Variation in total and volatile carbon concentration among the major tree species of the boreal forest

Understanding variation in carbon (C) concentration of live trees is essential for quantifying forest C stocks and validating forest C accounting models. Previous studies in boreal forests have assumed 50% C concentration or focused on species-specific C concentration estimation based on samples taken mostly from stemwood tissue of large trees. Yet, little is known about differences in C concentration between woody tissues or among trees of different sizes nor about the effects of life-history traits, such as shade tolerance and the role of volatile C on total C concentration in live trees. In this study, we examined variation in total and volatile C concentration in bark and stemwood tissues for trees of different sizes for six major North American boreal tree species. We found that bark had significantly higher total C and volatile C concentrations than stemwood and that both total C and volatile C concentration significantly varied among tree species. The average total C concentrations were 56.2% in the bark and 50.5% in the stemwood, and the average volatile C concentration were 5.8% and 3.0% for bark and stemwood, respectively. Furthermore, total C and volatile C concentration in stemwood and bark of almost all shade-intolerant species increased with tree size, whereas those of shade-tolerant species showed negative or neutral size-associated change. Our results show that volatile C concentration is a key driver of variation in total C concentration and highlights the importance of considering variation in C concentration when quantifying forest C stocks, which has important consequences for predicting future global C emissions scenarios.