Taiwan had established air quality enhancement zones (AQEZs) by planting trees, aiming to improve air quality and ecological environment. Trees in the AQEZs reduce the amount of carbon dioxide in the atmosphere by storing carbon in their tissues. Species diversity is a critical factor influencing the capacity of trees to capture carbon. In this study, we assessed tree species diversity and estimated the carbon storage of AQEZs of 98 sampling plots located in four regions. The zones examined contained 210 species from 145 genera and 61 families. Study results showed that despite this apparent diversity, at least one species represented more than 10% of the identified trees in the four regions. The overall proportion of non-native species was relatively high at 58%. The greatest individual tree carbon storage was 8.93 metric tons and the mean tree carbon storage was 0.05 tons C/tree. An overall carbon storage of 672.20 tons C in the sampling plots is estimated and carbon storage benefits are expected to increase as these trees mature. The research outcomes can be used for reference for authorities in carbon policy making. Due to the benefits of tree planting, more AQEZs are suggested to build considering the increasingly important global warming issue.