In 1929, the street trees in the protected area mainly consisted of P. stenoptera, S. sebiferum, P. orientalis, and S. japonica (Yang and Yan, 2013; Yan et al., 2012). Over the past century, the street tree species in the area have undergone constant change. However, due to their strong resistance and longevity, P. orientalis and P. stenoptera are still present and have become an important part of the historical and cultural heritage of the area. With the construction of high-density buildings, roads, and pipelines, the growth of street trees has become continually disrupted. Coupled with aging, diseases and insect pests, and other causes, these disturbances have caused the trees to become more prone to falling or having broken trunks and branches under extreme weather events such as typhoons ( Poulos et al., 2010; Jin et al., 2019). In this context, city managers must make accurate management decisions based on risk assessment. Conventional visual assessment methods mainly detect the growth of the tree canopy and trunk and pest infections in shallow root systems. However, it is difficult to accurately detect internal decay in trunks and the root system with these systems, affecting the accuracy of street tree safety assessments. Accurate detection of internal rot in tree trunks and the proportion of the internal decay in trunks is important; in addition, the distribution of the root system at different depths and ranges can be determined to provide more accurate quantitative indicators for subsequent risk assessments. In this study, we found that the risk level of tree branches of the majority of the street trees in the protected area is low, mainly due to timely management measures; for example, branches that may affect public facilities are annually removed, thereby lowering the possibility of large crown deviation, large dead branches, and diseases and insect pests. Most of the P. orientalis trees had a low trunk risk possibility level, while nearly half of the P. stenoptera trees had a high trunk risk possibility level, mainly due to internal decay in trunks caused by diseases and insect pests, fungal infestation, and aging(Jia et al., 2021a), which affect the stability of the trees and increase the likelihood that branches and trunks will break. Trees can enhance their wind resistance by moving the root fulcrum (Nicoll and Ray, 1996),. The root system risk level of most of the street trees in the study area is high, mainly because the area available for the trees is small, with poor soil fertility and inadequate growth space for root systems. Moreover, road adjustments and underground pipeline construction often directly cut the root systems of street trees, so certain soil layers contain no roots, and the root systems struggle to support the trees in all directions.
The framework of the street tree safety risk assessment system is based on accurate detection and is essentially similar to traditional visual assessment methods, although the method proposed here uses the internal decay and hollowness of tree trunks and the depth and scope of roots as important assessment indicators, which reduces the subjectivity of the assessment and improves the rationality of the evaluation system (Han, 2013; Jia et al., 2021b). Studies from Hong Kong, Xiamen, and other cities have shown that street trees are affected by the narrow growth space and municipal engineering projects, such as frequent construction and underground trenching (Jim, 2003; Tang, 2018), as well as excessive soil compaction caused by vehicle traffic, all of which can lead to repeated damage to branches and roots and a high probability of tree safety hazards (Jim and Zhang, 2013; North et al., 2017). In this study, we found that more than 74% of the street trees in the protected area have a risk consequence severity level of III or above, indicating severe consequences once the trees are broken. The main reason is that the study area is located in a downtown area with a high building density, high pedestrian and vehicle flows, and narrow sidewalks, and there are interactions among street trees, buildings, and power lines. As a result, cars, buildings, and pedestrians can be exposed to serious risk. However, more than 76% of the street trees have a risk level of II or below, indicating that maintenance and management measures for most of the street trees are in place, resulting in mostly moderate risk possibility levels that are negligible or acceptable. Roads with high risk levels are mostly those with a high road grade, a high building density, or a short distance to buildings as well as parks and green areas near roads with high daily traffic and pedestrian volumes. Taojiang Road, which has a low level of risk, is a small road in an area mainly used for pedestrian traffic, and there is a greater distance between street trees and buildings. The risk assessment method provides valuable information for street tree management in the area and can reduce the risk to a reasonable and acceptable level (He et al., 2021), thereby providing a basis for maintaining the desired vegetation structure of the preserved area in the future.
The results of the VTA-based street tree safety risk assessment of Shanghai City show that the trees in the downtown area of the city are mostly large trees with a heavy canopy load. Due to long-term development in the site environment, the trees exhibit large crown deviation, internal decay in the trunks, and leaning; these issues are concentrated in the branches and trunks (He et al., 2021). The precision diagnosis technique revealed that the main risk points of the street trees are the distribution depth and range of the root systems, belowground pipe gallery construction, and internal decay in the trunks and tree cavities. The results of the correlation analysis indicate that the street tree risk level is strongly correlated with internal decay in the trunks and tree cavities, diseases and insect pests, the distribution range and depth of the root systems, and t1ee leaning. Therefore, in future management of the preserved area, tree height and canopy width should continue to be controlled through daily pruning to avoid potential safety hazards due to excessive tree volume, and further measures should be taken to repair tree cavities and eliminate diseases and insect pests while ensuring that the growth environment of the roots of the street trees is not affected by the construction of pipelines and roads. For street trees with root damage, the range and depth of tree holes should be maximally extended so that the tree roots have sufficient growth space. Moreover, other measures should be used, such as adding supports, to promote healthy tree growth. In addition, regular observations should be conducted, which would allow multiyear detection data to be collected, so that more detailed support can be recommended to improve street tree safety, and a dynamic “planting-monitoring-evaluating-maintaining” management adjustment mechanism should be established.