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Chamaecyparis formosensis (Cupressaceae) is among the most precious endemic conifers in Taiwan. Field study was conducted on seasonal variations in emission rates and compositions of terpenoids from this tree species of two different ages. A total of 21 terpenoids were detected, of which there were 13 monoterpenoids (MTs), 4 sesquiterpenoids (STs), and 4 diterpenoids (DTs). MTs dominated the emissions in both saplings and adult trees and produced more than 80% of terpene emissions. Contrasting seasonal pattern between saplings and adult trees was found. Total actual emissions from saplings were higher in cold seasons (range, 64.40 ± 13.18 to 140.74 ± 18.90 ng g -1 h -1 ) than in warm seasons (range, 55.63 ± 15.84 to 63.48 ± 11.85 ng g -1 h -1 ). Photosynthetically active radiation (PAR) was found to be the most important factor affecting terpene emissions from saplings. On the contrary, higher emissions were found in warm seasons for adult trees (range, 101.49 ± 12.29 to 181.35 ± 80.15 ng g -1 h -1 ), and the emissions were mainly in response to temperature. Some compounds in C. formosensis of both ages (e.g., β-myrcene, α-terpinene, trans-β-ocimene, terpinen-4-ol, α-cedrene and trans-β-farnesene) showed comparably higher contents in cold seasons. Results presented here provide important fundamental information for better understanding of forest bathing and estimating air quality in Taiwan., (Copyright © 2019 Elsevier Masson SAS. All rights reserved.)