Impact of wild herbivorous mammals and birds on the altitudinal and northern treeline ecotones

Abstract

Wild herbivorous mammals may damage treeline vegetation an cause soil erosion at a local scale. In many high mountain areas of Europe and North America, large numbers of red deer have become a threat to the maintenance of high-elevation forests and attempts to restore the climatic treeline. In northern Fennoscandia, overgrazing by reindeer in combination with mass outbreaks of the autumnal moth are influencing treeline dynamics. Moose are also increasingly involved damaging treeline forest. In the Alps, the re-introduction of ibex is causing local damage to subalpine forests and tree establishment above the forest limit as well as aggravating soil erosion. High-elevation forests and treeline in Europe are susceptible to the deleterious impact of wild ungulate populations because of former extensive pastoral use. Rodents may damage tree seedlings and saplings by girdling, root cutting, bark stripping and burrowing. Hares damage young trees by gnawing. Large numbers of small rodents may occasionally impede tree regeneration by depleting the seed sources. Rodents do not contribute to forest expansion beyond the current treeline. Among birds, nutcrackers are highly effective in influencing tree distribution patterns and treeline dynamics. Without the nutcracker caching of stone pine seeds any upward advance of the trees in response to climatic warming would be impossible. Some bird species such as black grouse, willow grouse and ptarmigan can impair tree growth by feeding on buds, catkins and fresh terminal shoots.

Treelines—Approaches at Different Scales

“…Figure 1). The expansion of stone pines (e.g., Pinus cembra, P. sibirica, P. albicaulis, P. koraiensis; five-needled fascicles, wingless seeds, cones that remain closed at maturity) into, say, formerly grazed areas and the advancement to a potential climatic tree limit can be mediated only by seed dispersal and seed caching through nutcrackers (Nucifraga caryocatatces, Nucifraga macrorhyncos, Eurasia; Nucifraga columbiana, North America) [5,94]. On the other hand, animals' adverse effects (e.g., herbivory and trampling) on seedlings and saplings may locally overrule the influence of a warming climate that drives a treeline to a higher elevation and a more northerly position.…”

“…Soil material excavated by burrowing rodents and displaced over the ground surface may lastingly influence site conditions (e.g., [94,134,140,141]). Animals' effects, in particular on seedlings and saplings, should be more intensively studied [5,94,138].…”

“…Mass outbreaks of leaf-eating insects and bark beetles, for example, usually affect large forested areas at the regional (and landscape) scales, whereas wild and domestic mammalian herbivores and birds influence treelines usually at the local and microscale. Soil material excavated by burrowing rodents and displaced over the ground surface may lastingly influence site conditions (e.g., [94,134,140,141]). Animals' effects, in particular on seedlings and saplings, should be more intensively studied [5,94,138].…”

“…Thus, the distance from the seed source, the way of seed dispersal (anemochoric, zoochoric), the overlying of seeds, the availability of viable seeds, seed predation, and the availability of favorable seed beds (e.g., [5,85,87,88,[90][91][92][93]) are relevant factors that need further examination. However, discussion of whether reduced availability of viable seeds are playing a major role or not is controversial (e.g., [3,5,17,19,45,87,90,[93][94][95][96][97]). In any case, the performance of "new" trees that survived the seedling stage and became a sapling appears to be the most important factor for treeline advance in the medium and long term.…”

“…Comparatively little attention has been paid so far to the effects of mammalian herbivores, birds, and insects on treeline ecotones, with a few exceptions (e.g., [5,94,[132][133][134][135][136][137][138][139]). Animals may lastingly influence tree growth and physiognomy, regeneration, seed dispersal, soils, distribution pattern of trees, and treeline dynamics at the landscape and smaller scales (cf.…”