Temperature-sensitive biochemical 18O-fractionation and humidity-dependent attenuation factor of leaf water 18O-enrichment are needed to predict 18O composition of cellulose

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of cellulose (δ¹⁸O_cellulose). A new allocation-and-growth model was designed and added to the ¹⁸O-enabled soil-vegetation-atmosphere transfer model MuSICA to predict seasonal (April–October) and multi-annual (2007‑2012) variation of δ¹⁸O_cellulose and ¹⁸O-enrichment of leaf cellulose (Δ¹⁸O_cellulose) in a drought-prone, temperate grassland ecosystem. Modelled δ¹⁸O_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 δ¹⁸O_cellulose (R² = 0.57) and Δ¹⁸O_cellulose (R²= 0.74), and their negative relationship with canopy conductance, were improved significantly when both the biochemical ¹⁸O-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 ¹⁸O-enrichment (1 ‑ p_exp_x, range 0.23‑0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. These recently published results (Hirl et al. 2020, The New Phytologist, doi: 10.1111/nph.17111) demonstrate that disentangling the physiological and climatic information in δ¹⁸O_cellulose requires quantitative knowledge of direct climatic effects on p_exp_x and ϵ_bio.