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282 ELIEL STEENFig. 1. Birch heath on sandy soil. Torne Lappmark, SW of the little Lake Vuotnojarvi about 27 km S of the town ofKiruna. Altitude about 460 m. In the foreground Betula nana which in northernmost Lappland occurs on very dry soil.The lichens are kept short by the grazing of the reindeer. Aug. 16, 1961. Photo G. Lohammar.of about twenty plant communities for this purpose.This system was used in north Sweden aswell as in north Norway, where it was applied,especially for Troms county, in an extensive investigation,which resulted in a very accurate quantitativeevaluation of its vegetation. In later yearsSKUNCKE and SANDBERG (SKUNCKE 1958) used asystem based on newer Swedish plant sociology.In Norway a comprehensive study is in progress,founded on the earlier works of RoLF NoRDHAGEN(1943) and JoHANNES LID (unpubl.); see TvEITNES(1949) . For Finland, see a survey by ARTI (1961).In a recent Swedish investigation the gross annualproduction of about twenty important plantcommunities has been measured. Such an estimationimplies a number of methodical problems, whichcannot be treated here in full. The annual incrementis not always easy to separate from olderparts of perennial plants. This is especially difficultwith small trees, shrubs and lichens. Part of theannual increment can further be localized to subterrestrialor subaquatic organs of the plants.Finally the reindeer often eat perennial parts andconsequently consume the production of severalyears in one season. Of the lichens eaten in winteroften the increment of a decade or more is consumedin one piece.The next step is to calculate the actual amountof grazing, as feed units or caloric value eaten bythe reindeer. With knowledge of the energy requirementsof the reindeer, the number of animals andthe length of the grazing period these quantitiesmight be estimated in rough figures. The proportionbetween the net energy value of the organicnutrients eaten and that of the gross productionavailable indicates the grazing balance. This percentage,called degree of utilization, is approximatebut of great value as an indication of the grazingintensity within the different parts of the districtutilized by a Same community. Cases of overgrazingcan be detected and necessary corrections of thedisposition of the land suggested. Other useful figureseasy to calculate are number of reindeer perunit area and number of grazing days per unit area.The studies mentioned have shown that onlyabout 4 per cent of the new green growth in sum-Acta Phytogeogr. S£ec. 50