Layers of the Rainforest

Skyrail Nature Diary: August 2011

This month we will be looking at the structure of a typical rainforest.

Depending on the type of forest, there are normally three vertical layers, but occasionally there can be more. Unlike temperate forests, most activity in tropical rainforests takes place in the treetops. This is largely due to the close proximity of the trees which block out most of the light below. Light can however, penetrate through the canopy which is known as 'crown shyness' and occurs when the trees do not touch.

Experiments have demonstrated that trees which do have immediate contact start a chemical attack on one another in order to fight for growth. The result is that the trees literally push each other away. As a rule of thumb only 10% of the light at the top gets through to the branches just underneath the canopy, whilst only 1% or less reaches the forest floor. Since access to light drives the ecology in tropical rainforests, all life in the trees tends to divide the airspace into layers. This explains why rainforest trees are typically tall and slender with only limited branching under the canopy. Interestingly, quite a few trees dispense with branches altogether below a certain light level.

The Kauri Pine at Red Peak is a good example of this as there are no branches whatsoever underneath the crown. The top layer, known as the canopy, contains the most life out of all the layers. This is where you find all the mistletoes and epiphytes. The fern epiphytes tend to grow just below the surface of the tree crowns away from the wind. This is to avoid sunburn and to allow leaf litter to fall into the various types of naturally produced baskets which make compost.

Strangler figs often start their lives in these canopy gardens making competition fierce in the canopy. As a result, a number of vines have opted for life in the next layer, the sub-canopy which has a lower risk of dehydration from sun and wind.

Classic examples are the Candleberries, Native Monsteras and various species of grapevines. The forest floor is nearly always devoid of life due to the extreme lack of light however, various shade loving ferns can live here.