Quantifying and mapping urban trees' decay severity using thermal and spatial indices: implications for tree hazard assessment and management

Wood decay, a crucial factor in tree stability, is an internal long-term interaction between fungi and tree that leads to the disruption of energy flow, temperature abnormalities on the tree's surface, and possible tree mortality, especially when the decay extent is close to the threshold of 33%. In this study, arboreal vegetation species’ stability in two urban parks in the city of Mytilene, Greece, was evaluated, in accordance with the tree failure criterion, by measuring the trees’ morphological traits along with their decay severity. Thermal indices were developed by analyzing tree trunks’ temperature data, and strength loss equations associated with wood decay were applied for each tree. Temperature spatial dependence across each tree’s trunk was estimated using Moran's I index, while statistically significant spatial clusters were assessed using local spatial autocorrelation statistics. Relationships between tree stability, thermal, and spatial indices, were established using linear and logistic regression models. Finally, the Getis-Ord Gi* statistic was used for the recognition of hazardous tree hotspots in the urban parks, and the kriging geostatistical procedure was applied for mapping their spatial extension. The results have shown that thermal and spatial indices can sufficiently explain decay severity, identify hazardous trees, and contribute to tree health assessment for specialized park management.