Studies on lichen-dominated systems. XX. An examination of some aspects of the northern boreal lichen woodlands in Canada

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.

Influence of ground substrate on establishment of reindeer lichen after artificial dispersal

“…3), it was significantly larger in the inner parts of the quadrats with the moss substrate. According to several authors, high densities and thicknesses of matforming lichens may help them avoid desiccation (Kershaw andField 1975, Gauslaa andSolhaug 1998) and although the results of Gaio-Oliveira et al (2006) indicate that the density effect may be weaker than previously thought, the moss is likely to have provided more humidity than bare mineral soil. If the large ratios between inner-part cover and total cover (Fig.…”

Patterns of bryophyte diversity in humid coastal and inland cedar–hemlock forests of British Columbia

“…The result may become a patchy mosaic of highly disturbed and undisturbed forests; the latter are refugia for regenerating future forests (Kershaw 1976(Kershaw , 1985Larsen 1980). Indirectly, fire maintains forest ecosystem diversity (at the landscape level) because of the randomness, varying intensities, and frequencies of the fires (Connell and Slayter 1977;Zackrisson 1977;Lindholm and Vasander 1987;Arseneault and Payette 1992;de las Heras et al 1995).…”

Nitrogen fixation by associative cyanobacteria in the Canadian Arctic.

“…Differences in the capacity of vegetation types to retain moisture and make it accessible to cyanobacteria have been correlated with rates of N2-fixation (Zielke et al, 2002;. Shading vegetation can reduce the light intensities available to a N2-fixing association, and can limit the persistence of some lichens into later successional stages (Kershaw, 1976;Foster, 1985;Kurina & Vitousek, 1999). Shading of N2-fixing associations by shrubs may be particularly important given that remote sensing, repeat photography and experimental warming studies all suggest that current warming trends may be promoting shrub growth and expansion (Chapin et al, 1995;Sturm et al, 2001;Goetz et al, 2005;Walker et al, 2006).…”

“…Light limitation is often cited as the reason that some lichens do not persist into later successional stages (Kershaw, 1976;Foster, 1985;Kurina & Vitousek, 1999). Remote sensing, repeat photography, and warming experiments in combination with nutrient addition studies all suggest that current warming trends in the low Arctic may be promoting shrub growth and expansion within various topographic positions (Chapin et al, 1995;Sturm et al, 2001;Goetz et al, 2005).…”