Studies of anticipated effects of global warming tend to concentrate on the physiological limits of individual organisms, and imputed modifications to biome distributions expresed as climax ecosystems. Changes in distributions of individual species and of tree species associations in boreal and sub-alpine forests also depend, however, on disturbance events like forest fire. These alter the absolute and relative reproductive success of different species, thus promoting renewed succession. Forest fire occurrence increases during climate warming so that, it is hypothesised, fire will be a major if not dominant agent in global warming effects on boreal and sub-alpine vegetation. Two examples are considered: 1) In the boreal forest of Ontario, the zonation of pine and spruce-fir forests can be explained by the amount of stand-replacing forest fire. Jack pine (Pinus banksiana Lamb.) forests are fostered in southerly and westerly areas where fires are more common, and black spruce (Picea mariana (Mill.) B.S.P.), balsam fir (Abies balsamea (L.) Mill.) and white spruce (Picea glauca (Moench) Voss) are more common in the north and east, primarily because there is less fire there. 2) In sub-alpine forests of the Canadian Rocky Mountains, vertical zonation of lodgepole pine (Pinus contorta Dougl.) and alpine fir (Abies lasiocarpa (Hook) Nutt.)-Englemann spruce (Picea engelmannii (Parry) Engelm.) forests is affected by forest fire-return intervals. There is more fire at lower altitudes, so pine is dominant there. The return period and intensity of these fires is mediated primarily by summer climate, but also by avalanches which create natural fire breaks in mountain valleys. The return period and intensity of these fires is mediated primarily by summer climate, but also by avalanches which create natural fire breaks in mountain valleys. The return interval for avalanches is driven by winter climate, so that the emergent properties of the fire-avalanche system during climate warming are difficult to predict. It is concluded that climatically-driven disturbance events are a major way by which glocal warming will be manifested in boreal and sub-alpine vegetation change, and especially so where there is a spatial gradient in forest fire activity.