A Comparison of Alpine Timber Lines in New Zealand and North America

Abstract

Summary In New Zealand Nothofagus usually forms a close‐canopied, evergreen forest that halts at an abrupt timber line at 3,000‐5,000 ft, but under certain conditions the trees thin out unevenly or are stunted into krummholz . Subalpine scrub, diminishing in stature and extent with increasing altitude, occupies the subalpine belt when beeches are absent. On Iztaccihuatl, Mexico, erect, symmetrical trees of Pinus hartwegii Lindl. form an open timber line at c. 13,000 ft against tussock grassland. In Colorado Picea engelmanii (Parry) Engel. and associated species form timber lines exceeding 11,000 ft, and wind‐deformed krummholz can extend 500‐600 ft higher. Timber line at Garibaldi Park, British Columbia, is formed mainly by Abies lasiocarpa (Hook, f.) Nutt. Krummholz of Nothofagus solandri var. cliffortioides (Hook, f.) Poole results either from desiccation of exposed shoots in winter or from prevention of growth to windward. Near timber line, Nothofagus seedlings become established less frequently than at lower altitudes, the critical period probably being within four months of germination. Transplanted seedlings older than this have shown good survival even above timber line when shaded, but poor survival in the open. New growth of N. s. cliffortioides is liable to be damaged by late frosts, and above timber line is unlikely to be hardened in time to withstand the cold of autumn and winter. In Picea engelmannii winter desiccation appears to be related to failure to develop hardiness during the preceding summer. The snow pack provides protection that allows P. engelmannii seedlings to survive at elevations where they cannot develop into normal trees, but tends at the same time to curtail the critically short growing season. It is suggested that the upper limit for the esablishment of tree seedlings is set by their ability to produce shoots and leaves during summer and then to harden this new growth sufficiently to withstand the winter environment.