Tree-limit ribbons in the snowy mountains, Australia: characterization and recent seedling establishment

Tree-limit ribbons, isolated ribbons of trees growing above, but close to the alpine tree limit, have been described previously only for North America. Here, we describe such ribbons from the Snowy Mountains, Australia. Spread of trees above the treeline on lee slopes is generally as ribbons perpendicular to the prevailing wind, with snowdrifts accumulating downwind suppressing seedling establishment. The ribbons exhibit long-term stability, with estimated stem ages of snowgum (Eucalyptus pauciflora subsp. niphophila) up to 500 years, and the lignotubers considerably older. Windblown branches containing viable seed may allow initial establishment of trees above treeline leading to the formation of ribbons. Seedling establishment uphill of the highest ribbons is rare because snowgum has no inherent seed dispersal mechanism, depending on gravity for dispersal. However, seedling establishment immediately downslope of the highest ribbons, normally suppressed by snowdrifts, is more common and appears to have occurred mainly post-1970. Whether seedlings that established under snowdrifts post-1970 will remain as krummholz or proceed to full tree status will depend on the future snow regime and the persistence of regular snowdrifts. However, there are trees that have established below ribbons but outside the influence of snowdrifts, that exist now as younger ribbons, in clumps, or as individuals, in areas that previously did not support trees.