New Zealand has encouragingly “high” and “rich” diversity among its urban trees and Canterbury maybe leading the way, a new study has found. A snapshot of New Zealand plant nurseries showed 863 different tree species were listed for sale, according to the study led by Dr Justin Morgenroth of the School of Forestry at the University of Canterbury. Source: Stuff NZ
By comparison, a similar study in Utah found 262 tree species for sale. In the city of Minneapolis-Saint Paul in Minnesota, 387 species were listed. In Los Angeles County, it was 562 species. About 20% of the trees listed for sale across New Zealand were natives, the study found.
This could be “considered as relatively high”, Dr Morgenroth and his colleagues wrote in the scientific paper, but there was limited international research on the point.
Canterbury’s possible prominence arose from a statistical finding – the region’s nurseries listed 15 native trees that were not listed for sale in any other province. The nearest regions listed four unique native species.
Canterbury nurseries also listed 60 non-native trees that were not grown elsewhere, compared to an average of about 20 for other provinces.
“Growers are probably responding to the environment in which they sell,” Dr Morgenroth said. Canterbury had a diversity of ecologies – coastal Sumner to alpine Arthur’s Pass and nurseries had to predict and meet that demand for those places.
The study gathered the trees listed for sale by 75 nurseries in 2017-18. The purpose was understanding urban tree diversity, how it could be increased, and the roll that nurseries played.
There was an “abundance of evidence” that a large proportion of trees were sourced from nurseries and many of them were commercial enterprises. Market forces influenced tree diversity.
“Popular species that will sell (based on historical sales data) and those that are easy to grow and maintain are commonly produced, whereas unpopular species that are unlikely to sell are largely avoided,” wrote the authors.
“As such, the incentive for nurseries to trial new species can be low and the overproduction of a small number of popular species and cultivars can occur.”
Nonetheless, the 863 tree species listed by New Zealand nurseries a few years ago was “huge” and suggested that urban forest biodiversity was “rich”.
Dr Morgenroth thought native tree planting had increased markedly since he gathered the data. He had a companion study underway to understand how councils influenced tree demand and hoped to repeat the nursery list study in a few years to track changes.
