementarity that they are unable to utilize these additional resources, or (ii) insufficient time having elapsed for the remaining species to respond, because nutrients derived from neighbour removal probably became available later than nutrients added as fertilizer. 6 There may be a high potential for loss of available nutrients from the tundra ecosystem when species composition changes, if the remaining plants cannot adjust to use nutrients made available by the loss of their neighbours.