Urban development, land sharing and land sparing: the importance of considering restoration

1. At present, there is limited knowledge of how best to reconcile urban development with biodiversity conservation, and in particular whether populations of wild species would be greater under low-density housing (with larger gardens), or high-density housing (allowing more area to be left as undeveloped green spaces). The land sharing/sparing framework – originally developed in the context of farming – can be applied to address this question.
2. We sampled the abundance of trees in the city of Cambridge, UK, along a gradient of human density. We designed different scenarios of urban growth to accommodate the human population predicted in 2031. For each scenario, we projected the future city-wide tree population size and quantified its carbon sequestration potential. We also considered, for the first time in an urban sharing-sparing context, the implications of habitat restoration on degraded urban green space.
3. We found that the density of most native and non-native tree species is presently highest in areas of low human density, compared to both higher-density areas and green space (which is largely maintained with few trees). However, restoring woodland in green spaces would lead to far greater densities of native trees than on any existing land use. Hence provided >2% of green space is restored, native tree population sizes would be larger if urban growth followed a land-sparing approach. Likewise, carbon sequestration would be maximised under land sparing coupled with restoration, but even so only a maximum of 2.5% of the city’s annual greenhouse gas emissions could be offset.
4. Whilst both tree populations and carbon storage thus appear to benefit from land-sparing development, the risk that this might widen the existing disconnect between people and nature must also be addressed – perhaps through a combination of adding housing in low density areas while ensuring these are in close proximity to high-quality green space.
5. Synthesis and applications. In regions which have already been cleared of intact habitat, a combination of land-sparing urban development with the restoration of green space could accommodate urban population growth whilst dramatically improving the existing status of local tree populations. Where cities are expanding into intact habitat, the merits of urban development by land sparing may be even more pronounced. Studies in such regions are urgently needed.

1. 当前学界对于如何最优协调城市发展与生物多样性保护仍知之甚少，尤其是低密度住宅（搭配更大面积私家花园）与高密度住宅（可预留更多未开发绿地）两种模式下，野生物种种群规模是否会存在差异。原本针对农业场景提出的土地共享/土地预留（land sharing/sparing）框架，可被用于解答这一问题。
2. 研究团队沿英国剑桥市的人口密度梯度，对城区树木的丰度开展了采样调查。针对2031年预测的城市人口规模，研究团队设计了多套城市增长方案，针对每套方案推演了全市未来的树木种群规模，并量化了其碳固存潜力。此外，本研究首次在城市土地共享/预留场景中，探讨了退化城市绿地的栖息地修复所带来的影响。
3. 研究结果显示：相较于高密度居住区与以少量人工管护树木为主的绿地，当前多数本土与外来树种的种群密度在低人口密度区域最高。不过，在绿地中修复林地可使本土树种密度远高于现有任何土地利用类型。因此，只要对超过2%的绿地进行修复，采用土地预留模式的城市发展策略将能获得更大的本土树种种群规模。同理，结合栖息地修复的土地预留模式可最大化碳固存效益，但即便如此，其每年最多仅能抵消该市2.5%的温室气体排放。
4. 尽管树木种群与碳储存均似乎能从土地预留型开发中获益，但这一模式可能加剧当前民众与自然之间的疏离感，这一风险同样需要应对——或许可通过在低密度区域开发住宅，同时确保这些区域紧邻高品质绿地的组合方案来解决。
5. 综合与应用：在原生完整栖息地已被开发殆尽的区域，将土地预留型城市开发与绿地修复相结合，既可容纳城市人口增长，又能显著改善当地树木种群的现有生存状态。而在城市扩张进入原生完整栖息地的区域，土地预留型城市开发的优势可能更为显著，此类区域亟需开展相关研究。