Urban forest microclimates across temperate Europe are shaped by deep edge effects and forest structure

Abstract

The urban heat island (UHI) causes strong warming of cities and their urban forests worldwide. Especially urban forest edges are strongly exposed to the UHI effect, which could impact urban forest biodiversity and functioning. However, it is not known to what extent the UHI effect alters edge-to-interior microclimatic gradients within urban forests and whether this depends on the forests’ structure. Here we quantified gradients of air temperature, relative air humidity and vapour pressure deficits (VPD) along urban forest edge-to-interior transects with contrasting stand structures in six major cities across Europe. We performed continuous hourly microclimate measurements for two consecutive years and analysed the magnitude and depth of edge effects, as well as forest structural drivers of microclimatic variation. Compared to edge studies in rural temperate forests, we found that edge effects reached deeper into urban forests, at least up to 50 m. Throughout the year, urban forest edges were warmer and drier compared to forest interiors, with the largest differences occurring during summer and daytime. Not only maximum, but also mean and minimum temperatures were higher at the urban forest edge up to large edge distances (at least 85 m). Denser forests with a higher plant area index buffered high air temperatures and VPDs from spring to autumn. We conclude that urban forest edges are unique ecotones with specific microclimates shaped by the UHI effect. Both forest edges and interiors showed increased buffering capacities with higher forest canopy density. We advocate for the conservation and expansion of urban forests which can buffer increasingly frequent and intense climate extremes. To this end, urban forest managers are encouraged to aim for multi-layered dense forest canopies and consider edge buffer zones of at least 50 m wide.

Keywords

Air temperature; Edge effect; Forest structure; Microclimate; Urban heat island; Vapour pressure deficit

Published in

Agricultural and Forest Meteorology
2023, volume: 341, article number: 109632

SLU Authors

• Plue, Jan

  • Department of Urban and Rural Development, Swedish University of Agricultural Sciences
    

Associated SLU-program

SLU Swedish Biodiversity Centre

Global goals (SDG)

SDG11 Sustainable cities and communities
SDG15 Life on land

UKÄ Subject classification

Forest Science
Meteorology and Atmospheric Sciences

Publication identifier

• DOI: https://doi.org/10.1016/j.agrformet.2023.109632

Permanent link to this page (URI)

https://res.slu.se/id/publ/126478