New zealand timberlines. 3. A synthesis

Abstract

An experimental comparison of the native mountain beech (Nothofagus solandri var. cliffortioides) with exotic timberline species, and a study of the configuration of timberline in an alpine valley in relation to topography and microclimate are synthesised with the results of other timberline studies to develop general hypotheses about New Zealand timberlines.Although assimilation and growth in mountain beech decrease with increasing altitude, the abrupt nature and local altitudinal variations of beech timberlines mainly reflect the cold-tolerance limits of its seedlings. Winter death of beech twigs, that can lead to a krummholz form, is aligned with downslope winds, but sensitivity to low temperatures may also be involved. At least some of the tall shrubs that replace beech forest in cirque-form valley heads possess greater cold-tolerance. These shrubs also have endotrophic mycorrhizal associations, in contrast to the ectotrophic mycorrhizas of beech, which may have bearing on their lower stature and slower growth.On a regional scale, maximum tree limit, whether of beech or tall shrubs, is related to the regular altitudinal decrease in atmospheric temperatures, and not to microclimates as influenced by aspect and topography; and it is more likely related to how long temperatures lie within a range conducive to maturation and winter-hardening of shoots, than to annual carbon gain. Above tree limit, however, summer growth may not compensate for winter dieback. Low shrubs ascend far above tree limit, because the genetic restriction on their height growth protects them from repeated die-back.Species of northern hemisphere timberlines generally achieve far greater winter-hardiness than native trees, but many are limited in New Zealand because they are damaged by summer frosts. Pinus contorta, however, seems capable of forming trees 200 m above the natural tree limit. and krummholz at still higher altitudes. © 1985 Taylor & Francis Group, LLC.