Vegetation in the shadow of radiation: disentangling the cooling mechanism in a drained peatland ecosystem

Peatland ecosystems play a critical role in climate regulation by storing carbon and modulating energy fluxes. During evapotranspiration, radiative energy is converted into latent heat, which cools the atmosphere. Ecosystem energy fluxes which are strongly influenced by climate conditions can be tightly coupled to CO₂ fluxes through vegetation functioning. The relationship between carbon and energy fluxes in peatland ecosystems however remains relatively underexplored. Here we analyze eddy covariance measurements from a degraded raised bog in Amtsvenn-Hündfelder Moor (DE-Amv), located in North Rhine-Westphalia, Germany, to investigate the link between CO₂ uptake, canopy conductance and energy fluxes. DE-Amv has been part of the Natura 2000 network since 2004, with its flora dominated by Calluna vulgaris (L.) Hull and Molinia caerulea (L.) Moench. The dataset covers the entire year of 2023. We used the data to (1) examine the seasonal cycles of radiative and turbulent energy fluxes, and (2) evaluate the relationship between CO₂ uptake and energy fluxes. To investigate the ecophysiological drivers of latent heat flux (LE), we estimated canopy conductance (G_c) by inverting the Penman-Monteith equation and modelling a continuous time series of G_c over the study period.

Our results showed that the mean daily peaks of latent heat flux ranged from 8.5 W m⁻² to 215 W m⁻² in one year, with LE being strongly influenced by vegetation productivity (i.e., Gross Primary Productivity, GPP). Principal component analysis showed that GPP, vapor pressure deficit, and net radiation are the key drivers of LE dynamics (r > 0.85 for all variables). During the vegetation growing period (March to October) G_c ranged from a minimum daily value of 1.2 mm s^-1 in spring and autumn, to a maximum daily value of 15 mm s^-1 in August. While G_c was primarily driven by relative humidity during the colder months, it was mainly driven by net radiation from June to September, and it was not limited by VPD or soil moisture.

This study demonstrates how ecosystem eddy covariance flux measurements can quantify the stomatal regulation of energy fluxes in peatland ecosystems. By highlighting the strong coupling between energy and CO₂ fluxes, we emphasize the importance of understanding how environmental factors, particularly atmospheric vapor pressure deficit (VPD) and soil moisture, constrain G_c. Such insights are vital for predicting the effects of drier climatic conditions on the cooling capacity of drained peatlands, where vegetation type and management significantly influence their cooling potential.