| Popis: |
We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of cellulose (δ18Ocellulose). 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 δ18Ocellulose and 18O-enrichment of leaf cellulose (Δ18Ocellulose) in a drought-prone, temperate grassland ecosystem. Modelled δ18Ocellulose 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 δ18Ocellulose (R2 = 0.57) and Δ18Ocellulose (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 (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 18O-enrichment (1 ‑ pexpx, 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 δ18Ocellulose requires quantitative knowledge of direct climatic effects on pexpx and ϵbio. |
