Trees: structure and function and the challenges of urbanization

Abstract

Key message: Urban trees are passively subject to and actively mitigate urban environmental pressures. Suitable selection of tree species and appropriate management can make cities useful habitats for trees. Abstract: The urban environment is stressful not only because of pollution but also due to heat and drought, creating arid conditions. Trees are passively subject to the microclimate but are also actively modifying it, and they perform important urban ecosystem services. Trees function as bio-indicators of urban conditions by morphological features and by dendrochronology and dendrobiochemistry. Anchorage and mechanical stability need to be surveyed. Among the stresses in addition to deposition of metals, there is gaseous pollution of the atmosphere (SO2, NO2, and O3). Reacting to that are leaf anatomy as well as even the fine-tuning of biochemical pathways, e.g., terpene synthesis. The significant urban stresses are heat and drought. 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. Statistics of dendrochronology allow identification of pointer years with exceptionally wide and narrow tree rings. The prevalence of pointer years helps the selection of species suited for plantation in cities. Measurements of water potential at the turgor-loss point, πtlp, indicative of the permanent wilting point also helped identifying whole-plant drought tolerance. Infection by wood-decay fungi is a hazardous urban problem, as wood-decay builds up over many years increasing the danger of tree fall. 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. Management should be based on scientific investigations. Allometry modeling allows for quantifying ecosystem services. Surveying stability is possible for instance by noninvasive acoustic tomography. Practices are irrigation and mulching. With appropriate management, cities can be useful habitats for trees contributing to biological richness and the comfort of life in cities.