Winter leaf loss in the New Zealand woody flora

Abstract

Approximately 27 species (c. 5%) of the New Zealand woody flora have a marked loss of leaves in winter, although only 10 species are consistently fully deciduous and no extensive vegetation type is dominated by them. There are no summer deciduous species. The deciduous habit appears in most cases to have evolved independently within New Zealand from either ancestors or immigrants with short leaf life spans. New Zealand deciduous or semideciduous species typically lose substantial numbers of leaves throughout the growing season, and leaf loss is often gradual throughout winter but accelerated by frost. The deciduous habit in New Zealand tends to be a leaf- or shoot-level feature reflecting short leaf life spans and winter stress on individual shoots, rather than a systematic feature of the canopy-level organisation of the plant. Degree of deciduousness varies with taxon, plant age, climate, and soil fertility. A physiological study of co-occurring fully deciduous (Fuchsia excorticata) and semideciduous (Aristotelia serrata) trees in the South Island established that higher growing-season productivity of Fuchsia compensated for carbon gain forgone over winter. However, Fuchsia was significantly favoured over Aristotelia only on the coldest sites. Aside from some inland basins of the South Island, New Zealand has milder winters than those that favour deciduous taxa globally, and this is the primary reason for the low incidence of deciduous species. Even so, a number of deciduous species are common throughout. New Zealand deciduous trees and shrubs are typically fast growing and are characteristic of seral or forest marginal habitat on nutrient-rich soils developed in recent alluvium or debris. The relatively low nutrient status of most New Zealand forest soils makes a deciduous phenology, with its necessarily high turnover of nutrients, less competitive than a nutrient-conserving evergreen phenology. Deciduous species are often also divaricating, or have close relatives that are, and share a preference for nutrient-rich soils with this growth form. This strong relationship between the two habits suggests both are, in part, adaptations to stressful climates by plants with high-productivity leaves. © 2004 Taylor & Francis Group, LLC.