Bronson Davey, Balclutha Primary School, asks :-

Why do some trees lose leaves in Autumn and some trees don’t?

Roy Edwards, a horticultural botanist at Lincoln University, responded.

Trees that lose their leaves in autumn are called deciduous trees and are responding to the environmental signals of shorter day length and cooler temperatures. Most deciduous trees growing in New Zealand have been brought here from other countries and are termed exotics. Very few New Zealand native trees are deciduous.

New Zealand species have evolved in a predominantly mild climate, influenced by both mountains and moderated by the sea that surrounds it. Much of New Zealand has reliable rainfall and tree growth can continue for much of the year. Cool temperatures and short days will still influence growth rates.

Exotic deciduous trees have evolved in some countries where there are large land masses, with climates that are not moderated by the sea. Think of continental Europe, Asia and North America. In many of these areas there are much sharper, more clearly defined seasons than we experience in New Zealand and the climate is more predictable.

As trees evolved in different areas, it seems likely that the response by trees to drop leaves in autumn or not, is genetically controlled. If winter is too cold or dry for growth to occur, it is probably in the species best interests that leaves are shed and the tree enters a dormant state until conditions for growth resume again in spring. Evergreen trees generally have evolved thicker leaves with more protection than deciduous trees and accumulate sugars and starches more slowly. Some other trees have evolved in climates that have very marked wet or dry seasons and shed leaves annually at the start of the dry season.

While genes control leaf fall, day length and temperature are the triggers. It would be interesting to grow some deciduous trees in a greenhouse over a long period with warmer temperatures and long hours of daylight and observe their response.