IMPORTANCE OF BIOGEOGRAPHY AND ONTOGENY OF WOODY PLANTS IN WINTER HERBIVORY BY MAMMALS

Abstract Mammals can influence growth, reproduction, competitive ability, and survival of woody plants by virtue of selective browsing and gnawing of dormant shoots during winter. Apparently in response to this type of herbivory, plants have evolved chemical and mechanical deterrents to mammalian herbivores. We report on plant ontogeny and biogeography, which exert their influence on herbivory at different spatiotemporal scales. To evaluate how plant ontogeny influences herbivory, we conducted a meta-analysis of 128 studies, encompassing 37 plant and 10 mammal species, in which juvenile and mature growth stages of conspecific plants were made available to mammals during winter in temperate and northern latitudes. Mammals ate more of the mature-stage growth in 96% of the studies, and stage-specific differences in consumption were very large (d+ = 2.16). Plants characterized by rapid growth rates or low tolerances to resource limitation elicited the greatest degree of stage-specific discrimination by mammals, consistent with existing theories regarding tradeoffs governing plant growth and defense. The influence of a plant's growth rate and tolerance to resource limitation was dependent on climatic regime; plants grown in areas with harsh winter conditions tended to elicit greater discrimination of juvenile- and mature-stage growth by mammals than plants grown in more moderate climates. Further evidence for biogeographical variation in mammalian consumption came from 14 feeding studies, including 6 plant and 6 mammal species, that compared conspecific plants of the juvenile growth stage either grown or collected at different localities. In 86% of the studies, extent of herbivory by mammals varied inversely with latitude, and this yielded a moderate effect (Z+ = −0.46, r+ = −0.53). We discuss potential roles of life history, climate, and historical association of plants and mammals in shaping these biogeographical patterns.