Climate change and caribou: effects of summer weather on forage

In 1989, the Chisana caribou (Rangifer tarandus) herd in the northern Wrangell Mountains, Alaska, U.S.A., declined substantially in population size and productivity. Grasses, sedges, forbs, and willows (Salix spp.) are critical components of the diet of caribou in spring and summer, and the abundance and quality of forage are influenced by climate. To evaluate effects of climatic variation on caribou forage we conducted a field experiment in subarctic tundra where light, air temperature, and precipitation were manipulated. We used a plastic tarpaulin to increase air temperature and decrease precipitation. We also decreased light intensity with a shade cloth and increased precipitation by adding water to determine climatic effects on nutrient content and biomass of forage for caribou during the summers of 1994 and 1995. The most notable treatment effect on aboveground biomass was that shading resulted in higher nitrogen concentrations in all plant growth forms. In addition, shading consistently reduced biomass in forbs during mid and late season. Water treatment increased total plant biomass in the greenhouse plots during midseason in 1994 and in late spring in 1995. Water treatment also increased late-season biomass in control plots during 1994 but had no effect on biomass in shaded plots in either 1994 or 1995. A decline in nitrogen concentration in plants occurred throughout summer, a pattern that was not evident in in vitro dry matter digestibility. Climate variation and subsequent effects on forage plants have the potential to influence the population dynamics of caribou through effects on their food supply.