As a consequence of increasing greenhouse gas concentrations, climate change is predicted to be particularly pronounced, although regionally variable, in the vast arctic, sub-arctic and alpine tundra areas of the northern hemisphere. Here, we review winter foraging conditions for reindeer and caribou (Rangifer tarandus) living in these areas, and consider diet, forage quality and distribution, accessibility due to snow variation, and effects of snow condition on reindeer and caribou populations. Finally, we hypothesise how global warming may affect wild mountain reindeer herds in South Norway. Energy-rich lichens often dominate reindeer and caribou diets. The animals also prefer lichens, and their productivity has been shown to be higher on lichen-rich than on lichen-poor ranges. Nevertheless, this energy source appears to be neither sufficient as winter diet for reindeer or caribou (at least for pregnant females) nor necessary. Some reindeer and caribou populations seem to be better adapted to a non-lichen winter diet, e.g. by a larger alimentary tract. Shrubs appear to be the most common alternative winter forage, while some grasses appear to represent a good, nutritionally-balanced winter diet. Reindeer/caribou make good use of a wide variety of plants in winter, including dead and dry parts that are digested more than expected based on their fibre content. The diversity of winter forage is probably important for the mineral content of the diet. A lichen-dominated winter diet may be deficient in essential dietary elements, e.g. minerals. Sodium in particular may be marginal in inland winter ranges. Our review indicates that most Rangifer populations with lichen-dominated winter diets are either periodically or continuously heavily harvested by humans or predators. However, when population size is mainly limited by food, accessible lichen resources are often depleted. Plant studies simulating climatic change indicate that a warmer, wetter climate may cause an altitudinal upward shift in the production of mat-forming lichens in alpine, sub-arctic regions. This is due to an increased potential for lichen growth at high altitudes, combined with increased competition from taller-growing vascular plants at lower altitudes, where the biomass of Betula nana in particular will increase. Matforming lichens dominant on dry, windblown ridges are easily overgrazed at high reindeer densities. This has longterm effects due to lichens’ slow regeneration rate, but may also reduce competition from vascular plants in a long time perspective. Fires may act in a similar way in some forested areas. Accessibility of winter forage depends on plant biomass, snow depth and hardness; ice crusts or exceptionally deep snow may result in starvation and increased animal mortality. Calf recruitment appears to be low and/or highly variable where winter ranges are overgrazed and hard or deep snow is common. Population decline in several Rangifer tarandus spp. has been associated with snow-rich winters. Effects tend to be delayed and cumulative, particularly on calves. This is mainly ascribed to feeding conditions for young animals which later affect age at maturation. Global warming may increase the frequency of deep or hard snow on reindeer ranges in Norway, due to increased precipitation and more frequent mild periods in winter. We hypothesise that potential benefits from increased plant productivity due to global warming will be counteracted by shifts in the distribution of preferred lichen forage, reduction of the areas of suitable winter ranges, and generally reduced forage accessibility in winter.
