Nitrogen concentration and physical properties are key drivers of woody tissue respiration

Background and Aims Despite the critical role of woody tissues in determining net carbon exchange of terrestrial ecosystems, relatively little is known regarding the drivers of sapwood and bark respiration. Methods Using one of the most comprehensive wood respiration datasets to date (82 species from Australian rainforest, savanna and temperate forest), we quantified relationships between tissue respiration rates (R-d) measured in vitro (i.e. 'respiration potential') and physical properties of bark and sapwood, and nitrogen concentration (N-mass) of leaves, sapwood and bark. Key Results Across all sites, tissue density and thickness explained similar, and in some cases more, variation in bark and sapwood R-d than did N-mass. Higher density bark and sapwood tissues had lower R-d for a given N-mass than lower density tissues. R-d-N-mass slopes were less steep in thicker compared with thinner-barked species and less steep in sapwood than in bark. Including the interactive effects of N-mass, density and thickness significantly increased the explanatory power for bark and sapwood respiration in branches. Among these models, N-mass contributed more to explanatory power in trunks than in branches, and in sapwood than in bark. Our findings were largely consistent across sites, which varied in their climate, soils and dominant vegetation type, suggesting generality in the observed trait relationships. Compared with a global compilation of leaf, stem and root data, Australian species showed generally lower R-d and N-mass, and less steep R-d-N-mass relationships. Conclusions To the best of our knowledge, this is the first study to report control of respiration-nitrogen relationships by physical properties of tissues, and one of few to report respiration-nitrogen relationships in bark and sapwood. Together, our findings indicate a potential path towards improving current estimates of autotrophic respiration by integrating variation across distinct plant tissues.