Keegan Russell, of Hastings Intermediate, asks :-
Why are most of our native trees evergreen?
Janice Lord, a botanist at the University of Otago, responded.
Autumn highlights one of the most profound differences between native New Zealand trees and trees from temperate northern hemisphere regions. While visitors and locals alike marvel at the glorious colours of poplars, maples and many other northern hemisphere deciduous trees, our native trees mostly don't look that different.
Leaf fall in winter-deciduous trees is generally triggered by a combination of colder temperatures and shorter day lengths. The colour change we love occurs as a result of pigments and other compounds being dismantled or transported back into the stem as the plant actively cuts off its leaves. In regions where summers are predictably warm and suitable for growth and photosynthesis, but winters are predictably unsuitable for growth, leaf fall is thought to be economical as it reduces damage from frost, wind and snow and also conserves water, as leaves lose water through their pores (stomata).
Evergreen trees in cold climates, such as pines, have small leaves that are well protected with a thick waxy coating, and branching patterns that help shed snow. Our oceanic climate means New Zealand summers are not always reliably warm and New Zealand winters are not always consistently too cold for growth. This is thought to be the main reason why so few native trees have evolved to drop all their leaves in autumn - they might miss a chance to grow!
However, New Zealand trees don't keep their leaves forever; some species replace them more or less continuously, but other species such as the southern beeches (Nothofagus) have distinct spring or autumn periods of leaf fall. If you want to know when your favourite tree changes its leaves, mark some leaves with small dot of paint on the underside and watch them. You might be surprised to find our trees do drop leaves in autumn, just not all at the same time.