THE CHARCOAL EFFECT IN BOREAL FORESTS - MECHANISMS AND ECOLOGICAL CONSEQUENCES

Wildfire is the principal disturbance regime in northern Boreal forest s, where it has important rejuvenating effects on soil properties and encourages tree seedling regeneration and growth. One possible agent o f this rejuvenation is fire-produced charcoal, which adsorbs secondary metabolites such as humus phenolics produced by ericaceous vegetation in the absence of fire, which retard nutrient cycling and tree seedli ng growth. We investigated short-term ecological effects of charcoal o n the Boreal forest plant-soil system in a glasshouse experiment by pl anting seedlings of Betula pendula and Pinus sylvestris in each of thr ee humus substrates with and without charcoal, and with and without ph enol-rich Vaccinium myrtillus litter. These three substrates were from : (1) a high-productivity site with herbaceous ground vegetation; (2) a site of intermediate productivity dominated by ericaceous ground veg etation; and (3) an unproductive site dominated by Cladina spp. Growth of B. pendula was stimulated by charcoal addition and retarded by lit ter addition in the ericaceous substrate (but not in the other two), p resumably because of the high levels of phenolics present in that subs trate. Growth of P. sylvestris, which was less sensitive to substrate origin than was B. pendula, was unresponsive to charcoal. Charcoal add ition enhanced seedling shoot to root ratios of both tree species, but again only for the ericaceous substrate. This response is indicative of greater N uptake and greater efficiency of nutrient uptake (and pre sumably less binding of nutrients by phenolics) in the presence of cha rcoal. These effects were especially pronounced for B. pendula, which took up 6.22 times more nitrogen when charcoal was added. Charcoal had no effect on the competitive balance between B. pendula and P. sylves tris, probably due to the low intensity of competition present. Juveni le mosses and ferns growing in the pots were extremely responsive to c harcoal for all sites; fern prothalli were entirely absent in the eric aceous substrate unless charcoal was also present. Charcoal stimulated active soil microbial biomass in some instances, and also exerted sig nificant although idiosyncratic effects on decomposition of the added litter. Our results provide clear evidence that immediately after wild fire fresh charcoal can have important effects in Boreal forest ecosys tems dominated by ericaceous dwarf shrubs, and this is likely to provi de a major contribution to the rejuvenating effects of wildfire on for est ecosystems.