An explanation for alpine timberline

Abstract

Alpine timberline represents the highest altitude at which shoots of woody plants can grow and ripen under the air temperatures which prevail at the height of tree canopies. At higher altitudes, woody plants can occur as shrubs and krummholz, by benefiting from warmer day-time temperatures close to the ground. Inverted timberlines against valley floors are related to nocturnal temperature inversions and repeated daily freeze-thaw cycles. Evidence favouring these interpretations comes from experiments in the Craigieburn Range, New Zealand, in which seedlings of the local timberline species, Nothofagus solandri, and various overseas timberline species were grown in a series of gardens above and below the natural timberline. Although seedlings show decreasing growth with increasing altitude, tree limit is better explained in terms of the ripening of shoots so that they can withstand unfavourable winter conditions, than by the hypothesis relating it to the ability to achieve a positive CO₂ balance. Physiognomic differences among timberlines are related to other physiological characteristics such as light requirements, growth rates, and mycorrhizal associations, and to winter severity, which affects the floristic diversity and incidence of deciduousness. Above tree limit, predominance of large herbs and medium-sized shrubs in the lower part of the alpine belt in New Guinea and New Zealand reflects milder conditions than prevail in the Northern Hemisphere. Among woody plants, shrubby genera have shown the most speciation above timberline, but some tree species have evolved dwarf relatives. Nothofagus solandri seedlings grown on a valley floor below an inverted timberline fail to survive winter unless shaded, but North American conifers are fully winter hardy at the same site. In New Guinea, depressions supporting tussock grassland dotted with Cyathea tree ferns are considered to be basically related to frost.