Sulzer, M. | Search bioRxiv

bioRxiv2026

Seasonal dynamics and sun/shade heterogeneity of leaf gas exchange and VOC emissions inside a tall temperate forest canopy

Leaf gas exchange is the key driver of forest carbon uptake and directly determines forest carbon sink activity. Additionally, plants release a variety of biogenic volatile organic compounds (VOCs) acting as stress signals of trees. However, continuous hourly resolved measurements of leaf gas exchange and VOC emissions in tall tree canopies are challenging and remain scarce. To this end, we developed a sophisticated in-situ leaf gas exchange measurement system with 24 cuvettes deployed on mature Fagus sylvatica (n=3) and Pseudotsuga menziesii (n=3) individuals in a mixed temperate forest. We additionally measured sap flux density (Js), radial growth and tree water deficit (TWD) to gain a holistic picture of seasonal leaf and stem water and carbon flux dynamics during the summer of 2024. During midsummer, we found a gradual reduction of stomatal conductance (gs) and VOC emissions of sun, but not shade branchlets of P. menziesii in response to moderate atmospheric and edaphic drying. Decreased gs led to a downregulation of transpiration (E), Js, and carbon isotope discrimination accompanied by an increase in TWD and intrinsic water used efficiency. Leaf gas exchange of shade branchlets remained unaffected due to microclimatic buffering effects. Contrarily, sun leaves of F. sylvatica, profited from sunny midsummer conditions and increased leaf gas exchange, whereas shade leaves benefitted from more diffuse light during early summer exhibiting similar carbon assimilation, transpiration and VOC emissions as sun leaves. For both species we found a clear time lag of four to five hours between maximum leaf and stem water fluxes and a delay of up to 20 hours for the recovery of TWD, highlighting the role of stem water reserves. Pronounced seasonal and diurnal differences of leaf gas exchange, stem water fluxes and VOC emissions showed, that continuous data are essential to better understand variability of ecosystem flux dynamics.

plant biology↗

bioRxiv2025

Continuous chlorophyll fluorescence measurements trace sudden cold spell effects on photosynthetic efficiency in a temperate mixed forest

Air temperature extremes and fluctuations are expected to become more frequent and have been shown to affect the physiological functioning of temperate forests across Europe. The exposure to sudden cold spells during summer combined with high light intensities can lead to photoinhibition of photosystem II (PSII) and thereby reduce photosynthetic efficiency. In this study, we aimed to analyse the dynamics of photoinhibition as well as related protection mechanisms of tall tree canopies during such cold spells. Therefore, we continuously assessed leaf level chlorophyll fluorescence (ChlF) and ecosystem carbon fluxes by eddy covariance in a temperate mixed forest in southern Germany during growing season 2024. While the deciduous, broadleaved F. sylvatica indicated chronic photoinhibition as a response to sudden cold spells, the evergreen, coniferous P. menziesii showed higher tolerance to low air temperatures. Both species increased thermal energy dissipation at PSII during cold spells indicating the activation of protective mechanisms. Likewise, both species exhibited mainly the sustained form of non-photochemical quenching (NPQs) as a reaction to chilling temperatures and high light intensities which maintained elevated during the recovery phases. However, the dynamics of upregulation of photoprotection and recovery processes differed between the two tree species. Furthermore, the integration of ecosystem carbon exchange and continuous leaf level ChlF measurements gave valuable insights into the photosynthetic dynamics of the mixed forest canopy. This study emphasises the importance of the assessment of dynamic responses to future climate impacts on forest ecosystem on different scales.

ecology↗