Local government strategies and policies aimed at increasing tree planting and canopy-cover have become a familiar feature in many cities. However, the role of private urban land areas in a city’s ambitious plans to retain and increase the number of trees and canopy-cover is usually overlooked. In 2019, the University of Melbourne was funded by Horticultural Innovation Australia and partnered with a reference group of local experts, including academics, local government and industry partners, to investigate the mechanisms (regulations and incentives, or “sticks and carrots”) that cities have to retain, protect, and plant trees in private lands.
This academic literature review forms the first milestone of this project. The review highlights the importance of private property rights and planning laws for determining how cities influence what happens to trees on private land. Most urban jurisdictions where private property comes with strong rights and planning laws based on a hierarchical, top-down model, cannot protect trees over an owner’s right to protect their interests which may involve tree removal. However, many Canadian, Australian, US, and European cities have created local laws to protect private trees from being removed or altered. These provisions include regulatory mechanisms, such as requiring tree removal permits, maintaining significant tree registries, applying compensatory value formulas, or requiring arborist reports or building standards, as well as educational and social mechanisms, such as sponsoring volunteer programs and tree-give-away programs.
Some researchers have argued for jurisdictions to remove strict individual tree protections (i.e., those that protect specific trees to be removed, as in significant tree registries, or blanket laws that protect all trees from removal or alteration) because they are not effective. To support this, they have highlighted their limited coverage, such as exclusion of major land uses and medium/small trees, and the high approval rates of tree-removal permits. Enforcing existing regulations continues to be a challenge for many local governments. The effectiveness of existing regulations is dependent on the ability, willingness, and resourcing capacity of the authority that enforces it. Researchers have lauded the use of other mechanisms, such as education, to help protect trees in private lands. However, not only have these mechanisms not been described adequately, but their effectiveness has not been directly measured.
Only a few empirical studies have assessed the effectiveness of tree-protection laws in terms of increased tree numbers or tree-canopy cover. The usual approach is to compare tree-cover or tree numbers among cities with and without these protections between two points in time. These studies have shown mixed results. In the US, tree-protection laws appear to be effective, which means that cities with tree-protections have increased or retained tree-cover over two points of time. However, in other contexts, tree protections are not as effective, since increased tree-cover cannot be explained solely by the effect of these protections. The different context of cities and the different types of tree-protection specifications makes this type of research difficult to conduct.
This research will be complemented by a review of progressive case studies, mining information from non-academic sources, and by a synthesis of the opinions and experiences of international experts on the efficacy of tree-protection mechanisms through interviews and international workshops carried out in during 2019.