Studies On Water Isotopic Fractionations In Leaf-Canopy Of Mangrove Forests And Their Applications

Mangrove forests are major sinks of blue carbon and play a key role in mitigating climate change.These highly saline,tidally flooded environments seem unlikely to favor tree growth,yet mangrove forests are some of the most productive ecosystems on the planet and provide a range of ecosystem services.As a principal limiting resource for plant photosynthetic metabolism,water determines a trade-off balance between water loss and carbon gain,which has been extensively studied in mangroves at individual and species level but studies at ecosystem scale are still limited.Accurate predictions of mangrove ecosystem responses to environmental changes require good quantification of water exchange in mangrove ecosystems.Based on the abundant mangrove species diversity and long-term in-situ measurements of water vapor fluxes and isotopic vapor fluxes in mangrove forests in China,this paper addressed the following issues:to test the adaptabilities of isotopic fractionation models and the Two-source model in quantifying both the leaf water and transpiration processes of mangroves,and then to apply these modified models to partition evapotranspiration(ET).The main results showed:1)The investigation of leaf morphological traits and oxygen isotope of plant water in 15 species of true mangrove plants,14 species of adjacent freshwater trees,and 4 species of semi-mangrove plants at five study sites along the south-eastern coast of China showed that oxygen isotopic enrichment of leaf water(?L)was generally 3-4‰ lower for mangrove species than for neighboring freshwater or semi-mangrove species.The higher leaf water content and lower leaf stomatal density both played important roles in reducing ?L in mangroves relative to nearby freshwater plants.Both differences acted to elongate effective leaf mixing length(L)in mangroves by about 200%and lower stomatal conductance by about 30%.2)The direct measurements of transpired water(?T)showed ?Tvalues deviated from isotopic steady state throughout most days.The long turnover time of leaf water,accompanied by high response sensitivity of stomatal conductance to environmental factors,would result in a non-steady state.Peclet models based on leaf morphological traits could accurately predict oxygen isotope fractionations of both leaf water and transpired water vapor.3)The extensive two-source model modified in adding processes of tidal energy and salinity effects,and adjusting the photosynthesis-temperature curve showed simulations of ET and canopy transpiration(T)are highly correlated with eddy-covariance observations of ET(R2>0.93 at all three sites)and sap-flow gauge-based estimates of T(R2=0.93 at the Yunxiao site).The magnitudes of summer ET in these mangrove forests were similar to those in well-watered broadleaf forests,with average ET fluxes of 6.0?8.5 mm day-1 in three mangrove ecosystems,the winter ET fluxes were more comparable to those of semi-arid ecosystems,with average values of 3.0?4.5 mm day-1.Meanwhile,T had average fluxes of 2.0-2.9 mm day-1 in summer and 0.3-1.5 mm day-1 during winter.4)Unlike most well-watered terrestrial ecosystems,the T/ET ratios were larger in autumn or spring(35-53%)than in summer(30-38%);Mid-day T/ET ratios were lower than morning and late afternoon.These unexpected differences in T/ET of mangrove ecosystems arise from the suppression of canopy conductance but stimulation of evaporation rate at high temperatures,with tides effects superimposed.Despite the stable isotope approach in partitioning,ET presented a discrete duo to unstable air condition,and the partitioning results agreed with those of the two-source model.The new findings of this study lie in(1)The T and E processes in mangrove ecosystems are successfully quantified by the comprehensive means of direct measurements,stable isotope and the Two-source model.(2)On a large pool of mangrove species or long-term observation,the leaf morphological traits are found to control the isotope fractionations of both leaf water and transpired water vapor;and(3)Extension of the Two-source model permits reliable half-hourly simulations of response processes of stomatal conductance to tides and salinity in tidal mangrove ecosystems.These findings have not only potential applications in long-term paleoclimate reconstruction,but in predicting the response of mangrove ecosystems to climate change and sea level rise.