No abstract

KERSHAW, K. A. 1977. Studies on lichen-dominated systems. XX. An examination of some aspects of the northern boreal lichen woodlands in Canada. Can. J. Bot. 55: 39-10. The existence of two major types of lichen woodland in Canada, C/~dorlirr stelloris woodland and Stereoctrrrlot~p(rschr~l(~ woodland, is discussed in relation to their seral nature and their rarely developed theoretical climax type.Our own observations, coupled with previous descriptions from a wider area, suggest that Stereoctrrrlor~posd~rrle woodland replaces C I N L I O~I~O stelltrri~ woodland in a more or less continuous zone from just west of Churchill across to Great Slave Lake, immediately north and south of latitude 60" N. Both woodland types are often typical of sandy soils (pH 6 or less) and almost always represent the final recovery phase after fire. Rarely, the lichen surface is replaced by a continuous moss cover as the spruce canopy closes. The lichen surface is thus dependent on the lack of competition from higher plants, the absence of which is characteristic of the climate of this northern boreal region. CIodor~i(r stellrrris woodland also occurs on palsas and peat plateaux where, again, lackof higher plant competition and a suitable pH exist.The recovery sequence after fire is a highly complex process and as yet only the following parameters have been categorized. In the early recovery phases, limited soil moisture and hence a reduced summer latent heat flux enhance the sensible heat flux. The surface conditions are analogous to those of a hot desert with very high surface temperatures and extremely large diurnal temperature fluctuations. The physiology of these initial moss and lichen colonizers presumably enables them to tolerate these harsh conditions. The establishment of a few spruce seedlings and the subsequent development of open lichen woodland modulates the harsh summer temperature regime and allows the further development of a vegetated surface. After humus accumulation, which acts as an effective mulch, summer soil moisture is elevated, enhancing the latent heat flux and correspondingly reducing the sensible heat flux. This probably allows the full development of mature lichen woodland with its almost monospecific ground cover of either Cltrdot~irr stellrrris or Stereocarrlon poschnle. Limited data suggest that the net photosynthetic responses of these two species is favoured by the relatively warm mesic conditions established by the open spruce canopy. Good accumulation of snow in the winter is probably also important for protection of the lichen surface from low temperatures. The open nature of mature lichen woodland is apparently maintained by an active inhibition of spruce seedling establishment by the lichen mat, although the mechanism is not entirely clear.

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.

No abstract

The current belief that lichens are incapable of controlling their water relations is based largely on evidence collected from experiments testing for active metabolic controls only. In this study, controlled wind tunnel experiments designed to test for any passive morphological control of evaporation in four lichen species show that such mechanisms are important to the water relations of the plants and thus suggest that the observed morphological variability in lichens is of great ecological significance.Time-dependent and moisture-content-dependent changes in evaporation rate were examined using samples of lichen thalli from a number of different arctic and subarctic populations of Alectoria ochroleuca (Hoffm.) Massal., Alectoria nitidula (Th.Fr) Vain, Cetraria nivalis (L.) Ach., and Cladina alpestris (L.) Harm. The work has shown that the observed interspecific and intraspecific variability in evaporative resistance found in these plants is influenced by three separate morphological characteristics: variability in surface area to weight ratio; variability in thallus shape; and variability in the degree of clumping of thalli. When other aspects of the ecology and physiology of the test lichens were considered, it became clear that these lichens exhibited a potential to modify their morphological characteristics over long periods of time in such a way that greater photosynthetic productivity was assured.It was concluded that although the control of water relations in lichens is of a different nature from higher plants, the control is present nonetheless.

Results from studies of carbon dioxide assimilation in lichens cannot be tested independently against growth studies. To compensate for this, rapid, more rigorously controlled experimental systems must be used to sample a much wider range of environmental conditions.Methods in popular use are found to be inadequate in these respects.A new simple system is described which permits the response of net photosynthesis in lichens to be examined more efficiently than was previously possible.

No abstract

The water relations of Cladonia alpestris in spruce–lichen woodland in northern Ontario is described. The rate of drying of the lichen canopy was measured by resistance grids inserted into the canopy and monitored during the drying cycle. The effects of dew were measured in a similar fashion and shown to form an insignificant proportion of the total annual metabolism of the lichen. The lichen mat showed a very high stratified resistance to water loss and the effective mulching properties produce a high level of water availability under the lichen mat. This was confirmed using neutron attenuation techniques and the significance of this discussed in relation to the development of lichen woodland. The physiological response of the lichen to conditions of varying levels of saturation is also discussed.