New Zealand timberlines. 2. A study of forest limits in the Crow Valley near Arthur's Pass, Canterbury

Abstract

The Crow is typical of many mountain valleys in that the Nothofagus forest clothing the valJey sides narrows to a mid-slope wedge in the glacially broadened upper reaches. In a study of this phenomenon, temperatures near the ground were measured, and observations on winter death of shoots, seed fall, and distribution and growth of mountain beech (N. solandri var. cliffortioides) seedlings were made. On clear, calm nights, temperatures in the open close to the ground above the upper forest limits, or below an inverted timberline, are lower than those measured in openings within the forest, and can be lower than the experimentally determined limits of cold tolerance of beech seedlings. Other factors influencing the position of the alpine timberline are a tendency for growth rates of seedlings to decrease with increasing altitude, winter die-back of exposed shoots (especially where snow drifts into the forest margin), and poor seed dispersal beyond the forest margin. Below the inverted timberline, scattered beech seedlings become established on sites where frosts are less severe.

Mixed subalpine scrub grows beyond the up-valley limits of beech forest. Seedlings of two prominent species, Phyllocladus alpinus and Dracophyllum longifolium, occur only sparingly, but both species persist in mature scrub by virtue of their longevity and ability to layer.

Keywords:

• timberline
• mountain beech
• Phyllocladus alpinus
• Dracophyllum longifolium
• valley head
• krummholz
• frost