Population dynamics of dominant NZ trees
Forests were the dominant vegetation cover of prehistoric New Zealand and no other ecosystem arouses the same public concern about protection of natural values.
However, forests are constantly changing, often because of prolonged response to natural and human-related disturbances. For example, forests in the central North Island are still changing as a result of influences of the Taupo eruption of c. ad 130, while those in the western South Island are constantly adjusting to movements of the Alpine Fault. Biodiversity values in forests are threatened by human-related disturbances including introduction of mammalian herbivores and forest fragmentation. The introduced herbivores cause damage ranging from subtle changes in soil biota, to pervasive changes in understorey composition, to total collapse of canopies. Responses to herbivore control are often unpredictable because they are over-ridden by poorly known background processes. Our research focuses on some key processes that drive changes in populations of New Zealand's dominant trees. The aim is to provide a national framework for understanding of the complex drivers of change in forests.
Understanding background changes in tree populations requires investigations at different scales in time and space. Our research has determined changes in tree populations at the hundreds-of-years scale, revealed through pollen profiles in wetlands and forest soils, to decadal levels, revealed through repeated measurements of permanent forest plots. Resolution of change in forests takes place from individual stand-level studies to national overviews of tree populations informed by widespread forest plots. Our research addresses various stages of tree life-histories, from seedlings and what factors determine their establishment and onward growth (e.g. herbivory; competition; light and nutrient availability; interactions with mycorrhizal fungi) to adults and how their growth and mortality is related to soils, competition and natural disturbance. New Zealand has data available from permanently marked forest plots with tagged individual trees that can be used to determine the extent of regeneration of dominant forest trees. These forest plots can be used to interpret tree populations from local to national scales (e.g. using data from the Ministry for the Environment's Carbon Monitoring System), with repeated measurements allowing determination of tree growth and population trends.
Managers of New Zealand's indigenous forests, including the Department of Conservation, tangata whenua, private individuals and Trusts, want to know if there are population imbalances and likely long-term consequences of change. They especially want to know if intervention is required to maintain populations of some dominant trees, e.g. those palatable to introduced mammals or those dispersed by bird species that are declining, such as kererū. Our research can place such trends in a long-term context - for example, have such imbalances occurred in the past before human settlement? Our research can help put imbalances in perspective - are imbalances local or widespread? Our research also puts the consequences in a whole-ecosystem context - do increasing populations of unpalatable tree species with poor litter quality lead to conditions unsuitable for future recruitment of palatable trees? We work closely with indigenous forest managers to ensure that the research is used to inform management.