The final Editor’s Choice of 2017 is written by Associate Editor, Joseph Bennett. The article chosen is Urban development, land sharing and land sparing: the importance of considering restoration by Collas et al.

The land sparing versus land sharing debate is one of the best known and most controversial topics in conservation biology. Advocates for land sparing argue that ecological and human needs are best balanced by intensifying human use in some places and excluding it (or strictly controlling it) in others. Advocates for land sharing advocate allowing mixed use over wider areas as the best way to conserve biodiversity while meeting human needs.

Most of the debate has centred around agricultural production, while its applicability to urbanisation – one of the most important environmental challenges of the 21^st century – has been less explored. This month’s Editor’s Choice article by Collas et al. helps to rectify this issue, through a detailed examination of tree distributions within an urban centre, and projection of carbon sequestration by urban trees in various urban expansion scenarios.

In urban areas, the sparing-sharing debate hinges on whether cities should densify (while sparing some green space from development) or expand, allowing less dense development. Collas et al. examine these contrasting strategies using six urban growth scenarios (3 sparing, 3 sharing) for the city of Cambridge, UK, designed to accommodate the expected increase of 27,000 people by the year 2031. Specifically, they measure the abundance of native tree species and carbon sequestration along a gradient of human population density, and project the trends in these variables for each scenario. A key innovation of their study is that they include the possibility of restoring open green space to woodland, using nearby woodlands as reference sites for native tree density and carbon sequestration.

They find that native urban trees have relatively low density both in areas that are highly populated, and in dedicated green spaces. Projecting into the future, the answer to whether sparing or sharing scenarios would provide greater native tree density and carbon sequestration depends on whether green spaces can be restored to woodlands. If green spaces cannot be converted to woodland, then a land sharing approach of building relatively low density housing on present green spaces appears optimal for native trees and carbon. However, if such spaces could be converted to woodland, a sparing approach whereby current housing areas are densified is optimal. Indeed, if all undeveloped green space could be converted to woodland, populations of native trees in the city could be over 12 times higher than current levels.

The authors recognise that complete conversion of green spaces to forest is likely unrealistic. Urban green spaces have many human uses, a large portion of which are incompatible with forest. Thus, the authors also investigate the implications of partial restoration to forest, and compare projected native tree populations for land sharing and land sparing approaches. They find that only 2% of green space needs to be restored to woodland for the sparing scenarios to outperform the sharing scenarios.

These results have fascinating – and nuanced – implications. In general, if modest restoration of green spaces to forests is allowed, then a city can accommodate its growth while conserving or growing native tree populations and carbon sequestration. This suggests that a flexible strategy of densification while restoring some green space is a better strategy than allowing less dense housing to proliferate. However, such a strategy would need to carefully balance the benefits of increased trees with potential impacts on public use, as urban green spaces are cherished by the public, and in some cases urban forests can be perceived as being unusable or unsafe.

Although land sparing approaches did allow native tree populations to expand, none of the scenarios they examined could substantially offset the city of Cambridge carbon emissions. There is no single strategy for offsetting the environmental impact of urban population increase. Encouraging a flexible approach to using and restoring urban green spaces is just one of many necessary strategies for designing truly ‘green’ cities.

The full article, Urban development, land sharing and land sparing: the importance of considering restoration is available in issue 54:6 of Journal of Applied Ecology.