MAIKAWA, E . , and K . A. KERSHAW. 1976. Studies on lichen-dominated systems. XIX. The postfire recovery sequence of black spruce -lichen woodland in the Abitau Lake Region, N.W.T. Can. J. Bot. 54: 2679-2687.The postfire recovery sequence in spruce woodlandgrowing on drumlins in the Abitau-Dunvegan Lakes area of the Northwest Territories is described. Four phases are recognized: year 1 to ye,ar 20, the Polytrichrrtn phase dominated by P. pilifer~rm, with Leciden grnnirlosn and L. rrlrginosn as associated species; year 21 to about year 60, the Clodor~in phase dominated by Clrrrlot~irr stellnris and C. ~rncinlis; year 61 to about year 130, the spruce-Stereoca~rlon phase with Stereocrr~rlon pnscl~nle forming an almost pure lichen ground cover; after year 130, the canopy closes and the lichen cover disappears and is replaced by a moss cover forming the final phase, spruce-moss woodland. These phases are confirmed by component analysis.The existence of spruce-Stereocarrlon woodland in the area is thus dependent on cyclic burning. In the absence of fire the spruce canopy would close and the lichen cover would largely disappear. This event is rare in the area with a rebuln cycle of about 100 years. Fire is thus an important vector in the maintenance of this extensive barren-ground caribou winter range.

SUMMARYThe nitrogenase activity in Peltigera polydactyla was examined during varying periods of darkness and after re-illumination. There was an immediate decrease in activity of about 50% when the lichen was darkened, followed by a progressive further decrease of activity with time. Recovery to the original level of activity on re-illumination showed an increasing lag as the dark period was extended. The effect of an external carbon source on nitrogenase activity was also examined and very large increases in activity were induced in the light, by both 2% glucose and mannitol. The decrease during a dark period was, however, still very marked.

Comparative rates of nitrogen fixation in the lichen Peltigera canina from subarctic and temperate habitats have been examined using the acetylene reduction method. Maximum acetylene reduction at thallus saturation and with 20000 lx illumination takes place at 16 °C in subarctic material and at 21 °C in temperate material. This adaptation of nitrogenase activity to temperature in P. canina is discussed in relation to low levels of soil nitrogen in arctic systems.