What is under these dying trees? An urban soil environment survey*

Abstract

Urban disturbances cause severe challenges to street tree growth. Street trees, as a vital part of urban green infrastructure, are subject to multiple and severe constraints that influence their survival and growth. However, not all street trees respond similarly to these adverse growth conditions, and it has been observed in the older suburbs of Canberra that trees of the same species on street verges planted at the same time are displaying very different health and growth characteristics. Trees established in relatively short stretches of street verge will have similar aboveground conditions (e.g. weather, roadside disturbances), and I hypothesise that the belowground soil environment could be a key factor contributing to the different growth and health of street trees. The highly disturbed urban soils and their poor characteristics have been shown in many studies to have negative impacts on urban tree establishment and development. Soil compaction and its associated low soil porosity, high soil bulk density and low water infiltration rate are the common soil constraints that trigger tree health decline. On the basis that soil physical properties can cause damage to street tree growth, the aim of this thesis was to test the hypothesis that healthy street trees grow in soils with better soil properties (mainly soil infiltration rate) than unhealthy street trees. Paired observations (including two healthy and two unhealthy trees in each street) were made for eight streets in Canberra. Tree health conditions were assessed using a crown scoring system. Tree height and diameter at breast height (DBH) were also measured and it was found that healthy trees had relatively larger diameters than unhealthy trees. Conversely, some unhealthy trees of a given DBH were taller than healthy trees. This morphological difference may be due to long-term water stress, and further work is recommended to determine if these results are due to larger-diameter trees storing more water resources for tree growth and taller trees facing more severe hydraulic resistance under drought conditions. Soil infiltration observations did not show that healthier trees occurred on soils with better water infiltration rates. Most trees studied were on soils with low surface infiltration rates and subsoil hydraulic conductivity. Our field observations and measurements indicate that the presence of soil compaction, soil water repellency and soil surface crusting contributed to these low soil water infiltration capacities. Such poor soil characteristics can influence street tree growth. With continuing urbanisation and its associated disturbances on urban soils, urban trees will face more challenges. It is necessary to improve our understanding of urban soil constraints and their impacts on urban tree health. First, the monitoring of street tree health and growth, and more measurements about soil infiltration, soil compaction and soil water repellency, are needed. Further investigation is needed on the effects of urban soil chemical and biological properties on street tree performance. Also, considering soil constraints and that some disturbances in urban areas can cause synergistic effects of tree health decline, future studies about street tree dieback mechanisms are needed, which will assist us in enhancing tree resilience and resistance.