The role of effective leaf mixing length in the relationship between the δ 18 O of stem cellulose and source water across a salinity gradient

Previous mangrove tree ring studies attempted, unsuccessfully, to relate the δ 18 O of trunk cellulose ( δ 18 O CELL ) to the δ 18 O of source water ( δ 18 O SW ). 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 (Δ LW ) can interfere with the δ 18 O SW signal as it is recorded in the δ 18 O CELL of mangrove (saltwater) and hammock (freshwater) plants. We selected two transects experiencing a salinity gradient, located in the Florida Keys, USA. The δ 18 O CELL throughout both transects did not show the pattern expected based on that of the δ 18 O SW . We found that in one of the transects, biochemical fractionation interfered with the δ 18 O SW signal, while in the other transect Δ LW differed between mangrove and hammock plants. Observed differences in Δ 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 δ 18 O CELL to record not only variations in the δ 18 O SW but also in Δ LW making it impossible to isolate the δ 18 O SW signal.