Introduction

The boreal zone is a Northern Hemisphere, circumpolar annulus ranging from around 50° to as far as 80°, including land and the Atlantic, Pacific, and Arctic Oceans. To the north, in summer and including much of the Arctic Ocean, associated archipelago, and 80% of Greenland, ice covers on the order of 10 Tm2 (10 million km2; Barry, 1995; Fig. 1). Low temperatures are thus one feature of the boreal zone. Vast areas of the zone sometimes become covered by ice, which happened most recently only ca. 10,000 years ago. This influenced the soil, some still underlain by permafrost, and the biodiversity of the vegetation (McGlone, 1996). Vegetation covers most of the boreal zone's land today and its physical features grade from south to north; notably, height and cover decrease significantly. The vegetation and terrestrial ecosystem evaporation rates during summer, the subjects of this chapter, are profoundly influenced by climate near the earth's surface. Because of the thermal properties of water, proximity to the open ocean moderates climate. In summer, the maritime climate is wetter (i.e., more water supply) and cooler (i.e., less atmospheric demand). A maritime influence can also delay the ends of cold and warm seasons. The magnitude of this effect varies throughout the boreal zone depending on the timing and extent of sea ice formation, if present nearby, and the range of sea-surface temperatures (Hertz-man, 1997). Energy is carried to and from the ocean by the wind in a process known as advection. A vortex of westerly winds tends to encircle the north pole although it is significantly interrupted by a perpendicular cordillera in western North America (Hare, 1997) and the Ural Mountains in western Siberia. Orographic effects greatly increase the precipitation rate on the windward side. Leeward, in the absence of water vapor advection from elsewhere, precipitation decreases toward the east until another ocean is approached.

The boreal zone's, remoteness and harsh winter climate have led to much of it being sparsely populated by people, especially in Siberia. Although not completely isolated from anthropogenic influence, of which fire control may be most significant, the boreal zone is relatively pristine. This attribute is significant for a baseline study of natural processes in the terrestrial biosphere, an opportunity that is becoming increasingly difficult to realize in much of the world. Evaporation is an important natural process because of its role in the surface-energy balance and the close linkages between plant water use and carbon gain. Beyond climate, elucidating the physics of evaporation is a necessary prerequisite to its study. An example is the interpretation of the recently decreasing evaporation rates during summer from pans of water located in European Russia, Siberia, and some other places. Peterson et al. (1995) attributed this trend to increasing cloudiness and, paradoxically, suggested that it indicated a decreasing terrestrial ecosystem evaporation; apparently failing to see how more (not

GLOBAL BIOCEOCHEMICAL CYCLES IN THE CLIMATE SYSTEM

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Pacific Ocean

Europe

Europe

Pacific Ocean

FIGURE 1 Sketch map of the boreal zone and polar ice cap (a cross marks the North Pole) with land, open ocean and ice illustrated with light and heavy shaded patterns and white, respectively. The outer limit of the map is approximately 50°N and a 1000 km scale is given. The ice extends to an average southern limit during summer after Barry (1995).

FIGURE 1 Sketch map of the boreal zone and polar ice cap (a cross marks the North Pole) with land, open ocean and ice illustrated with light and heavy shaded patterns and white, respectively. The outer limit of the map is approximately 50°N and a 1000 km scale is given. The ice extends to an average southern limit during summer after Barry (1995).

less) evaporation is required to satisfy the converse of their argument. Because Russia and most terrestrial ecosystems are not well supplied with water in summer, pan and terrestrial ecosystem evaporation rates tend to be inversely, not proportionally, related (Brutsaert and Parlange, 1998). Hence, decreasing pan evaporation rates indicate increasing terrestrial ecosystem evaporation in accordance with the recent trend of increasing precipitation (Brutsaert and Parlange, 1998). This example also illustrates how the short but hot and dry summers in the boreal zone involve significant surface-atmosphere energy exchange. Our purpose is to examine the regulation of this exchange in terms of evaporation from vegetation and soil by utilizing available field data, about half of which has been published in the past two years, with some new information from Siberia. As a synopsis, we distill the relevant physics, including the physics of the atmosphere and soil, and connect with the boreal zone's hydrology and its vegetation via plant physiology and the availability of nitrogen, a commonly limiting nutrient.

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