Ectomycorrhiza is hampered in urban soils having a low inoculum potential due to chemical and physical soil qualities regarding cation-exchange capacity, permanent wilting point, and availability of water.

Infection by wood-decay fungi is a hazardous urban problem, as wood-decay builds up over many years increasing the danger of tree fall. Measurements of water potential at the turgor-loss point, π tlp, indicative of the permanent wilting point also helped identifying whole-plant drought tolerance. The prevalence of pointer years helps the selection of species suited for plantation in cities. Statistics of dendrochronology allow identification of pointer years with exceptionally wide and narrow tree rings. They increase top dieback and lead to a decrease of life crown-top heights from the ground (LCTH), where water relations remain similar in the crowns at various heights. The significant urban stresses are heat and drought.

Reacting to that are leaf anatomy as well as even the fine-tuning of biochemical pathways, e.g., terpene synthesis. Among the stresses in addition to deposition of metals, there is gaseous pollution of the atmosphere (SO 2, NO 2, and O 3). Anchorage and mechanical stability need to be surveyed. Trees function as bio-indicators of urban conditions by morphological features and by dendrochronology and dendrobiochemistry.

Trees are passively subject to the microclimate but are also actively modifying it, and they perform important urban ecosystem services. The urban environment is stressful not only because of pollution but also due to heat and drought, creating arid conditions. Suitable selection of tree species and appropriate management can make cities useful habitats for trees. Urban trees are passively subject to and actively mitigate urban environmental pressures.

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