Are pavements a major cause of tree decline?

This research investigated the effects of pavements on growth, physiology, and root mycorrhizal colonization in Celtis australis and Fraxinus ornus plants over a 9-year period. In March 2012, 48 trees were planted into 1 m2 planting pit, and assigned to one of the four pavement treatments: 1) impermeable pavement; 2) permeable pavers; 3) permeable concrete; 4) unpaved control. The experimental design was a randomized complete block with 6 blocks. Soil moisture and temperature were monitored using FDR and temperature probes, respectively. CO2 efflux from soil and soil O2 concentration were measured using a soil respiration chamber. Stem DBH, tree height, crown radius, and shoot growth were measured using a diameter tape and a graduated post. Leaf gas exchange and water relations were measured using an infra-red gas analyser and a pressure chamber, respectively. Root density and rooting depth were estimated using a ground-penetrating-radar. The diversity and composition of arbuscular mycorrhizal fungal communities were assessed by PCR denaturating gradient gel electrophoresis of partial 18S rRNA gene, AMF taxa were identified by amplicon sequencing and mycorrhizal colonization was evaluated after root clearing and staining. Impermeable pavements and permeable pavers increased soil temperature and CO2, compared to permeable concrete and control. Before tree establishment, which occurred in 2015, soil moisture was higher under all types of pavements than in bare soil. After tree establishment, impermeable pavements delayed both soil dehydration during summer and soil rehydration during winter and early spring. Leaf gas exchange, water relation, and above-ground growth were unaffected by pavement treatments. Conversely, impermeable pavements affected root system characteristics and mycorrhizal fungal communities associated to the roots. The evidence that some fungal symbionts were only found under impermeable pavements suggests the opportunity to develop specific mycorrhizal inocula for trees growing in sealed soils. Overall, results of this research show that the quality of the soil beneath the pavements is far more important to tree health than the pavement itself. Authors are grateful to the Tree Fund for funding this research with Grants 13-JK-01 and 14-RF-01.