New Zealand Forest to Alpine Transitions in Global Context

Abstract

ABSTRACTNew Zealand high-altitude tree limits are formed either abruptly by evergreen Nothofagus or by low forest of more frost-tolerant small trees reaching similar maximum altitudes. Whereas most tree limits are contiguous with low-growing alpine vegetation, in New Zealand a belt dominated by tussock grasses intervenes that is vulnerable to invasion by hardy introduced trees and seems ecologically equivalent to fire-maintained high-altitude tropical grasslands. New Zealand tree limits coincide with warmer growing-season temperatures than other tree limits, including deciduous Nothofagus in the southern Andes. They also correlate with coldest-month mean temperatures around 0°C, in accordance with the limits of broadleaved evergreen trees globally, unlike north temperate subalpine trees that withstand extreme winter cold. Adverse environments lead to krummholz that in temperate regions can form an attenuated belt above the forest limit, but in New Zealand Nothofagus krummholz develops only at or below the forest limit, in accordance with absence of Nothofagus seedlings beyond a few meters above the forest limit. The relatively low altitudes attained by New Zealand trees are related to isolation and the recent uplift of high mountains, and the differentiation between Nothofagus forest and low forest reflects historical and geological events.