Temperature-sensitive biochemical 18 O-fractionation and humidity-dependent attenuation factor are needed to predict δ18 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 (δ18 Ocellulose ) 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 δ18 Ocellulose and 18 O-enrichment of leaf cellulose (Δ18 Ocellulose ) based on the Barbour-Farquhar model. Modelled δ18 Ocellulose 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 δ18 Ocellulose (R2  = 0.57) and Δ18 Ocellulose (R2  = 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 (εbio , 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 - pex px , range 0.23-0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. Understanding physiological information in δ18 Ocellulose requires quantitative knowledge of climatic effects on pex px and εbio .