Vegetation Mortality in the Kaimai Ranges, North Island, New Zealand!

Areas ofvegetation mortality have been recognized in the upland forests of the Kaimai Ranges, North Island, New Zealand. The mortality is not confined to any particular vegetation type, but occurs above a critical altitude that differs at different points along the ranges. The mortality is episodic and has occurred at intervals over a period of several hundred years, with severe droughts implicated as the causative factor. The numerous mortality events have resulted in forests of many age classes where trees do not reach potential maturity. The coincidence between the occurrence of cloud and the mortality is suggested to make the forests drought-sensitive through poorly developed root systems caused by prolonged soil waterlogging. VEGETATION MORTALITY HAS BEEN EVIDENT in the Kaimai Ranges, North Island, New Zealand, for at least 40 yr, but it was only reported after a period of severe erosion triggered by an increased storm frequency (Dale and James 1977). Initially, it was suggested that introduced animals were responsible for both the vegetation mortality and the erosion, but in the ensuing investigations this was largely refuted and other causes sought (Jane and Green 1983a, 1983b). The Kaimai Ranges are located in the northern North Island, range in altitude from 100 m to 950 m, and form three distinct prominences, Te Aroha, Te Rere, and Te Hunga, in which study areas were established (Figure 1). Lowland temperatures average 17°C in summer, 7°C in winter, and maxima rarely exceed 30°C in lowland areas or 20°C at upland sites. Rainfall ranges from 1500 mm in the foothills to the west to over 3000 mm on the southern prominences. Fog occurs on over 200 days a year and often persists for much of the day (Jane and Green 1983a). The average cloud base is at about 150 m I This paper is a partial summary of research carried out for a doctorate thesis at the University of Waikato by G. T. Jane. Manuscript accepted 5 October 1983. 2 Waikato University, Department of Biological Sciences, Hamilton, New Zealand. below the ridge crest and differs in altitude among study areas. Upland soils are waterlogged for much of the year and rarely fall below field capacity (Jane and Green 1983a). Basement rock throughout the study areas is andesite or dacite overlain by fine-textured tephra up to 1 m deep. Soils in upland areas (derived from andesitic lava and breccia) are stony clays less than 1 m deep and low in bases and phosphates; these are further reduced in fertility by the presence of allophane.