Urban trees contribute substantially to numerous ecosystem services. Here we quantify the threat to carbon stored by urban trees from increased heat and drought arising from climate change. We use data from tree inventories in 22 European cities, spread across five Köppen-Geiger climatic zones, that record ∼1.2 million trees from 188 species. We calculate species’ climatic niches using global tree distribution data and estimate species-specific thermal and hydraulic safety gaps and margins for each city in 2050 and 2070 using the RCP 8.5 emissions scenario. This scenario provides the best match for emissions to at least 2050 under current and stated policy plans, and highly plausible emission levels to 2100. We then assess the proportion of current carbon storage at risk from changes in temperature (associated with thermal stress) and precipitation changes (associated with hydraulic stress). By 2070 a substantial amount of the current carbon storage in urban trees is projected to be threatened by climatic stress. Average values (depending on the precise methods used for calculating climatic niches) are: 99.96 % - 99.98 % in the cold semi-arid climate zone; 82.97 % - 92.61 % in the humid subtropical zone, 69.72 % - 72.00 % in the warm Mediterranean zone, 44.18 % - 55.06 % in the humid continental zone and 29.60 % - 43.22 % in the temperate oceanic zone – although within each climatic zone risks are lower in some cities. In each climatic zone the vast majority of this threat is associated with thermal stress, with precipitation changes projected to be a comparatively minor threat. Our analyses highlight individual species which are particularly vulnerable to future climatic conditions, and more resilient species that if rapidly planted on mass could improve resilience of urban tree stocks to climate change. Our findings inform the development of climate-ready urban forestry and planning strategies that will facilitate long term carbon storage capacity of Europe’s urban forests, and emphasise the urgency of doing so.

中文翻译：

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气候变化威胁到欧洲城市树木的碳储存


城市树木为众多生态系统服务做出了重大贡献。在这里，我们量化了气候变化引起的高温和干旱增加对城市树木储存的碳的威胁。我们使用来自欧洲 22 个城市的树木清查数据，这些城市分布在 5 个 Köppen-Geiger 气候区，记录了来自 188 个物种的约 120 万棵树。我们使用全球树木分布数据计算物种的气候生态位，并使用 RCP 8.5 排放情景估计 2050 年和 2070 年每个城市特定物种的热和水力安全差距和边缘。根据当前和既定的政策计划，该情景为至少 2050 年的排放量提供了最佳匹配，并且到 2100 年的排放水平非常合理。然后，我们评估了当前碳储存中受温度变化（与热应力相关）和降水变化（与水力应力相关）风险的比例。到 2070 年，预计城市树木中目前的大量碳储存将受到气候压力的威胁。平均值（取决于用于计算气候生态位的精确方法）为： 99.96 % - 99.98 % 在寒冷的半干旱气候区;湿润的亚热带地区为 82.97 % - 92.61 %，温暖的地中海地区为 69.72 % - 72.00 %，潮湿的大陆地区为 44.18 % - 55.06 %，温带海洋区为 29.60 % - 43.22 % – 尽管在每个气候区内，一些城市的风险较低。在每个气候区，绝大多数威胁都与热应力有关，预计降水变化是一个相对较小的威胁。 我们的分析突出了特别容易受到未来气候条件影响的单个物种，以及更具适应力的物种，如果迅速大规模种植可以提高城市树木存量对气候变化的适应能力。我们的研究结果为制定气候就绪型城市林业和规划策略提供了信息，这些策略将促进欧洲城市森林的长期碳储存能力，并强调了这样做的紧迫性。