The role of respired CO2 recycling in tree carbon allocation

Carbon (C) allocation is an important determinant of the C cycle of plants and ecosystems and its response to climate change and therefore its understanding is of paramount importance for global change science. C allocation has been generally described as the transfer of photosynthate from source to sink tissues, where partitioning between growth, respiration and storage occurs. The traditional view considers leaves as the only source of assimilated C, overlooking that respired CO2 can dissolve in the xylem sap and be transported to and recycled in remote plant organs, thus representing a potentially relevant C source. During periods when leaf photosynthesis is reduced, for instance during drought stress, recycling of respired CO2 could become increasingly important for the C balance and C allocation dynamics of trees. However, despite its potential importance, recycling of respired CO2 has never been considered in studies working on the response of C allocation to climate change. The main goal of the proposed research is to understand the role of the recycling of respired CO2 in the allocation of respiratory substrate to metabolic activity in trees, and how it is affected by drought. This will be achieved by applying stable C isotopes to trace the role of recycled CO2 as respiratory C source and to assess its importance for tree C allocation, and how it is affected by drought. As model plants we will use young beech trees. In a first stage of the proposed project, we will develop and refine a novel approach for tracing the recycling of respired CO2 into different organs. In a second stage, isotopic labeling experiments will be performed to analyze the contribution of respired CO2 recycling to the metabolic activity of different organs of control and drought-stressed trees. We will trace allocation dynamics of recycled CO2 to respiration processes at high resolution using laser spectroscopy and will analyze the fate of recycled CO2 in non-structural carbohydrate pools using compound specific isotope analysis. In a final stage we will modify an existing C allocation model to account for CO2 recycling as an additional source of respiratory substrate, and will apply it to quantify the role of CO2 recycling for tree C allocation dynamics in response to drought.

Carbon (C) allocation defines the flows of C between plant organs and their storage pools and metabolic processes and is therefore considered as an important determinant of forest C budgets and their responses to climate change. In trees, assimilates derived from leaf photosynthesis are transported via the phloem to above- and belowground sink tissues, where partitioning between growth, storage, and respiration occurs. At the same time, root- and aboveground respired CO2 can be dissolved in water and transported in the xylem tissue of the tree, thereby representing a secondary C flux of large magnitude. This secondary C flux of large magnitude is also identified as CO2 recycling mechanism, as it may contribute to the uptake of C by tree by recycling CO2 within tree tissues. In this project we provide the first detailed results of the upward transport of Cboth with the water taken up by the roots as well as the downward transport of C with the sugars derived from leaf photosynthesis. Moreover we tested a method to see whether we can study the recycling of CO2 in trees under drought, which is believe to gain importance as leaf photosynthesis is reduced under these conditions.In this project we used the C isotope 13C to trace the fate of C either transported with the xylem and phloem pathway based on labeling experiments. In a 1st experiment we infused the 13C label in the stem base of different 3 m tall saplings red oak to simulate the ascent of C within the tree. Based on detailed measurements of the 13C label diffusing from the stem and soil to the atmosphere we could understand how C was transported with the xylem water. The results showed that a large fraction of the C transported with the xylem diffused to the atmosphere on a very short term after labeling, which illustrates that researchers should be cautious when using chamber-based CO2 efflux measurements to estimate stem and soil respiration at different parts of the tree. Moreover detailed analysis of the 13C isotope in nonstructural carbohydrates hints that xylem-transported C can potentially be directly transferred in the phloem. In a 2nd experiment, we used a large labeling chamber installed over tree canopy of the same type of trees to study the transport of C from leaf (where photosynthesis occurs) to sink tissues. The observed patterns in the transport of C were very different as the ones observed for the xylem transport as this transport is more based on physiological processes. The decline in the label was much more lagged and showed that less 13C tracer was lost to the atmosphere on a short temporal scale. Finally, the method to test the recycling of C with trees under drought was performed based on the use of stem chambers, in which the 13C label concentration was increased. The main outcome of the project is that on one hand under well-watered conditions xylem C transport provides C assimilated in nonstructural carbohydrates in the leaf petiole and veins and that phloem and xylem C transport have different properties related to physical vs. physiological processes affecting transport rates, while on the other hand under drought conditions respired CO2 can be recycled to a lesser extent.