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Article

Characteristics of Suspended Solid Responses to Forest Thinning in Steep Small Headwater Catchments in Coniferous Forest

by
Honggeun Lim
,
Qiwen Li
,
Byoungki Choi
,
Hyung Tae Choi
and
Sooyoun Nam
*
Forest Disaster and Environmental Research Department, National Institute of Forest Science, Seoul 02455, Republic of Korea
*
Author to whom correspondence should be addressed.
Water 2024, 16(24), 3610; https://doi.org/10.3390/w16243610
Submission received: 16 October 2024 / Revised: 15 November 2024 / Accepted: 13 December 2024 / Published: 15 December 2024
(This article belongs to the Special Issue Non-Point Source Pollution and Water Resource Protection)
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Abstract

:
We examined the responses of suspended solids to forest thinning in steep small headwater catchments, PT (0.8 ha) and PR (0.7 ha), that drain a Korean pine (Pinus koraiensis) plantation forest. Based on a paired-catchment design, the relationship between total suspended solids (TSS) and the time differential of water runoff (dQ/dt) indicated a difference in the characteristics of TSS in the rising and falling stages within the initial two years after forest thinning. The relatively high initial TSS responded to the concentration-based first flush criterion in the early stage of the rainfall event concentrated in this initial period after the thinning. The rate of TSS event loads in the PT catchment was 4.3-fold greater than that in the PR catchment within the initial two years after forest thinning. This was induced by the low disturbance of soil surface by forest workers using chainsaws and non-heavy machinery. Three years later, the TSS event loads in the PT catchment appeared to decrease due to trapping and settling by protective vegetation. Therefore, mitigating accelerated TSS events during forest thinning requires appropriate site-specific land preparation, particularly for improving stream water quality in forested catchments.

1. Introduction

Stream water quality is determined by a range of current and historical impacts on natural and artificial catchments and is an important indicator of aquatic ecosystem health [1,2,3,4]. Due to ecosystem linkages, the dynamic characteristics of materials are closely related to the quality and quantity of materials supplied from upstream source areas [5]. Upstream forested areas are headwater catchments with first- and second-order channels, which are water, nutrient, and sediment sources with downstream systems [6]. Considering that the total area of headwater catchments comprises a major part of a watershed [6,7], elucidating material dynamics in headwater catchments is vital for predicting the influences of material transports on their downstream counterparts. Boggs et al. [8] stated that forests offer the highest water quality compared to other land uses, and forest cover minimizes the risk of water quality degradation. Additionally, Cooper and Thomsen [9], Quinn et al. [10], and Foley et al. [11] indicated that forests generally had higher water quality compared with agriculture and urban areas. In this context, the lower load of total suspended solids (TSS) in forests are partly attributed to better soil infiltration and a variety of soil physical and biogeochemical processes filtering particles and chemicals from the forested stream [12]. The combination and interaction of these processes can be influenced by temporally and spatially complex and dynamic systems, both within landscapes for freshwater supply and management [13,14].
In total, 63.1% of the land is covered by forests, and streams (i.e., tertiary streams) account for 88.9% of the total length of streams nationally in the Republic of Korea [15,16]. Topography consisted of steep hills with narrow valleys attributed to rapid flood responses and high peak flows [17,18]. Streams had a significant role in water supply and management [19]. The streams located in the upper position of catchments have relatively low flow rates with ephemeral streams, when compared with rivers and lakes located in the lower position of the catchment [20,21]. Kim et al. [22] highlighted that annual flood damage came from the East Asian monsoon and clarified that the flood damage costs related to these events in the Korean Peninsula. Kim et al. [23] reported that heavy rainfall mainly occurs in the summer season (June–September) as a result of the East Asian monsoon as affected by the passage of severe tropical typhoons. Gillham [24] confirmed that the streams can occur rapidly in response to the started rainfall events during the East Asian monsoon. Thus, catchment management practices are vital for spatial–temporal variability and are vital for mitigating direct and indirect negative consequences, water yield decline, flooding, and water quality degradation [25,26].
Due to this spatial–temporal variability, forest thinning needs to consider the longer routing processes of water and greater storage capacity for water resources [27,28]. Forest thinning is generally applied to forest plantations with closed canopies to create suitable spaces among trees and to mitigate competition for light (especially for the lower crown), water, and nutrients, thus enhancing the growth of the remaining trees and timber production [29,30,31]. Forest thinning is also an important forest management strategy for preserving forest biodiversity and maintaining ecosystem functions [32]. For example, forest thinning may lead to various hydrological concerns associated with overland flow and soil erosion under dense conditions, such as unmanaged forests [33,34,35]. Similarly, Prima et al. [36] and Del Campo et al. [37] reported that changes in hydrological processes and stand structure can affect soil water conditions, which are indirectly regulated by forest thinning. Thus, forest thinning is recommended to enhance forest management with forest water use efficiency, along with hydrological processes, for minimizing ecological impacts in catchments [38,39].
Although forest thinning is an important management strategy, information on how the thinning alters changes on material transport (e.g., TSS and nutrients) [40,41] and hydrological processes [42,43] in forested headwater catchments has not been fully investigated. Current understanding for the impact of forest-thinning-accelerated TSS event loads has primarily been derived from soil surface disturbance [44,45]. It appears that factors such as the intensity and extent of soil surface disturbances induced by thinning operations are major determinants of TSS event loads instead of the extent of thinning [46]. High increase rates of TSS event loads were possibly associated with soil surface disturbance by thinning operations (e.g., skid trail and road constructions) under high and intense precipitation conditions [41]. To interpret TSS event loads in light of thinning operations, it is vital to consider long-term periods in single-catchment analysis, including the effects of climate variations (e.g., precipitation) after thinning [47,48]. We, thus, aimed to examine TSS responses with the period after forest thinning and to determine TSS event loads by comparing two different catchments in a paired-catchment design with 50% thinning.

2. Materials and Methods

2.1. Study Site

This study was conducted in two small steep headwater catchments, PT (treted catchment, 0.8 ha) and PR (reference catchment, 0.7 ha), in Pocheon-si, 27.0 km north of Seoul (37°45′ N, 127°09′ E). These catchments are managed by the National Institute of Forest Science (NiFoS) (Figure 1). Based on the Köppen–Geiger classification system [49,50], the climate types in this study area are Dwa (snow, winter dry, and hot summer). Climate classes are further sub-classified by the precipitation and temperature conditions [51]. The mean annual precipitation (mm ± standard deviation; SD) from 2002 to 2021 at the Pocheon Automatic Weather Station (AWS) was 1349.0 ± 356.2 mm (minimum–maximum values: 869.5–2329.0 mm), with 68% occurred from July to September. The mean annual temperature ± SD was 10.3 ± 0.5 °C (9.2–11.0 °C).
The elevation of the monitored catchments ranged from 165 to 306 m above sea level. The hillslope gradients ranged from 0.7 to 34.5° in the PT catchment and from 0.5 to 33.0° in the PR catchment. The underlying geology comprises metamorphic rocks. The soil is a slightly dry brown forest soil (Inceptisol, USDA Soil Taxonomy) originating from gneiss with a sandy loam texture. The catchments were vegetated by 41–50-year-old Korean pine (Pinus koraiensis) stands (V) based on a forest-type map (1:5000). The mean tree height was 18.4 ± 1.5 m (26.5 ± 5.2 cm in diameter of breast height (DBH)) in the PT catchment and 17.1 ± 2.1 m (22.7 ± 4.9 cm DBH) in the PR catchment. The two catchments, located in the upper part of the basin, have relatively low flow rates and/or exhibit characteristics of ephemeral streams during normal periods. Stream channels were 0.034 and 0.063 km in length, with a slope of 0.26 and 0.30 m/m in the PT and PR catchments, respectively, based on digital topographic maps.
The catchments were originally managed as hillslope agricultural land with slash-and-burn farming [43]. The catchments were reforested by planting coniferous trees in 1976. A total of 45% of stems were uniformly thinned within the two catchments in 1996. The PT catchment was thinned to 50% of the stems using chainsaws and non-heavy machinery from December 2016 to March 2017 (Figure 1). Tree density was 558 trees/ha (with 63% in crown density) in the PT catchment and 1192 trees/ha (with 92% in crown density) in the PR catchment (Figure 1).

2.2. Field Observation and Water Sampling

The observation period included 18 rainfall events concentrated in the summer sea-son (July–September) for 5 years (2017–2021), which did not include a period with forest thinning operations. Logged trees were removed in March 2017. Stream gauging stations managed by the NiFoS were used to monitor the streams.
Precipitation was obtained by the Pocheon Automatic Weather Station in the Korea Meteorological Administration (KMA) Weather Data Service. The total precipitation (PT), maximum 1 h precipitation (P1), and 7- and 30-day antecedent precipitation indices (API7 and API30) were calculated as rainfall parameters [52]. API7 reflects soil moisture conditions near the surface, whereas API30 represents moisture in the deep soil matrix [53,54]. The water level in the sharp-crested weirs (90° V-notch weirs) was measured using a capacitance water level recorder (OTT-Orpheus Mini Water Level Logger, OTT Messtechnik, Kempten, Germany). The water level was measured at 10 min intervals for each catchment outlet. The runoff (mm) was divided by the projected catchment area.
TSS were measured by an automatic water sampler (Teledyne ISCO Inc., Lincoln, NE, USA) collecting the samples at 1 h intervals for the duration of the event [55]. All water samples were immediately transported to a laboratory. Some events included the peak runoff and the recession limb of the hydrograph as stormflow samples [56]. TSS were measured using filtration methods employing membrane filters (Whatman GF/F 47 mm glass fiber filter) and a vacuum pump (Gravimetric Method).

2.3. Data Calculation and Analysis

The first flush included concentrated first flush and mass first flush. Dimensionless mass/volume curves representing the distribution of TSS mass versus volume in rainfall event water discharge are used to evaluate TSS discharge patterns for different rainfall events and catchments [57]. The concentration of the first flush refers to the TSS in the initial runoff, which is significantly higher, whereas the mass first flush refers to the cumulative transport rate of the TSS at the initial stage, which is greater than that of the runoff volume. Geiger [58] defined the first flush phenomenon according to TSS mass and runoff volume as follows:
M F F n = M ( t ) V ( t ) = ∫ 0 t Q ( t ) C ( t ) d t   / ∫ 0 T Q ( t ) C ( t ) d t ∫ 0 t Q ( t ) d t   / ∫ 0 T Q ( t ) d t  
where MFFn is the mass first flush ratio, M(t) is the total suspended solids (TSS) transport rate at time t, V(t) is the observed rainfall–runoff volume transport rate at time t, Q(t) is the runoff flow rate at time t, C(t) is the TSS of the runoff at time t, t is the observed rainfall event duration, and T is the total duration of the observed rainfall event. At the early stage of a rainfall event, the TSS emission rate in the initial runoff is greater than the runoff discharge rate when M(t)/V(t) > 1 or the curve slope > 45°, which indicates the occurrence of the first flush. Here, the first flush effect was defined when at least 80% of the TSS mass is emitted in the first 30% of the runoff volume, 50% TSS mass with 25% runoff volume, and 40% TSS mass with 20% runoff volume [59,60].
The event mean concentration (EMC) is useful for interpreting complex patterns and comparing different events under conditions for different times or sites [61]. The EMC was calculated using the relationship between the TSS mass and total runoff volume relations during each rainfall event:
E M C = Mass of TSS contained in the runoff event Total volume of flow in the event = ∑ Q i T S S i ∑ Q i
where EMC is the event mean concentration (mg/L), Qi indicates the discrete flow coordinates on the event hydrograph, and TSSi is the corresponding discrete TSS on the mass graph. It should be noted that, instead of measuring the discrete flows and concentrations throughout the EMC, it is represented as the TSS of a flow-weighted composite sample of the runoff event [62,63].
The TSS event load was then calculated according to the relationship between TSS and discharge using the following equation [64]:
T S S   e v e n t   l o a d = ∑ i = 1 n T S S i Q i A
where TSS event load is the total suspended solids event load (kg/ha) and A is the catchment area (ha). All data analyses were performed using SPSS (Statistical Package for Social Sciences, version 29) and R (version 4.1.2).

3. Results and Discussion

3.1. Total Suspended Solids Response to Event Precipitation and Runoff

Figure 2 presents the temporal changes in TSS and both event precipitation and runoff in the thinned and reference catchments during 18 observed rainfall events from 2017 to 2021. The PT, API7, and API30 during the rainfall events were 5.5–95.0, 6.5–224.5, and 79.0–814.0 mm, respectively. The event total runoff ranged from 0.2 to 42.5 mm in the PT catchment and 0.1 to 97.5 mm in the PR catchment (Figure 2b). Mean TSS ranged from 1.3 to 38.6 mg/L in PT catchment and from 1.2 to 16.9 mg/L in PR catchment (Figure 2c). The thinning experiment (removing 50% of stems) increased the mean TSS (Figure 2 and Figure 3). Increases were revealed in the TSS, but the total runoff did not increase during the observed rainfall events. However, a slight (but not significant) increase in the total runoff was observed after forest thinning. While we monitored overland flow on the hillslope, discontinuous overland flow could also have occurred, in which part of the flow infiltrates into the soil matrix [33,65]. Here, our small forested catchments (0.7 and 0.8 ha) were located in mountainous terrain with steep slopes (>30°), in which overland flow could propagate much faster into channel flow [66] with a low capacity for water to infiltrate [67]. Our study was conducted in the flood season, which is consistent with the growing season. Available water in the growing season may, therefore, be limited. Thus, increased overland flow generated by increased net precipitation did not contribute directly to increases in total runoff in our catchments after forest thinning.
We investigated the potential relationships between TSS and the normalized difference vegetation index (NDVI) at the two catchments. Figure 3 presents TSS and the NDVI according to each of the five investigated years after forest thinning. Small variations in TSS may be reflected in the NDVI, and increases in TSS event loads within one and two years were associated with soil surface disturbance induced by loss of surface cover and raindrop splash erosion [68,69]. However, newly established vegetation may reduce the effect of splash erosion from three years after thinning onward [70,71,72,73]. Thus, we divided suspended solid responses to thinning into two periods, the initial two years and the three years after thinning.

3.2. Characteristics of Event Total Suspended Solids with Time After Forest Thinning

We observed seven and twelve rainfall events within the initial two years and the three years after thinning, respectively (Table 1). The PT was 5.5–87.5 mm with a 4.0–34.5 mm of P1 within the initial two years after forest thinning. Three years after forest thinning and onward, PT and P1 were 7.5–95.0 and 4.0–17.0 mm, respectively. The API7 and API30 were 7.5–224.5 and 117.5–434.5 mm for the initial two years and 6.5–177.5 and 79.0–814.0 mm from three years after thinning onward, respectively. Under this precipitation condition, the total runoff ranged from 0.4 to 37.7 mm for the PT catchment and from 0.1 to 77.6 mm for the PR catchment within the initial two years after forest thinning. The total runoff in the PT and PR catchments was 0.2–42.5 and 0.3–97.5 mm from three years after forest thinning and onward, respectively (Table 1). The mean TSS were 1.4–38.6 mg/L for the PT catchment and 1.2–14.3 mg/L for PR catchment within the initial two years after forest thinning. From three years after that, the mean TSS were 1.3–12.8 mg/L for the PT catchment and 1.2–16.9 mg/L for the PR catchment (Table 1).
As the characteristics of rainfall events can affect the TSS responses with runoff, two rainfall events (i.e., E4 and E10) with similar rainfall intensities and durations were selected in this study (Figure 4a,b). For instance, Hicks et al. [74] showed that TSS responses were related to rainfall amount and intensity. Similarly, Old et al. [75] showed that TSS concentrations corresponded to rainfall amount and intensity, with an increased suspension of coarse sediment particles with an increasing flow. Miller et al. [76] reported that the TSS concentration from upland areas was controlled by total event precipitation and maximum hourly rainfall intensity. Thus, when the runoff receded progressively after the rainfall had ceased, no distinctive time lag was observed between the peak rainfall input and peak flow, except for the peak of TSS. The peak runoff at 10.0 mm of PT with an intensity of 4.0 mm/h in the first year after forest thinning (27–28 July 2017) was 0.03 and 0.05 mm/h in the PT and PR catchments, respectively. The peak TSS were 14.3 mg/L in the PT catchment and 2.6 mg/L in the PR catchment (Figure 4a). After four years (2–3 October 2019), the peak runoff was 0.02 and 0.08 mm/h in the PT and PR catchments, respectively, at 15.0 mm of PT with an intensity of 4.0 mm/h. The peak TSS decreased to 5.7 mg/L in the PT catchment and increased 136.0 mg/L in the PR catchment (Figure 4b).
Figure 5 presents the relationships between the time differential of water runoff (dQ/dt) and runoff (Figure 5a), and the relationships between dQ/dt and TSS (Figure 5b) for the two periods after forest thinning. The relationship between dQ/dt and water runoff produced nearly symmetrical V-shaped distributions with an axis of dQ/dt = 0 (Figure 5a). On the other hand, the TSS presented a different trend, which depended on whether dQ/dt was positive (rising stage of water runoff) or negative (falling stage of water runoff) (Figure 5b).
The TSS were scattered in relation to dQ/dt during the rising stage, particularly in the initial two years after forest thinning. In this period, the dQ/dt in the rising stage positively correlated with precipitation, runoff, and TSS (correlation coefficient: 0.613–0.950, p < 0.01), while dQ/dt in the falling stage negatively correlated with both runoff and TSS (correlation coefficient: −0.897–−0.208, p < 0.05) for the two catchments (Table 2). The TSS were almost constant, and the values decreased with decreasing dQ/dt during the falling stage. Although we did not exactly separate catchment and channel contributions to suspended solid flows, our data indicated the relationship between TSS and dQ/dt values in the rising and falling stages in the two periods after forest thinning. That is, dQ/dt in the PT catchment was significantly correlated with runoff (correlation coefficient: −0.494 and 0.457, p < 0.01), while dQ/dt in the PR catchment was significantly correlated with precipitation and runoff (correlation coefficient: −0.634–0.762, p < 0.05) (Table 2). As presented in Figure 5, the origin of TSS possibly depends on whether the stage of water runoff increases or decreases (Figure 5a,b). For instance, Stott et al. [77] reported that TSS associated with water discharge was compared with those calculated without considering the stage difference. This was because forest thinning may affect TSS differently during rising and falling stages. Similarly, Hotta et al. [78] indicated that TSS were influenced by different processes during rising and falling stages, suggesting that forest thinning could affect these processes differently in the two stages. Gomi et al. [79] also reported that erodible material on the surface was flushed at rising water levels, while less material was transported at declining water levels. This pointed to the overland and streamflow transport of potential sources in the upstream sections of the catchment, and to the pre-existing low contributions of groundwater sources to the stream channel.

3.3. First Flush Effects of Forest Thinning on Total Suspended Solids

This study was indicated to assess the event water TSS in first flush runoff using a time series of forest-thinning impacts (Figure 6). As presented in Figure 6a,b, the TSS loads for the PT catchment were above the 1:1 line compared to those in the PR catchment. Hence, there were strong impacts on first flush consistent with the results of previous studies [80,81,82]. The concentration of TSS also decreased as a response to an increased rainfall and runoff [69]. Deletic [83] explained that the first flush effect (i.e., the initial runoff being more polluted than the subsequent runoff) may occur in runoff events but not in others at the same site. Gupta and Saul [84] found in their study that the first flush load correlated well with the peak rainfall intensity, the storm duration, and the antecedent dry weather period. Taebi and Droste [85] observed that the TSS loads in storm water runoff increased when the intensity and duration of a rainfall event increased. Bach et al. [81] reported that the first flush is often more polluted than the subsequent runoff, even if there is no agreement regarding the quantification of first flush.
In Table 3, the TSS of the first flush ranged from 32.1 to 57.6% and 22.4 to 61.0% of the average cumulative load in the PT and PR catchments, respectively, and MFFn ranged from 1.13 to 1.62 in the PT catchment and from 1.09 to 1.22 in the PR catchment within the initial two years after forest thinning. From three years after thinning onward, the TSS of the first flush ranged from 23.8 to 63.1% and from 32.6 to 73.9% of average cumulative load in PT and PR catchments, respectively (Figure 6b). MFFn ranged from 1.23 to 1.28 in the PT catchment and from 1.45 to 1.80 in the PR catchment (Table 3). The cumulative TSS loads relative to 20 and 30% of runoff flow (except for 50% of runoff flow) with MFFn of PR catchment were greater than those in PT catchment because of the additionally concentrated initial TSS responses in the first flush runoff within the initial two years after forest thinning. In contrast, all cumulative TSS loads relative to 20, 30, and 50% of runoff flow with MFFn in PT catchment were greater than those in PR catchment from three years after forest thinning onward (Table 3).
Flow peaks can be directly link to rainfall and runoff, and the characteristics of rainfall–runoff can change after thinning [86]. Some studies also indicated the effect of thinning on rainfall–runoff processes only based on rainfall events [38,87,88]. In Table 3, MFFn may be effectively understood with TSS transports created in the first 20 and 30% of runoff volume and rainfall and runoff characteristics, particularly in the initial two years after forest thinning. Saget et al. [59] and Bertrand-Krajewski et al. [60] also indicated similar patterns of MFFn, which may be effectively understood with pollutant loads created in the first 30% of runoff volume. Through the first flush analysis of TSS, we obtained information on the timing of the concentration peak and/or the peak discharge during an observed event, which in turn revealed potential flow pathways and sources for solutes and particulates [89]. Stream water concentrations during this flushing period also depended on the timing and intensity of flushing storms [90,91].

3.4. Suspended Solid Responses to Forest Thinning

We compared the EMCs of TSS in the PT and PR catchments after forest thinning (Figure 7). The mean EMC of TSS ± SD was 17.3 ± 22.8 and 5.8 ± 8.2 mg/L in the PT and PR catchments, respectively, within the initial two years after forest thinning (Figure 7a). From three years after thinning onward, the mean EMC of TSS ± SD was 3.6 ± 3.1 and 5.3 ± 6.0 in the PT and PR catchments, respectively (Figure 7b). The EMCs of TSS for the PR catchment tended to be greater than those for the PT catchment within the initial two years after forest thinning. In contrast, the EMCs of TSS were similar in the two catchments from three years after forest thinning onward.
The TSS event loads after 50% forest thinning were increased based on the paired-catchment design (Figure 8). The TSS event load ranged from 0.01 to 18.9 kg/ha in the PT catchment and from 0.001 to 19.5 kg/ha in the PR catchment within the initial two years after forest thinning. From three years after thinning onward, the TSS event load ranged from 0.002 to 1.8 and from 0.01 to 2.7 kg/ha in the PT and PR catchments, respectively (Figure 8). The TSS event loads were relatively similar between the two catchments from the three years after forest thinning onward (Figure 8). The temporal variability in precipitation during the observation period is critical in determining increases or decreases in the TSS response to vegetation changes after a given treatment [41]. Thus, our paired-catchment design is useful in that it reduced changes in response to climate variability and allowed for examining the effects of changes in vegetation conditions on hydrological processes [92,93,94]. It is also effective for investigating changes in TSS event loads [95,96].
The increased rate of TSS event loads in the PT catchment was on average 4.3-fold, ranging from 0.2- to 13.1-fold, greater than that in the PR catchment in the initial two years after forest thinning (Figure 9). The maximum 13.1-fold increase in TSS event loads occurred with 225 and 428 mm for API7 and API30, respectively (in July 2017). Kasran [44] demonstrated that suspended solid loads after thinning had increase 2.0-fold in hill dipterocarp forested catchments due to enforced regulations regarding road installations, buffer strips, and the construction of cross drains. Webb et al. [96] also indicated that thinning conducted in a eucalyptus forested catchment produced only a 0.3-fold increase in SS loads because of careful site management. In contrast, although our thinning operation using chainsaws and non-heavy machinery seriously impacted soil surface disturbance, there was a maximum 13.1-fold increase in TSS event loads (Figure 8 and Figure 9). This was similar to the 11-fold increase in TSS loads by shifting cultivation with steep land as geomorphic in the tropical rainforest of the Southeast Asian region [97,98]. Since the headwater was located in the highest position of the catchment in those studies, storm flows could respond to intense rainfall by small storage and shorter pathways for TSS event loads [6,99,100,101]. Increases in TSS event loads after thinning were associated with soil surface disturbance by the loss of surface cover and splash erosion, particularly during the flood season (June–September) [68,69]. Summer monsoons may, additionally, deliver seasonal rainfall and runoff, affecting sediment deposition on hillslopes and the transport of fluvial sediment [69,102,103]. Vaughan et al. [104] also found that changes in land cover can alter the response to rainfall events, from supply to transport. These results indicated that TSS event loads could increase due to hydrological connectivity via the magnitude of the soil surface disturbance area in the immediate post-thinning period.
In addition, the TSS event loads demonstrated relatively similar rates in the two catchments from three years after forest thinning onward (Figure 9). This was because the thinning operations induced minimum impact due to the use of chainsaws and non-heavy machinery; episodic mass movements could, therefore, have slightly elevated the TSS event loads. For instance, Brown and Krygier [105] demonstrated that suspended solid loads in a forested catchment recovered within the initial two years due to a lack of the effects of soil surface disturbance by logging operations. Marryanna et al. [106] demonstrated that the SS loads in plantation forests started to decrease again within two years after felling as the forest recovered. Kastendick et al. [107] indicated that, similar to suspended solid loads, nutrient concentrations in runoff have been proven to be strongly influenced by thinning operations. Gomi et al. [108] reported that the rates of recovery were variable and appeared to depend on the nature of the disturbance, in particular, whether increased TSS loads were associated with the primary disturbance from logging operations or secondary disturbance such as mass movements. Thinning operations tended to induce a lower rate of increase in TSS transport because of the minimized disturbance of the soil surface [109,110]. Indeed, thinning may lead to enhance light conditions under the forest canopy, which can increase the growth of understory vegetation [111]. This growth can develop forest floor conditions by altering infiltration capacities [112] and the potential for shallow flow pathways [113].
Our results indicated that TSS event loads could increase slightly due to the soil surface disturbance area during the immediate post-thinning period. Increased TSS event loads also could decrease rapidly three years after thinning and onward due to the protective effect of trapping and settling by the vegetation that could rapidly reestablish due to the low impact of thinning operations with chainsaws and non-heavy machinery. However, because we focused on our observed rainfall events, our results are limited in resolving whether the observed trends were influenced by the accumulation of water flow and TSS loads.
Therefore, forest thinning requires appropriate site-specific land preparation and water conservation management. Moreover, practical guides for sustainable forest thinning and the reduction in suspended solids need to be developed based on site-specific information on soil disturbances and the understanding of processes related to the transport of suspended solids from hillslopes to streams in forested catchments.

4. Conclusions

The suspended solid responses to 50% forest thinning were examined using a paired-catchment design within the initial one to five years after forest thinning. Our main findings were as follows: (1) The total suspended solids (TSS) responses differed depending on the rising or falling stage during rainfall events by the time series of thinning impacts. (2) The cumulative TSS loads in the PT catchment were greater than those in the PR catchment by higher TSS of the first flush period due to forest thinning. (3) The event mean concentration with loads of TSS also increased in the PT catchment during this period. (4) The rate of TSS event loads in the PT catchment was 4.3-fold greater than that in the PR catchment within the initial two years after forest thinning. (5) The TSS event loads were relatively similar between the two catchments from three years after forest thinning onward. These results indicated that TSS event loads only slightly increased due to the low disturbance of soil surface, and that increased TSS loads decreased rapidly due to the protective effects of vegetation trapping and settling, which were possible because forest workers used chainsaws and non-heavy machinery. Hence, for mitigating increases in TSS, appropriate site-specific management and land preparation strategies have to be implemented. Furthermore, it may be reasonable to promote regional, sometimes local, and sustainable water conservation and management with differentiated views for establishing proper forest management practices.

Author Contributions

Conceptualization, H.L. and S.N.; methodology, H.L. and S.N.; software, H.L., H.T.C. and S.N.; validation, S.N.; formal analysis, H.L., Q.L. and S.N.; investigation, H.L.; resources, H.L., H.T.C. and S.N.; data curation, S.N.; writing—original draft preparation, H.L. and S.N.; writing—review and editing, Q.L., B.C. and S.N.; visualization, H.L., B.C. and S.N.; supervision, H.T.C. and S.N.; project administration, H.L., B.C., H.T.C. and S.N.; funding acquisition, H.T.C. and S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not available publicly as they are from a project for obtaining specific research results and because of the intellectual property rights at the National Institute of Forest Science.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Location of the study site and monitoring station in forested headwater catchments. The photos show forest thinning operations using chainsaws and non-heavy machinery in the PT catchment during the thinning period. Vertical distribution and crown density in the PT and PR catchments are shown as well.
Figure 1. Location of the study site and monitoring station in forested headwater catchments. The photos show forest thinning operations using chainsaws and non-heavy machinery in the PT catchment during the thinning period. Vertical distribution and crown density in the PT and PR catchments are shown as well.
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Figure 2. Event characteristics of (a) precipitation, (b) total runoff, and (c) mean of total suspended solids (TSS) in PT and PR catchments during observed 18 rainfall events from 2017 to 2021.
Figure 2. Event characteristics of (a) precipitation, (b) total runoff, and (c) mean of total suspended solids (TSS) in PT and PR catchments during observed 18 rainfall events from 2017 to 2021.
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Figure 3. Time variable mean of the normalized difference vegetation index (NDVI) and total runoff mean of total suspended solids (TSS).
Figure 3. Time variable mean of the normalized difference vegetation index (NDVI) and total runoff mean of total suspended solids (TSS).
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Figure 4. Total suspended solids (TSS) responses and changes in runoff during selected rainfall events, (a) one year after thinning (27–28 July 2017) and (b) four years after thinning (2–3 October 2019). API7 and API30 indicate 7- and 30-day antecedent precipitation indices.
Figure 4. Total suspended solids (TSS) responses and changes in runoff during selected rainfall events, (a) one year after thinning (27–28 July 2017) and (b) four years after thinning (2–3 October 2019). API7 and API30 indicate 7- and 30-day antecedent precipitation indices.
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Figure 5. Relationships between the differential of water runoff (dQ/dt) and (a) runoff and (b) total suspended solids (TSS) in the PT (cross) and PR (open circle) catchments in different years after forest thinning.
Figure 5. Relationships between the differential of water runoff (dQ/dt) and (a) runoff and (b) total suspended solids (TSS) in the PT (cross) and PR (open circle) catchments in different years after forest thinning.
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Figure 6. Cumulative load ratio for total suspended solids (TSS) between the (a) RR and (b) PT catchments after forest thinning.
Figure 6. Cumulative load ratio for total suspended solids (TSS) between the (a) RR and (b) PT catchments after forest thinning.
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Figure 7. Box plots of the event mean concentration (EMC) in total suspended solids (TSS) for the PT and PR catchments (a) within the initial two years and (b) from three years after forest thinning onward.
Figure 7. Box plots of the event mean concentration (EMC) in total suspended solids (TSS) for the PT and PR catchments (a) within the initial two years and (b) from three years after forest thinning onward.
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Figure 8. Relationship between total suspended solids (TSS) event loads from the PR and PT catchments after forest thinning. Thick and broken lines were calculated using regression analysis within initial two years and three years after forest thinning onward.
Figure 8. Relationship between total suspended solids (TSS) event loads from the PR and PT catchments after forest thinning. Thick and broken lines were calculated using regression analysis within initial two years and three years after forest thinning onward.
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Figure 9. Rate of total suspended solids (TSS) event loads from the PR to PT catchments during observed 18 rainfall events from 2017 to 2021.
Figure 9. Rate of total suspended solids (TSS) event loads from the PR to PT catchments during observed 18 rainfall events from 2017 to 2021.
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Table 1. Total suspended solids (TSS) with rainfall events and runoff from two catchments in the post-thinning period.
Table 1. Total suspended solids (TSS) with rainfall events and runoff from two catchments in the post-thinning period.
Post-ThinningNo. EventPT
(mm)		P1
(mm)		API7 (mm)API30
(mm)		Total Runoff (mm)Mean of TSS (mg/L)
PTPRPTPR
≤One and two years 75.5–87.54.0–34.57.5–224.5117.5–434.50.4–37.70.1–77.61.4–38.61.2–14.3
≥Three years 117.5–95.04.0–17.06.5–177.579.0–814.00.2–42.50.3–97.51.3–12.81.2–16.9
Notes: PT: total precipitation, P1: maximum 1 h precipitation, TSS: total suspended solids. PT: treated catchment, PR: reference catchment.
Table 2. Correlation analysis between differential water runoff (dQ/dt) and precipitation, runoff, and total suspended solids (TSS).
Table 2. Correlation analysis between differential water runoff (dQ/dt) and precipitation, runoff, and total suspended solids (TSS).
Post-Thinning PT CatchmentPR Catchment
Rising StageFalling StageRising StageFalling Stage
≤One and two years Precipitation0.633p < 0.01−0.093p = 0.3970.613p < 0.01−0.181p = 0.080
Runoff0.837p < 0.01−0.897p < 0.010.798p < 0.01−0.711p < 0.01
TSS0.950p < 0.01−0.299p = 0.0050.902p < 0.01−0.208p = 0.043
≥Three years Precipitation0.061p = 0.616−0.074p = 0.3260.762p < 0.01−0.182p = 0.011
Runoff0.457p < 0.01−0.494p < 0.010.624p < 0.01−0.634p < 0.01
TSS0.068p = 0.5720.026p = 0.7320.036p = 0.8020.005p = 0.948
Notes: TSS: total suspended solids; significant correlations are shown in bold.
Table 3. Average cumulative load relative to cumulative runoff in the two catchments after forest thinning.
Table 3. Average cumulative load relative to cumulative runoff in the two catchments after forest thinning.
Post-ThinningCumulative
Runoff (%)		Cumulative Load (%)MFFn
PT CatchmentPR CatchmentPT CatchmentPR Catchment
≤One and two years2032.122.41.621.09
3039.734.71.381.18
5057.661.01.131.22
≥Three years2023.832.61.231.65
3037.552.41.281.80
5063.173.91.271.45
Notes: MFFn indicates mass first flush ratio.
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MDPI and ACS Style

Lim, H.; Li, Q.; Choi, B.; Choi, H.T.; Nam, S. Characteristics of Suspended Solid Responses to Forest Thinning in Steep Small Headwater Catchments in Coniferous Forest. Water 2024, 16, 3610. https://doi.org/10.3390/w16243610

AMA Style

Lim H, Li Q, Choi B, Choi HT, Nam S. Characteristics of Suspended Solid Responses to Forest Thinning in Steep Small Headwater Catchments in Coniferous Forest. Water. 2024; 16(24):3610. https://doi.org/10.3390/w16243610

Chicago/Turabian Style

Lim, Honggeun, Qiwen Li, Byoungki Choi, Hyung Tae Choi, and Sooyoun Nam. 2024. "Characteristics of Suspended Solid Responses to Forest Thinning in Steep Small Headwater Catchments in Coniferous Forest" Water 16, no. 24: 3610. https://doi.org/10.3390/w16243610

APA Style

Lim, H., Li, Q., Choi, B., Choi, H. T., & Nam, S. (2024). Characteristics of Suspended Solid Responses to Forest Thinning in Steep Small Headwater Catchments in Coniferous Forest. Water, 16(24), 3610. https://doi.org/10.3390/w16243610

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