Trees and ecosystems

An ecosystem consists of all of the living organisms in an area and the interactions between them and the physical environment. New Zealand has a wide range of ecosystems, and trees play a major role in many of them.

Since many of our trees are unique, so are the ecosystems they help to create. The ecology of New Zealand has developed in isolation from the rest of the world, and some of the oldest species are recognisable as descendants of Gondwanaland. For example, New Zealand is famous for its podocarp forests. These forests create ecosystems that contain the highest biodiversity of any terrestrial (land) New Zealand ecosystem. The 17 New Zealand podocarp species include trees such as kahikatea, miro, rimu and tōtara. Over time, many of these species have become uniquely adapted to life in New Zealand and are now endemic.

As the trees and other plants have become adapted to life in New Zealand, so have animals. Many of our animals and plants have co-evolved – they have developed adaptations that mean they rely closely on each other.

Energy transfer

All ecosystems involve the transfer of energy. Energy flows into terrestrial ecosystems as light energy from the Sun. Trees catch this light energy and convert it into chemical energy through photosynthesis. The big canopy trees are specifically designed to capture light at the highest levels of the forest. Smaller trees tend to grow on the edges of an established forest or have leaves designed to make the most of the light available underneath the canopy.

Energy transfer continues in the form of food webs. Many animals such as birds, lizards and invertebrates depend on plants for their food.

Mutualism

Native birds interact with trees in a mutualistic way – they both benefit from the relationship. While the birds receive food – nectar or fruit (usually berries) – from the trees they visit, the trees benefit by having other organisms help with pollination and/or seed dispersal. Many native trees cannot perform these processes without the help of animals, and in New Zealand, this is mainly birds.

Over 70% of plants in New Zealand forests have fleshy fruit. Many of the seeds within these fruits have coats that must be weakened by chemicals as they pass through a bird’s digestive system before they will germinate. After being eaten, seeds will often be dropped far away from a tree’s location, enabling the tree to grow in a new area. Trees such as miro, pūriri, tawa and taraire rely almost solely on the kererū for seed dispersal. The seeds within smaller fruit are often able to be dispersed by tūī and bellbirds.

In some cases, particular plants rely on specific species of animals for pollination. For example taurepo (Rhabdothamus solandri), an understorey shrub in the bush, relies on bellbirds (korimako) and stitchbirds (hihi) for pollination. Native mistletoe species depend on bellbirds and tūī. As these birds have declined in numbers, so have these plants.

Stratified habitats

New Zealand native forests – ngahere – are complex ecosystems. Trees offer shelter and nesting materials for birds. They also provide habitats for many smaller species such as lizards and thousands of invertebrate species.

Inside the ngahere, there is often a diversity of trees, providing the structure of the forest. A healthy forest will have many layers of plants – from seedlings and small plants such as ferns and mosses on the ngahere floor to the tall canopy trees at the top. This layering is called stratification. The different layers in the ngahere often create different microclimates, providing specialised habitats – almost mini-ecosystems for a range of different species.

Forested wetland systems have zones that perform similar functions.

Trees and insects

One unique ecosystem in the beech forests is formed around honeydew. Honeydew is a sweet, sticky substance produced by small scale insects. It is an important part of a complex food web that supports fungi, insects and birds like the kākā and tūī.

Insects are important because they provide food for a range of other organisms, pollinate plants, form soils and help with nutrient cycling.

Creation of soil

Around 25% of everything alive on the Earth uses soil as a habitat. Soil is made of inorganic matter (such as minerals), humus (leaf litter and other organic matter) as well as billions of microorganisms. Trees depend on soil for air, water, minerals and anchorage, but they are also the main contributors to the leaf litter that is eventually broken down and decomposed by insects and microorganisms and added back to the soil.