A 338-year oxygen isotope record from teak tree-ring cellulose collected from Mae Hong Son province in northwestern Thailand was presented. The tree-ring series preserves the isotopic signal of the regional wet season rainfall and relative humidity. Tree-ring δ 18 O correlates strongly with regional rainfall from May to October, showing coherent variations over large areas in Southeast Asia. We reconstructed the summer monsoon season (May to October) rainfall based on a linear regression model that explained 35.2% of the actual rainfall variance. Additionally, we found that in the 19 th century, there was a remarkable drought during many years that corresponded to regional historic drought events. The signals of the June to September Indian summer monsoon (ISM) for the period between 1948 and 2009 were clearly found. Spatial correlations and spectral analyses revealed a strong impact of the El Niño-Southern Oscillation (ENSO) on tree-ring δ 18 O. However, ENSO influenced the tree-ring δ 18 O more strongly in the 1870-1906, 1907-1943, and 1944-1980 periods than in the 1981-2015 period, which corresponded to periods of weaker and stronger ISM intensity.

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We developed two tree-ring chronologies of teak (Tectona grandis L.f.) from Mae Tuen (462-year, 1555–2016) and Umphang (165-year, 1852–2016) in Tak province, northwestern Thailand. The chronologies were based on 67 and 71 living teak trees, respectively. We used crossdating methods to check and verify the tree-ring width data and tree-ring chronology construction using the ARSTAN program. In this study, the two teak tree-ring chronologies from two different growth areas could not be crossdated. The relationship among these chronologies is, thus, relatively low (r = 0.33, n = 165, p < 0.01). This result shows that the growth of tree-ring structure from two sites can be affected by a variety of non-climatic patterns due to site variation, such as topography, nutrient, light, and internal factors. However, these chronologies have a significant positive correlation with rainfall, during the pre-monsoon season (April to May). As demonstrated by the spatial correlation patterns, these chronologies represent April to May rainfall, which was a limiting factor of teak growth from northwestern Thailand. While the difference in surface temperatures of the Indian Ocean Dipole (IOD) might not be affected by rainfall, its unstable relationship with the El Niño-Southern Oscillation (ENSO) was noted to have occurred.

The two hemispherical photographs (THP) method was used to calculate gap area. The areas of nine forest gaps were measured. Meanwhile, non-gap areas were selected as control groups with areas of 225 m2. Plots with areas of 25 m2 in five different directions within gap and non-gap areas were conducted for collecting leaf samples. To determine the effect of gap size on leaf traits the selected traits were leaf area (LA), leaf dry mass (LDM), specific leaf area (SLA), Chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (TChl), and carotenoid (CAR). Leaves were collected from the regeneration layer (<2 cm DBH, height 2–5 m) to measure the leaf traits in winter and summer seasons. Results confirmed significant positive correlations between LA and LDM in the small, medium, large gap sizes, and non-gap areas (r2 = 0.913, 0.827, 0.897, and 0.939, p < 0.01, respectively). On the contrary, relationships between LDM and SLA in the small, medium, large gap sizes, and non-gap areas have significant negative correlations (r2 = −0.269, −0.259, −0.417, and −0.505, p < 0.05, respectively). The effect of gap size on the average Chl a, Chl b, TChl, and CAR varies by the season. During the summer season, the highest chlorophyll contents were recorded in the small gap size and the lowest in the non-gap area, while during the winter season, the highest values of these chlorophyll contents appeared in the medium gap size. Moreover, the directions within the gap in the medium gap size of the summer season had an effect on the Chl a and TChl.

Teak ring-width is one of the promising paleoclimate proxies in the tropical region. Tree-ring chronology spanning from 1840 to 2016 (177 years) was derived from seventy-six trees from Phrae Province, northern Thailand. A total of 141 core samples were cross-dated, a standardized master was constructed, and the tree residual master chronology was developed by ARSTAN program. The tree-ring chronology has a significant positive correlation with the monthly rainfall and relative humidity during the monsoon season (May - June). In addition, the growth of tree-ring width also significantly inversely correlated with Niño 3, Niño 3.4, and Niño 4 indices during the second half of the dry season (January - March). We reconstructed summer monsoon season (May - June) rainfall based on a linear regression model which explained 21.95% of the actual rainfall variance. The trend of the reconstructed rainfall record shows a decrease of 0.6 mm per decade and substantially showed four wet periods and five dry periods. These results suggest that this teak chronology has a good potential to be a high-resolution proxy for reconstructing the past local climate in northern Thailand.

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