Forest ecosystems
When people first arrived in New Zealand about 800 years ago, forests covered most of the country.
These forests are dominated by trees unique to New Zealand. They are now much reduced in area since human settlement, especially in the lowlands, but still cover much of the landscape. Over time, these forests have provided important resources for people, from food, timber and water supplies to tourism. Public campaigns to protect and conserve forests have been important in New Zealand’s political development.
Forests change in composition and structure. Some changes result from human activities: introduction of alien plants, animals and pathogens. Other human influences are those of past clearance and pervasive effects of climate change. Understanding how these novel influences affect forests and what might be done to mitigate their effects is not straight-forward. Novel influences need to be evaluated in the context of natural agents of change in forests. New Zealand’s forests continue to adjust to major past disturbances, such as glaciation that ended about 12,000 years ago, or the effects of volcanic eruptions, such as the Taupo eruption, the world’s largest of the past 2000 years. Mountain building and tectonism, erosion, flooding and cyclones all influence forest pattern. All occur within the influences of climate, underlying geology, and soil nutrient availability and soil biota.
New Zealand’s forests are often composed of very long-lived trees, sometimes over 1000 years old. Therefore understanding change in forests requires research based on long-term records. This can be achieved using pollen records: these can reveal whether current observed changes have occurred in the past, before the influence of humans. Tracking change in forests can also be achieved using areas of known disturbance history, such as forests on sites that differ in age since retreat of the Franz Josef Glacier. Here limitations to forest growth and where palatable species dominate the influence can be linked to change in soil nutrients and biota over time. Decadal-level records of change in New Zealand forests can be determined through repeated measurements of tagged trees. New Zealand has a world-leading network of permanent forest plots throughout the country and many have over 30 years’ of records of tree growth and mortality. Using these records it is possible to project nationally the population trends of tree species known to be palatable to introduced animals such as deer and possums. New Zealand also has internationally outstanding records of seed production by several canopy trees. These data inform models of the climatic cues that initiate heavy seed production and which can instigate outbreaks of introduced seed predators, especially mice and rats.
Experimental studies are also important in determining effects of introduced animals and plants. For example, replicated deer exclosures can reveal when and where palatable tree species can be expected to regenerate and what are the effects of altered tree communities on soils and soil biota. This research can help inform managers about where should be priority areas for introduced animal control and when these changes might be reversible.
A synthesis across the various pieces of research takes place in models at local scales (e.g., SORTIE/NZ) and national scales. These models will inform land managers and owners to make realistic projections for changes in forests. Models will help inform when, or if, active management (such as wild animal control) is necessary and what it might achieve, as well as identifying stages of forest development and tree life histories, or which parts of the forested landscape, would be most likely to benefi