Optimizing the structure of urban woody plantings for a warming climate: a frequency-weighted exposure framework for woody species in Chongqing, China

Résumé fourni par la source

As climate warming intensifies, woody plants in urban green-space systems are increasingly exposed to heat stress. Climate-adaptive assessment of urban woody species has often focused on species-level thermal tolerance, but system-level exposure also depends on how widely each species is used within existing planting structures. We used Chongqing, a major inland megacity in subtropical China, as a case study to demonstrate a frequency-weighted heat-stress exposure (HSE) framework for urban woody species. By integrating species-specific thermal niche thresholds with plot-level occurrence frequency, we developed a frequency-weighted HSE index to quantify the contribution of individual woody plant species to system-level heat exposure under future climatic conditions. Using survey data from 257 woody plant species across 310 plots in Chongqing, we found that HSE is highly concentrated among a subset of species in the current planting structure of the surveyed woody-plant system. The top 10 species with the highest HSE values accounted for nearly 30% of total system HSE, and the Herfindahl-Hirschman Index (HHI) of exposure concentration was 0.01525. To address this issue, we developed a strategy to optimize patterns of species use while maintaining the current proportions of species from various functional groups (evergreen trees, deciduous trees, etc.). Specifically, we reduced the occurrence frequency assigned to high-HSE species and analytically reallocated the released frequency to lower-deficit species from the same functional groups. Under the default optimization scenario, the optimized configuration reduced total system HSE by 2.02%, Top10_share by 3.26%, and HHI by 0.00254, demonstrating a reduction in reliance on a small number of commonly occurring high-HSE species. This study provides a quantitative basis for species selection, nursery stock procurement, and renewal strategies, supporting the climate-responsive management of woody-plant systems in urban green spaces.

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