348 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



often evenly spaced dominants, poverty in associated vascular plants 

 (apart from xeromorphic ' heaths', such as Crowberry and species 

 of Vaccinium, in the shelter of the dominants), and richness of the 

 lichen carpet at least in dry places (Fig. 98). The Lichens are 

 usually intricately mixed and inclusive of so-called Reindeer-mosses 

 {Cladonia spp.) and Iceland-mosses {Cetraria spp.), being typically 

 aggregated into a pale but dense sward several centimetres thick. 

 In damp depressions and especially along the courses of rivers, there 

 occur faciations approaching the ordinary northern coniferous forest 

 of the region. These may project as timbered tongues containing 

 well-grown trees, or even form outliers in the tundra — so con- 

 stituting the so-called ' forest-tundra '. Such better growth often 

 seems to be correlated with better aeration of the roots where there 

 is active drainage of water (as confirmed by Professor Harold J. 

 Lutz, voce). In general, however, the dominants of the open 

 ' taiga ' are of poor development, often gnarled and only a few feet 

 high though ancient,^ conditions for growth being here largely 

 unfavourable. These dominants are usually the one or more hardiest 

 tree types of the forest lying to the south — for example, White or 

 Black Spruce and/or Tamarack across most of northern Canada, 

 Dahurian Larch in much of Siberia, and a scrubby Birch [Betula 

 odorata) in northernmost Scandinavia (Fig. 96). 



3. The Pacific ' coast forest ' of western North America, developed 

 chiefly from southern British Columbia to northern California, but 

 with some of the main dominants extending much farther north- 

 wards as well as southwards. The region is one of equable climate 

 with high rainfall and atmospheric humidity, and supports the 

 densest coniferous forest of the world as well as some of the biggest 

 and tallest of all trees — e.g. Coastal Redwood {Sequoia sempervirens). 

 Big-tree [Sequoiadendron giganteum), and Douglas Fir {Pseudotsiiga 

 taxifolia). These may reach heights of around 100 metres, or, in 

 the case of the first-named, no metres, with trunk girths often over 

 20 metres in the first two cases. A forest of Coastal Redwood is 

 shown in Fig. 18, D, Various further Conifers belonging to several 

 different genera constitute the other main dominants, etc. Although 

 Ferns, including the common Bracken {Pteridium aqiiiliniim agg.) and 

 Hard Fern {Blechnum spicant), are widespread, herbs in general are 

 little in evidence. However, many characteristic shrubs occur, 



^ Near the northern limit of forest in the Northwest Territories of Canada, 

 the author has counted more than 100 growth-rings in Black Spruces with trunks 

 barely three inches in diameter at the base and five feet high. 



