POSITIVE PLANT INTERACTIONS IN TUNDRA VEGETATION AND THE IMPORTANCE OF SHELTER

(1) The hypothesis that positive plant interactions, arising from species aggregations, are important in harsh environments was tested on the sedge Carex bigelowii in the alpine/subarctic tundra of Swedish Lapland. (2) Association analyses showed that, in contrast to records for the High Arctic, aggregations of the low-growing species (< 15 cm tall) were not well developed at the study site. However, when C. bigelowii was growing in close proximity to Cassiope tetragona or Empetrum hermaphroditum, two evergreen dwarf shrubs, or in mats of the moss Racomitrium lanuginosum, increased leaf lengths and culm heights were recorded, compared with plants growing > 2 cm away from the dwarf shrubs or outside the moss mat. (3) The effect of the dwarf shrubs was interpreted as a result of above-ground shelter because shoot height, as well as number of shoots, also increased when plywood shelters were erected on the windward side of C. bigelowii. The effects of this above-ground shelter on shoot growth could result from increased snow cover, protection against strong desiccating winds early in the growing season, and a warmer micro-climate. (4) An alternative explanation for the increased growth of C. bigelowii growing near other species is that their patchy distributions, particularly those of the dwarf shrubs, may be related to localized increased soil nutrient status. When fertilizer was added to C. bigelowii, there were significant increases in shoot height and number. The benefits to the growth of C. bigelowii when growing close to other species may be a combination of shelter effects and a patchy distribution of nutrients.