The decomposition of forest litter is a process mainly driven by environmental conditions and the quality of the substrate (Sariyildiz and Kucuk 2008; Berg and McClaugherty 2014; Karishna and Mohan 2017). Silvicultural treatments, such as thinning and seed cutting, can alter environmental conditions as well as production (Prescott et al. 2004; Tian et al. 2010) and composition of litterfall. Besides, the quantity and quality of the forest floor can be changed by residues left in the stand after the cutting treatments. In our study, lower remaining litter mass in young and open canopied forests compared to dense ones indicated that stand structure influenced on the decomposition rate of the forest floor by changing the environmental conditions, namely temperature and moisture in the stands. Besides, thinning operations can affect litterfall composition as suggested by Blanco et al. (2006). Specifically, needle litter was more dominant in young stands, leading to a high decomposition rate due to easily decomposable chemical composition. Cutting in forest results in a decrease of canopy closure and the basal area left, leading to more rainfall and light to reach the soil surface (Prescott et al. 2004; Tian et al. 2010). Thus, the activity of decomposers accelerates, and consequently decomposition rate of the litter increases. Despite low needle litterfall, an easily decomposable fraction due to low lignin and high nitrogen content, but a higher cones fraction, decomposition rates were higher in the open canopied stands, likely due to the prevailing effect of microclimatic condition on soil biota. Unlike the needles, cones have a woody structure, indicating that they include more lignin than needles do (Taylor et al. 1991; Font et al. 2009). High lignin content causes a slow break down in the forest floor (Sariyildiz and Kucuk 2008; Wang et al. 2018). Our results clearly showed that cuttings accelerated the decomposition of litter in a mature stand, meaning an increased carbon emission.
The remaining mass of litter in young stands which had a needle fraction of 87% of the total litterfall was close to some studies regarding needle litter decomposition (Berg and Staaf 1980; Laiho et al. 2004), likely due to similar climatic conditions and a high needles fraction of our study. On the other hand, Sariyildiz and Kucuk (2008) reported a higher decomposition rate compared to our results, likely because their results were for needle litterfall. Pausas (1997) reported that the k values varied between 0.221 and 0.283 for Scots pine stands, which were close to our results for moderately dense and open canopied forests but lower for young and higher for dense ones. An additional explanation may be the suitable climate of their study area with more moist conditions. On the other hand, Sariyildiz (2008) observed a closer decomposition rate to our results for young stand stands, with k values from 0.3 to 0.4. Janušauskaité et al. (2013) reported a lower decomposition rate for northern sites. This may be related to the retarding effect of low temperature on decomposition (Berg 2014; Krishna and Mohan 2017). Enez et al. (2015) found a lower decomposition rate of needle litter on scalped mineral soil than on non-harvesting activity areas under Scots pine stand due to the less microbial activity. Thus, litter decomposition dynamics of young stand might differ from those of mature ones due to the differences in composition of the litterfall and the changes in environmental conditions of the stands.
Many researchers reported that the slope facing affected decomposition rate with the faster on the northern slope than on the southern slope (Sariyildiz and Kucuk 2008; Jasińska et al. 2019). On the other hand, some researchers reported a faster decomposition of litterfall on the south aspects than on the north (Mudrick et al. 1994; Qualls 2016), which was in line with our results. Furthermore, in open canopied mature stands, decomposition rate was found to be faster on southern slopes in comparison to the north in contrast to in the case of the dense canopy, likely due to age- and light-related changes in litterfall composition.
We did not determine the decomposition rates of the litter fractions separately in our study since all litterfall fractions make up the forest floor and decompose together. To take into account only a certain fraction of litterfall such as needles or leaves could result in an overestimation of litter decomposition rate.
Temporal changes in k constant and estimation models
Decomposition constant trends showed different patterns due to changes in the chemical composition of litter and microbial diversity during decomposition process (Yue et al. 2018), indicating that k constant would be stabilized after approximately 1000 days in dense and moderately dense forest, while not in open as well as young stands. In addition, k constant might continue to increase slightly until 1400 days in open-canopied and young stands. Therefore, k values might be underestimated in young stands, as well as heavily thinned stands, in case of incubation time shorter than 1400 days. Our results partly confirmed the Berg et al. (2010), who proposed that 1100 to 2000 days of the period were sufficient for limit values of litter decomposition. Still, there is a need for further studies on the decomposition rate in young and mature stands with heavily treated.
Basal area is an important parameter that can be used for evaluating the stand structure and decreased by silvicultural interventions as well as natural or human-induced disturbances. A decline in the basal area, meaning also a decrease in canopy cover, leads to more sunlight and rainfall to reach the forest floor, favoring the environmental conditions for microbial activity. Therefore, the basal area gave a robust fit with k constant.
Nitrogen and carbon releasing
Our results showed that initial N concentrations of litter were related to the decomposition rate. Higher N concentrations in young stands also supported this relationship. An increase in the N concentration causes to decrease in C: N ratio, which is an index for litter decomposition. Sun et al. (2016) also reported a decreasing trend in N concentration of senesced pine needles from young to older stands. The decreasing trend in N with stand age might be related to changes in soil nutrient availability and forest growth rates during stand development, as pointed out by Sun et al. (2016). Besides, the concentrations tended to increase slightly with the canopy structure of the stands became more open, indicating that cuttings might result in changes in nutrient withdrawal before needle shed in pines in contrast to Blanco et al. (2009) reported. But our results confirm the findings of Berg et al. (1995), who reported that N concentration in needle litter of Scots pine was mainly related to climatic conditions. On the other hand, the initial N concentration of the litter for the dense forests of our study was similar to the results of Blanco et al. (2006) for needle litter of Scots pine stands in sites with a cold wet Mediterranean climate. However, they reported lower values for sites with a cold wet continental climate than our results. They also suggested that thinning affected N concentration of needle litter.
In this study, higher initial N concentration of litter resulted in a higher decomposition rate as revealed by many researchers (Gao et al. 2019; Sariyildiz 2003; Zhang et al. 2008) in contrast to Berg (2000). The enhancing effects of N on decomposition rate may be related to rich microbial diversity supported by the high nutritious value of the litter (Gao et al. 2015), which has a high proportion of needles that contains higher nutrients compared to other litter fractions in young stands. On the other hand, despite the lower nutrient content likely due to the inclusion of a higher portion of fine woody litter, the decomposition rate in open-canopied stands also was higher than in the moderately thinned and un-thinned stands likely because of the prominent effects of temperature and humidity on the decomposition process.