Close-up of glass containers tied to a tree trunk.

Respiration and carbon flow in trees and soils

I study how carbon flows through trees and soils. I am particularly interested in linking processes to gas exchange measurements, especially in respiration. Gases provide us a “window” through which we can see the respiratory substrates, the active enzymes, and other reactive processes. For example, in my PhD, I developed the measurement of CO2/O2 fluxes ratio. By theory, this ratio shows which substrate fuels respiration. In practice, I found that in tree stems and soils, the ratio is often below substrate values. In the tree stems the low ratios can be explained by non-respiratory processes that consume CO2 like dark CO2 re-fixation, while in soils oxidative breakdown of macromolecules that consume O2 can explain the observations. Currently, I am exploring the use of this ratio in separating respiration of roots and mineral soil.

Another area I research is how aboveground tissues and fine roots use storage compounds. Fine roots consume a large portion of forests carbon budget. But because they are hidden belowground, we know surprisingly little about their carbon cycles. Here, I use radiocarbon measurements of different carbon pools to understand turnover rates and storage compounds use. The research is taking place in the Alpine tree line and in temperate forest. 


Journal Article (9)

Journal Article
Rog, I.; Hilman, B.; Fox, H.; Yalin, D.; Qubaja, R.; Klein, T.: Increased belowground tree carbon allocation in a mature mixed forest in a dry versus a wet year. Global Change Biology 30 (2), e17172 (2024)
Journal Article
Salomón, R. L.; Helm, J.; Gessler, A.; Grams, T. E. E.; Hilman, B.; Muhr, J.; Steppe, K.; Wittmann, C.; Hartmann, H.: The quandary of sources and sinks of CO2 efflux in tree stems - new insights and future directions. Tree Physiology (accepted)
Journal Article
Helm, J.; Muhr, J.; Hilman, B.; Kahmen, A.; Schulze, E. D.; Trumbore, S. E.; Herrera-Ramirez, D.; Hartmann, H.: Carbon dynamics in long-term starving poplar trees—the importance of older carbohydrates and a shift to lipids during survival. Tree Physiology, tpad135 (2023)
Journal Article
Helm, J.; Salomón, R. L.; Hilman, B.; Muhr, J.; Knohl, A.; Steppe, K.; Gibon, Y.; Cassan, C.; Hartmann, H.: Differences between tree stem CO2 efflux and O2 influx rates cannot be explained by internal CO2 transport or storage in large beech trees. Plant, Cell and Environment 46 (9), pp. 2680 - 2693 (2023)
Journal Article
Hilman, B.; Weiner, T.; Haran, T.; Masiello, C. A.; Gao, X.; Angert, A.: The apparent respiratory quotient of soils and tree stems and the processes that control it. Journal of Geophysical Research: Biogeosciences 127 (3), e2021JG006676 (2022)
Journal Article
Helm, J.; Hartmann, H.; Göbel, M.; Hilman, B.; Herrera, D. A.; Muhr, J.: Low-cost chamber design for simultaneous CO2 and O2 flux measurements between tree stems and the atmosphere. Tree Physiology 41 (9), pp. 1767 - 1780 (2021)
Journal Article
Hilman, B.; Muhr, J.; Helm, J.; Kuhlmann, I.; Schulze, E. D.; Trumbore, S. E.: The size and the age of the metabolically active carbon in tree roots. Plant, Cell and Environment 44 (8), pp. 2522 - 2535 (2021)
Journal Article
Pries, C. H.; Angert, A.; Castanha, C.; Hilman, B.; Torn, M. S.: Using respiration quotients to track changing sources of soil respiration seasonally and with experimental warming. Biogeosciences 17 (12), pp. 3045 - 3055 (2020)
Journal Article
Weiner, T.; Gross, A.; Moreno, G.; Migliavacca, M.; Schrumpf, M.; Reichstein, M.; Hilman, B.; Carrara, A.; Angert, A.: Following the turnover of soil bioavailable phosphate in mediterranean savanna by oxygen stable isotopes. Journal of Geophysical Research: Biogeosciences 123 (6), pp. 1850 - 1862 (2018)


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