The role of effective leaf mixing length in the relationship between the δ18O of stem cellulose and source water across a salinity gradient

Previous mangrove tree ring studies attempted, unsuccessfully, to relate the d18O of trunk cellulose (d18OCELL) to the d18O of source water (d18OSW). Here, we tested whether biochemical fractionation associated with one of the oxygen in the cellulose glucose moiety or variation in leaf water oxygen isotope fractionation (?LW) can interfere with the d18OSW signal as it is recorded in the d18OCELL of mangrove (saltwater) and hammock (freshwater) plants. We selected two transects experiencing a salinity gradient, located in the Florida Keys, USA. The d18OCELL throughout both transects did not show the pattern expected based on that of the d18OSW. We found that in one of the transects, biochemical fractionation interfered with the d18OSW signal, while in the other transect ?LW differed between mangrove and hammock plants. Observed differences in ?LW between mangroves and hammocks were caused by a longer effective leaf mixing length (L) of the water pathway in mangrove leaves compared to those of hammock leaves. Changes in L could have caused the d18OCELL to record not only variations in the d18OSW but also in ?LW making it impossible to isolate the d18OSW signal.