Modeling biophysical controls on canopy foliage water 18O enrichment in wheat and corn

School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut, USA
Global Change Biology (Impact Factor: 8.04). 04/2012; 18(5):1769 - 1780. DOI: 10.1111/j.1365-2486.2012.02648.x


Leaf water 18O enrichment is an important factor controlling the H218O, C18OO, and O18O exchanges between the biosphere and the atmosphere. At present, there is limited capacity to explain the enrichment mechanisms in field conditions. In this study, three models of varying complexity were used to simulate the leaf water 18O enrichment at the canopy scale. Comparisons were made among the models and with high-frequency isotopic measurements of ecosystem water pools in wheat and corn. The results show that the steady state assumption was a better approximation for ecosystems with lower canopy resistance, that it is important to consider the effect of leaf water turnover in modeling the enrichment and not necessary to deal with time changes in leaf water content, and that the leaf-scale Péclet effect was incompatible with the big-leaf modeling framework for canopy-air interactions. After turbulent diffusion has been accounted for in an apparent kinetic factor parameterization, the mean 18O composition of the canopy foliage water was a well-behaved property predictable according to the principles established by leaf-scale studies, despite substantial variations in the leaf water enrichment with leaf and canopy positions. In the online supplement we provided a discussion on the observed variability of leaf water 18O composition with leaf and canopy positions and on the procedure for correcting isotopic measurements for organic contamination.

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    • "In this study, the slightly contaminated (BB < 1.2) xylem water ıD were corrected based on our calibration Fig. 7. Seasonal variations of ıD in soil stratum (0–100 cm) and xylem water (mean value of xylem water in P. massoniana, P. elliottii and C. lanceolata ± SE) between July 2011 and October 2013 at the experimental site. curves (Xiao et al., 2012). However, the correction curve for ıD (y = −0.23x "
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    • "(Chinese Ecosystem Research Network, CERN) (37°53′N, 114°41′E, 50 m[19])[6,8],[20],[17]Δδ>0, ; Δδ<0, 1 (Test1 Test2 Test3) "

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    • "where P represents the Péclet effect and d/dt is the time rate deriva- tive. WhileBarbour et al. (2000)provide compelling evidence for the Péclet effect at the leaf scale, recent works byXiao et al. (2010Xiao et al. ( , 2012) have shown that optimization of the Péclet parameters (effective length scale) results in anomalously small values, effectively zero, when applied at the canopy scale. They concluded that the leafscale physics do not have an analog at the canopy scale (i.e. this same diffusional pathway cannot take place within the canopy airspace, but rather is restricted to individual leaves). "
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