Mangrove wetlands play an important role in global carbon cycle due to their strong carbon sequestration resulting from high plant carbon assimilation and low soil respiration. However, temporal variability of carbon sequestration in mangrove wetlands is less understood since carbon processes of mangrove wetlands, located at the interface between terrestrial and marine systems, are influenced by many complicated and concurrent environmental controls including tidal activities, site climate and soil conditions. Compared to soil respiration, mangrove carbon assimilation by photosynthesis has stronger temporal variability and contributes more to temporal variability of net carbon exchange. Canopy light use efficiency (LUE), is the most important plant physiological parameter that can be used to describe the temporal dynamics of canopy photosynthesis, and therefore a better characterization of temporal variability of canopy LUE will improve our understanding in mangrove photosynthesis and carbon balance.
One of our aims is to study the temporal variability of canopy LUE and its environmental controls in a subtropical mangrove wetland. Half-hourly canopy LUE is derived from eddy covariance (EC) carbon flux and photosynthesis active radiation observations, and half-hourly environmental controls we measure include temperature, humidity, precipitation, radiation, tidal height, salinity, etc. We will explore potential qualitative relationships between canopy LUE and environmental controls to improve our understanding in how these environmental factors affect the temporal variability of canopy LUE. Another aim is to explore the links between canopy LUE and spectral indices derived from near-surface tower-based remote sensing, and then identify potential quantitative relationships for developing remote sensing-based estimation methods of canopy LUE. We are planning to mount multiple radiometer sensors on flux tower to measure and calculate half-hourly spectral indices, including normalized difference vegetation index, enhanced vegetation index, photochemical reflectance index, solar-induced chlorophyll fluorescence, etc.
At present, some instruments in our in-situ observation system have not yet been installed (planned in next months) and therefore we don’t have enough measurements to support our analysis. However, a preliminary analysis of our historical EC and climate observations in past several years indicates that canopy LUE shows strong temporal variability and is greatly affected by environmental factors such as tidal activity. Detailed and systematic analyses of temporal variability of canopy LUE and its environmental controls and potential remote sensing estimation methods will be conducted when our in-situ observation system is ready in near future.
