The study generally aimed to identify the top three oldest trees in 10 stations at Clark Special Economic Zone and its adjoining areas which may serve as biomonitor. Specifically, it aimed to determine the estimated age of the trees, the different manganese concentrations in the different tree barks collected in different stations, the manganese concentrations in terms of tree types and the relationship between the estimated age of tree and the manganese content of its bark. A total of 30 bark samples from 11 different tree species were collected with their ages ranging from 30-113 years old. The manganese concentrations were determined by Atomic Absorption Spectrophotometer (ASS). As a result of the study, the Mn trace element investigated revealed that the accumulation of Mn ranged from 2-135 ppm. Evaluation of the results taken from the study showed that all stations had Mn value. Twenty one out of 30 bark samples had a manganese value that exceeded the acceptable Mn content limit of 20-700 ppm for plants. Results showed that there was no significant difference among the estimated ages of the top three oldest trees in 10 stations, while there was no significant difference among the concentrations of manganese in the different tree barks collected in the different stations.There was also no significant difference among the manganese concentrations in terms of tree types. Statistical analysis for the test of independence between the estimated age of the tree and its manganese content showed that the two variables were independent.