Temperature‐sensitive biochemical <sup>18</sup>O‐fractionation and humidity‐dependent attenuation factor are needed to predict δ<sup>18</sup>O of cellulose from leaf water in a grassland ecosystem
Regina T. Hirl, Jérôme Ogée, Ulrike Ostler, Rudi Schäufele, Juan C. Baca Cabrera, Jianjun Zhu, Inga Schleip, Lisa Wingate, H. Schnyder
Abstract
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 .