ABSTRACT
INTRODUCTION The boreal climatic zone occurs in the northern hemisphere and lies roughly in the latitudinal zone between the Arctic tundra to the north and temperate broadleaved forests and grasslands in the south. Its natural vegetation is dominated by forests of needleleaved conifers, but various species of broadleaved trees such as Betula (birch), Populus (poplar), Salix (willow), and Alnus (alder) often occur in wetter areas. Here, the nitrogen-fixing actinomycete Frankia occurs in the roots of alder (Chapter 1). Under natural conditions, these boreal forests cover the region except where interrupted by swamps, lakes, river flood plains, high mountains rising above the climatic tree line, or by the oceans. Despite human impacts through logging and conversion, much of this broad forest belt still remains intact across North America (Canada and Alaska), northern Scandinavia, and Siberia.
