Savanna ecosystems play a crucial role in the global carbon cycle. However, there is a gap in our understanding of carbon fluxes in the savanna ecosystems of Southeast Asia. In this study, the eddy covariance technique (EC) and the biometric-based method (BM) were used to determine carbon exchange in a savanna ecosystem in Southwest China. The BM-based net ecosystem production (NEP) was 0.96 tC ha −1 yr −1 . The EC-based estimates of the average annual gross primary productivity (GPP), ecosystem respiration (R eco ), and net ecosystem carbon exchange (NEE) were 6.84, 5.54, and −1.30 tC ha −1 yr −1 , respectively, from May 2013 to December 2015, indicating that this savanna ecosystem acted as an appreciable carbon sink. The ecosystem was more efficient during the wet season than the dry season, so that it represented a small carbon sink of 0.16 tC ha −1 yr −1 in the dry season and a considerable carbon sink of 1.14 tC ha −1 yr −1 in the wet season. However, it is noteworthy that the carbon sink capacity may decline in the future under rising temperatures and decreasing rainfall. Consequently, further studies should assess how environmental factors and climate change will influence carbon-water fluxes.Savanna ecosystems are characterized by distinct wet and dry seasons, the codominance of C3 trees and C4 grasses 1 , and their location mainly in the tropics and subtropics. They cover almost 60%, 50%, and 45% of the areas of Africa, Australia, and South America, respectively, and more than 10% of the area of Southeast Asia 2 . They play an increasing role in the carbon cycle and energy fluxes in the context of climate changes (e.g., decreasing precipitation and increasing temperature), as they cover approximately 20.0% (2.7 billion ha) of the global land surface 1,3-5 and account for ~30% of the net primary production (NPP) of the terrestrial ecosystem 6 . Savannas also have a large and rapidly growing human population: about one-fifth of the global population is supported by savanna ecosystems 3,4,7 . The carbon exchange of savanna ecosystems, therefore, has a significant influence on global carbon cycling. Consequently, research on the spatiotemporal characteristics of carbon exchange and its responses to biotic and abiotic controls on savanna ecosystems is of great importance, not only for improved fundamental ecological understanding of the impact of global change on carbon fluxes but also for the improved protection and management of this vulnerable ecosystem type for sustainable development and provision of better ecosystem services (e.g., the management of resources, water, biodiversity, and climate change).

The potential impact of drought on the carbon balance in plants has gained great attention. Non-structural carbohydrate (NSC) dynamics have been suggested as an important trait reflecting carbon balance under drought conditions. However, NSC dynamics under drought and the response mechanisms of NSC to drought remain unclear, especially in water-limited savanna ecosystems. A precipitation exclusion experiment was performed to simulate different drought intensities in a savanna ecosystem in Yuanjiang valley in southwestern China. Growth, total NSC concentration and diurnal change of NSC were determined for the leaves and non-photosynthetic organs of three dominant species (Lannea coromandelica, Polyalthia cerasoides and Heteropogon contortus) throughout the growing season. Drought significantly reduced the growth of all the three species. Total NSC concentration averaged ~8.1%, varying with species, organ and sampling period, and did not significantly decrease under drought stress. By contrast, the diurnal change of NSC in these three species increased under drought stress. These results indicate that these three dominant species did not undergo carbon limitation. Thus, relative change in NSC is a more sensitive and effective indicator than carbon reserves in evaluation of plant carbon balance. These findings provide new insights for the understanding of carbon balance and the mechanisms of carbon starvation.