Studies on Growth, Reproduction and Population Ecology in Relation to Microclimate in the Bipolar Moss Polytrichum alpestre

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Abstract
Data on growth and reproduction in relation to microclimate in boreal forest populations of P. alpestre Hoppe at Elma and Pinawa, Manitoba, and in sub-Arctic populations at Churchill, Manitoba, Canada, are compared with previous results from Antarctic regions. Thermistor readings at 15-60 min intervals for over 2 yr indicated that moss level temperatures at Pinawa were relatively stable between 0.degree. C and -10.degree. C beneath winter snow cover but showed wide diurnal fluctuation in summer. Mean daily maxima were commonly 40.degree. C from May through Aug., but mean daytime temperatures were normally approximately 15 degrees lower. Freeze-thaw cycles in spring and autumn were less frequent, with higher minimum temperatures, at moss level than 200 cm above the ground. Shoots of P. alpestre show annual growth segments due to variation in the length of leaves produced at different times during the summer growing season. Stem elongation at Pinawa plotted against time showed a sigmoid curve. Both stem elongation and leaf production occurred principally during June and July, but current season growth segments may continue to increase in weight until Sept. The mid-summer cessation of stem elongation showed no strong correlation with temperature or moss water content. A similar sigmoid pattern was recorded when plants were grown under constant environmental conditions, at photoperiods of 12 h or 17 h, suggesting that growth may be partially under endogenous control. Shoots less than 4 yr old predominated in samples from colonies at Pinawa and at Churchill. Replacement occurred through the development of lateral shoots from buds commonly located 5-10 cm behind the apex of the parent shoot. The young shoots were initially non-photosynthetic, bearing only white tomentum and scale leaves until the apices reached the surface of the turf. Net annual shoot production in 2 colonies at Churchill was estimated at 97 and 148 g/m2, respectively, of which 15-18% was composed of the non-photosynthetic portion of 1st year shoots. The seasonal developmental cycles of gametangia and sporophytes in the Manitoban populations were similar to those previously reported from the sub-Antarctic. Development during several phases of the reproductive process was more rapid at Churchill than under warmer conditions at Pinawa and Elma.