In this Latitude post, we get to know Robbie Mallett, a new member of the PLOS Climate editorial board. About Robbie After…
Jihwi Jang, PhD student, University of Canterbury, New Zealand
The importance of forest conservation in global efforts to fight climate change is recognized in an article of the Paris Agreement, and the ecosystem services provided by urban forests in particular have gained prominence in the era of the Covid-19 pandemic. For many urban dwellers, restrictions on travel and changes in lifestyle habits have fundamentally altered their relationship with urban green spaces.
In the case of New Zealand, the role of urban trees in greenhouse gas (GHG) mitigation has been recognised in the country’s Zero-Carbon Act, which was enacted in response to the Paris Agreement. However, despite this legal recognition, there has been relatively little scientific research into the contributions of trees- and especially indigenous trees- to urban ecology and ecosystem services.
The decline of indigenous trees in New Zealand began with the arrival of Maori pioneers around 1000AD, with deforestation associated with land-use conversion. From around 1840, with the arrival and establishment of the first European settlers, the loss of natural forest intensified as settlements were developed and large-scale agricultural activity began. The forested natural estate in New Zealand is now dramatically reduced compared to its original pre-settlement extent.
Since the 1990s, exotic tree species have had a significantly higher afforestation rate and reached higher forest coverage in New Zealand. Historically, the use of exotic tree species has been preferred in urban garden planning and in the timber industry. The exotic tree species Pinus radiata now accounts for a considerable majority of the total forestry plantation area across New Zealand. The introduction (and escape) of exotic species in New Zealand cities has also contributed to severe declines in the size and connectivity of urban populations of indigenous trees over time.
Urban development is the major driver of the fragmentation of urban tree populations. The overwhelming majority of New Zealanders currently reside in urban environments, and the drift of the human population towards urban centres is expected to continue over the coming decades. The GHG inventory reports prepared by New Zealand’s government have identified the energy and transport sectors as the key drivers of increased GHG emissions since 1990. Emissions from road transportation grew steadily over the past two decades as the private vehicle fleet expanded. Private vehicle deterioration is also likely to have a profound effect on New Zealand’s GHG emissions, with the average age of New Zealand’s vehicle fleet thought to be older than that of most OECD countries. Urban areas have thus come to experience very high emissions and concentrations of GHGs.
In one of the few studies to explore the carbon sequestration potential of indigenous trees, focusing on Kānuka (Kunzea ericoides), Karaka (Corynocarpus laevigatus), Tarata (Pittosporum eugenioides) and Kohuhu (Pittosporum tenuifolium) growing in an Auckland urban park, researchers estimated potential sequestration to be in the range of 70–290 kgC with a carbon concentration of 44.9–49.6% . Even if these sequestration rates are lower than for exotics such as Pinus radiata, New Zealand indigenous trees may have significant potential to mitigate GHGs if they are competitive in long-term woody succession.
There has been continuous debate as to whether indigenous trees are resilient to urban abiotic stresses and whether they should be planted in cities rather than exotic trees. These societal and ecological debates have revealed a lack of knowledge about how indigenous tree species growing in urban areas of New Zealand will respond to projected GHG concentrations and climate change. Given this lack of a strong evidence base, it is difficult to say whether or not they hold substantial promise for ecosystem service provision over time. A concerted effort to develop a better understanding of their tolerances to the extreme conditions of urban environments would allow us to conserve and deploy indigenous trees to maximum advantage.
- Schwendenmann, L., Mitchell, N.D. 2014. Carbon accumulation by native trees and soils in an urban park, Auckland. New Zealand Journal of Ecology 38: 213-220.