The combined study of C and O isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional responses to environmental change. The approach relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to infer changes in photosynthetic assimilation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humid-ity, temperature, nutrients). We review the mechanistic basis for a conceptual model, in light of recently published research, and discuss where isotopic observations don’t match our current understanding of plant physiological response to environment. We demonstrate that 1) the mod-el was applied successfully in many, but not all studies, 2), while originally conceived for leaf isotopes, the model has been applied extensively to tree ring isotopes in the context of tree physiology and dendrochronology. Where isotopic observations deviate from physiologically plau-sible conclusions, this mismatch between gas-exchange and isotope response provides valuable insights on underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. The dual isotope model helps to interpret plant responses to a multitude of environmental factors.

The combined study of C and O isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional response to environmental change. The approach relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to draw inferences about changes in photosynthetic assimi-lation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humidity, temperature, nutrients). We review the mechanistic basis for model and research to date, and discuss where isotopic observations don’t match our current under-standing of plant physiological response to environment. We demonstrate that 1) the model has been applied successfully in many, but not all studies, and 2), while originally conceived for leaf isotopes, the model has been applied extensively to tree ring isotopes in the context of tree phys-iology and dendrochronology. Where isotopic observations deviate from physiologically plau-sible conclusions, this mismatch between gas-exchange and isotope response provides valuable insights on underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. Thus, the dual isotope model helps to interpret plant responses to a multitude of environmental factors.

No abstract

<p>∙ To assess how belowground mycorrhizal networks may share resources, we used δ<sup>13</sup>C, δ<sup>15</sup>N, and C/N measurements to calculate spatial and temporal dynamics of carbohydrate and amino acid movement through ectomycorrhizal networks of mature trees.</p><p>∙ Canopies of 14 deciduous trees were continuously labeled with <sup>13</sup>C-depleted CO<sub>2</sub> from 2001-2005 (Swiss Forest FACE) and the <sup>13</sup>C label traced into ectomycorrhizal sporocarps.</p><p>∙ Sporocarps derived 69±5%, 30±6%, and 16±7% of their carbon from labeled trees in the elevated (beneath labeled trees), 0-6 m, and 6-12 m distances, respectively. Sporocarp δ<sup>13</sup>C correlated positively with C/N under elevated CO<sub>2</sub> and negatively elsewhere, reflecting that high-δ<sup>13</sup>C carbohydrates from surrounding trees contributed to sporocarps under elevated CO<sub>2</sub> and low-δ<sup>13</sup>C carbohydrates from elevated CO<sub>2</sub> trees contributed to sporocarps elsewhere. Sporocarp δ<sup>15</sup>N increased in <em>Cortinarius </em>with decreasing δ<sup>13</sup>C, suggesting that greater hyphal growth with elevated CO<sub>2</sub> sequestered <sup>15</sup>N-depleted N from sporocarp formation. Sporocarp log<sub>e</sub> C/N decreased during the 2004 growing season and the contribution of <sup>13</sup>C-depleted carbon from elevated CO<sub>2</sub> plants decreased at the 0-6 m and 6-12 m distances, suggesting decreased carbohydrate availability and network transport that year. In contrast, sporocarp log<sub>e</sub> C/N increased during the 2005 growing season and the contribution of <sup>13</sup>C-depleted carbon from elevated CO<sub>2</sub> plants increased at the 0-6 m distance, suggesting increased carbohydrate availability and network transport that year. Relative to other taxa, elevated CO<sub>2</sub> reduced C/N by 15% and ambient CO<sub>2</sub> increased C/N by 5% in taxa exclusively associated with deciduous trees, suggesting increased carbohydrate sharing by the deciduous-associated taxa.</p><p>∙ These patterns indicated that 1) carbohydrates (high C/N), not amino acids (low C/N), were preferentially transferred between regions differing in source δ<sup>13</sup>C, 2) sporocarp C/N reflected yearly plant productivity, 3) network transport was influenced by climate, and 4) taxonomy influenced transport dynamics belowground.</p>