Forest re-establishment after wind throw How and when can the young keep up with the old?

Climate change and the potential increase of natural hazards make it necessary to regard the effect of wind throws in forest on CO2-sinks in order to arrive at reliable estimates of future CO2-sinks. In 2007 a severe winter storm resulted in large wind throw areas in many forests in Germany, leading to clear-cuts and re-planting of forests. We present data of an oak plantation, which was established by planting seedlings of Quercus robur and Quercus rubra at such an area in 2008. In 2009, a tower was installed at the site (DE-Hzd) and measurements of the CO2 exchange started based on eddy-covariance and complemented by additional meteorological measurements. This early start of measurements after planting made it possible to observe the different stages of the young oak plantation. Additionally, we take advantage of an existing long-term eddy-covariance site above a spruce forest (Picea abies; DE-Tha), which represents an old mature forest (> 120 years old). Both sites are situated in southeast Germany in a large forested area (6000 ha). DE-Hzd is situated approx. 5.5 km west of DE-Tha only. Therefore, general climatic characteristics are quite similar but marked differences concerning soil exist. In the first eight years of measurement, the oak site did act as a CO2 source, i.e. more CO2 was released than sequestered by the young trees and any additional pioneering vegetation. This CO2 source could be as large as the CO2 sink of the old spruce forests at an annual basis. Therefore, the CO2 sink per area of the old spruce forest was almost totally offset by the CO2 source per area of the young oak plantation. The transition of the oak plantation from a CO2 source to a CO2 sink took place very quickly (from one year to the other) without the source having steadily decreased in previous years. Since 2017 the oak plantation has act as CO2 sink but the sink remained smaller than those of the old spruce forest. We hypothesise that very warm years (2018, 2019, 2020) and vegetation growth (canopy closure) contributed to the shift. This stresses the extremely important role of disturbances such as wind throws for regional CO2 budgets. Fig. 1: (a) Yearly sums of net ecosystem exchange (NEP) for the oak plantation and the spruce stand, (b) scatter plot and (c) ratio NEPoak/NEPspruce.