INDUCTION OF VERTICAL ZONES IN SUB-ALPINE VALLEY FORESTS BY AVALANCHE-FORMED FUEL BREAKS

Vertical zonation of forests in high mountains is normally explained i n terms of climatic variation, but avalanche tracks can act as fuel-br eaks in southern Alberta Rocky Mountain forests. This is an additional way of explaining the spruce-fir (Picea engelmannii (Parry) Engelm. - Abies lascioparpa (Hook.) Nutt.) and lodgepole pine (Pinus contorta D ougl. var. latifolia Wats.) communities of the upper and lower sub-alp ine zones. The hypotheses are that: 1) Fires start more often at lower than high altitude and that, as they spread into high valleys, 2) the y are halted where avalanche tracks reach the valley bottom from both slopes. Also, (3) the average return interval of fires will be greater above this ''avalanche block'', and 4) vegetation above the block wil l consist primarily of near-climax, fire-intolerant communities. These hypotheses were tested using the Highwood Pass (50-degrees-21'N, 114- degrees-26'W) in the Rocky Mountains of southern Alberta, Canada. Maps of avalanche tracks and past fires, a point-centered quarter survey o f forest stands, and disturbance histories established by increment co ring were used to test the hypotheses which were all upheld. Thus aval anche tracks are one of the complex of factors limiting fires in the s ub-alpine zone of the Alberta Rocky Mountains. The results, additional ly, imply that vertical vegetation zones in temperate high mountains a re influenced, not only by climatic factors, but also by avalanches an d other landforms.