Climate is critical for restoring rainforest to the floodplain of Ourimbah Creek. In the 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) kills the foliage of many of our rainforest
seedlings. As a result, seedlings of 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.
The open grassy areas are frosty, but the forested areas are not. The trees keep warmth in and frost out. Much of a tree is water, which 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 shaded seedlings are not scorched by sunlight and they have a cooler and more humid environment. It's hardly surprising then that rainforest seedlings grow much
better under a protective canopy than 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.