Increasing midday depression of mangrove photosynthesis with heat and drought stresses

Midday depression of photosynthesis (MD) refers to the phenomenon that vegetation's photosynthetic rate decreases at midday experiencing environmental stresses. Mangrove MD and its responses to heat and drought stresses offer valuable insights into understanding the impact of climate change on mangrove blue carbon. However, the temporal variability of mangrove MD and its interactions with these stresses across short time scales remain less investigated. Here, we quantified mangrove MD using two diurnal metrics, relative midday depression (RMD) and diurnal centroid shift (DCS), and examined its responses to heat (air temperature) and drought (vapor pressure deficit (VPD) and rain) stresses in a subtropical estuarine wetland of Southeast China, based on six-year eddy covariance measurements from 2017 to 2022. The results indicate: (1) mangrove MD occurred at air temperature or VPD above a certain threshold but became severe when the stresses co-existed; (2) RMD performed better than DCS in measuring mangrove MD; (3) monthly RMD had a clear seasonal pattern peaking in summer (up to 26.1 %), while annual RMD (5.0 similar to 10.2 %) changed with gross primary productivity (GPP) in the opposite direction; (4) RMD increased with both air temperature (1.01 similar to 1.35 %/ degrees C) and VPD (8.41 similar to 13.79 %/kPa) for each year but with different sensitivities; (5) larger annual sensitivities of RMD to both air temperature and VPD tended to occur in drier years with less rain. This study highlights the importance of heat and drought stresses in affecting mangrove MD and GPP, implying that future warmer and drier climates are likely to weaken mangrove carbon uptake. Future empirical and model studies on mangrove blue carbon should explicitly consider sub-daily interactions between mangrove MD and environmental stresses to reduce the uncertainty in assessing mangrove carbon budget in the context of climate change.