The Role of Hydrology in Controlling the Inter-annual Variability of Net Ecosystem Carbon Exchange in a Tropical Peat Forest in Sarawak, Malaysian Borneo

Tropical peat forests store 11-14% of the world's peat carbon, but the sensitivity of this globally important carbon pool to changes in the soil-vegetation-atmosphere continuum is uncertain. To improve our understanding of the carbon dynamics of tropical peat forests, an eddy covariance tower was erected in a protected peat swamp forest in Sarawak, Malaysia and instrumented to continuously measure the net ecosystem exchange of CO2 (NEE) between the forest ecosystem and atmosphere. NEE was estimated to be 587, 486, 163 and 347 g C m-2 y-1 for 2011, 2012, 2013 and 2014, i.e. the forest was a net source of CO2 to the atmosphere during every year of measurement. Carbon fluxes were highly sensitive to hydrology via the water table depth and precipitation. Rainfall was highest in 2011 (3890 mm) and NEE was negative (indicating a carbon sink) during the wet season until late June, after which the declining water table depth coincided with a large C loss event of > 500 g C m-2 over a three month period. Rainfall in 2012 and 2013 was close to the long-term average (2798 mm) and the differences in NEE between these two years was related to seasonal differences in precipitation and water table depth. 2014 experienced the lowest rainfall (2272 mm) but rates of C loss similar to the average of the other three years. Our results suggest the carbon balance of this tropical peat forest was not only sensitive to the annual sum of rainfall, but also to its seasonal distribution, with important implications for understanding the response of tropical peat forests to future hydrologic disturbances.