The Effect of Structural and Environmental Changes on Litter Decomposition of in Pinus Sylvestris Stands


 Background: Decomposition of litter has an important role in primary production with its influence on nutrient release for plant uptake and carbon flux in forest ecosystems. Thus, understanding the effects of the intervention on litter decomposition is crucial for sustainable forest management. In this study, the effect of structural change and facing slope on litter decomposition in Scots pine stands (Pinus sylvestris L.) were investigated.Results: The decomposition rate of litter decreased as the stand age increased. Litter decomposed more rapidly on northern slopes than southern slopes. Cutting caused to accelerate the decomposition at a rate of up to 58% depending on its intensity. The k values were found to fluctuate though the time from 0.189 in moderately dense canopied stands to 0.317 in open canopied over-mature stands. Stand basal area, incubation time, and remaining carbon concentration of the litter accounted for 75% of the variation in the decomposition constant. Conclusions: Cutting-induced stand structural changes affected the litter decomposition process in forest ecosystems due to the micro environmental change as well as the change in litterfall composition and chemistry. Heavy treatments can change the litter decomposition process drastically, while moderate thinning may not have a clear effect in the long run. The stand specific k value should be considered to use in forest carbon models for more accurate estimation. Decomposition constant should be calculated by considering stand structure and incubation time of at least 1050 days. Besides, due to the significant effect of canopy closure on decomposition rate, stand specific or recalculated k constants according to stand basal area, incubation time, and remaining carbon concentration of the litter should be recommended to use in forest carbon models for more accurate carbon budget estimation.

Conclusions: Cutting-induced stand structural changes affected the litter decomposition process in forest ecosystems due to the micro environmental change as well as the change in litterfall composition and chemistry. Heavy treatments can change the litter decomposition process drastically, while moderate thinning may not have a clear effect in the long run. The stand speci c k value should be considered to use in forest carbon models for more accurate estimation. Decomposition constant should be calculated by considering stand structure and incubation time of at least 1050 days. Besides, due to the signi cant effect of canopy closure on decomposition rate, stand speci c or recalculated k constants according to stand basal area, incubation time, and remaining carbon concentration of the litter should be recommended to use in forest carbon models for more accurate carbon budget estimation.

Background
Plant litter decomposition through microbes and fungi is the only process allowing recycle of chemical elements in forest ecosystems, and thus crucial for sustaining life on earth (Berg and Laskowski 2005). The decomposition of litter is an important process critical to understand the nutrient dynamics as well as the development of nutrient de ciency in a forest ecosystem and responses of the ecosystems to environmental disturbances (Prescott 2010). The most important product of litter decomposition in uences soil productivity and microbial biodiversity (Berg and McClaugherty 2014). In the sense of climate change, decomposition of litter has an important role in primary production with its in uence on nutrient release for plant uptake and carbon ux (Rodríguez et al. 2009). Organic matter decaying is an important source of carbon dioxide returned to the atmosphere which is the main greenhouse gas with a role in potential global climate change (Berg and McClaugherty 2014).
Silvicultural cutting treatments, that change the structure of stand by reducing canopy cover and basal area, have been applied in forest stands to maintain the growth or to establish a new generation by forest management. Seed cut, a silvicultural harvesting method, which takes all trees but the seed trees, has been widely used for natural regeneration of trees (Youngblood 1991;Schönenberger and Brang 2004) although much of the forests are regenerated by clear-cutting (Dougherty and Duryea 1991). Scots pine stands are commonly regenerated by using the seed cut method in Turkey. As another silvicultural method, thinning has been a common silvicultural practice applied in the forest to maintain the growth rate of stands for decades in forestry (Pourmajidian et al. 2010;Tian et al. 2010). All kinds of cutting alter the balance of the ecosystem. Although the effects of thinning on the stand growth, litter production, soil properties, and carbon sequestration were well documented, very few studies were conducted on the thinning effect on litter decomposition dynamics in Pinus sylvestris forests (del Rio et al. 2017). However, contrasting results have been presented by the studies. For example, Blanco et al. (2011) carried out a study in which two degrees of thinning with a control in young Scots pine stands at two different sites, namely the Mediterranean and continental sites, suggesting that decomposition rate decreased with increasing thinning intensity. In contrast, Kunhamu et al. (2009) suggested that high thinning intensities accelerated the decay rate of litter in an Acacia mangium stand. However, Will et al. (1983) reported a non-signi cant effect of thinning on the decomposition rate of Pinus radiata litter.
The effect of climatic factors such as precipitation, temperature, and evapotranspiration and litter chemistry on litter decomposition was determined by many researchers (Johansson 1994; Sariyildiz 2003; Prescott 2010; Kim 2012). However, less study has been carried out to determine the silvicultural cutting in uence on the decomposition rate (Cortina and Vallejo 1994;Kunhamu et al. 2009). On the other hand, litterfall components, including needles, cones, branches, differ in chemical composition and thus have different decomposition patterns (Hristovski et al. 2001;Berg 2014). However, most of the studies on litter decomposition have focused on the foliar litterfall so far. Therefore there is a gap of knowledge on the decomposition of total litterfall. The objective of this study was to evaluate the effects of stand structure and facing slope on litter decomposition. We hypothesized that mass loss and nutrient release are higher i) in sparse canopy cover than in dense canopy, ii) in young stands than in older, and iii) on the southern slope than on the northern slope.

Study Site
The study was conducted at pure Scots pine stands spreading over Sundiken Mountain range in the inner Anatolia region of Turkey (39º57 N, 31°09 E) on a well-drained luvisol (IUSS Working Group WRB 2015) developed from mica schist, with a sandy loam texture and pH of 5.6-6.0. Sampling was performed in naturally regenerated stands. The study area has a continental climate with low rainfall, with a mean temperature and rainfall of 9.5 °C and 486 mm, respectively. A detailed description of the study area was given

Sampling procedure
To cover a wide range of life cycle of the forest and to understand the stand structure effect on litter decomposition, sample plots were chosen from three developmental stages, i.e. young, mature, and overmature stages; and three classes of canopy cover, i.e., open forest (10-40%), moderately dense (40-70%) and very dense canopy (> 70%), except young stands because of the lack of open and moderately dense canopied stands, with four replications, 28 in totally (Table 1). To understand the effect of slope aspect on the decomposition rate, two of the replications were established on the northern slope, while the others on the southern slope. Mature and over-mature stands studied were exposed silvicultural treatments 5 to 8 years before the measurements by the local authority. Litterfall samples were collected by 0.5 × 0.5 m traps for two years to attain enough amounts of litterfall samples to be placed in litter bags. After having collected the samples in each sample plot, litter samples with an air-dried weight of approximately 10 g including needles, bark, branches, and cones, taking into account their share in the total litterfall were lled into the litter bags to determine the overall litter decomposition. The shares of litterfall components are given in Table 1. To determine the litter decomposition rate, 60 litter bags, made from nylon nets with 1 × 1 mm mesh and 20 × 15 cm in size, were lled with litter and placed on the ground of each sample plot. Four litter bag samples were collected from each plot every two to ve months for four years. The mean mass loss rate was calculated from the four litter bags of each plot. Laboratory methods Litter bag samples were oven-dried at 70 ºC until constant weight before the weighting. Mass loss was calculated by Eq. (1): Where M R is remaining mass (%), M 0 is the initial mass of litter (g), M t is the litter mass t time later (g) Accumulated carbon (C) and nitrogen (N) release were calculated by Eq. (2) Where k is decomposition constant, M 0 is the initial mass of litter (g), M t is the litter mass t time later (g) The samples were analyzed for carbon and nitrogen by the Dumas method using the LECO CNH analyzer (Leco Corporation, St. Joseph, Michigan). Nitrogen and carbon concentrations were standardized to initial concentrations by multiplying their concentrations by remaining mass percent.

Statistical analysis
Data was controlled by the Shapiro-Wilk test for the normal distribution, while the homogeneity of variance by Levene's test. When data normally distributed, variations in the mass loss by time and stand type evaluated by Repeated Measure ANOVA, taking the maturity, canopy, and facing slopes as between-subject factors. All data showed normal distribution and homogeneous variance. The Scheffe test was used to determine the homogenous groups.
Decomposition constant was estimated by multiple regression analysis, using the basal area of the stands m 2 ha − 1 ), stand age (year), the length of the incubation period of the litter (days), and accumulated C and N concentration (%) as independent variables. Equations with the highest R 2 , the lowest standard error, and constants with a signi cant level at P < 0.05 were given in this study.

Litter Mass Loss
The remaining mass of the litter and k constant varied signi cantly with time and stand canopy, as well as slope facing. Interactions between the time and stand canopy affected the litter decomposition. Canopy closure and slope facing also affected signi cantly decomposition dynamics (Table 2). At the end of the incubation time of 1367 days, 30% of the initial litter mass was remained undecomposed in young stands, while that of almost half in moderately dense and dense ones. Although the remaining mass was higher in the moderately dense forest in mature stands, it was low in dense over-mature stands. Open canopied stands had a lower remaining mass in both mature and over-mature ones. As the canopy became more open, the remaining mass of the litter decreased, except for moderately dense mature stands. An ascending trend was observed in stands from young to over-mature stage in remaining litter mass, from 30% in young stands to 49% in over-mature stands. Decomposition constant was found to be highest in young stands while the lowest in moderately dense mature stands with an increasing tendency from the dense canopy to open and young forest, with the exception of moderately dense mature stands (Table 3). Remaining masses of the litter were found to be very close to each other between northern and southern slopes in young stands, while those of northern slopes were lower than southern ones in both mature and over-mature stands. Litter was decomposed more in open canopied stands than in both moderately dense and dense canopied ones, except in mature stands on the northern slope. The decomposition process occurred more rapidly on the northern slopes (Fig. 1).
Decomposition constant showed an increase towards to end of the rst year in all stands, with the highest in young and the lowest in over-mature dense-canopied stands. After the third year of the incubation, k constants of the moderately dense and dense stands have remained steady, while those of young and over-mature open forests have continued to increase slightly. Differences in k constant between open and moderately dense canopied forests became pronounced after 687 days of incubation (Fig. 2).
Stand basal area alone accounted for 47% of the variation in k constant that calculated for 1367 days, while for 75% of that along with incubation days and remaining C. Basal area, incubation days, and remaining C in the litter were negatively related to k constant (Table 4). 1137. Subsequently, a slight decrease trend was observed after 1137 days ( Fig. 3a-b).
Carbon concentrations of the litter uctuated from 50-55% through the study period, with a signi cant difference (P < 0.01) according to repeated measures of ANOVA. Initial carbon concentrations among the stands were not signi cantly different, ranging from 51 to 54%. Cumulative carbon concentrations showed a steady decrease from 53-21%, approximately. At the end of 1367 days of incubation, 24% of the initial carbon remained undecomposed in the dense stands, while that of 15% in young stands ( Fig. 3c-d).
Initial N concentrations of the litters were signi cantly different among the stands, with the highest in young stands and the lowest in dense stands (P < 0.01). An increase in the N concentrations from dense stands to the open-canopied as well as the young ones was observed. However, there were no signi cant differences in initial carbon concentrations among the stands, although litter from dense stands had the highest carbon concentration with a mean of 54% and the moderately dense stands had the lowest with 51% (Table 5). Remaining litter mass was correlated with the initial N concentration at a signi cance level of P < 0.01. The remaining mass decreased with increasing N concentration of the litterfall (Fig. 4).

Litter decomposition
The decomposition of forest litter is a process mainly driven by environmental conditions and the quality of the 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 oor, favoring the environmental conditions for microbial activity. Therefore, the basal area gave a robust t 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.  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 ne 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.

Conclusions
In this study, it was concluded that cutting-induced stand structure change affected the litter decomposition process in forest ecosystems due to the micro environmental change as well as the change in litterfall composition and chemistry. Although moderate thinning could not change the litter decomposition process drastically, heavy treatments could do in the long run. Relatively higher precipitation reaching the forest oor with together temperature due to the decreasing of canopy might cause a rapid N mineralization, thus, leading to needle fall with reach in N concentration. However, there is a need for further researches on the effect of thinning on the nutrient re-translocation process. On the other hand, it is important to use decomposition constant which relies on long term incubation, with at least 1050 days. Besides, due to the signi cant effect of canopy closure on decomposition rate, stand speci c or recalculated k constants according to stand basal area should be recommended to use in forest carbon models for more accurate carbon budget estimation. The relationship between remaining mass in April 2016 relative to the initial mass of litter