RELATIONSHIP OF CAMBIAL ACTIVITY AND XYLEM PRODUCTION IN TEAK (<em>TECTONA</em> <em>GRANDIS</em>) TO PHENOLOGY AND CLIMATIC VARIABLES IN NORTH-WESTERN THAILAND
Keywords:Tropics, rainfall, xylogenesis, phenology, tree ring width
Seasonal cambial activity and xylem production were studied in teak trees (Tectona grandis) at Mae Sariang, Mae Hong Son Province, Thailand. Block samples were collected every month from October 2017 to October 2018 and from March to October 2019. Indicators of cambial activity and xylem production were correlated with climate factors. The number of cambial cell layers (CCL) was significantly and positively correlated with rainfall, relative humidity, temperature and soil moisture (r = 0.730, 0.515, 0.464 and 0.657, respectively). Both cambial zone width (CZW) and xylem differentiation zone width (Xydf) were significantly and positively correlated with rainfall, relative humidity and soil moisture (CZW: r = 0.862, 0.409 and 0.647, respectively; Xydf: r = 0.747, 0.436 and 0.556, respectively). Additionally, CCL was significantly and positively correlated with CZW and Xydf (R2 = 0.564 and 0.740, respectively), and CZW was significantly and positively correlated with Xydf (R2 = 0.771). Overall, in agreement with previous studies, the cambial activity of teak coincided with its leaf phenology; trees were active when new budding was occurring and dormant when leaves were falling. However, the cambial activity in this study started later, in accordance with the last rainfall. Indeed, rainfall in April was significantly and positively correlated with the ring-width index of teak, indicating that rainfall is the limiting factor that triggers cambial activation. The fluctuation in Asian monsoons appears to affect cambial activity, xylem productivity and ring width. This knowledge improves the understanding of the seasonal cambial activity of teak and provides useful data for teak plantation management in northwestern Thailand.