Spontaneous tree growth and carbon dynamics during the first decade after removal of a coniferous monoculture at a humid temperate forest site

In September 2013, 8.6 hectares of a 70-year old Norway spruce (picea abies) monoculture were cleared in the newly founded national park Eifel (Germany) and left to spontaneous regrowth of the expected deciduous forest matching the site’s climate and soil conditions. The site is part of the 38.5 hectare experimental catchment “Wüstebach” (50° 30’N, 6° 19’E, 595 to 630 m a.s.l.), one of the core investigation sites of TERENO (TERrestrial ENvironmental Observatories, https://www.tereno.net). Most of the rest of the catchment is still covered by the original spruce monoculture. Its energy and matter exchange with the atmosphere, most notably of CO2, is monitored by an ICOS associated eddy-covariance station (DE-RuW) since 2010. In 2013 after the partial deforestation, a second flux station was installed near the centre of the clearcut. Due to an overpressure of game (boar and deer) in the area, 2 hectares of the central clearcut area are protected against grazing by a fence. CO2 budget and albedo results from the first four growing periods after the clearcut were presented by Ney et al. in 2019 (https://doi.org/10.1016/j.agrformet.2019.04.009). Here, we will give an update covering the first ten growing periods after deforestation (2014-2023). Most notably, regrowing vegetation on the initially almost bare clearcut turned it from a source back into a sink of atmospheric CO2 eight years after the deforestation. We will give an overview on how flux components (soil) respiration and gross primary productivity, season length and peak fluxes contributed to the difference between the spruce forest and the early and recent stages of the regrowing forest. For the last eight years, we recorded the species, height and partly the diameter of all spontaneous regrowing trees in the deforested area in a 10 m corridor both inside and outside the fence. Regrowth was strongly dominated by rowan (sorbus aucuparia, >1200 trees), a pioneer species propagated through their berries by birds that was present with at least one adult tree already before the deforestation beside further trees in distant surroundings. The next two important species were spruce and birch (betula pendula), whose seeds are propagated by wind. Rowan and birch grew in height approximately twice as fast as spruce. The presence of the protective fence affected all species, especially rowan, which grew more than twice as fast on the inside of the fence.