Does healthy soil matter more than fungal diversity for the health of city trees?

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Urban trees provide city residents with ecosystem services including carbon sequestration, air quality improvement, storm water attenuation and energy conservation. However, the typically poor quality of urban soils may jeopardize these services.

Ectomycorrhizal fungi (EcM), known for their multifunctional role in forest ecosystems, may also contribute to urban tree health and the ecosystem services trees provide. The researchers noted that no studies had yet directly related EcM community composition in situ to urban tree health indicators.

This observational study combined two previously collected datasets: health data for 175 Tilia tomentosa trees in three European cities—Leuven, Strasbourg and Porto—and data on the ectomycorrhizal diversity of those trees. Tree health was estimated using a range of reflectance, chlorophyll fluorescence and physical leaf indicators; ectomycorrhizal diversity was characterized using next-generation sequencing. The researchers used canonical redundancy analysis to relate tree health indicators to soil characteristics and EcM diversity.

Soil organic matter significantly explained variation in tree health indicators, whereas no significant relationship was found between mycorrhizal diversity variables and tree health indicators.

The authors concluded that soil organic matter is a main positive influence on urban tree health, interpreting its role as promoting soil aggregate formation and porosity and thereby indirectly improving water availability to trees. Their results suggest that urban planners should not overlook soil quality and its water holding capacity when considering urban tree health and, potentially, the ecosystem services trees deliver.

The authors also called for research on other soil microbiota that may influence tree performance in urban settings, either independently or through interactions with ectomycorrhiza.