Climate control of cambial dynamics and tree-ring width in two tropical pines in Thailand

Tropical forests are affected by recent climate change, but detailed knowledge how forests in different regions of the tropics respond to changing climate conditions is not sufficiently studied. Our objectives were to evaluate climatic effects on the monthly cambial activity of Pinus latteri and Pinus kesiya and to determine the relationships between climatic variables (precipitation, temperature, and relative humidity) and the tree-ring widths of these two pine species. Cambial dynamics of P. latteri and P. kesiya in subtropical Thailand were studied for two consecutive growing seasons. We observed that monthly rainfall, relative humidity, soil moisture, monthly minimum temperature, and monthly mean temperature were positively related to the number of cambial cell layers and cambial zone width. Moisture in the late dry season and in the beginning of the rainy season was very important in stimulating cambial cell division after a long-term moisture deficit for more than three months. The growth responses to climate in a 128-year-long chronology of P. latteri and an 88-year-long chronology of P. kesiya were similar. Importantly, the amount of rainfall and relative humidity in April and May had positive effects on the tree-ring widths of both pine species. On the other hand, both mean monthly maximum temperature and mean monthly temperature had negative effects on tree-ring widths. P. kesiya is more susceptible to drought, as shown by its correlation with the self-calibrated Palmer drought severity index. The rising temperatures during the recent 30-year observation period clearly show that higher temperatures covered the northern and central regions of Thailand and are likely to reduce the productivity of both pine species.