Structure and composition of the subalpine forest on Mt Hauhungatahi, North Island, New Zealand

Abstract

Tree stumps of Holocene age were identified and measured at three sites above the present tree-line on Mt Hauhungatahi, Tongariro National Park, New Zealand. Wood and peat were radiocarbon dated and annual rings in stump cross-sections counted for growth-rates. Comparative measurements were made in three plots in the modern subapline forests.

Stem densities and basal areas in the ancient forest were within the range of the modern subalpine forest samples. The conifers Halocarpus biformis, Phyllocladus aspleniifolius var. alpinus, and Libocedrus bidwillii trees were present in both sets of samples, but in different proportions. Modern forest plots also contained Podocarpus hallii and angiosperm trees. Tree-line forest, composed predominantly of Halocarpus bidwillii, reached 1340 m by c. 10 000 cal. BP and c. 1390 m by 8700 cal. BP. These early-Holocene H. bidwillii trees were slower-growing and longer-lived than the same species in current subalpine forests. From c. 5400 to 3800 cal. BP woody vegetation was again preserved. The current irregularly depressed timberline appears to have been initiated before the Taupo eruption (c. 1700 cal. BP). Charcoal indicates infrequent fires in the subalpine bogs and scrublands in post-Taupo time.

Keywords

Timberline, subfossil wood, climatic change, forest history, vegetation structure, subalpine, Taupo Tephra, Tongariro National Park, New Zealand, Holocene.