Associate Prof Janice Lord, of the University of Otago botany department, said warmer temperatures in March and early April, effectively tricked deciduous trees into thinking it was still summer, despite the days growing shorter and the season officially being autumn.
The warmer temperatures were left over from the marine heatwave which brought Otago one of the hottest summers on record, she said.
Niwa meteorologists said summer temperatures were 2.3degC above average.
The previous hottest New Zealand summer on record was in 1934-35 when the temperature was 1.8degC above average.
The warmer-than-average Tasman Sea was a signature of La Nina, and was associated with higher-than-normal air pressure over the region during the late spring and early summer.
It prevented mixing of deeper, cooler sea water with warmer surface water.
In addition, warm winds from the north pushed warm water toward the country from the sub-tropics.
Prof Lord said the recent burst of cold weather had signalled to trees in the area, that winter was not far away.
She said many people thought the red and yellow colours of autumn leaves was the result of the leaves dying.
``What is actually happening is, in response to temperature and light cues, the photosynthetic machinery is dismantled and all the nutrients from the leaves is stored in the tree over winter.
``The pigments are produced in the leaf - the reds and the yellows - to stop insects from over-wintering on the tree.
``Reds and yellows are less visible to insects. It's a deliberate thing the tree is doing.''
She said the trees were basically giving themselves a ``detox'' before winter.
``They're packing away all the goodies and storing them away, and then during that phase when they're a bit vulnerable, they're making themselves less attractive to insects.
``Otherwise they'll end up with aphids laying eggs and over-wintering on the trees and attacking the buds.
``It would mean they would reach spring in worse shape.''
