Harvesting intensity and tree species affect soil respiration in uneven-aged Dinaric forest stands
Introduction
The net carbon (C) flux in terrestrial ecosystems results from the balance between photosynthetic CO2 fixation and its release by ecosystem respiration. Temperate forest ecosystems act as a carbon sink (Lal and Lorenz, 2012) with soil respiration accounting for half of total ecosystem respiration (Yuste et al., 2005, Giasson et al., 2013). The rate of C loss from soil through respiration is, to some extent, a function of temperature and water availability, so the soil C balance is likely to be a sensitive indicator of climate change. This makes soil respiration a crucial component of carbon flux on Earth (Weiman, 2015), as it may release huge amounts of C stored in soil (Jian et al., 2018).
Soil respiration, measured often as CO2 efflux from the soil surface (Rs) (Maier et al., 2011), consists of respiration from roots, microbes, and soil fauna (Kuzyakov, 2006). The abiotic carbonate-derived CO2 contribution to soil efflux has been considered of minor importance (Werth and Kuzyakov, 2008, Schindlbacher et al., 2015), but in semiarid and arid environments with carbonate-rich soils, efflux could be significant (Emmerich, 2003, Huxman et al., 2004). Soil CO2 efflux varies temporally from seconds to inter-annual scale, and spatially on the plot to landscape level. Seasonal variations in Rs, observed in almost all ecosystems, have often been associated with changes in temperature, moisture, photosynthetic production, root growth and their combination (Yan et al., 2011, Acosta et al., 2018, Makita et al., 2018, Zhang et al., 2018). On the other hand, the spatial variability in Rs results from a large variability in soil physical properties, soil chemistry, fine-root biomass, fungi and bacteria, nutrient availability and others (disturbance and weathering) (Hanson et al., 1993, Fang et al., 1998, La Scala et al., 2000, Xu and Qi, 2001, Merbold et al., 2011, Allaire et al., 2012, Dore et al., 2014, De Carlo et al., 2019, D'Andrea et al., 2020). The differences in Rs among stands result also from the prevailing tree species, age and management practices (Peng et al., 2008, Akburak and Makineci, 2013, Wang et al., 2013).
Forest management, especially thinning and harvesting measures, has a significant impact on the forest carbon balance and may even result in altering the forest ecosystem from CO2 sink to CO2 source (Larson and Axelrod, 2017). As the forest floor becomes more open to light and precipitation, these conditions can increase the organic matter decomposition, causing substantial loss of carbon soil accumulated for decades. The most affected are organic soil horizons which, according to different studies on clear-cuts of a variety of forests, may lose around 30% of carbon (Nave et al., 2010, James and Harrison, 2016). The enhanced decomposition of soil organic matter may lead to higher soil CO2 efflux compared to that before the harvest despite a decrease in root respiration resulting from tree harvest (Londo et al., 1999, Darenova and Čater, 2020). A combination of decreased fresh organic input after harvest and increased decomposition of the existing organic material may result in carbon and nitrogen losses (Mayer et al., 2020), changes in C:N ratio (Rosikova et al., 2019), and contributes to an increase in CO2 concentration in the atmosphere (Denman et al., 2007). Studies about ecosystem respiration and net ecosystem balance of CO2 development after the harvest (Humphreys et al., 2006, Amiro et al., 2010, Williams et al., 2014) investigated the harvesting effect with soil CO2 efflux, as a main CO2 source, but have been performed during only one growing season. Knowledge of the longer-time changes in soil CO2 efflux after harvest is rare, especially for temperate European areas on sensitive forest soils. The period when CO2 efflux returns to the original levels after applied silvicultural measures and the time of primary production recovery are crucial factors of the disturbed forest ecosystem to become CO2 sink again.
Mixed uneven aged forests with predominating silver fir (Abies alba Mill.), beech (Fagus sylvatica L.), and Norway spruce (Picea abies Karst.) in the Dinaric region represent the largest continuous forest area in Central Europe (Horvat et al., 1974). Most of these forests were gradually transformed from old-growth conditions and have never experienced clear-cut silvicultural systems and extensive planting ((Boncina et al., 2013). They were managed with close-to-nature continuous-cover silvicultural systems like selection systems, irregular shelterwood or their combination, freestyle technique (Mlinšek, 1969), mimicking natural processes. Such approach helped to preserve a higher share of conifers and forest structure (Diaci et al., 2011) on sensitive, shallow high karst soils. So far, little evidence has been provided as to how disturbances and forest management with different harvesting intensities in these sensitive high karst Dinaric ecosystems affect soil processes, particularly soil CO2 efflux. The aims of this study were: 1) to determine soil CO2 efflux shortly after 50% and 100% harvesting intensity; 2) to compare harvesting effect in stands with three different predominating tree species, and 3) to determine the time when soil CO2 efflux after harvesting would return to the control state.
Section snippets
Site conditions
The research area, located in the southern part of Slovenian high karst Dinaric region (Habič, 1978), is distributed along three forest sites: Trnovo, Snežnik and Rog (Fig. 1), all belonging to Omphalodo-Fagetum association (Surina and Dakskobler, 2013) with silver fir, Norway spruce and European beech as predominating tree species with comparable forest structure, soil and climatic conditions (Table 1). All studied forest areas are included in the NATURA 2000 network.
The entire high karst
Light conditions
Forest stands indicated similar forest cover all over three forest complexes before harvesting, showing in all plot values of Gap Fraction (%), Openness (%) and Indirect Site Factor (ISF, %) (Table 2) without any significant differences between the studied plots and sites.
Light conditions on control plots were after the applied intervention similar to the conditions before harvesting, without significant differences: on plots with 50% harvesting intensity ISF increased up to 44–49% ISF and on
Soil CO2 efflux
The study sites are situated in the high karst Dinaric region, where CO2 can be released not only from organic matter decomposition but also form parent material. Plestenjak et al. (2012) reported that the vast majority (78–99%) of soil CO2 during warm periods between May and October (2008–2010) originated from the organic sources in a Mediterranean karst area and inorganic CO2 contributed 12% on average. Similarly, contribution of an inorganic pool of about 17% to total soil CO2 efflux was
Conclusions
The study was carried out at sites with long-term continuous forest coverage. Our results point out that soil respiration, and therefore soil carbon stock, of this type of forests is very sensitive to any harvesting operations. We observed a significant increase in soil CO2 efflux in plots with both 50% and 100% harvesting intensity. In beech stands, the initial increase in soil CO2 efflux was much bigger that in spruce and fir stands, but the LP when soil CO2 efflux returned to the level of
CRediT authorship contribution statement
Matjaž Čater: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Formal analysis, Supervision. Eva Darenova: Validation, Formal analysis, Writing - review & editing. Primož Simončič: Conceptualization, Methodology, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P4-0107 Program research group ‘‘Forest Biology, Ecology and Technology” at the Slovenian Forestry Institute, and Ministry of Agriculture, Forestry and Food (CRP projects V4-1820) and Man-For C. BD. (LIFE 09 ENV/IT/000078). Presented work was also supported by the Ministry of Education, Youth and Sports of the Czech Republic within the CzeCOS program (LM2015061). We are indebted to the
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