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Received: 5 Aug 2019 Received in revised: 3 Feb 2020 Accepted: 19 Feb 2020 Published online: 26 May 2020 DOI: 10.32526/ennrj.18.3.2020.22 The research clarifies which climatic factors induce annual and intra-annual ring formation in merkus pine (Pinus merkusii) growing in the low lying plains of central Thailand and reconstructs the past climate by using climate modelling derived from climate-growth response. Not only are climate variations longer than a century in central Thailand explained, but the study also explores for the first time the variability in climate using the formation of intra-annual rings in Thai merkus pines. The tree-ring analysis of wood core samples indicated that the pine stand was more than 150 years old with the oldest tree being 191 years old. The annual variation in tree growth significantly correlated with local climate variables, the number of rainy days in each year (r=0.520, p<0.01) and the extreme maximum temperature in April (r=-0.377, p<0.01). The regional climate of the Equatorial Southern Oscillation in March (EQ_SOIMarch) also highly correlated with the pine growth (r=0.360, p<0.01). The climate reconstruction indicated a declining trend in the number of rainy days during the 20th century and a decline in the number of rainy days was observed during the first and second decades of the 21st century, respectively, while the past climate reconstruction of maximum temperature in April and EQ SOIMarch indicated a decline during the previous century and an increase in this century. A multiple regression analysis indicated that the extreme maximum temperature, which declined at the beginning of the wet season and increased around the transitional period of the late rainy and the cold seasons, influenced the formation of intraannual rings (r2=40.5%, p<0.05). It can be summarized that the number of rainy days increasing in each year associated with the declining temperature at the beginning of the wet season indicated a rapid growth in P. merkusii, while the anomalous temperature declining at the beginning and increasing at the end of the wet season was the main factor inducing the intra-annual ring formation. Therefore the activity of forest and planation management, especially in the watering at the beginning of the wet season when anomalous increased temperature occurred, shall be specified in the forest management plan in order to increase annual pine growth and wood formation.

Effects of Climate Variability on the Annual and Intra-annual Ring Formation of Pinus merkusii growing in Central Thailand