Influence of nitrogen fertilization on abundance and diversity of plants and animals in temperate and boreal forests

Abstract

Aerial and land-based applications of nitrogen-based fertilizers to enhance forest growth makes nutrients potentially available to all trees, plants, and wildlife in a given ecosystem and, therefore, may have direct and indirect effects on wildlife and biodiversity. A scientific review of these potential effects was conducted with 106 published studies covering vascular and nonvascular plants, amphibians, birds, mammals, terrestrial invertebrates, and soil animals associated with fertilization in temperate and boreal forests, primarily in North America and Scandinavia. In terms of direct effects, amphibians and domestic mammals appear to be the most sensitive to urea used in fertilization programs. The avoidance behaviour and (or) mortality of amphibians in laboratory studies was species dependent. Ruminant animals, including wild ungulates, rapidly convert urea to ammonia and are susceptible to toxicity following ingestion of large amounts of urea. Feeding on urea pellets by small mammals or gallinaceous birds appears to be minimal as granules are unpalatable. In terms of indirect effects, the majority of responses of understory herbs to nitrogen fertilization showed an increase in abundance. Some shrubs in repeatedly fertilized stands eventually increased in abundance in long-term studies, whereas dwarf shrubs and abundance of bryophytes (mosses and terrestrial lichens) declined. In general, species richness and diversity of understory herbs and shrubs declined, or were unaffected, in fertilized stands. Response in abundance and species richness-diversity of vascular plants to a single application of nitrogen showed either an increase or no change. Repeated applications (2–5 and >5) usually resulted in declines in these responses. Relative abundance of mule deer (Odocoileus Rafinesque spp.), moose (Alces alces L.), and hares (Lepus L. spp.), and forage quantity and quality were usually increased by fertilization. Small mammal species generally showed increases or no change in abundance; decreases may be related to fertilizer-induced changes in food sources. Forest fertilization may provide winter feeding habitat for coniferous foliage-gleaning insectivorous birds in some cases. Six species of forest grouse showed no response to fertilizer treatments. Responses of soil animals to nitrogen fertilization appeared to be species- and dose-specific and are ameliorated by surrounding micro- and macro-habitat characteristics.