Temperature-sensitive biochemical O-fractionation and humidity-dependent attenuation factor are needed to predict δ O of cellulose from leaf water in a grassland ecosystem.

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ O ) in a drought-prone, temperate grassland ecosystem. A new allocation-and-growth model was designed and added to an O-enabled soil-vegetation-atmosphere transfer model (MuSICA) to predict seasonal (April-October) and multi-annual (2007-2012) variation of δ O and O-enrichment of leaf cellulose (Δ O ) based on the Barbour-Farquhar model. Modelled δ O agreed best with observations when integrated over c. 400 growing-degree-days, similar to the average leaf lifespan observed at the site. Over the integration time, air temperature ranged from 7 to 22°C and midday relative humidity from 47 to 73%. Model agreement with observations of δ O (R  = 0.57) and Δ O (R  = 0.74), and their negative relationship with canopy conductance, was improved significantly when both the biochemical O-fractionation between water and substrate for cellulose synthesis (ε , range 26-30‰) was temperature-sensitive, as previously reported for aquatic plants and heterotrophically grown wheat seedlings, and the proportion of oxygen in cellulose reflecting leaf water O-enrichment (1 - p p , range 0.23-0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. Understanding physiological information in δ O requires quantitative knowledge of climatic effects on p p and ε .