Carbon dioxide and water vapour fluxes in a heathland and an evergreen forest in the temperate zone under contrasting annual and seasonal meteorological and environmental conditions

Temperate heathlands represent an important and protected biome in Europe. Yet, their environmental regulation in terms of carbon (C) and water vapour fluxes is understudied. Using the eddy covariance (EC) technique, we analyzed the seasonal and inter-annual CO 2 and water vapour fluxes of a temperate heathland (dominated by Calluna vulgaris L. ca. 45 years old) and compared them to the ones of an evergreen forest (dominated by Pinus sylvestris L. ca. 90 years old) in Flanders, Belgium, between 2021 and 2023. The year 2022 presented a severe summer drought whereas the years 2021 and 2023 presented wet summers but dry springs. The forest was a consistent CO 2 sink (-216 ± 61 gC m -2 y -1 ) compared to the heathland which was on average a small source (29 ± 9 gC m -2 y -1 ). Conversely, because of the lower evapotranspiration, the heathland showed larger water drainage than the forest (519 vs. 403 mm y -1 , respectively) and therefore larger ground water recharge. Despite the similar meteorology and soil, the heathland presented consistently lower soil water content than the forest and marked drought impacts such as sharp decline in CO 2 uptake and in evapotranspiration. The forest was less affected by drought, likely because Scots pine trees can access deep soil water thanks to their deep root system. These conditions resulted in the water use efficiency for the growing season being higher for the forest. Overall, this study describes a unique datasets of CO 2 and water vapour fluxes for the temperate heathland ecosystem. It shows that, compared to the forest, the heathland presented an expected larger water rechange, lower than expected C sequestration potential and larger than expected sensitivity of CO 2 fluxes to drought.