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The following relationships describe how species interact and serve as biotic factors which affect plant growth.
1) Competition - Plants compete for sunlight and water, both required for photosynthesis. As described above, some behavioural adaptations account for competitive strategies such as the broccoli shaped canopy trees blocking out the sunlight for plants below. Plants also compete for water and the roots of neighbouring trees battle for nutrient rich soil and area where they can absorb rainfall. In one extreme version of competition, the kauri actually loses its leaves which are highly acidic, making the soil around them impossible for seeds of other plants to grow.
Other forms of competition in a forest ecosystem include herbivory, which is when animals eat parts of a plant, which is damaging to the plant, e.g.: leaf damage reduces the surface area available to capture sunlight for photosynthesis.
Certain plants have adaptations to help them avoid herbivory, for example, the physiological adaptation of producing secondary compounds that make the leaves bitter to taste.
2) Commensalism - Some plants benefit from relying on other plant species without causing harm to the neighbouring plant. Epiphytes often arrive at their location through the help of birds. Seeds high in the tree-tops germinate and the plants growing there benefit from greater light intensity. The host tree is not harmed in this type of relationship, nor do they benefit.
3) Amensalism - Unlike commensalism, one organism is harmed while the other is not affected at all. A key example of this is the relationship between canopy trees and the sub-canopy, shrub, and forest floor plants below. The taller trees have access to full sun, they also shade out all the other plants and trees below. A lack of light can cause issues with the rate of photosynthesis. To combat this understory plants and trees have structural adaptations that allow them to tolerate low light levels.
4) Mutualism - This is a relationship where both species benefit from their interactions. In a forest ecosystem, this is particularly evident with insects and birds and plants. Most plants and trees in New Zealand are pollinated by insects and birds. Insects and birds are also important for seed dispersal. Seeds generally have to travel through the gut of a bird or an insect undamaged to then germinate on the forest floor. Examples of this include tree weta dispersing kotukutuku (tree fuschia seeds) and kereru dispersing karaka and tawa seeds.
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