Tropical rainforests harbor a great diversity of species and play a significant role in the global carbon cycle due to their huge biomass and high productivity. In the studies on tropical rainforests over the past decade, there were two popular hypotheses, i.e., CO₂ fertilization–related accelerating growth hypothesis (AGH) and remote sensing–based drought resilience hypothesis (DRH). The hypotheses were necessarily to be testified.

A few researchers from Xishuangbanna Tropical Botanical Garden (XTBG) used 10 years of annual recensus inventory data collected in a tropical rainforest in the National Nature Reserve of Xishuangbanna (21°55′39″N, 101°15′55″E, 750 m above sea level) to examine the AGH and the DRH. Since both enhanced photosynthesis and aboveground carbon allocation could contribute to increased tree growth, 10 year eddy flux data were compiled as an independent approach to discern the relative contributions of these parameters. Interestingly, a severe, once-a-century drought occurred in our study site, providing an opportunity to examine the seldom-addressed drought recovery issue. They also reported a 10 year interannual variation in ecosystem respiration and its environmental controls.

Inventory data showed that the growth rate of the studied tropical rainforest has decreased over the past 10 years, a phenomenon that was inconsistent with the commonly suggested accelerating growth hypothesis (AGH). The studied forest was highly sensitive to water variability, with low canopy photosynthesis, slow stand growth, and high mortality rate in dry years, especially in the severe drought. Ecosystem respiration was not correlated with net water balance within years, but significant correlations were found between these parameters with a time lag of 10–15 months. A boom of photosynthesis in 1 year post drought was most probably a result of nutrient pulse–related drought. In general, neither AGH nor DRH was supported by the study.

The researchers suggested that much attention should be paid to the potential fate of carbon sink in tropical rainforests.

 The study entitled “High sensitivity of a tropical rainforest to water variability: Evidence from 10 years of inventory and eddy flux data” has been published online in Journal of Geophysical Research: Atmospheres.

Xishuangbanna tropical rainforest in nature reserve (Image by TAN Zhenghong)