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.

中文翻译：

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这些垂死的树下是什么？城市土壤环境调查*

城市干扰对行道树的生长造成了严峻挑战。行道树作为城市绿色基础设施的重要组成部分，受到多种严重制约，影响其生存和生长。然而，并不是所有的行道树对这些不利的生长条件都有类似的反应，在堪培拉的老郊区观察到，在街边同时种植的同一物种的树木表现出非常不同的健康和生长特征。在相对较短的街道边缘建立的树木将具有相似的地上条件（例如天气、路边干扰），我假设地下土壤环境可能是导致街道树木不同生长和健康的关键因素。许多研究表明，高度扰动的城市土壤及其不良特征对城市树木的建立和发展产生负面影响。土壤板结及其相关的低土壤孔隙度、高土壤容重和低水分渗透率是引发树木健康下降的常见土壤制约因素。基于土壤物理性质会对行道树生长造成损害，本论文的目的是检验健康行道树生长在土壤性质（主要是土壤入渗率）比不健康行道树更好的土壤中的假设。对堪培拉的 8 条街道进行了配对观察（包括每条街道上的两棵健康树和两棵不健康的树）。使用树冠评分系统评估树木健康状况。还测量了树高和胸高直径 (DBH)，发现健康的树木比不健康的树木具有相对更大的直径。相反，给定 DBH 的一些不健康的树木比健康的树木高。这种形态差异可能是由于长期缺水造成的，建议进一步开展工作以确定这些结果是否是由于较大直径的树木为树木生长储存了更多的水资源，而较高的树木在干旱条件下面临更严重的水力抵抗。土壤渗透观察并未显示更健康的树木生长在具有更好水渗透率的土壤上。所研究的大多数树木都生长在具有低表面渗透率和底土导水率的土壤上。我们的实地观察和测量表明，土壤压实的存在，土壤拒水性和土壤表面结痂导致土壤水分渗透能力低。这种贫瘠的土壤特征会影响行道树的生长。随着城市化的持续推进及其对城市土壤的相关干扰，城市树木将面临更多挑战。有必要提高我们对城市土壤约束及其对城市树木健康影响的理解。首先，需要对行道树健康和生长进行监测，以及更多关于土壤入渗、土壤压实和土壤防水性的测量。需要进一步研究城市土壤化学和生物特性对行道树性能的影响。此外，考虑到土壤限制以及城市地区的一些干扰会导致树木健康下降的协同效应，