**1. Introduction**

Terrestrial ecosystems, such as forests, play a significant role in the global carbon cycle by sequestering carbon dioxide (CO2) through photosynthesis and the conversion of biomass into stable soil organic compounds [1–6]. However, carbon uptake and carbon allocation can be reduced by high vapour pressure deficits (VPDs) and soil water

**Citation:** Mensah, C.; Šigut, L.; Fischer, M.; Foltýnová , L.; Jocher, G.; Acosta, M.; Kowalska, N.; Kokrda, L.; Pavelka, M.; Marshall, J.D.; et al. Assessing the Contrasting Effects of the Exceptional 2015 Drought on the Carbon Dynamics in Two Norway Spruce Forest Ecosystems. *Atmosphere* **2021**, *12*, 988. https://doi.org/ 10.3390/atmos12080988

Academic Editors: Agnieszka Ziernicka-Wojtaszek and Andrzej Walega

Received: 21 June 2021 Accepted: 24 July 2021 Published: 31 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

deficits, commonly observed during periods with increased temperature and lack of precipitation [7,8]. These physiological responses to drought in plants vary at both local and global scales, depending on the type of plant species (based on the different levels of resilience to water stress), the local climatic condition, and other additional factors [9–13]. It has also been widely demonstrated that climate change increases the likelihood and severity of such drought events, especially within the temperate regions [14–18]. Thus, despite the resilience of some tree species, the projected increase in the occurrence and severity of extreme drought events will further reduce forest ecosystem photosynthesis in many parts of Europe, especially at unsuitable locations with sub-optimal climatic conditions [19].

Drought does not have a single definition and can be used in different contexts. However, typically, it is used to describe the periods with high VPD (meteorological drought) and low soil moisture (edaphic drought) [20]. The severity of the drought impact on carbon exchange depends on the site characteristics, duration and intensity of the drought periods [18,21,22]. In Europe, the 2003 summer drought period resulted in an estimated loss of about 30% in GPP (0.5 P gC y−1) over both the northern Mediterranean forests and temperate deciduous beech forests [23,24]. This was the result of the prolonged abnormal reduction in the soil moisture and high air temperatures below the wilting point and high air temperatures recorded across Europe during the summer period in 2003. The summer drought of 2015 was considered one of the most severe drought events in Europe after the 2003 summer drought [25]. During this period, much of the European continent (Poland, the Czech Republic, Slovakia, Western Ukraine, and Belarus) was severely affected in June and July 2015, due to the exceptionally high maximum daily air temperatures and the prolonged rainfall shortage by 31% since April [26–29]. Consequently, this had a significant effect on forest growth, as observed in both spruce and beech forests, within the Czech Republic. Nonetheless, spruce forest showed a higher sensitivity to drought as compared to beech forests [30,31].

The Norway spruce (*Picea abies* (L.) H. Karst) has significant economic and ecological importance within Europe [32]. However, its shallow root system makes it vulnerable to drought stress, especially at lower altitudes [33]. Since spruce thrives well in cold and humid regions, a significant decrease in soil moisture coupled with high air temperatures could have severe consequences on the spruce forest growth [34]. Therefore, the 2015 summer drought episode provided a good opportunity to further examine the response of spruce forest stands with different climatic conditions to extreme drought stress events.

The eddy covariance (EC) technique provides direct measurements of CO2 exchange between the atmosphere and the underlying ecosystem [35]. It is a convenient and widely used approach for observation of a forest stand carbon uptake and its dynamic response to environmental variables. Obtained CO2 fluxes are integrated over hundreds of square meters and resolved to half-hourly intervals [35,36]. Continuous micrometeorological and EC measurements over multiple years allow to detect drought periods and evaluate their impacts on CO2 exchange.

In this study, we hypothesized that the impact of extreme drought conditions will be more severe on the Norway spruce forest stand located in dry and hot climates, due to higher VPD and the sub-optimal supply of soil moisture in such drier climates. To test this hypothesis, we sought to evaluate the different effects of the 2015 drought during the main growing season in the wet and dry spruce forest ecosystems, compared to normal climatic conditions within the same period of two adjacent years (2014 and 2016). Secondly, we aimed to determine the influence of critical site-specific environmental factors (such as the VPD and soil volumetric water content) on the drought stress response at each of the spruce forest stands located in mountainous (around 900 m a.s.l) and highland (up to 600 m a.s.l) regions within the Czech Republic, using long-term eddy covariance CO2 flux data.
