Regional Variability in the Maximum Water Holding Capacity and Physicochemical Properties of Forest Floor Litter in Anatolian Black Pine (Pinus nigra J.F. Arnold) Stands in Türkiye

Abstract

Forest litter plays a critical role in regulating the water balance of forest ecosystems, particularly in semi-arid regions where hydrological stability is under pressure due to climate change. This study investigates the maximum water holding capacity (MWHC) of litter layers across three ecologically distinct regions in Türkiye—Kastamonu, Kütahya, and Muğla—to evaluate how structural and physicochemical characteristics influence the maximum water holding capacity (MWHC) of litter layers. Litter samples classified into humus, fermenting debris, and needles were analyzed for MWHC, pH, electrical conductivity (EC), and total dissolved solids (TDSs). The results revealed that both the type of litter and regional ecological conditions significantly affect MWHC, with humus layers and moist environments exhibiting the highest water holding capacity. Additionally, MWHC showed moderate positive correlations with EC and TDS, highlighting the importance of chemical composition in water dynamics. The findings underscore that forest litter should be regarded as a dynamic and functional hydrological component, not merely residual biomass. This perspective is vital for sustainable watershed planning and adaptive forest management. The study supports the development of integrated management strategies aligned with the United Nations Sustainable Development Goals (SDGs), particularly SDG 6 (Clean Water and Sanitation), SDG 13 (Climate Action), and SDG 15 (Life on Land).

1. Introduction

Forest ecosystems are vital components of the Earth’s terrestrial systems, providing essential ecosystem services such as water cycle regulation, carbon sequestration, biodiversity conservation, and climate moderation [1,2,3]. However, the sustainability of these functions is increasingly threatened by climate change and anthropogenic disturbances, including deforestation, land degradation, and altered precipitation regimes [2,3,4].

Forest litter, an important component of forest ecosystems, has received significant scientific attention due to its role in soil and ecosystem dynamics [5,6]. Composed of plant leaves, twigs, and accumulated organic matter, forest litter acts as a buffer between aboveground and belowground systems by forming a protective layer on the soil surface [7,8]. This layer is essential for hydrological functions such as water holding, nutrient cycling, habitat provision for soil organisms, and erosion control [9,10,11]. One of the key metrics used to evaluate its ecological function is the maximum water holding capacity (MWHC), which reflects the ability of litter to absorb and retain water, moderate soil moisture, and reduce evaporation [12,13,14].

The MWHCs of forest litter are influenced by several physicochemical parameters, including organic matter content, bulk density, pH, electrical conductivity (EC), and salinity. For instance, low pH can increase the solubility of organic materials, enhancing water holding capacity, whereas high salinity often restricts water availability [5,9]. Furthermore, local climatic conditions and forest development stages may cause significant variability in litter structure and function across ecological gradients [15,16].

The diverse ecoregions of Türkiye—namely the Mediterranean, Black Sea, and Central Anatolia—show marked differences in rainfall [17]. High-rainfall areas like the Black Sea support thick, porous litter layers, whereas Mediterranean climates see higher evaporation and salt accumulation, reducing hydrological function [18]. Climatic extremes like droughts and erratic precipitation further modulate litter’s hydrological performance across regions [19,20,21].

Although some studies have investigated litter nutrient dynamics and decomposition in Turkish forests (e.g., [22]), there is a notable lack of research systematically assessing MWHC and related physicochemical parameters across different ecological zones and stand developmental stages. Previous studies have mostly focused on litter quantity or chemical traits in specific forest types, and few have explored pure coniferous stands across multiple climate zones. While recent studies have compared WHC between pure and mixed stands of different species [23], understanding the dynamics within a single dominant species across ecological gradients remains underexplored.

This study focuses on Anatolian black pine (Pinus nigra J.F. Arnold subsp. pallasiana (Lamb.) Holmboe var. pallasiana), a native conifer widely distributed across Türkiye that produces dense, slowly decomposing litter with high lignin content—traits associated with strong water holding capacity and interactions with nutrient cycling and microbial communities [24,25,26,27]. This makes it an ideal model for hydrological and physicochemical analysis under varying ecological conditions.

Anatolian black pine is a widely distributed native conifer species in Türkiye that plays a critical ecological role [28]. Its needle-like, lignin-rich litter decomposes slowly and accumulates in thick layers, which significantly influences soil moisture conservation and nutrient cycling over time [29]. These characteristics make it an ideal model species for studying forest floor hydrological behavior under various environmental conditions. Furthermore, the resilience of Anatolian black pine to different climatic conditions, particularly its response to drought, highlights its importance in forest management and conservation strategies in Türkiye. Research has shown that while this species can thrive in diverse environments, its growth is primarily influenced by precipitation levels, especially during critical months like May. Insufficient rainfall during this time can severely limit its radial growth [24]. This adaptive capacity not only supports local biodiversity but also enhances the forest’s ability to regulate water cycles and reduce soil erosion, thereby playing a vital role in maintaining ecological balance [30]. Understanding these dynamics is essential for developing effective afforestation and regeneration practices that address the challenges posed by climate change, ensuring the sustainability of forest ecosystems in the region [31,32].

The forest floor litter (FFL) plays various significant roles in the ecosystem, such as regulation of hydrological processes, maintaining soil temperature, and nutrient cycling. It accepts precipitation and decreases runoff; it similarly stores water temporarily where applicable, with a reported storage of 1.9–3.1 mm (0.075–0.12 in), representing up to 18% of the yearly precipitation rhythm in some kinds of woodland [21]. These hydrological functions are primarily dependent on the stratification of litter into L, F, and H layers; the stages of decomposition and the physicochemical properties of these layers, such as pH, EC, and the organic content, have a significant impact on the maximum water capacity of the layer (MWHC) [33,34,35]. Recent research has emphasized the importance of understanding the MWHC and chemical composition of the soil in relation to climate change adaptation and forest management, as the litter increases water storage capacity, decreases the risk of wildfire, and improves the soil’s fertility [36,37]. Despite the recognized capabilities, studies of the MWHC and physical–chemical properties of the forest in different ecological contexts have been limited, particularly in the Anatolian black pine (P. nigra) forest. This study assesses the hydrological and ecological importance of forest floor litter in semi-arid Anatolia, focusing on its maximum water holding capacity and associated ecosystem functions.

This research also contributes to the Sustainable Development Goals associated with water sources, climate action, and terrestrial ecosystems (SDGs 6, 13, and 15) by providing information on the role of litter in hydrological functions and the value of forest services to society.

This study aims to evaluate the maximum water holding capacity (MWHC) and associated physicochemical properties (moisture content, pH, EC, and salinity) of forest litter in P. nigra stands across three distinct ecological regions of Türkiye: Kastamonu (humid), Kütahya (semi-arid), and Muğla (Mediterranean). This investigation provides the first interregional analysis of the maximum water capacity of forest floor litter in different areas of Türkiye that are ecologically distinct, located at the westernmost extent of Asia. The ecological role of litter as a functional hydrological layer has long been recognized, particularly in studies of humus forms and organic horizons in forest soils [38,39,40,41]. Our study provides novel quantitative data and insights regarding the variability of this function across diverse climatic and management contexts.

By examining the interplay between environmental factors and forest stand age classes, the study provides critical insights for sustainable forest management, climate change adaptation, and the conservation of ecosystem services in Mediterranean-type forest systems.

2. Materials and Methods

2.1. Study Area

This research was conducted in three regions of Türkiye—Kastamonu, Kütahya, and Muğla—each representing distinct climatic and ecological conditions. These regions exemplify humid, semi-arid, and arid climate types, respectively, thereby enhancing the ecological diversity of the study. All sampling sites are located in pure P. nigra stands that have been regularly managed every five to ten years. Due to this silvicultural regime, no significant understory vegetation (shrubs or herbaceous species) was present, ensuring uniform litter sampling conditions.

The soils in the study areas exhibit varying ecological properties [42]. Kütahya soil types are characterized by their shallow nature and development on clay, marl, and limestone, which hinders the movement of water and nutrients. Kastamonu soil types are characterized as brown forest soils with elevated water and organic content. In contrast, Muğla soil types are highly weathered, acidic red Mediterranean soils situated on a karstic platform, which facilitates enhanced root development. The discrepancies influence the structure of vegetation and the functioning of hydrological systems.

As described by the FAO/WRB (2022) [43], these soils are classified as Leptosols, Calcisols, and Cambisols in Kütahya; Cambisols and Luvisols in Kastamonu; and Chromic Luvisols, Rhodic Cambisols, and Rendzinas/Leptosols in Muğla, which are associated with different vegetation structures and hydrological functions.

The studied forests are dominated by P. nigra J.F. Arnold and represent the Pinion nigrae alliance within the Braun–Blanquet phytosociological system [44]. Bioclimatically, these stands correspond to the Mediterranean pluviseasonal–oceanic regime and its transitional temperate sub-Mediterranean variant, according to the worldwide bioclimatic classification of Rivas-Martínez et al. [45].

The selection of these ecological regions was based on the classification proposed by Atalay [46], which integrates climate, topography, soil, and vegetation characteristics into regionally distinct ecological units. Accordingly, the Aegean Mountain (Black Pine) Zone from the Aegean Region—AR (28°27′–28°33′ E, 37°07′–37°22′ N), the Arid Forest–Anthropogenic Steppe Zone from the Central Anatolia Region—MATR (39°23′–39°25′ E, 37°07′–37°22′ N), and the Cold Semi-Humid Coniferous Forest Division of the Black Sea Ridge Plateau and Mountains from the Black Sea Region—BSR (33°52′–34°15′ E, 41°14′–41°27′ N) were selected as the primary sampling zones (Figure 1).

Kastamonu: Located in the Black Sea Region, Kastamonu experiences a humid climate due to its proximity to the sea, with an average annual precipitation of approximately 485 mm [46,47]. The region is characterized by high humidity, organic-rich soils, and forest stands dominated by black pine (P. nigra) and oriental beech (Fagus orientalis Lipsky). The terrain is gently sloped, and the soils are moderately permeable, favoring the accumulation and persistence of forest litter.

Kütahya: Situated in the Inner Western Anatolia Region, Kütahya has a continental and semi-arid climate with an annual precipitation of about 563 mm [48]. The forests are primarily composed of black pine (P. nigra) and oak species (Quercus spp.). The soils are moderately permeable, contain limestone, and lie on slightly sloping topography. These features influence the physical and chemical characteristics of the forest litter in the area.

Muğla: Located in the Mediterranean Region, Muğla is characterized by a typical Mediterranean climate with high annual rainfall averaging 1206.1 mm [46,47]. Despite high precipitation, high evapotranspiration rates and well-drained soils limit the water holding capacity of the substrate. The forests are predominantly composed of brutian pine (Pinus brutia Ten.). The terrain consists of rocky, highly permeable soils that restrict water holding and affect the hydrological functions of the forest floor.

These three regions offer optimal ecological variability to assess differences in the maximum water holding capacity (MWHC) of the forest floor litter. Topography, microclimate, and vegetation composition are essential factors in evaluating the hydrological functions of the litter layer.

2.2. Methods

The research process started by establishing the study objectives and selecting three representative sites (Kastamonu, Kütahya, and Muğla). A strategy for sampling was employed that included at least 10 plots with different levels of development in stand. Fieldwork involved the collection of sample litter (20 × 20 m plots, 10 × 10 cm subsamples) and their classification into F, L, and H layers. Laboratory analyses involved the measurement of saturation, oven-dry weight, at least MWHC, and physicochemical properties (pH, EC, and TDS). Data were organized and analyzed in order to assess regional differences and structural differences, then management and environmental recommendations were made (Figure 2).

2.2.1. Litter Sampling Design

Stratified random sampling was employed in this study, as recommended for ecological research under heterogeneous conditions [49,50]. The sampling design was structured to capture both intraregional and interregional variability. To address the main objective of the project—understanding the dynamics of ecological succession and litter cover [4]—two stratification factors were considered: stand development stages (a, b, c, d) and ecological regions (Muğla, Kütahya, and Kastamonu).

The sampling strategy was intended to collect FFL in order to represent the diversity of both ecological habitats and developmental stages. Three different ecological areas were chosen: Muğla (Mediterranean), Kütahya (semi-arid), and Kastamonu (humid). Within each region, sampling was stratified by the 4 stages of stand development (a, b, c, d) which resulted in 12 different combinations (4 stages of stand development × 3 ecological regions). For each stage of development in each region, 10 samples of 400 m² (20 m × 20 m) were created. In each plot, three different subsamples of forest floor litter were collected (10 cm × 10 cm); these were collected at 10 m intervals and accounted for the spatial diversity of trees in relation to their distance from the ground. This method produced a minimum of 360 samples from the litter that were representative of both the intraregional and the interregional (

Table 1. Sampling design summary.

Ecological Region	Stand Development Stage	Number of Plots	Number of Subsamples
Muğla	a, b, c, d	≥40	≥120
Kütahya	a, b, c, d	≥40	≥120
Kastamonu	a, b, c, d	≥40	≥120
Total	12 combinations	≥120	≥360

).

The subsamples were carefully placed in appropriately sized plastic containers with lids to preserve their structural integrity during transport to the laboratory [7]. Thus, for each ecoregion, 10 sample plots and at least 30 subsamples from different stand development ages were collected.

2.2.2. Laboratory Analyses

Laboratory analyses were conducted to determine the maximum water holding capacity (MWHC), pH, electrical conductivity (EC), and total dissolved solids (TDSs) of the litter samples. These parameters were selected to assess the hydrophysical and chemical properties of the forest floor litter, which are critical for understanding its water holding and filtering functions in different ecological conditions.

For sample preparation, organic material that was fresh and had not been decomposed was designated as the L layer. The remaining material that was partially decomposed and still had a humic component was passed through a 1 mm mesh sieve: the fraction that was retained on the screen was considered the F layer (fermentation), and the remainder was considered the H layer (humus). This experimental-based operational distinction follows the classical concept of the humus line of demarcation [51] and is methodologically similar to Abdalmoula’s [52], who separated the forest floor material into a litter layer plus a fermentation layer (L + F) and a humus layer (H).

Maximum Water Holding Capacity (MWHC)

Litter subsamples were initially air-dried under laboratory conditions until a constant weight was reached, referred to as the air-dry weight (ADW). These samples were then saturated in distilled water for 24 h and allowed to drain for 20 min to obtain the water-saturated weight (WSW). Subsequently, the samples were oven-dried at 105 °C for 24 h to determine the oven-dry weight (ODW).

The maximum water holding capacity (MWHC) of each sample was calculated using the following formula:

$$MWHC\left(\%\right)={\displaystyle \frac{WSW-ODW}{ODW}}\times 100$$

(1)

Determination of Selected Physico-Chemical Properties of Forest Floor Litter

To determine the pH, electrical conductivity (EC), and salt (total dissolved solids, TDSs) content of the litter samples, subsamples representing the needle, fermentation, and humus layers were immersed in distilled water and allowed to equilibrate for 24 h, as recommended by Shi et al. [33]. Following the equilibration period, pH measurements were performed on air-dried 1:5 (w/w) litter-to-water suspensions using the multiparameter probe (Hach HQ40d, Hach Lange GmbH, Düsseldorf, Germany) of a Hach-Lange instrument according to Standard Methods for the Examination of Water and Wastewater (Clesceri et al. [53]). EC and TDS values were measured using the EC-TDS electrode of the same device.

Statistical Analyses

Statistical analyses were employed to compare the variations in the MWHC of forest litter across different ecogeographical zones and stages of stand development. In the preliminary analyses, the groups exhibited the normal distribution (Kolmogorov–Smirnov test, p > 0.05) and had a homogeneous variance (Levene’s test, p > 0.05). Between-group differences were evaluated using a one-way Analysis of Variance (ANOVA), followed by a Least Significant Difference (LSD) post hoc test for pairwise comparisons. LSD was selected due to its efficacy in facilitating direct and interpretable comparisons of group means, particularly in instances of unequal sample sizes [54]. This method facilitated improved disambiguation of statistically significant differences across spatial and developmental gradients. Relationships among subsample properties, including pH, electrical conductivity, salt content (TDS), and MWHC, were examined using correlation analysis.

All statistical analyses were performed using R (version 4.3.2; R Core Team, Vienna, Austria) and Python software (version 3.11.5; Python Software Foundation, Wilmington, DE, USA). Data visualization techniques such as boxplots, violin plots, and heatmaps were employed to support the interpretation of results.

3. Results

This study comparatively analyzed physicochemical parameters, including pH, electrical conductivity (EC), total dissolved solids (TDSs), and maximum water holding capacity (MWHC), of dead cover across three distinct ecological regions in Türkiye: Muğla, Kütahya, and Kastamonu. Descriptive statistics indicated that regional climatic conditions and microhabitat characteristics significantly influenced these parameters. Despite the Muğla region being influenced by a Mediterranean climate, it attained the highest maximum MWHC value (3496.7%) recorded in the study. The average MWHC was 376.8%, with elevated skewness (8.1) and kurtosis (110.6) values suggesting that the distribution in this area contains significant outliers. This signifies structural heterogeneity, as certain samples exhibit exceptionally high water holding capacity. The coefficient of variation of 58.3% signifies moderate variability among the samples in Muğla. The average pH value of the region is 5.0, indicating slight acidity. The averages for EC and TDS were 135.0 µS/cm and 0.1‰, respectively; the elevated coefficients of variation (72.3% and 75.5%) for both parameters suggest that the chemical properties in the region are heterogeneous. The Kütahya region is situated in a semi-arid continental climate zone and possesses the highest mean water holding capacity (467.4%). Nonetheless, this value must be evaluated in conjunction with a notably high standard deviation (657.1). The skewness (8.6) and kurtosis (87.4) values suggest the presence of extreme MWHC values in the area, indicating a significant deviation from homogeneity in the litter structure. The coefficient variation of 140.6% signifies substantial variability among samples within the region. The mean pH value recorded in the study is 5.8, and is nearer to neutral. The highest pH value (9.8) was observed at one Muğla site on carbonate-rich limestone bedrock, typical of karstic terrain, which can induce locally alkaline conditions. The values of EC (112.4 µS/cm) and TDS (0.1‰) are comparable to those of Muğla; however, their distribution is irregular, as indicated by elevated skewness and kurtosis values. This may indicate variations in weathering intensity attributable to microclimatic differences and soil composition. The Kastamonu region is situated in a humid, temperate climate zone, with an average MWHC value of 402.5%. This region possesses the lowest coefficient of variation (46.9%) and demonstrates a relatively uniform distribution among the samples. Nonetheless, the skewness (3.8) and kurtosis (25.6) values continue to signify the existence of certain outliers. The pH level in the area is marginally acidic at 5.3, aligning with standard coniferous litter ecosystems. The average EC value of 135.3 µS/cm is comparable to that of Muğla; however, the standard deviation and skewness are elevated. The average TDS is 0.1‰, yet the coefficient of variation of 99.7% signifies substantial variability (

Table 2. Regional descriptive statistics of forest floor litter hydrophysical and chemical properties.

Region	Variables	Min	Max	Mean	Std Dev	Median	Skewness	Kurtosis	CV (%)
Mugla	MWHC (%)	88.7	3496.7	376.8	219.7	362.2	8.1	110.6	58.3
	pH	3.1	9.8	5.0	0.8	5.1	0.5	2.1	16.1
	EC (µs/cm)	5.3	435.0	135.0	97.7	96.9	0.9	−0.1	72.3
	TDS (‰)	0.0	0.2	0.1	0.0	0.0	0.9	−0.1	75.5
Kütahya	MWHC (%)	44.4	7582.2	467.4	657.1	324.7	8.6	87.4	140.6
	pH	4.5	7.7	5.8	0.7	5.7	0.2	−0.8	11.6
	EC (µs/cm)	8.3	457.0	112.4	82.6	115.7	1.1	2.0	73.5
	TDS (‰)	0.0	0.2	0.1	0.0	0.1	1.2	2.6	75.8
Kastamonu	MWHC (%)	190.8	1976.9	402.5	188.8	370.3	3.8	25.6	46.9
	pH	3.9	7.6	5.3	0.9	5.0	0.7	−0.5	16.9
	EC (µs/cm)	5.3	672.0	135.3	128.5	99.9	1.3	1.3	95.0
	TDS (‰)	0.0	0.4	0.1	0.1	0.1	1.4	1.8	99.7

).

Figure 3 illustrates that a moderate positive correlation was observed between MWHC and EC and TDS, with a noticeable dispersion of the values among these responses. Especially in the samples with higher EC and TDS values, the wide range and high levels of MWHC are evident. This means that the above chemical parameters may be involved in MWHC. With respect to the pH variable, distribution mainly focuses on the range between 5 and 8 and exhibits a symmetric pattern. This limited variability agrees with the low correlation between pH and MWHC.

3.1. Differences According to Ecological Regions

There were statistically significant differences in the maximum water holding capacity (MWHC) of litter samples collected from the provinces of Kastamonu, Kütahya, and Muğla (F = 4.62, p = 0.0101). These differences, identified through one-way ANOVA performed for parametric comparison among groups under the homogeneity of variance (Levene’s statistic: 16.217), were further examined using the Least Significant Difference (LSD) post hoc test at the 0.05 significance level. According to the LSD results, Kütahya samples formed a statistically distinct group (a), while Kastamonu and Muğla were classified within the same group (b) (a > b). Based on the mean MWHC values, Kütahya exhibited the highest water holding capacity (467%), whereas Kastamonu (402%) and Muğla (376%) had lower but statistically similar mean values (Figure 4).

The pH values of litter samples showed statistically significant differences among the ecoregions (F = 76.03, p < 0.05). One-way ANOVA followed by the Least Significant Difference (LSD) post hoc test (Levene’s statistic: 12.221) revealed that the Kütahya samples (mean = 6.05) were statistically different from the Kastamonu (mean = 5.11) and Muğla (mean = 4.87) samples. According to the LSD test, Kütahya belonged to group “a”, Kastamonu to group “b”, and Muğla to group “c” (a > b > c). The lowercase letters in Figure 5 indicate statistically distinct groups based on the LSD test at the 0.05 significance level.

Total dissolved solids (TDS, ‰) values showed statistically significant differences among the ecoregions (F = 6.01, p = 0.0025). One-way ANOVA followed by the Least Significant Difference (LSD) post hoc test (Levene’s statistic: 35.465) revealed that the Kastamonu samples (mean = 0.094‰) formed a statistically distinct group (a), whereas Kütahya (mean = 0.056‰) and Muğla (mean = 0.058‰) were statistically similar and grouped as “b”. The lowercase letters in Figure 6 indicate statistically distinct groups based on the LSD test at the 0.05 significance level.

Electrical conductivity (EC, µS/cm) values exhibited statistically significant differences among the ecoregions (F = 4.93, p = 0.00074). According to the results of the Least Significant Difference (LSD) post hoc test (Levene’s statistic: 28.864) applied following one-way ANOVA, the Kütahya samples (mean = 122.5 µS/cm) were classified in a distinct group (b), while the Kastamonu (mean = 158.3 µS/cm) and Muğla (mean = 160.7 µS/cm) samples were grouped together (a). The lowercase letters in Figure 7 indicate statistically distinct groups based on the LSD test at the 0.05 significance level.

3.2. Differences According to Litter Types

Maximum water holding capacity (MWHC, %) differed significantly among litter types (F = 54.56, p < 0.05). According to the LSD (Least Significant Difference) post hoc test (Levene’s statistic: 41.850) results, humus (H) samples exhibited the highest mean MWHC (512%), statistically classified as group (a). Fragmented (F) litter followed with a mean of 453%, forming group (b), while leaf (L) litter had the lowest capacity (mean = 367%) and was categorized as group (c) (Figure 8). The lowercase letters in Figure indicate statistically distinct groups based on the LSD test at the 0.05 significance level.

4. Discussion

4.1. Interpretation of Differences Between Ecological Regions

The water holding capacity of the forest floor is not a static trait; rather, it is a dynamic ecological function regulated by a complex set of biophysical factors such as climate, topography, species composition, and soil conditions. In our study, significant differences in MWHC were seen between three different ecological regions—Muğla, Kastamonu, and Kütahya—demonstrating the sensitivity of litter hydrology to ecological gradients. The greater increases in Kastamonu (up to 350%) than in Muğla (c. 200%) reinforce the effect of macro- and microclimatic conditions in litter dynamics. In addition, the findings of Floriancic et al. [21] and Floriancic et al. [55] suggest that the water holding capacity of the litter layer (not just precipitation, but also potential evapotranspiration and local microclimates) modifies other micro-environmental conditions within the litter layer. Bioclimatic zonation is believed to be an important factor in the regulation of litter hydrology. Wang et al. [56] found that forests in moist climates store more organic matter and retain more water in the litter than those in drier climates. These results serve to support the inference that climate-controlled litter formation processes (e.g., decomposition rate, biomass productivity, leaf litterfall) are strongly connected with hydrological function. Gao et al. [57] demonstrated that forest productivity (NPP) is strongly influenced by climatic conditions. Given that high NPP typically results in greater biomass accumulation and canopy density, it is reasonable to infer that such conditions may also lead to increased litter input on the forest floor. This, in turn, could enhance the thickness and absorptive capacity of the litter layer, thereby supporting higher maximum water holding capacity (MWHC). Although Gao et al. did not directly assess litter properties, their findings provide an indirect framework for understanding how climate-driven productivity could influence litter hydrology.

In drier areas like Muğla, elevated evapotranspiration may lead to salinity buildup in soil and litter, reducing the litter’s capacity to hold water. This may be supported by the work of Zhang et al. [34], who identified that accumulation of salt in forage may be raised in aridity and subsequently reduce the absorption capability of forest floor litter. The high EC and TDS levels determined in Muğla samples in our study also corroborate this mechanism, representing not only osmotic stress, but also variations in the hydrophysical status of the litter layer. In addition to climate, vegetation structure and species composition play strong roles in controlling litter water storage. As Shi [33] showed, species produce litter with more water holding capacity than conifers like F. hodginsii or C. lanceolata. These interspecific variations are related to the morphology of the leaves, litter chemistry, and decomposition dynamics—and determine the porosity and capillarity of the litter matrix. Furthermore, as demonstrated by Tang et al. [58] and Ilek et al. [23], mixed broadleaf–coniferous stands have the greater ability to maintain effective hydrological functioning in litter layers because of the existence of species with low C:N ratios and greater litter density [59]. And the spatial differentiation in hydrological relations is not limited to the surface. Subterranean traits such as rootzone water storage also show ecological differentiation. Nijzink et al. [60] also highlighted that rooting depth and density are very important in the mediation of water uptake during dry periods, and that regional variation in vegetation architecture affects both surface and subsurface hydrological resilience.

Finally, the effect of high elevation as an ecological filter must not be overlooked. Topographic gradients in temperature and precipitation affect rates of decomposition, microbial activity, and organic matter inputs. Kolb et al. [61] underscored that montane forests exhibit greater infiltration and lower evapotranspiration rates with elevation, which could provide a mechanism of maintaining litter moisture similar to that detected at our Kastamonu site. Infiltration dynamics may also be influenced by the interaction between slope and litter density, although further studies are needed to clarify this relationship across different forest types and topographic conditions. In conclusion, the capacity of litter to retain water is a result of interactions among climate, vegetation, soil, and topography. Its recognition and quantification are important not only for the development of ecohydrological models but also have immediate applications to forest management. Furthermore, incorporation of litter hydrology into drought management, fire danger forecasts, and climate change adaptation will be beneficial to the resilience of forest ecosystems. It is, thus, suggested that the monitoring and control of litter dynamics across the entire ecological range are used as the driving forces of sustainable watershed governance under the influence of climate change.

4.2. Interpretation of Variations in MWHC Based on Litter Type

These findings indicate that the MWHCs in forest ecosystems have significant correlation with litter types. Higher MWHCs in mid- and high-elevation zones may result from a more humid micro-environment and the accumulation of low-decomposed organic matter. Humus (h) samples showed the highest MWHC of the various litter types analyzed, followed by fragmented litter (f) and undecomposed needle litter (L). This trend is largely contributed to the fine texture, porosity, and fibrous nature of the humus which further stabilize water. On the other hand, the lower values of MWHC in needle-type litter are associated with its high permeability and low degree of decomposition. Litter with intermediate levels of fragmentation showed moderate values, indicating that water holding capacity increases with the degree of decomposition.

In forest systems, the capacity of litter to retain water is not only based on the function of litter material, but also on the ecological environment in which it is developed. In this sense, litter quality seems to be an environmental driver of water holding capacity.

Water holding capacity is strongly influenced by various physical and chemical traits of the litter, including texture, porosity, and decomposition stage. A meta-analysis by Shi et al. [62] showed that the MWHC of partly decomposed litter layers was markedly higher than that of undecomposed layers, consistent with the present results. Similarly, Kim et al. [63], found that the litter of Pinus koraiensis Sieb.et Zucc exhibited higher water holding capacity compared to Quercus acutissima Carruth, indicating species-dependent variability. This highlights the importance of dominant species and litter type in regulating ecohydrological processes.

In support of global findings by Liu et al. [37] in which climate factors are prioritized as direct driving forces of forest water storage capacity, the contribution of micro-environmental factors (e.g., elevation and litter type) at a local scale is noted in this study. The interplay of these controls constitutes an important axis controlling hydrological behavior in forested systems. Under this perspective, the diversity and composition of litter should be given more attention in forest and watershed management, notably in semi-deciduous woodlands.

4.3. Relationships Between Physicochemical Properties and Maximum Water Holding Capacity

In forest ecosystems, dead cover is not only a static organic layer but also a dynamic element, fulfilling important ecohydrological roles, including water holding, delay in evaporation, and temporarily storing dissolved ions. The water holding capacity of this layer follows not only the structural properties but also chemical environmental conditions. Dead cover hydrological behavior is largely influenced by the presence of soluble ions (EC and TDS). The literature suggests that high ionic content and salinity reduce the rate of pore water movement and slow down the evaporation processes, which helps water storage underneath the cover [61,64]. The dead cover can also indirectly influence water retention time through physical processes such as decomposition rates and microbially driven postmortem chemistry. Activation of microbes in an acidic to slightly acidic medium enhances water retention time by increasing the solubility of organic compounds, as reported by Debnath et al. [65] and Ilek et al. [16]. In this regard, pH is a signal that drives hydrological processes (either directly or via microbe-mediated pathways). Physical parameters such as the porosity, bulk density, and organic matter content of the litter layer are determining factors in water holding capacity. More open, lower-density structures are able to house free water as well as dissolved substances more effectively [66]. In fact, the water holding capacities of dead organic matter originating from pure stands of conifers are different from those resulting from broadleaf or mixed stands [23,58]. The above discussion serves to underscore the importance of treating non-living organic matter as more than a physical surface cover and as an active hydrological buffer zone. When viewed from the perspective of ecosystem services, the role of this layer in controlling water and flood distribution over space and time, reducing flood magnitudes and durations, and regulating changes in soil moisture distribution is fundamental, especially in semi-arid and arid areas [61,67]. Consequently, active management of dead cover should not be based solely on fire risk or nutrient cycling, but also on the, as yet, unaddressed sustainability of the forest water budget. In that sense, management tools such as fostering mixed species-stocking, enriching leaf composition, or quantifying dead cover density to improve water holding capacity and drought resistance of ecosystems are recommended [23,58].

4.4. Comparison of Findings with the Literature

The results of our study demonstrate that litter MWHC also significantly differed among ecological regions, a result that is consistent with previous studies in similar ecological locations [68]. The results of this study demonstrate that litter MWHC also significantly differed among ecological regions, a pattern consistent with findings from other forest types. For example, in Xinglong Mountain (Gansu), the undecomposed layer’s maximum water holding capacity ranged between 4.52 and 18.72 t·hm⁻² across tree species, with broadleaf species generally exceeding conifers in their water holding capacity [68]. An earlier study on the same site also reported similar variation in water holding traits among forest types, emphasizing the influence of species composition and litter characteristics [69]. The high MWHC values observed in Muğla might be attributed to the vegetative structure—particularly the composition and accumulation of broadleaf and conifer litter—which constitutes a substantial proportion of the forest floor organic stock [70,71]. In addition, slower decomposition rates in warmer and more humid areas lead to thicker litter layers, which increase water holding capacity, as previously described by Sayer [13] and Zhang et al. [72]. Furthermore, the observed elevation-induced rise in MWHC also suggests that elevation can exert control over organic layer buildup and litter architecture. Comparison to other montane ecosystems from the Balkans and the Black Sea region revealed altitudinal patterns of litter hydrological function (see also findings by Xing et al. [73] in high-elevation Larix plantations). The differences among different litter types in our study provided support for the conclusion that structure and composition are factors governing water holding capacity [9]. Needle litter types are the quickest to dry, in contrast to decomposed and humified layers, which retain more water and whose higher porosity and organic matter content delay the drying process [69,74]. Furthermore, spatial variations in pH, TDS, and EC values indicate that the litter layer is responsive to micro-environmental and geochemical input. These parameters depend on climatic and pedological contexts [29,75]. High EC and TDS values in Muğla may be attributed to increased evaporation under the Mediterranean climate, where hot, dry summers accelerate surface water loss and concentrate salts in the soil and litter layers. This process, further modulated by organic matter inputs and decomposition dynamics, has been observed to significantly influence soil salinity patterns [76,77,78].

Together, these results indicate that regional ecological factors—such as vegetation composition, topography, and climatic conditions—play a critical role in shaping the hydrological characteristics of forest floor materials, beyond litter type alone. Similar patterns have been reported in various forest ecosystems worldwide, underscoring the need to consider localized ecological drivers in understanding litter hydrology [79,80,81]. Rather than applying a one-size-fits-all approach, our findings highlight the importance of region-specific adaptations and the interactions between physiographic and vegetative conditions in shaping the spatial variability of litter water holding performance across ecological gradients.

4.5. Management and Ecological Recommendations

The water holding capacity of forest litter plays a crucial role in both the ecological functions and sustainable management of forest ecosystems. Our findings revealed significant differences in the water-holding performance of the litter layer across ecological regions and structural characteristics. This highlights the need to integrate water-sensitive approaches into ecosystem-based forest management practices. From an ecological perspective, surface cover components—commonly referred to as litter and deadwood—are critical in hydrological functions such as water conservation, evaporation reduction, erosion control, and microclimate regulation [55,82,83]. This role becomes particularly evident in semi-arid regions, where the litter layer enhances ecosystem resilience by retaining moisture under conditions of water scarcity [78]. In fact, forest floor litter layers can retain up to 3.1 mm of precipitation (spruce needles) and 1.9 mm (beech leaves), with the litter cycling around 18% of annual precipitation [55], underscoring its significance for regional water balance. Therefore, the physical preservation of the litter layer contributes not only to enhancing water holding capacity but also to maintaining soil moisture and supporting understory vegetation development. Moreover, the reintroduction of organic debris following deforestation or intensive management may support the reorganization of hydrological cycles [84].

Operationally, the regional-scale product of water holding capacity of dead fuel helps in building prediction models at the planning scale. Accordingly, the choice of land use and tree species can be made in line with the criteria established for water management, as supported by studies emphasizing species-specific water holding capacity and the role of afforestation in regulating hydrological dynamics under regional constraints [85,86,87,88]. In afforestation and reforestation activities in particular, tree species should be chosen according to litter productivity and hydrothermal regulating ability to increase ecosystem resilience to water stress [83]. The management of deadwood must take into consideration several elements, including not only the state of the forest aboveground but different parameters like soil physics and chemistry, topography, and microclimate. In this sense, the results from this study have significant managerial and ecological implications, especially for forest hydrological function conservation and water-saving management in semi-arid areas.

Additionally, the findings of this study have implications beyond local forest hydrology, aligning with global sustainability targets. By demonstrating how forest floor litter contributes to regulating water holding capacity, mitigating soil erosion, and supporting ecosystem resilience, these results address key elements of SDG 6 (Clean Water and Sanitation), SDG 13 (Climate Action), and SDG 15 (Life on Land). Improved understanding of litter-mediated hydrological functions and their physicochemical controls can support evidence-based forest management strategies that enhance water security, reduce climate-related risks such as drought and wildfires, and conserve biodiversity.

5. Conclusions

This study strongly demonstrated that the litter layer is not only a passive ground cover but also an active component that provides hydrological buffering, water regulation, and secondary microsite services in forest ecosystems. Findings from three different ecoregions showed that elevation gradients, forest floor layers (litter, fermentation, humus) and physicochemical variables (especially EC and TDS) were particularly influential on maximum water holding capacity (MWHC).

The most striking finding was that humus- and litter-type litter can retain up to 60% more water than litter with a needle structure. This suggests that the loss of these layers during deforestation, post-fire interventions, or mechanical clearing may directly weaken the soil moisture balance and microclimate resistance. Increasing humus formation at higher elevations enhances water holding capacity, reduces runoff, and improves infiltration balance.

The findings also showed that beyond physical structure, chemical properties such as dissolved ion content and electrical conductivity have significant effects on MWHC. This was supported by the high EC-TDS-MWHC association observed in semi-arid regions and emphasized that the dead cover should be considered as a matrix that plays an active role in water–solution relations. In this context, the key outcome of this study is the following: “Dead cover is a functional water management unit where the water holding capacity of forest ecosystems should be evaluated by considering both physical and chemical components.” This finding offers a new perspective for watershed-based integrated management strategies and has the potential for direct application in areas such as sustainable use of water resources, climate change-resilient forest planning, and consideration of soil moisture balance in tree species selection.

Future studies should be expanded to include the seasonal dynamics of the litter layer, its relationship with microbial decomposition processes and its integration into infiltration modeling. Furthermore, predictive models focused on the MWHC-EC-TDS relationship will provide critical contributions both in terms of preparation for drought scenarios and optimization of forest–watershed–water cycle relationships.