A method to estimate the impact of clear-cutting on nutrient concentrations in boreal headwater streams

Abstract

Large-scale forestry operations, like clear-cutting, may impair surface water quality if not done with environmental considerations in mind. Catchment and country level estimates of nutrient loads from forestry are generally based on specific export values, i.e., changes in annual exports due to the implemented forestry operations expressed in kg ha⁻¹. We introduce here a specific concentration approach as a method to estimate the impact of clear-cutting on nutrient concentrations and export in headwater streams. This new method is potentially a more dynamic and flexible tool to estimate nutrient loads caused by forestry, because variation in annual runoff can be taken into account in load assessments. We combined water quality data from eight boreal headwater catchment pairs located in Finland and Sweden, where the effect of clear-cutting on stream water quality has been studied experimentally. Statistically significant specific concentration values could be produced for total nitrogen, nitrate, ammonium, and phosphate. The significant increases in the concentrations of these nutrients occurred between 2 and 6 years after clear-cutting. Significant specific concentration values could not be produced for total phosphorus and total organic carbon with the whole dataset, although in some single studies significant increases in their concentrations after clear-cutting were observed. The presented method enables taking into account variation in runoff, temporal dynamics of effects, and the proportional size of the treated area in load calculations. The number of existing studies considering large site-specific variation in responses to clear-cutting is small, and therefore further empirical studies are needed to improve predictive capabilities of the specific concentration values.

Introduction

Clear-cutting, soil preparation, ditch cleaning, and fertilization are commonly practised forestry operations. They increase nutrient concentrations and loads in receiving waters (Ahtiainen and Huttunen 1999; Kreutzweiser et al. 2008; Nieminen et al. 2010) which may result in degradation of water quality, eutrophication, and formation of harmful algal blooms (Conley et al. 2009). In Fennoscandia, several hundred thousand hectares of forests are clear-cut annually and the majority of clear-cut areas are prepared mechanically before regeneration operations (Swedish Forest Agency 2013; Ylitalo 2013).

Nitrogen (N) and phosphorus (P) concentrations and loads increase in receiving waters after clear-cutting because the removal of trees decreases water and nutrient uptake and increases runoff (Vitousek et al. 1979; Stednick 1996; Kreutzweiser et al. 2008). In addition, increased soil temperatures following clear-cutting accelerate mineralization and nitrification in the soil (Paavolainen and Smolander 1998; Smolander et al. 2001) and nutrients are released from decomposing logging residues (Palviainen et al. 2004). Clear-cutting may also increase total or dissolved organic carbon (TOC, DOC) export (Lamontagne et al. 2000; Schelker et al. 2012, 2014), which have implications for catchment carbon budgets (Schelker et al. 2012), the structure of aquatic food webs (Jansson et al. 2000), the acid–base chemistry of surface waters (Buffam et al. 2008), and the mobility, toxicity, and bioavailability of trace metals and organic pollutants (Porvari et al. 2003; Bergknut et al. 2011). The impacts on water quality are long-term and they are generally at its greatest during the first years after clear-cutting (Rosén et al. 1996; Ahtiainen and Huttunen 1999; Palviainen et al. 2014). The negative effects are reduced by using buffer zones along the streams which are obligatory according to the current forest management guidelines and certification systems.

In order to improve water quality, a number of political programmes and targets have set levels for nutrient loading in Europe (e.g., EU’s Water Framework and Marine Strategy directives and HELCOM’s Baltic Sea Action Plan). The implementation of EU’s Water Framework Directive (WFD, 2000/60/EC) necessitates that European waters achieve good ecological and chemical status and requires the assessment of anthropogenic impacts on water bodies. To meet targets of the directives, we need to know the impact of forestry operations on stream water quality and develop methods to estimate the increase in nutrient export to watercourses.

The impact of forest management practices on nutrient export has been determined in many single-paired catchment studies (Ahtiainen and Huttunen 1999; Löfgren et al. 2009; Palviainen et al. 2014). In the paired catchment method, runoff and water quality of two similar catchments are monitored during several years after which one of the catchments is treated and the other remains untreated. The nutrient loads for the catchments are calculated by multiplying the runoff with the nutrient concentrations measured from the runoff water. The relationship in nutrient load between the catchments during the pre-treatment period is used to predict the behavior of the treated catchment during the post-treatment period as if it had not been treated. The treatment effect is determined as the difference between the measured and predicted nutrient load values. This additional load caused by treatment is referred to specific export value i.e., change in annual export due to the forestry operation in kg ha⁻¹ of land operated (Ahtiainen and Huttunen 1999; Kenttämies 2006). Specific export values derived for total nitrogen and total phosphorus are widely used as tools for environmental management of lakes, rivers, and coastal waters in Finland. For that mean nationwide specific export values have been derived based on several paired catchment studies, and are in use by forest and water managers and policy makers to estimate the impacts of the different forest operations on water quality and to plan the operations to meet the targets of EU’s water policy (Kenttämies 2006; Finér et al. 2010). The mean nationwide specific export values expressed as kg ha⁻¹ are not sensitive to inter-annual variations in climate and runoff conditions, although hydrological and weather conditions have been shown to be one of the key factors influencing nutrient concentrations in watercourses and their fluxes from the catchment (Andersson and Lepistö 2000; Köhler et al. 2009). In addition to weather conditions, also the proportion of the catchment being harvested affects runoff (Idé et al. 2013) and stream water nutrient concentrations and loadings (Lamontagne et al. 2000; Palviainen et al. 2014; Schelker et al. 2014).

Specific concentration values could be an alternative and potentially more dynamic and flexible method to estimate nutrient loadings of forestry, because specific concentration values enable taking into account variation in annual runoff in load assessments. The aim of this study was to produce general specific concentration values that facilitate easy estimation of annual nutrient load following clear-cutting under different runoff conditions. The model approach takes into account temporal variations during 10 years period after clear-cutting, and the proportional size of the treated area. The specific concentration values were derived by combining the data from existing studies concerning the effects of clear-cutting on stream water quality in Fennoscandian boreal forests receiving low N deposition.

Materials and methods

Study areas

Data from eight paired catchments from Fennoscandia, in which the effects of clear-cutting on stream water quality have been studied, were included in the study. Only catchments in which an uncut forest buffer strip was left along the stream were included in the study. The catchments are located in different parts of Finland and Sweden between latitudes 61 and 64°N (Fig. 1). The mean annual precipitation of the study catchments is 550–700 mm and mean annual temperature 0.6–5°C. The total (wet + dry) annual bulk deposition of N in the study catchments is 2–4 kg ha⁻¹. The areas of the treated catchments varied from 5 to 176 ha and the areas of the control catchments from 12 to 165 ha. Forests are dominated by Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.). The proportion of peatlands varies between 1 and 50 % in the treated catchments and between 16 and 48 % in the control catchments. Bedrock is formed of gneiss granite, granodiorite and aplite and is overlain by till. The soils are ferric or haplic podzols and fibric histosols. The study catchments are described in detail by Finér et al. (1997), Ahtiainen and Huttunen (1999), Porvari et al. (2003), Haapanen et al. (2006), Löfgren et al. (2009) and Palviainen et al. (2014).

Fig. 1.

Location of the study catchments

Clear-cutting was performed after the pre-treatment period of 2 years at Balsjö, 3 years at Paroninkorpi, 4 years at Lehmikorvenoja, and 5 years at the remaining five catchments. The proportion of the clear-cut area of the total catchment area varied from 11 to 76 %. Only merchantable stems were removed and logging residues were left on site. In all catchments, unmanaged buffer zones were left along the streams and their minimum width varied from 5 to 33 m. Clear-cut areas were prepared by ploughing (Kivipuro) or disk-trenching (all the other sites) in the second year after clear-cutting. One-year-old Norway spruce or Scots pine seedlings were planted in the next year following soil preparation except at Balsjö where Scots pine seeds were sown during the soil preparation. The length of the post-treatment period was 6 (Porraskorvenoja, Lehmikorvenoja, and Iso-Kauhea), 7 (Balsjö), 9 (Paroninkorpi), or 10 years (Kivipuro, Vanneskorvenoja, and Kangasvaara).

Sampling and chemical analysis

Runoff and water quality data were derived from the Finnish Environment Institute’s (SYKE) databases (https://wwwp2.ymparisto.fi/scripts/oiva.asp, downloaded 20.5.2013), and Högbom et al., provided partly unpublished data from Swedish Balsjö catchment (Löfgren et al. 2009). Runoff from the catchments was measured using V-notch weirs equipped with continuous water-level recorders. Water samples for chemical analyses were taken a few meters upstream from the weir. The sampling intensity ranged from 7 to 42 samples per year. Samples were taken weekly or bi-weekly during the spring and autumn high flow periods and once a month or every second month at other times. The water samples were analyzed with accredited methods at the Finnish Environment Institute, in the laboratories of the Regional Environment Centres (Finland) or at the Swedish University of Agricultural Sciences. Total N concentration was determined colorimetrically or by chemiluminescence detection after oxidization with K₂S₂O₈, NH₄-N was determined spectrophotometrically with hypochlorite and phenol, and the sum of NO₃-N and NO₂-N (hereafter referred to as NO₃-N) by the cadmium method. Total P was analyzed by the molybdenum blue method after digestion with K₂S₂O₈. PO₄-P was measured by the molybdenum blue method. The acidified TOC samples were bubbled with N to remove inorganic carbon (CO₂) and TOC concentrations determined using high-temperature oxidation followed by infra-red gas measurements.

Calculations and statistical analyses

Flow-weighted annual mean concentrations of analyzed nutrients were calculated by dividing the sum of observed daily fluxes (g ha⁻¹ day⁻¹) with the sum of runoff (l ha⁻¹ day⁻¹) at the days of sampling (Eq. 1). The daily fluxes were calculated by multiplying the measured concentrations with the runoff measured on the sampling day.

$$C_{\text{w}}=\frac{{\sum }_{j=1}^{N_{\text{obs}}}{c}_j{q}_j}{{\sum }_{j=1}^{N_{\text{obs}}}{q}_j},j\in \left\{1,\dots ,N_{\text{obs}}\right\}$$ 1

where C_w is the flow-weighted annual concentration (µg l⁻¹ or mg l⁻¹), c_j is sampled concentration (µg l⁻¹ or mg l⁻¹), q_j is runoff (l ha⁻¹ day⁻¹) at the sampling day and N_obs is the number of observations (Table 1).

Table 1.

Characteristics of the treated (T) and control (C) catchments

Catchment	T/C	Location	Area (ha)	Elevation (m a.s.l.)	Peatlands (%)	Clear-cutting (ha/%)a	Removed timber (m3 ha−1)
1. Iso-Kauhea	T	63°53′N, 28°37′E	176.0	200	50	20/11	190
	C	63°52′N, 29°10′E	72.0	182	16
2. Balsjö	T	63°49′N, 20°15′E	11.0	265	2	3.3/30	237
	C	63°49′N, 20°15′E	20.0	297	19
3. Kangasvaara	T	63°51′N, 28°58′E	56.0	187	8	19/34	226
	C	63°52′N, 29°10′E	72.0	182	16
4. Lehmikorvenoja	T	61°52′N, 23°42′E	7.2	127	14	2.8/39	301
	C	61°52′N, 23°43′E	25.0	135	18
5. Vanneskorvenoja	T	61°51′N, 23°42′E	32.8	125	14	13.1/40	220
	C	61°52′N, 23°43′E	25.0	135	18
6. Porraskorvenoja	T	61°52′N, 23°41′E	5.2	125	13	2.1/40	234
	C	61°52′N, 23°43′E	25.0	135	18
7. Kivipuro	T	63°52′N, 28°65′E	54.0	230	32	30/56
	C	63°47′N, 28°29′E	165.0	240	48
8. Paroninkorpi	T	61°00′N, 24°45′E	7.1	130	1	5.4/76	197
	C	61°01′N, 24°49′E	12.3	150	30

^aThe size of the clear-cut area as hectares and as percentage of the catchment area

Annual flow-weighted concentration data from all eight catchment pairs were combined in the same analysis which allows the wider generalization of the results than individual catchment studies. To consider the different proportions of clear-cut area, the treatment effect is expressed as specific concentrations which are defined as the detected change from the background level divided by the proportional area of the clear-cutting. Annual treatment effects (b₁…b₁₀) for the nutrient concentrations were calculated using the following linear mixed model:

$$f_j^{-1}{T}_{ij}=a_j{C}_{ij}{f}_j^{-1}+b_1{I}_1+b_2{I}_2+b_3{I}_3\dots b_{10}{I}_{10}+{\text{e}}_{ij},i=1,2,3,4,5,6,7,8,9,10,$$ 2

where f_j is the proportion of the treated area from the treatment catchment area, T_ij is the annual concentration from the treatment catchment (µg l⁻¹ or mg l⁻¹), i is the year index, j is the catchment pair, a_j is the slope coefficient, C_ij is the annual concentration from the control catchment, b₁,…,b₁₀ are the parameters determining specific concentrations for the post-treatment years from 1 to 10, I₁,…,I₁₀ are the dummy variables for post-treatment years, and e_ij is the error term. The dummy variable I_k is assigned with a value of 1 when the year index i is equal to nc + k, else I_k is equal to zero (nc is the number of pre-treatment years, k is the index of post-treatment years). The concentrations for the treatment catchments as if they had not been treated are calculated as f_j(a_jC_ijf⁻¹_j). This approach is based on the work by Laurén et al. (2009) and Nieminen et al. (2010) and further information can be found therein. The advantage of this approach, as compared to more conventional methods is that the random variability between treatment and control catchments during the calibration period and the proportional size of the treated area can be taken into account in the interpretation of the treatment effects.

The effect of clear-cutting on runoff concentrations in individual catchments were studied by fitting the following linear regression model between control and treatment catchments:

$$T_i=a_1{C}_i+b_1{I}_1+b_2{I}_2+b_3{I}_3+\cdots +b_m{I}_m+e_i,i=1,2,3,\dots n,$$ 3

where i is the year index, n is the total number of years in the dataset, nc is the number of years in the calibration period, m is the number of years in the post-treatment period, k is the post-treatment year index, T_i is the observed mean annual flow-weighted nutrient concentration (µg l⁻¹, mg l⁻¹) for the treatment catchment in year i, C_i is the observed mean annual flow-weighted nutrient concentration (µg l⁻¹, mg l⁻¹) in the control catchment in year i, a₁, b₁, b₂,…,b_m are regression coefficients, I₁,…,I_m are the dummy variables for post-treatment years. The dummy variable I connects the observation C_i to the treatment effect b_i for each year separately. The dummy variable I is assigned with a value of 1, when the year index i is equal to nc + k, else I is equal to zero. e_i is the error term. Errors (e_i) were assumed to be uncorrelated and to have homogenous variance. Estimates for coefficients b₁, b₂,…,b_m represent annual treatment effects.

Long-term (1970–2012) annual runoff varies from 200 to 425 mm in Finland (Statistical Yearbook of Finland 2013). If 20, 40, 60, 80 and 100 % of the catchment area is clear-cut annual runoff increases in Fennoscandia approximately by 0, 55, 110, 170 and 220 mm respectively (Idé et al. 2013). The produced specific concentration values and the above-mentioned ranges in runoff were used to calculate excess N exports caused by clear-cutting (Table 4) and the values were compared to specific export values that are currently used in Finland (Finér et al. 2010).

Table 4.

Excess nitrogen export (kg ha⁻¹ a⁻¹) caused by clear-cutting in boreal catchments according to specific export values (Finér et al. 2010) and specific concentration values (this study) if 20, 40, 60, 80 and 100 % of the catchment area is clear-cut

Years after clear-cutting	Specific concentration values					Specific export values
	20 %	40 %	60 %	80 %	100 %
1	0.06–0.13	0.16–0.30	0.29–0.50	0.46–0.73	0.65–1.00	0.95
2	0.08–0.17	0.21–0.39	0.38–0.66	0.60–0.97	0.86–1.32	0.82
3	0.18–0.39	0.47–0.88	0.86–1.48	1.36–2.19	1.94–2.97	0.82
4	0.15–0.32	0.38–0.72	0.70–1.21	1.11–1.79	1.58–2.43	0.77
5	0.19–0.40	0.47–0.89	0.87–1.50	1.37–2.20	1.95–3.00	0.62
Total	0.67–1.41	1.70–3.19	3.11–5.35	4.89–7.88	6.98–10.73	3.98

Results

During the pre-treatment period, the mean annual flow-weighted total N concentrations varied between 171 and 675 µg l⁻¹ in the study catchments (Table 2). The concentrations of NO₃-N, NH₄-N, total P, PO₄-P and TOC varied in the ranges of 3–78, 3–34, 6–127, 1–9 μg l⁻¹ and 7–35 mg l⁻¹, respectively.

Table 2.

Flow-weighted (mean ± standard error of the mean) annual total nitrogen, nitrate, ammonium, total phosphorus, phosphate and total organic carbon concentrations in the treatment (T) and control (C) catchments during the pre-treatment period

Catchment	Total N µg l−1	NO3-N µg l−1	NH4-N µg l−1	Total P µg l−1	PO4-P µg l−1	TOC
1. Iso-Kauhea
T	481 ± 12	14 ± 1.6	7 ± 0.5	16 ± 0.8	2 ± 0.2	24 ± 0.9
C	248 ± 17	5 ± 0.5	4 ± 0.3	8 ± 0.6	1 ± 0.1	11 ± 0.8
2. Balsjö
T	347 ± 24	14 ± 6.0	23 ± 0.5	12 ± 0.9	–	26 ± 3.3
C	409 ± 64	22 ± 7.5	24 ± 0.8	16 ± 2.2	–	28 ± 2.6
3. Kangasvaara
T	171 ± 13	5 ± 0.6	3 ± 0.3	6 ± 0.7	1 ± 0.1	7 ± 0.6
C	248 ± 17	5 ± 0.5	4 ± 0.3	8 ± 0.6	1 ± 0.1	11 ± 0.8
4. Lehmikorvenoja
T	471 ± 55	23 ± 7.8	33 ± 3.6	16 ± 0.9	3 ± 0.7	19 ± 1.4
C	473 ± 54	6 ± 0.6	9 ± 1.0	26 ± 2.4	4 ± 0.8	23 ± 1.1
5. Vanneskorvenoja
T	376 ± 27	27 ± 6.1	32 ± 10.4	127 ± 39.4	8 ± 0.5	16 ± 1.3
C	476 ± 42	5 ± 0.9	9 ± 0.9	24 ± 2.5	4 ± 0.7	23 ± 0.9
6. Porraskorvenoja
T	472 ± 21	78 ± 18.1	34 ± 4.3	27 ± 1.1	9 ± 1.2	15 ± 0.9
C	476 ± 42	5 ± 0.8	9 ± 0.9	24 ± 2.5	4 ± 0.7	23 ± 0.9
7. Kivipuro
T	504 ± 67	3 ± 0.3	6 ± 0.7	21 ± 1.4	4 ± 0.5	24 ± 0.4
C	478 ± 27	3 ± 0.6	6 ± 1.5	25 ± 1.5	6 ± 1.0	25 ± 0.5
8. Paroninkorpi
T	632 ± 28	34 ± 28.3	6 ± 2.0	15 ± 4	1 ± 0.03	28 ± 2.8
C	675 ± 58	16 ± 2.4	7 ± 0.4	16 ± 3	1 ± 0.1	35 ± 3.3

Total N and NO₃-N concentrations increased significantly (P < 0.05) after clear-cutting in five of the eight treatment catchments, whereas NH₄-N concentrations increased only in two catchments (Fig. 2). Significant increases in PO₄-P concentrations were observed in three catchments. Total P and TOC concentrations increased significantly only in two catchments (Kivipuro and Porraskorvenoja).

Fig. 2.

Treatment effects for total nitrogen, nitrate, ammonium and phosphate concentrations in the study catchments. Lines indicate model-predicted treatment effects if 20, 40, 60 and 80 % of the catchment area is clear-cut

The paired catchment analysis combining data from the eight catchment pairs indicates that clear-cutting increased significantly the concentrations of total N, NO₃-N, NH₄-N and PO₄-P but did not affect total P and TOC concentrations (Table 3). Total N concentrations increased significantly during the second, third, fourth and fifth year after clear-cutting. The specific concentration estimates (b₂, b₃, b₄ and b₅) indicated that every 10 % increment in the proportion of clear-cutting increased total N concentrations by 21, 46, 38 and 47 µg l⁻¹ during the second, third, fourth and fifth year after clear-cutting, respectively (Fig. 2; Table 3). The increase in NO₃-N concentrations was significant in the second and fourth year after clear-cutting (Table 3). A 10 % increase in the proportion of clear-cutting caused NO₃-N concentrations to increase by 4 and 19 µg l⁻¹ in the second and fourth year after clear-cutting, respectively (Fig. 2). Ammonium-nitrogen concentrations increased significantly only in the second year after clear-cutting, and the increase was 0.4 µg l⁻¹ per 10 % increase in the proportion of clear-cut area (Fig. 2; Table 3). Phosphate-phosphorus concentrations increased in the third, fourth, fifth and sixth year after clear-cutting (Table 3). Every 10 % increment in the proportion of clear-cutting increased PO₄-P concentrations by 1, 0.5, 0.9 and 0.5 µg l⁻¹ during the third, fourth, fifth and sixth year after clear-cutting, respectively (Fig. 2; Table 3).

Table 3.

Parameter estimates and their 95 % confidence intervals for annual total nitrogen, nitrate, ammonium, total phosphorus, phosphate and total organic carbon concentrations. Parameter estimates b ₁…b ₁₀ are annual specific concentrations i.e., treatment-induced excess concentrations above the background level is divided by the proportion of the treated area from catchment area. Bold values are statistically significant (P < 0.05)

	Total N	NO3-N	NH4-N	Total P	PO4-P	TOC
a j	1.01 ± 0.36	3.39 ± 3.92	2.03 ± 1.26	1.70 ± 1.04	1.22 ± 0.70	0.94 ± 0.39
b 1	154.64 ± 199.76	8.05 ± 34.73	6.74 ± 22.21	79.91 ± 175.88	10.83 ± 20.81	−1.05 ± 13.76
b 2	205.48 ± 200.27	35.02 ± 34.80	35.18 ± 32.22	−0.99 ± 47.73	4.36 ± 5.89	6.18 ± 8.98
b 3	460.75 ± 292.68	163.95 ± 181.61	123.04 ± 201.11	7.98 ± 48.07	10.11 ± 5.63	7.30 ± 14.53
b 4	377.40 ± 204.88	186.48 ± 114.23	26.25 ± 31.74	10.23 ± 47.37	5.22 ± 3.80	−3.40 ± 9.50
b 5	465.08 ± 294.76	167.92 ± 191.78	32.21 ± 34.25	14.35 ± 47.21	8.84 ± 6.58	9.01 ± 16.13
b 6	162.96 ± 203.43	−41.01 ± 267.45	22.34 ± 35.02	16.10 ± 47.47	4.84 ± 3.64	−2.68 ± 9.29
b 7	128.99 ± 277.82	72.30 ± 149.29	1.12 ± 30.91	−2.29 ± 67.66	1.07 ± 5.58	−1.77 ± 12.49
b 8	47.44 ± 279.97	36.56 ± 47.20	−13.55 ± 31.37	−43.47 ± 144.33	0.87 ± 4.90	−0.27 ± 12.92
b 9	−32.85 ± 284.23	−10.73 ± 225.42	−32.19 ± 150.97	−112.38 ± 352.53	−21.09 ± 75.61	0.68 ± 12.60
b 10	−51.25 ± 317.42	−18.74 ± 60.08	−63.54 ± 338.71	−80.14 ± 341.50	−13.47 ± −62.08	1.44 ± 14.32
var(a j)	0.18	19.85	1.76	1.19	0.46	0.22
var(e i)	64 594.4	2097.2	847.6	4004.9	20.1	130.9

The load calculations indicated that excess N export during the first 5 years after clear-cutting may vary considerably depending on the inter-annual variation in runoff and the proportion of the clear-cut area (Table 4). Our model predicts that the increase in N concentrations and export is at its greatest between 3 and 5 years after clear-cutting, whereas previously used specific export values derived for Finnish conditions assume that the treatment effect is greatest during the first year after clear-cutting after which it gradually decreases with time (Finér et al. 2010).

Discussion

Specific concentrations

Statistically significant specific concentration values could be produced for total N, NO₃-N, NH₄-N and PO₄-P. For total P and TOC specific concentration values could not be produced although in some single studies significant increase in their concentrations after clear-cutting have been observed (Ahtiainen and Huttunen 1999; Haapanen et al. 2006). The changes in nutrient concentrations after clear-cutting varied considerably among the catchments (Fig. 2) and, consequently, 95 % confidence intervals of the specific concentration estimates were rather wide (Table 3). The magnitude and duration of the changes in concentrations depend on several factors such as catchment topography, soil properties, site fertility, atmospheric deposition, vegetation recovery, the timing of management practices and weather conditions after clear-cutting (Ahtiainen and Huttunen 1999; Mattsson et al. 2003; Gundersen et al. 2006; Palviainen et al. 2007; Kreutzweiser et al. 2008; Löfgren et al. 2009; Futter et al. 2010; Palviainen et al. 2014). In addition to above-mentioned factors, the location of clear-cut area in relation to the stream may cause variability in treatment effects among studies (Kreutzweiser et al. 2008; Abdelnour et al. 2011). Also the properties of buffer zones may affect responses (Gundersen et al. 2010). The efficiency of buffer zones to reduce nutrient export depends on soil and topographical characteristics of buffer zone, hydrological pathways, vegetation and microbial activity and dimensions of buffer zones in relation to the upslope area (Gundersen et al. 2010). The number of catchment pairs in the present study is not sufficient to fully analyze and explain observed variation, and further studies are needed to clarify the role of catchment characteristics in different responses to clear-cutting. The highest increase in nutrient concentrations occurred in Paroninkorpi, the southern, fertile, spruce dominated catchment where the largest proportion (76 %) of the catchment was clear-cut. In contrast, at more northern Kivipuro and Balsjö catchments the concentrations of N fractions and PO₄-P did not change although the significant proportion of the catchments were clear-cut (Ahtiainen and Huttunen 1999; Löfgren et al. 2009). At Kangasvaara catchment, which also situates rather north, only NO₃-N concentrations increased after clear-cutting (Palviainen et al. 2014). Possibly clear-cutting increase stream water nutrient concentrations less in northern than in southern Fennoscandia due to slower mineralization rates and lower deposition fluxes (Akselsson et al. 2004; Kortelainen et al. 2006; Futter et al. 2010; Palviainen et al. 2014), and in the northern catchments the observed rise in nutrient loading is largely due to the increased runoff (Ahtiainen and Huttunen 1999; Palviainen et al. 2014).

The increase in total N, NO₃-N, NH₄-N and PO₄-P concentrations after clear-cutting is in accordance with the results from other boreal catchments (Grip 1982; Rosén et al. 1996; Lamontagne et al. 2000; Kreutzweiser et al. 2008). The increases in the concentrations of N fractions in stream water could result from higher deposition loads due to the lack of N retention by tree canopy (Piirainen et al. 2002), reduced nutrient uptake by trees and understory vegetation (Palviainen et al. 2007), and increased nitrification in litter layer and soil (Paavolainen and Smolander 1998). Nitrate is poorly retained in the soil by sorption and can be easily leached to surface waters (Rosén et al. 1996; Kreutzweiser et al. 2008). The increases in NO₃-N concentrations (maximum 190 µg l⁻¹ for the 100 % clear-cut) were considerable smaller than those reported for temperate streams in high N deposition areas, where mean annual NO₃-N concentrations have been shown to increase by 500–3800 µg l⁻¹ after clear-cutting (Adamson et al. 1987; Wiklander et al. 1991; Reynolds et al. 1995; Feller 2005; Gundersen et al. 2006). The elevated PO₄-P concentrations may be due to reduced nutrient uptake and mineralization from logging residues and dead ground vegetation (Palviainen et al. 2004, 2007). Furthermore, elevated ground water levels and more superficial water flow paths after clear-cutting may also have promoted PO₄-P solubility and transport to the stream (Piirainen et al. 2004; Kreutzweiser et al. 2008). The TOC concentration changes have been more variable with some studies showing increases (Kreutzweiser et al. 2008; Laudon et al. 2009; Schelker et al. 2014) and others decreases (Meyer and Tate 1983; Palviainen et al. 2014) after clear-cutting. Increased microbial activity in response to the higher temperatures and the decomposition of logging residues may increase TOC concentrations after clear-cutting, whereas the decline in litter and the throughfall input of TOC may have an opposite effect (Kalbitz et al. 2004). The decline in TOC concentrations after clear-cutting may also result from the dilution effect caused by the increased water fluxes (Mattsson et al. 2003; Kalbitz et al. 2004). Water flow paths regulate surface water TOC concentrations (Laudon et al. 2011) and clear-cutting does not necessarily increase TOC concentrations and export in boreal catchments because runoff increases mainly in spring (Idé et al. 2013) when the soil can be frozen and runoff is generated with little contact with organic soil horizons (Mattsson et al. 2003; Köhler et al. 2009). In other times, downward-percolating TOC fluxes are effectively retained in the B-horizon even after clear-cutting in boreal podzol soils (Piirainen et al. 2002).

Stream water nutrient concentrations peaked 3 years after harvesting and returned to the initial levels within 6 years. In most other studies, stream water chemistry has returned to pre-harvest levels within 5 years (Feller and Kimmins 1984; Reynolds et al. 1995; Gundersen et al. 2006), whereas nutrient export has remained elevated for a longer time (Rosén et al. 1996; Ahtiainen and Huttunen 1999; Palviainen et al. 2014), mostly because it takes up to 20 years until the runoff returns to the pre-cutting level (Idé et al. 2013). There can be a time lag before stream water N concentrations start to rise, because N is immobilized in logging residues during the first years of decomposition (Palviainen et al. 2004) and there is a delay in nitrification response (Vitousek et al. 1979; Ring 2007). Generally in high deposition areas or in catchments with fertile soils, stream water N concentrations increase rather quickly, whereas in nutrient-poor catchments or low deposition areas such as in Finland and central and northern Sweden, the response can be delayed and small (Bredemeier et al. 1998; Gundersen et al. 2006; Ring 2007). Our results are in agreement with the other studies, covering more limited number of catchment pairs, that even a spatially rather limited (<20 % of the catchment area) clear-cutting has a small but detectable effect on stream water quality (Andersson and Lepistö 2000; Palviainen et al. 2014; Schelker et al. 2014).

Reliability of results

Combining data from eight catchment pairs allowed quantification of the effects of clear-cutting on water quality in a more general level than in former individual case studies. The observed concentrations in the streams during the pre-treatment period were of the same order of magnitude as previously reported for unmanaged boreal forested catchments in Finland (Mattsson et al. 2003; Kortelainen et al. 2006). The study catchments are typical of extensive areas of northern Fennoscandian boreal forests with respect to climate, tree stands (Norway spruce and Scots pine dominated), soil type (podzols, medium rich site types) and atmospheric deposition (≤4 kg N ha⁻¹ a⁻¹). Hence, the presented specific concentration values can be assumed to be representative for large parts of the boreal forests in northern Europe. However, some uncertainty is related to the specific concentration values. The accuracy of these results may be somewhat affected by the limitations associated with the weekly or monthly samplings that missed potential daily or hourly concentration fluctuations. Particularly water flow paths and the transit time through the soil regulate the transport and retention of nutrients. Due to variations in water flow paths, concentrations can change rapidly causing uncertainty in the estimates of annual average concentrations (Arheimer et al. 1996; Laudon et al. 2011). The effect of runoff was endeavored to take into consideration by using flow-weighted concentrations (Arheimer et al. 1996; Andersson and Lepistö 2000). There is also uncertainty in the relationship between the treatment and control catchments during the pre-treatment period. Unlike many other studies, we took into account the uncertainty in the regression between the pre-treatment concentrations from the control and the treatment catchments and avoided the overinterpretation of the results (Laurén et al. 2009). The number of existing studies is small considering large site-specific variation in responses to clear-cutting but produced specific concentration values cannot be improved without new data. The method itself is easy to apply for estimating the excess nutrient loads caused by clear-cutting and it takes into account the variation in runoff, temporal dynamics of effects, and the proportional size of the treated area in load calculations. The presented method can also be introduced for estimating the effects of other forest management practices.

Runoff can vary considerably from year to year and, therefore, the use of specific nutrient concentrations can potentially give more accurate estimates of nutrient loading than static specific export values (Table 4). Previously used specific export values which do not take into account the variation in runoff may overestimate excess N export in dry years and correspondingly underestimate the excess N export in wet years. Annual background leaching of N from unmanaged forested catchments in Finland has been shown to vary from 0.29 to 2.3 kg ha⁻¹ (Kortelainen et al. 2006). It should, however, be noted that the presented approach does not take into account the possible differences in water retention times, flow regimes and other processes related to nutrient cycling and retention between wet and dry years.

This study describes clear-cutting induced changes in stream water quality in headwater catchments. When water from headwater streams merges to form larger streams, nutrients originating from various source areas are mixed and processed (Lepistö et al. 2006; Futter et al. 2010; Schelker et al. 2014). Some of the nutrients entering headwater streams are lost or retained in lakes via biological uptake, sedimentation, respiration and denitrification before they reach larger rivers and the marine environment and this should be keep in mind when the results are up-scaled to larger drainage basins (Lepistö et al. 2006; Futter et al. 2010; Schelker et al. 2014).

Conclusions

These specific concentrations values generated for the northern Fennoscandian boreal forests based on total of eight paired catchment studies show that clear-cutting with buffer zones along the streams increases the concentrations of total N, NO₃-N, NH₄-N, and PO₄-P in stream water, and the treatment effect disappears within the first 6 years. The produced specific concentration values can be used to estimate the nutrient loading caused by clear-cutting and enables taking into account variation in runoff, temporal dynamics of effects, and the proportional size of the treated area in load calculations. The widespread application of these specific concentration values includes uncertainties due to heterogeneity and variable responses of the catchments. To improve predictive capabilities of the specific concentration values, more catchment-level empirical data are needed.

Biographies

Marjo Palviainen

has a PhD in forestry and she works as a researcher in the Department of Forest Sciences at the University of Helsinki. Her research focuses on the biogeochemistry of forest ecosystems.

Leena Finér

is a Professor of Silviculture in the Natural Resources Institute Finland (Luke). Her research focuses on nutrient pools and fluxes in forest ecosystems and the environmental impacts of forestry practices.

Ari Laurén

has a PhD in forestry and he works as a senior researcher in the Natural Resources Institute Finland (Luke). His research focuses modeling hydrology and nutrient fluxes in catchments.

Tuija Mattsson

has a PhD in aquatic sciences and she works as a senior researcher in the Finnish Environment Institute. Her research interests include nutrient leaching from boreal catchments and the diffuse loading from forestry.

Lars Högbom

is an Associate Professor in Soil Science and Works at Skogforsk—The Forestry Research Institute of Sweden. His research focuses on the environmental effects of forestry mainly on soil and water.