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Tropical forests are characterized by vast biomass, complex structures and mega-biodiversity. However, the adaptation processes of these forests to seasonal water availability are less understood, especially those located in the monsoonal and mountainous regions of tropical Southeast Asia. This study used four representative tropical forests spanning from 2° N to 22° N in continental Southeast Asia to address dry-condition photosynthesis at the seasonal scale. We first provided novel and reliable estimations of ecosystem photosynthesis (gross primary production; GPP) seasonality at all four sites. As expected, both evergreen and deciduous seasonal forests exhibited higher GPPs during the rainy season than during the dry season. A bimodal pattern corresponding to solar radiation occurred in the GPP of the perhumid forest. The surface conductance (G s) was consistently lower both in the dry season and during dry spells (DSPs) than during the wet season and non-dry spells. However, this did not prevent GPP from increasing alongside increasing irradiance in the perhumid forest, suggesting that other ecosystem physiological properties, for example, the light-saturated photosynthetic rate, must have increased, thus surpassing the effect of G s reduction. Thus, perhumid forests could be defined as light-demanding ecosystems with regard to their seasonal dynamics. Seasonal forests are water-stressed ecosystems in the dry season, as shown by the reductions in GPP, G s and related ecosystem physiological properties. At all four forest sites, we observed a lack of consistent adaptive strategy to fit the water seasonality due to the diversity in leaf phenology, soil nutrient availability, root depth and other potential factors. [ABSTRACT FROM AUTHOR]

Rubber (Hevea brasiliensis) plantations have been facing a double challenge of land degradation and seasonal drought in Southeast Asia. Various cash crops are recently interplanted with rubber trees to face these issues. However, the water relations between rubber trees and the intercrops remain poorly understood. This study aims to evaluate the influences of three cash intercrops, namely two herbaceous plants (Amomum villosum and Alpinia oxyphylla) and a woody beverage (Coffea arabica), on rubber water utilization through both spatial and temporal scales. We investigated the plant water-absorption dynamics, root biomass, and intrinsic water use efficiency (WUE i) throughout a whole year (2017–2018). The results showed that rubber trees (43.5 ± 2.6%) and intercrops (69.1 ± 3.2%) highly depended on soil water from the 0–20 cm depths. An interspecific water competition occurred in all the rubber-based agroforestry practices, because of their similar water source and root distribution in the vertical soil profiles. Overall, the WUE i of rubber trees was relatively higher during the dry season (δ 13C: − 30.79 ± 1.12‰) compared to the rainy season (δ 13C: − 31.65 ± 0.99‰). Coffee (C. arabica) better facilitated the soil water availability than the other intercrops, suggesting its suitability as an intercrop for rubber trees. Alpinia-oxyphylla (A. oxyphylla) played a moderate role on soil water retention. Amomum (A. villosum), however, aggravated the soil water deficit in the agroforestry practice. Given the differences in water relations to rubber trees, the introduction of woody crops rather than herbaceous crops can improve the resistance of rubber plantation to the frequent drought stress in this region. • Three prevailing intercrops have interspecific water competition with rubber. • Uptake depths of plants are consistent with their vertical root distribution. • Water use efficiency of rubber maintains a high level under drought stress. • Intercropping coffee trees plays a beneficial role in soil water availabilities. [ABSTRACT FROM AUTHOR]

• Interspecific water competition existed between rubber trees and intercrops. • Observed water uptake complementarity differed from that of previous reports. • Intercrops could promote surface soil water availability for rubber trees. • Facilitative effects were most marked in rubber-galangal agroforestry. Despite the development of rubber agroforestry systems for ecological and economic benefits in Southeast Asia, knowledge of their water uptake dynamics and interspecific water interactions remains limited. The objective of this study is to reveal the water relations (i.e., competition/complementarity) between rubber (Hevea brasiliensis) trees and different kinds of intercrops. We investigated the stable isotopes (δ D and δ 18O), fine root length density, and soil water content (SWC) under three agroforestry practices and one rubber monoculture across a year (2017/2018). Our results indicated that rubber trees acquired more than 40.5 ± 21.3 % of their water from shallow (0−20 cm) soil stratum, as do perennial galangal (Alpinia officinarum), tea (Camellia sinensis), and cocoa (Theobroma cacao). The complementarity hypothesis was not supported for rubber trees and the intercrops. In the dry season (November to April), there was strong interspecific competition for shallow water resources where the intercropping was practiced. However, intercropping increased the available soil water, enabling rubber trees to acquire more (9.4–24.3 %) shallow soil water. In the wet season (May to October), interspecific water competition was less pronounced based on the relative difference in soil water content (RDSW). Higher relative water content, in the order rubber-galangal > rubber-tea > rubber-cocoa, further showed that facilitative effects dominated interspecific water competition in all the agroforestry practices. This information regarding water relations between rubber trees and their intercrops will be essential to optimize land and water resource utilization in this region. [ABSTRACT FROM AUTHOR]