Leaf Litter Decomposition Rate in the Karst Ecosystem of the Community Forest in Wonodadi Village, Pracimantoro, Wonogiri, Indonesia

2.3.1. Litter decomposition

The measurement of the litter decomposition rate was carried out using the litter bag method. Data were collected with a litter bag weighing 20 grams/bag by arranging the litter, especially on large leaves, so that layering does not occur, thus slowing down microbial respiration in decomposition (Berg & McClaugherty, 2008; Devianti & Tjahjaningrum, 2017). And then, a litter bag was placed on the forest floor (Devianti & Tjahjaningrum, 2017; Susanto et al., 2021) and taken every two weeks (Salim & Budiadi, 2014; Karina et al., 2022). Furthermore, the litter will be dried using an oven at a temperature of 105 °C for 24 hours until the weight is constant (Kusmana & Yentiana, 2021), then the weight reduction is calculated. The analysis of the decomposition rate uses the Olson (1963) equation as shown in Eq. (1) (Aerts, 1997; Kusmana & Yentiana, 2021). Data analysis was carried out using SPSS version 16.

(1)

Notes: k = litter decomposition rate; T = observation time (weeks)

2.3.2. Environmental Parameters

Environmental parameter measurements were conducted every 15 days in the morning (07:00 - 09:00 WIB), afternoon (12:00 - 14:00 WIB), and evening (16:00 - 18:00 WIB) (Annisa et al., 2015) along with the day the samples were taken. Measurement of temperature and humidity using a thermohygrometer. The results obtained were then presented as a daily relative average table. Data analysis was calculated using Eq. (2) (Setyowati, 2008; Annisa et al., 2015; Simbolon et al., 2022).

(2)

Notes: E = environmental parameters; E1 = morning measurement; E2 = afternoon measurement; E3 = evening measurement

3.2.1. Temperature

The environmental parameters at the study site are presented in Table 3. The average daily air temperature in the Pracimantoro karst community forest is 30.5°C. This value is slightly higher than the average air temperature in karst ecosystem forests in China, which was 30.3°C (Zhu et al., 2021). Based on the classification index by Setyowati (2008), the average air temperature in the Pracimantoro karst community forest indicates that the area has a hot microclimate.

Community forests with a monoculture pattern have the highest average daily temperature of 31.65°C. The high temperature is due to the species planted on monoculture land having light canopies. The thin canopy allows high light intensity to penetrate to the forest floor, resulting in low air humidity under the stand (Siagian et al., 2021). Indirectly, air temperature significantly affects the reduction of litter mass because it also influences microbial activity (Deviani & Tjahjaningrum, 2017).

3.2.2. Relative humidity

Based on Table 3, the average relative humidity in the karst community forest was 62.55%, with the highest average of 64.82% in agroforestry land. Susanti & Halwany (2017) stated that air temperature in an area is inversely proportional to its air humidity. This is consistent with the results of this study, showing that community forests with higher humidity have relatively lower air temperatures. The classification of climate conditions based on air humidity by Setyowati (2008) indicates that the Pracimantoro karst community forest has a dry climate, as it has a relative humidity of <70%.

Air humidity is a critical environmental factor in the litter decomposition process because it affects the presence of decomposing microorganisms. Appropriate humidity (>70%) can accelerate litter decomposition even in areas with high air temperature (Deviani & Tjahjaningrum, 2017). Jupri et al. (2024) stated that air humidity in an area is influenced by air temperature, light intensity, air pressure, water availability, and vegetation. Agroforestry community forests have vegetation with medium to dense canopies, creating higher air humidity. However, the karst community forest, with air humidity below 70%, experiences a slower rate of decomposition.

3.2.3. Soil pH

Based on the results of field analysis, the soil pH in the Pracimantoro karst community forest ranged from 7.0 to 8.0. The research by Prabowo et al. (2024) also showed that the soil pH in the Pracimantoro karst community forest ranged from 7.56 to 8.03. This indicates that the soil in the karst community forest is neutral to slightly alkaline. The karst structure, consisting of limestone (CaCO₃), results in high base cations, making the soil alkaline. Muhsin et al. (2017) stated that the optimal soil pH for litter decomposition is 6.5 to 7.0. Furthermore, Nursal et al. (2015) added that alkaline soil can interfere with the activity of decomposing microorganisms, causing the decomposition process to proceed slowly. In their research, Susanto et al. (2021) concluded that soil pH has a 50% influence on the litter decomposition process when combined with appropriate humidity.

3.2.4. Rainfall

Rainfall data obtained from the Central Statistics Agency of Wonogiri Regency over a period of 10 years (2014–2023) show that Wonogiri Regency has rainfall ranging from 1,108 to 3,066mm/year, with an average of 2,084.1mm/year. According to the classification by Schmidt and Ferguson, Wonogiri Regency falls under climate type C (rather wet), with a Q value of 55.89%.

Based on data from the Ministry of Public Works and Spatial Planning, Directorate General of Water Resources, BBWS Bengawan Solo (2024), rainfall in Wonogiri Regency averaged 134.43mm, and during observations from May to July, it averaged 25.89mm. According to the Meteorology, Climatology, and Geophysics Agency (BMKG), the average rainfall in that year indicates that Wonogiri Regency experiences medium rainfall, while the average monthly rainfall during that period is classified as low. Deviani & Tjahjaningrum (2017) stated that rainfall affects humidity and environmental temperature, and high rainfall can facilitate litter breakdown, accelerating decomposition. The low rainfall during the research period made temperature and humidity less supportive of the decomposition process.

3.2.5. Light Intensity

The light intensity in the three types of planting patterns in the community forest ranged from 78.11% to 87.55%, with an average of 83.06% (Table 3). Light intensity in different types of community forests is influenced by canopy density. Siagian et al. (2021) stated that a dense canopy creates shade, preventing sunlight from reaching the forest floor. The light intensity produced by solar radiation can also accelerate the photodegradation of recalcitrant chemical compounds (Huang et al., 2024).

Community forests with an agroforestry planting pattern have the lowest light intensity. The diverse strata and wide canopy of the constituent stands are influencing factors (Siagian et al., 2021). In contrast, monoculture land has the highest light intensity because the planting pattern includes Acacia and Bintaro, both of which have light canopies. The temperature and humidity of the air beneath the stand are also impacted by light intensity (Nurjanto et al., 2016). Denser stands block more sunlight from reaching the forest floor, resulting in lower light reception (Simbolon et al., 2022), which in turn increases humidity levels.

3.3.1. Litter Weight Reduction

The process of leaf litter decomposition can be seen directly by observing the color changes of the leaves along with changes in their physical structure (Fig. 2). The initial stage of litter decomposition is the occurrence of physical changes in the litter, which will become more intensive over time (Karina et al., 2022). Color changes in leaf litter can occur due to the breakdown of the green leaf substance (chlorophyll) in the leaves, which were originally green, turning yellow, brown, and changing to other darker colors (Berg & McClaugherty, 2008). Physical changes in leaf litter during observation did not show any striking changes (Fig. 2). However, the structure of the litter began to become brittle and break and crack. Changes in litter structure are presented in Fig. (2a) (monoculture), Fig. (2b) (polyculture), and Fig. (2c) (agroforestry). Damage to the leaf litter structure on monoculture and agroforestry is more visible when compared to damage to the leaf litter structure on polyculture land. Siagian et al. (2021) stated that the decomposition process will change the structure of leaf litter to become more brittle and then form flakes.

Dominantly, the litter decomposition process is accompanied by a decrease in the dry weight of the litter. The diverse composition of the stand causes variation in litter weight loss rates in the field. Litter weight loss occurs fluctuatingly depending on supporting factors over time (Nursal et al., 2015). The decrease in litter weight in the Pracimantoro karst community forest is presented in Fig. (3).

The average final litter weight, based on Fig. (3), in agroforestry, polyculture, and monoculture community forests was 13.04 g, 14.23 g, and 12.90 g, respectively. The most significant decrease in litter weight was observed in monoculture community forests, at 35.50%. This value is slightly higher than the weight loss in agroforestry community forests, which reached only 34.78% of the initial weight. The magnitude of litter weight loss can be influenced by various factors, including environmental and climatic conditions, as well as the quality of the litter from the standing species.

The results shown in Fig. (4) indicate that the average percentage of dry litter weight loss in all types of community forests over 2 months was 33.05%. The decrease in dry litter weight increased over time. The percentage decrease in litter weight observed in this study is smaller than that reported by Rumambi et al. (2018), who found a weight loss of 32.37% during one month of decomposition, but larger than that reported by Karina et al. (2022), who observed a 10.4% decrease over 3 months.

Polyculture community forests with environmental conditions that are almost similar to agroforestry forests tend to experience the slowest decrease in litter weight (Fig. 4). The decrease in litter weight is only 28.85% of its initial weight (Fig. 3). This is due to the dominance of stands with low litter quality so that they are difficult to decompose. These stands include: mahogany, teak, and acacia. Susanto et al. (2021) stated that the loss of litter weight is caused by differences in litter chemistry.

Based on Table 4, the three dominant species in the study site have high lignin and phenol chemical content, so they have low litter quality and are difficult to decompose. Salim & Budiadi (2014) emphasized that a land will experience a slow decomposition process if it is dominated by stands that have high lignin and polyphenol content in their litter. According to Hairiah et al. (2006), high-quality litter is litter with a C/N ratio of <25%, lignin <15%, and polyphenol content <3% which will be easily decomposed. Lignin and polyphenol content are difficult to decompose because of their resistance to decomposer bacteria (Darmayanti & Rindyastuti, 2012). Aerts (1997) stated that the lignin content of evergreen stands is relatively high, so that it can control the speed or slowness of the litter decomposition process. Mahogany and Acacia have higher lignin than teak because both are included in evergreen stands, while teak is not (Table 4).

In high concentrations, lignin and polyphenol content can inhibit the rate of decomposition, whereas the N and P content in the litter can accelerate the rate of decomposition (Aerts, 1997; Huang et al., 2024). Therefore, the litter weight loss in monoculture community forests was highest because mahogany and acacia, as their constituent species, have higher N and P content than teak, which is not their constituent species (Table 4). Furthermore, agroforestry community forests have the second-highest weight loss below monoculture forests because, in the agroforestry land, the three stands above are not the primary constituent stands but are combined with other stands.

3.3.2. Litter Decomposition Rate

The rate of leaf litter decomposition in several forms of community forest land management is presented in Fig. (5). The litter decomposition rate was illustrated by the decomposition constant (k). The results of the analysis show that the average decomposition rate is 0.0268. The highest decomposition rate is on land with a monoculture planting pattern of 0.0293, then in an agroforestry planting pattern of 0.0277, and the smallest decomposition rate of 0.0229 in community forests with a polyculture planting pattern (Fig. 5). The decomposition rate is influenced by environmental conditions such as vegetation, temperature, air humidity, and rainfall. [16] Vegetation density and the amount of rainfall are the main factors causing high or low temperatures and air humidity in an area.

The decomposition rate was calculated from the weight loss that occurred during the research process. The litter decomposition process occurs in two stages, namely the reduction in water content of the litter along with dissolved chemicals in water, such as glucose and phenol compounds, and further decomposition with the breakdown of complex litter chemicals such as lignin and cellulose (Nursal et al., 2015).

Based on the measurements carried out, Wonodadi Village has low rainfall, a dry and hot climate, so the decomposition process is slow. It was recorded that the Pracimantoro karst community forest had a very low k value when compared to research at the Nglanggeran Gunungkidul Ancient Volcano by Moro et al. (2016), which had a positive k value ≤3 as a weathering constant. However, the results of the measurement of the litter decomposition rate are higher than the research of Karina et al. (2022) at KHDTK, Bengkulu University, whose highest decomposition constant value was <0.02. Even so, the decomposition rate that occurs in the Pracimantoro karst community forest is difficult to observe because it has a very small k value (<0.1). Moro et al. (2016) stated that the decomposition constant value (k) <0.1 will be difficult to observe, especially during the dry season.

Based on Fig. (5), the decomposition rate that occurs in the karst community forest has a number that is not much different from one another. The k value tends to increase in all forms of land management as the observation time increases. Deviani & Tjahjaningrum (2017) stated that the suppression effect of litter decomposition will occur since the litter shrinks by 30 - 40% of the initial weight of organic material. Microclimate conditions in an area will affect the presence of decomposing microorganisms and affect their speed in decomposing litter (Karina et al., 2022).

An environment that is always wet and humid will accelerate the decomposition process of litter on the forest floor. The hot and dry environmental conditions in the Pracimantoro karst forest have an impact on the slow rate of decomposition in the area. Berg & McClaugherty (2008) said that the value of the litter decomposition rate can be interpreted as the rate of return of nutrients in an ecosystem. The rate of return of soil nutrients depends on the rate of litter destruction and the decrease in dry weight of litter over a certain period of time.