Drought years of 2018 and 2019 affect CO2 balance of urban forest ecosystems in the Ruhr Metropolitan Region (Germany) differently

Forests are important ecosystems for mitigating CO₂. However, droughts affect the vitality of forests and alter CO₂ uptake. In worst cases, forest ecosystems can even turn from a carbon sink to a source in consequence of water shortage. Forest stands in urban areas are more prone to droughts because of elevated temperatures in comparison to rural land and unfavorable growth conditions such as limited rooting depth and low soil carbon content.

The drought years 2018 and 2019 in the Ruhr Metropolitan Region (Germany) were characterized by a 0.6 K higher mean annual temperature as normal and only 75 % of the normal annual precipitation. During this period, we investigated the CO₂ balance of urban forest ecosystems, considering annual changes in carbon stocks of tree biomass and litterfall and annual CO₂ effluxes from soil respiration, at eleven monitoring sites across the Ruhr Metropolitan Region by combining measuring and modelling approaches. The chosen sites represent the different urban forest types found here: old-grown semi-natural forests (beech, oak, maple), autochthon non-managed succession forests of birch, poplar or willow on brownfields and allochthone mixed forest stands planted in urban parcs and on heaps (urban greening forests).

Tree growth, leaf expansion, and CO₂ efflux decreased at nearly all sites in 2019 in comparison to 2018 in consequence of the ongoing drought. While the semi-natural forests were able to increase CO₂ uptake by 11 % in 2019, the urban greening forests decreased their CO₂ uptake by 62.9 %. The succession forests were CO₂ sources in both years but increased the CO₂ release in the second year by 85 % in comparison to the first year. Two sites turned from carbon sinks in 2018 to carbon sources in 2019. Correlation analyses showed that the soil hydraulic properties such as depth of the rooting zone, soil carbon content, and plant available water were the main influencing factors describing the decrease in tree growth and leaf development. Overall, the results indicate that, semi-natural forests on mesophilic sites are more resilient against droughts due to unlimited rooting zone, high soil carbon content, which favor the amount and accessibility of plant available water, while urban greening and succession forests are more vulnerable to droughts due to limiting rooting zone, low soil carbon content, and low plant available water. More vulnerable to droughts are also semi-natural forests on more extreme sites, like an examined Stellario-Carpinetum, which turned from a carbon sink in 2018 to a source in 2019. Furthermore, two patterns of seasonal changes in soil respiration were found in reaction to the drought. i) those of elevated soil respiration associated to elevated temperature in 2018 and decrease of soil respiration in 2019 in consequence of thermal denaturation of the microbial community, and one ii) those where, the mineralization activity was shifted to winter when the upper soil layer was rewetted, leading to larger soil respiration during the cold season.

Urban planners should ensure a deep rooting zone and carbon rich soils by establishing new urban forest stands to tackle drought periods.