Effect of elevated atmospheric CO2 concentration on greenhouse gas exchange of common hazel trees and soils

Trees are known to emit and consume methane (CH₄) and nitrous oxide (N₂O), important greenhouse gases (GHGs). Most studies have focused on stems, whereas the role of tree leaves in forest CH₄ and N₂O exchange remains unknown. In recent decades, forests have been responding to changing environmental conditions, including increasing/elevated atmospheric carbon dioxide (eCO₂). However, the long-term effect of eCO₂ on tree CH₄ and N₂O exchange is almost unknown.  

We suggested that the conserved stomatal behavior under eCO₂ may directly affect N₂O and CH₄ fluxes from leaves or stems by altering transpiration, carbon assimilation and allocation, and indirectly soil N₂O and CH₄ fluxes by altering soil moisture and root exudation patterns.

At the Birmingham Institute of Forest Research’s Free Air CO₂ Enrichment (BIFoR-FACE) facility, we studied i) CH₄, N₂O and CO₂ exchange from soils, and stems and shoots of mature Common Hazel (Corylus avellana), and ii) the long-term effect of eCO₂ on this GHG exchange. The facility dominated by English Oak with sub-canopy hazel includes three arrays with +150 ppm CO₂ enrichment above the ambient (eCO₂) and three arrays under ambient CO₂ (aCO₂).

We measured GHG exchange from three hazel trees and three soil positions in each array and in one sunny aCO₂ plot in June 2025. Hazel trees at all arrays grow under low photosynthetically active radiation (PAR). Photosynthesis and transpiration were measured in parallel to GHG fluxes at all studied trees. The gas exchange was studied using static chamber systems and portable LiCOR analysers. 

The soil was a sink for CH₄ and a source for N₂O and CO₂. The nine years of eCO₂ enrichment tended to reduce the soil CH₄ uptake by 55%, and significantly increased soil N₂O and CO₂ emissions by 93 and 62%, respectively. The stem emissions of CH₄, N₂O and CO₂ were not affected by eCO₂. However, trees growing under sunny conditions showed significantly higher stem CO₂ efflux than shaded trees. The shoots were CH₄ sources irrespective of eCO₂ treatment. The shoots turned from being an N₂O source under aCO₂ to a weak N₂O sink under eCO₂ (non-significant change). The leaves exposed to eCO₂ showed higher CO₂ assimilation and transpiration rates compared to aCO₂. However, the leaves growing under sunny ambient conditions demonstrated much higher physiological activity than leaves under shaded ambient conditions. The eCO₂ seems to partly compensate the low PAR intensities at arrays, and approximates the light curve to the sunny leaves.

Concluded, eCO₂ seems to affect the GHG fluxes from soils rather than from hazel stems and shoots. The tree CO₂ exchange tends to be more related to PAR conditions than to the atmospheric CO₂ levels, mainly due to shaded conditions at arrays.  

 

Acknowledgement

This research was supported by the Ministry of Education, Youth and Sports of CR within programs LU-INTER-EXCELLENCE II [LUC23162] and CzeCOS [LM2023048], and project AdAgriF-Advanced methods of greenhouse gases emission reduction and sequestration in agriculture and forest landscape for climate change mitigation [CZ.02.01.01/00/22_008/0004635]. We thank Robert Grzesik and Kris Hart from BIFoR-FACE for all their field support.