It is important to understand how resilient primary evergreen subtropical forests are against regional expressions of climate change. Evapotranspiration is one key indicator of this resilience and plays a central role in eco-hydrological systems.

Thus, it is critical to understand the drivers of evapotranspiration, and how they might be affected by climate or land-use changes in the near future. 

Prof. ZHANG Yiping and his team of Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences observed evapotranspiration over a 5-year period at a primary subtropical evergreen forest in in the Ailaoshan National Nature Reserve in southwest China.

They used the eddy covariance method and quantified the precipitation (P), throughfall (TF), stemflow (SF), soil evaporation (E_s), and routine meteorological parameters to analyze the contributions of stand-level transpiration (E_t) and canopy interception loss (E_i) to the total evapotranspiration.

They aimed to determine how these factors vary by season and how they vary between years. Overall, they were interested in how resilient this system is against climate change.

They found that the precipitation always exceeded potential evapotranspiration and was, on an annual basis, almost twice as large as evapotranspiration. Precipitation varied largely: in one year it was only 10% larger than potential evapotranspiration.

This shows that the relatively guaranteed resilience of the eco-hydrologic system towards climate change is rather small. The water supply is only guaranteed as long as there is enough precipitation.  

The relative contributions of the stand-level tree transpiration, soil evaporation, and canopy interception to evapotranspiration exhibited large inter-annual fluctuations. The stand-level transpiration accounted for the largest contribution to evapotranspiration. 

The seasonal change in evapotranspiration is influenced by net radiation and vapor pressure deficit. Although the study did not collect specific data for the effect of fog on evapotranspiration, fog likely plays an important role in the eco-hydrologic system and deserves further investigation. 

The study entitled “Evapotranspiration from a primary subtropical evergreen forest in Southwest China” has been published online in Ecohydrology.