Sapwood area is an important parameter for estimating canopy transpiration in the forest water cycle. However, sapwood area highly varies across species and forest ecosystems and is difficult to measure directly. Therefore, species- and site-specific allometric equations are needed to estimate the sapwood area of all trees in a forest. Here, we conducted a comprehensive campaign to measure sapwood thickness and to estimate the sapwood area of 14 common tree species in a successional forest in Thailand. These data represent the first comprehensive measurements of sapwood area in south-east Asian successional forests growing under diverse environmental conditions in terms of soil moisture and canopy density. The results show that a power function can significantly explain the relationship between sapwood area and stem size, represented by diameter at breast height (DBH), in all species in both primary and secondary forests. Interestingly, a single equation could describe the sapwood area~DBH relationship in all species and forest stages, except for Dipterocarpus gracilis, an emergent, dominant species in the primary forest. The latter showed slower growth in sapwood area once the trees reached a DBH of ~30 cm. Overall, our results can benefit future studies that estimate canopy transpiration of tropical forests with similar conditions as in our study sites.
Soil respiration (SR) in forests contributes significant carbon dioxide emissions from terrestrial ecosystems and is highly sensitive to environmental changes, including soil temperature, soil moisture, microbial community, surface litter, and vegetation type. Indeed, a small change in SR may have large impacts on the global carbon balance, further influencing feedbacks to climate change. Thus, detailed characterization of SR responses to changes in environmental conditions is needed to accurately estimate carbon dioxide emissions from forest ecosystems. However, data for such analyses are still limited, especially in tropical forests of Southeast Asia where various stages of forest succession exist due to previous land-use changes. In this study, we measured SR and some environmental factors including soil temperature (ST), soil moisture (SM), and organic matter content (OM) in three successional tropical forests in both wet and dry periods. We also analyzed the relationships between SR and these environmental variables. Results showed that SR was higher in the wet period and in older forests. Although no response of SR to ST was found in younger forest stages, SR of the old-growth forest significantly responded to ST, plausibly due to the nonuniform forest structure, including gaps, that resulted in a wide range of ST.
Large-scale abandoned agricultural areas in Southeast Asia resulted in patches of forests of multiple successions and characteristics, challenging the study of their responses to environmental changes, especially under climatic water stress. Here, we investigated seasonal variation in leaf water status and drought tolerance of dominant tree species in three multi-aged tropical forests, ranging from 5 to > 200 years old, with contrasting soil moisture in Thailand. Seasonal variation in leaf water status differed among the forests with trees in young and intermediate sites demonstrating larger differences between seasons than the old-growth forest. Although vulnerability to embolism curves revealed that trees in old-growth forest were potentially more sensitive to declining leaf water status than others, they were predicted to lose < 5% of their hydraulic capacity as opposed to 13% for the trees in the younger sites. Our results suggest that the responses to water stress of tree species in different forest ages greatly vary with a tendency of trees in younger sites to be more resilience than those in older sites. Such information would benefit the selection of tree species that could adapt well to specific environments, thus improving the strategies for managing forests of different ages under a warmer future.
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