The local climate has to be considered in devising strategies for restoring rainforest. In our river valley, night frosts are common during winter. Cold air drains down from the surrounding hills and settles in the valley, making a frost hollow that is colder in the valley than on the hills, especially on nights that are clear of cloud. This
effect (the “temperature inversion”) also condenses moisture as mist.
Even a mild frost (-1oC
a metre or so above ground level) can kill the foliage of seedling rainforest
species. Species such as lilly pilly (Acmena smithii), red
ash (Alphitonia excelsa) and euodia (Melicope micrococca) benefit
from the shelter of hardier natives, such as water gum and wattles, because they keep the frost off.
While any open grassy areas tend to be frosty, this changes once they have trees to keep the warmth in and frost out. Each tree contains a great deal of water, which is an effective medium for storing heat - it warms during
the day then releases the warmth at night. As well as that, the tree's foliage slows
down the cold air which streams down from the surrounding hills at night.This same foliage shields the ground from radiating its heat to the cold sky.
In the summer, the protective canopy of trees has the reverse effect. The shade protects seedlings from being scorched by sunlight and provides them with a cooler and more humid environment. It's hardly surprising then that rainforest seedlings regenerate well under a protective canopy, while they struggle to survive in open fields. This is why we have
planted “pioneer” species such as wattles in the more open areas.
Frost on the leaves of a water gum Tristaniopsis laurina. This is one of the plants on our site that is most resistant to frost. Unfortunately for us, swamp wallabies love to browse on its seedlings, so we have to protect them with tree guards.