The nature of the lower North Island floristic gap

Abstract

Patterns of regional endemism and disjunction of higher plants in the lower North Island are reviewed in relation to the plants' habitats. A redefinition of the lower North Island floristic gap emerges. A northern boundary in the southern central North Island uplands replaces the previous one at 39°S latitude. Southern boundaries occur either in Cook Strait or in southern Wellington-southern Wairarapa. Most endemic and disjunct species defining the gap occur in non-forest habitats. Previous explanations for these distribution patterns centre either on the disruptive effects of glacial ice and harsh climates of the Last Glaciation or on the tectonic modification of the New Zealand land mass since the Oligocene. It is suggested that regional discontinuities in the availability of non-forest habitats account for many disjunctions. Furthermore, patterns of endemism and disjunction most closely equate with the tectonic alteration of lower North Island. In particular, fault displacement of large land masses, obliteration of habitats by marine transgression and the regionally differential effects of mountain building have acted to preserve habitats of elements of an old flora in technically stable regions.