Introduction

The Arctic LTER site is located at 68°38'N and 149°43'W, at an elevation of 760 m in the northern foothills of the Brooks Range, Alaska. The location, 208 km south of Prudhoe Bay, was chosen for accessibility to the Dalton Highway, which extends along the Trans-Alaska Oil Pipeline from north of Fairbanks to Prudhoe Bay on the Arctic Ocean (figure 5.1). The rolling foothills at the site are covered with low tundra vegetation (Shaver et al. 1986a), which varies from heaths and lichens in dry sites to sedge tussocks on moist hillslopes to sedge wetlands in valley bottoms and along lakes. Riparian zones often have willow thickets up to 2 m in height. Small lakes are frequent; the best studied such lake is the 25-m-deep Toolik Lake (O'Brien 1992), the center of the LTER research site. Some 14 km from Toolik Lake, the Dalton Highway crosses the fourth-order Kuparuk River, the location of much of the LTER stream research (Peterson et al. 1993).

Climate records at Toolik Lake have been kept since the early 1970s when a pipeline construction camp was established. On completion of the road in 1975, climate stations were set up by the U.S. Army Cold Regions Research Laboratory (CRREL, climate reported in Haugan 1982 and Haugen and Brown 1980). Since 1987, the LTER project has maintained climate stations at Toolik Lake (http:// ecosystems.mbl.edu/arc/) whereas the Water Resources Center of the University of Alaska has continuous records beginning in 1985 from nearby Imnavait Creek. An

Figure 5.1 Location of Arctic LTER site at Toolik Lake, Alaska.

automatic station at Imnavait now reports every few hours to the Natural Resources Conservation Service-Alaska of the U.S. Dept. of Agriculture.

The characteristics of the climate in northern Alaska are summarized by Zhang et al. (1996), who pointed out the strong influence of the ocean during both summer and winter months. They reported that the mean annual air temperature is coldest at the coast (-12.4°C), where there are strong temperature inversions in the winter, and warmest in the foothills (-8.0°C). At Toolik Lake, snow covers the ground for about eight months, and some 40% of the total precipitation of 250-350 mm falls as snow. However, snowfall can occur on any day of the year. Summer temperatures at Toolik Lake are significantly warmer than at Prudhoe Bay on the coast.

Important climate factors that affect the ecology of the ecosystems at Toolik Lake include (1) the low temperatures in the air and soil that affect the metabolism of all the biota but especially cause a reduction in the microbial decomposition rates, (2) the 8-month snow cover that allows only a very short growing season for plants, (3) the reduced amount of light energy for photosynthesis because plant growth does not begin until after half of the annual radiant energy input has occurred, (4) the completely frozen streams from mid-September until mid-May that reduce the fish diversity to one species, and (5) the long duration of the ice cover of lakes (from the end of September until mid-to-late June) that reduces the light available for photosynthesis.

One consequence of the climate at Toolik Lake is the lack of significant trees in the vegetation. The northern limit for spruce is a hundred kilometers away on the south side of the Brooks Range. Another consequence is the presence of permafrost, frozen soil and rock extending some 200 m into the ground. On top of the permafrost is the active layer, soil that thaws each summer to a depth of 30-50 cm. Not only does permafrost restrict the rooting zone of plants to the active layer, it also seals the soils to water penetration. The result is that water from snowmelt and rain is held in the active layer, especially in the organic matter-rich upper 10-20 cm, thus the soils are usually moist despite the low precipitation. When there is enough precipitation to saturate the soil, the resulting runoff is "flashy"—that is, there is a quick peak of flow in the streams, but there is little water storage so the peak decreases quickly (Stieglitz et al. 1999).

The preceding illustrations show how the fundamental ecology at the Arctic LTER site is set by the long-term climate that determines such things as the makeup of the plant communities, the length of the growing season in tundra, streams, and lakes, and the hydrologic cycle. But important clues about ecosystem function and controls also arise from observations of the ecosystem response to short- and long-term climate changes. In the Arctic, there are many aspects of short-term climate variability, including year-to-year snow cover duration, the variation in lake temperatures from year to year, the effects of air and soil temperature changes from year to year, the ecosystem changes caused by stream flow and stream temperature differences from one summer to another, and the changes within a lake related to irregular stream flows caused by rain events. Of the possible long-term changes in climate, an increase in air and permafrost temperatures is the only one detected thus far.

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