Background Global ecosystem deprivation is linked to reduced microbial diversity and diminished immunological resilience. Urban rewilding and decomposing plant matter have been suggested to reverse this deprivation and support human health.

Methods We rewilded 21 urban private yards with vegetation and deadwood. Control yards (15) were analyzed for comparison. We collected microbial samples and used vegetation and deadwood inventories, landcover data and questionnaires to determine the effects of rewilding and living environment on skin and salivary microbiota, gene pathways and cytokine levels (IL-6, IL-10). Samples were collected before the rewilding in summer and three months later in autumn.

Findings Rewilding preserved skin microbial richness in comparison to control group, including previously health-associated Alpha- and Gammaproteobacteria, despite the normal seasonal decline and less outdoor time in autumn. Deadwood abundance within 200-m radii associated directly to beta diversity of skin microbiota and Gammaproteobacterial taxonomies.

In saliva, deadwood was directly associated with the diversity of functional gene pathways, which in turn was negatively associated with pleiotropic IL-6 levels. Rewilding was associated with a decrease in L-histidine degradation and an increase in Mycobacteriaceae.

Interpretation Since both yard rewilding and neighborhood deadwood preserved rich commensal microbiota and reduced pro-inflammatory signals, decomposing plant matter, not just plant richness, seems to be crucial for ecosystem services that contribute to health. Since deadwood abundance was associated to reduced pro-inflammatory signals, it may be a suitable indicator of environment supporting immunological resilience.

Our findings provide an incentive for future strategic investments for planetary health.

Funding Strategic Research council Finland.

Aki Sinkkonen and Olli Laitinen are co-inventors in a patent "Probiotic immunomodulatory compositions" (U.S. patent no. 11318173, U.S. Patent and Trademark Office) and shareholders and members of the board of Uute scientific LtD which develops immunomodulatory treatments.

NCT06353035

This work was supported by Research Council of Finland, Strategic Research Council funding (grant numbers 346136 to A.S., 346137 to J.R. and 346138 to O.H.L.).

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ethics Committee on Medical Research Ethics of Helsinki University Hospital District gave HUS/1002/2022 ethical approval for this work.

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Skin bacterial sequence data were accessioned into the Sequence Read Archive (BioProject ID: PRJNA1102262). All other data needed to support the conclusions of this manuscript are included in the main text and supplementary appendix.

The sensitive data that support the findings of this study are available from Natural Resources Institute Finland, but restrictions defined in General Data Protection Regulation (EU 2016/679) and Finnish Data Protection Act 1050/2018 apply to the availability of these data, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission from the ethical committee of the local hospital district (Helsinki and Uusimaa Hospital District, Finland).