Competition causes regular spacing of alder in Alaskan shrub tundra.

Alders (Alnus crispa) in shrub tundra in northern Alaska showed significant regularity of spacing. Removal of neighboring alder shrubs stimulated nutrient accumulation and growth of remaining alders but did not stimulate nutrient accumulation or growth of any other shrub species. This demonstrates that neighboring alders competed with one another and that, when alders were removed, the resources made available were used preferentially by remaining alders rather than by the community in general. Neither patterns of seedling establishment nor patterns of frostrelated features could explain the regular distribution of alder. We suggest that regular patterns of plant distribution are restricted to sites of low-resource availability, because in these habitats (1) there is strong competition for a scarce resource, and (2) there are only one or a few dominant species to compete for these resources in a given canopy height or rooting depth.