Short-term dynamics of evaporative enrichment of xylem water in woody stems: implications for ecohydrology
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In ecohydrology, it is generally assumed that xylem water reflects the water source used by plants. Several studies have reported isotopic enrichment within woody tissues, particularly during dormancy periods or after long periods of inactivity. However, little is known about the short-term dynamics
of this process. Here we assessed the magnitude and dynamics of xylem isotopic enrichment in suberized twigs of pines and oaks. We performed a series of laboratory experiments, in which we monitored hourly changes in water content and isotopic composition under two contrasting scenarios of sap flow restriction. First, we simulated the effect of extreme hydraulic failure by excising twigs to restrict sap flow, while sealing the wounds to ensure that water loss took place only through the leaves or bark, as would be the case for evaporation in attached stems. Second, we studied the effect of reduced leaf transpiration by darkening with aluminium foil all the leaves of healthy, well-watered saplings growing in pot conditions. We found evidence of fast evaporative enrichment in metabolically active stems, as a consequence of a temporal decline in sap flow rates, and not necessarily linked to a traceable decline in stem water content. The excision experiments showed significant isotopic changes (~+1‰ in oxygen) appearing in <1 h. Similarly, the pot experiment showed a progressive increase in isotope composition (up to +8‰ in oxygen in a 3-day cycle) when the leaves were covered, and a rapid recovery to initial values when sap flow rates were re-established. We conclude that evaporative enrichment of xylem water in stems is a highly dynamic process that may have significant effects even during short periods of restricted water flow. This has important implications for the study of plant water uptake, as well as for ecosystem- and global-scale hydrological models.