Temperature‐sensitive biochemical 18 O‐fractionation and humidity‐dependent attenuation factor are needed to predict δ 18 O of cellulose from leaf water in a grassland ecosystem

Summary We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ 18 O cellulose ) in a drought‐prone, temperate grassland ecosystem. A new allocation‐and‐growth model was designed and added to an 18 O‐enabled soil–vegetation–atmosphere transfer model (MuSICA) to predict seasonal (April–October) and multi‐annual (2007–2012) variation of δ 18 O cellulose and 18 O‐enrichment of leaf cellulose (Δ 18 O cellulose ) based on the Barbour–Farquhar model. Modelled δ 18 O cellulose 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 18 O cellulose ( R 2  = 0.57) and Δ 18 O cellulose ( R 2  = 0.74), and their negative relationship with canopy conductance, was improved significantly when both the biochemical 18 O‐fractionation between water and substrate for cellulose synthesis ( ε bio , 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 18 O‐enrichment (1 –  p ex p x , range 0.23–0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. Understanding physiological information in δ 18 O cellulose requires quantitative knowledge of climatic effects on p ex p x and ε bio .