Losses of nitrate from gaps of different sizes in a managed beech ...

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Losses of nitrate from gaps of different sizes in a managed beech (Fagus sylvatica) forest E. Ritter, M. Starr, and L. Vesterdal

Abstract: In the ongoing discussion about sustainable forestry, gap regeneration is suggested to reduce nitrate (NO3–) losses from forest ecosystems. The effect of gap formation and gap size on soil moisture and NO3– leaching was studied in two managed beech (Fagus sylvatica L.) stands in Denmark for about 2 years after formation of four gaps (approx. 20 and 30 m in diameter). Soil moisture content, soil solution NO3-N concentrations, and nitrogen (N) concentrations in throughfall and precipitation were measured along transects from the gaps into the surrounding forests. Losses of NO3-N were estimated using the water balance model WATBAL. Soil moisture content in gaps remained close to field capacity throughout the year, while it decreased to 50%–70% of field capacity under the closed canopy during the growing season. Drainage water fluxes, soil solution NO3-N concentrations, and NO3-N losses were increased in the gaps as compared to under the canopy. For the whole study period, losses of NO3-N were 3- to 13-fold higher in the gaps than in the surrounding forests. However, a significant effect of gap size was not found within the range of the investigated gap diameters and canopy heights. Presumably, not only the aboveground canopy gaps, but also the belowground root gaps affected soil moisture and thus drainage water fluxes and NO3– losses. Résumé : Les discussions actuelles sur la foresterie durable suggèrent de régénérer les trouées pour réduire les pertes de NO3– dans les écosystèmes forestiers. L’effet des trouées et de leur dimension sur la teneur en eau du sol et le lessivage de NO3– a été étudié dans deux peuplements aménagés de hêtre (Fagus sylvatica L.) au Danemark. L’étude s’est poursuivie pendant environ deux ans après la formation de quatre trouées d’environ 20 et 30 m de diamètre. La teneur en eau du sol, la concentration de N-NO3 dans la solution de sol et la concentration d’azote (N) dans la précipitation au sol et la précipitation incidente ont été mesurées le long de transects allant des trouées vers la forêt environnante. Les pertes de N-NO3 ont été estimées à l’aide du modèle du bilan hydrique WATBAL. La teneur en eau du sol dans les trouées est demeurée près de la capacité au champ pendant toute l’année tandis qu’elle a diminué jusqu’à 50% – 70% de la capacité au champ sous le couvert forestier pendant la saison de croissance. Les flux d’eau de drainage, la concentration de N-NO3 dans la solution de sol et les pertes de N-NO3 étaient plus élevés dans les trouées que sous couvert. Pendant toute la durée de l’étude, les pertes de N-NO3 ont été 3–13 fois plus élevées dans les trouées que dans la forêt avoisinante. Cependant, aucun effet significatif de la dimension des trouées n’a été observé dans la gamme de diamètres des trouées et de hauteurs de la canopée considérée dans cette étude. La teneur en eau du sol et par conséquent les flux d’eau de drainage et les pertes de NO3– n’étaient pas seulement affectés par la discontinuité aérienne du couvert dans les trouées mais vraisemblablement aussi par la discontinuité souterraine des racines. [Traduit par la Rédaction]

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Introduction Forest management affects the nutrient balance of forest ecosystems. A central problem in this context is the increased loss of nitrogen (N) following the disturbance of forests, particularly after harvesting (Vitousek et al. 1979; Piirainen et al. 2002). There is cause for concern not only because N is an essential element for plants and its availability in the soil is often growth limiting (Date 1973; Santa Regina et al. Received 24 May 2004. Accepted 14 October 2004. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 24 February 2005. E. Ritter1,2 and L. Vesterdal. Department of Applied Ecology, Danish Centre for Forest, Landscape and Planning, KVL, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark. M. Starr. Department of Forest Ecology, P.O. Box 27, FI00014 University of Helsinki, Finland. 1 2

Corresponding author (e-mail: [email protected]; [email protected]). Present address: The Agricultural University of Iceland, Hvanneyri, IS-311 Borgarnes, Iceland.

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1997), but also because of potential environmental consequences of nitrate (NO3–) leaching (Nihlgard 1985; Agren and Bosatta 1988; Büttner 1996). The formation of NO3– involves the release of hydrogen ions (acidification), and the leaching of NO3– is often associated with the leaching of base cations, which are also important plant nutrients. In addition, high NO3– concentrations in ground water and stream water negatively affect water quality (Attiwill and Leeper 1987; Gundersen et al. 1998; Myrold 1999). In regions of high N deposition, such as Denmark, the risk of soil N saturation only exacerbates the potential for NO3– leaching. Therefore, in the ongoing discussion about sustainable forestry, management practices should focus on mitigating the leaching of NO3–. Losses of NO3– after tree felling are related to increased substrate availability (logging residues) and mineralization rates and reduced nutrient uptake by plant roots (Likens et al. 1969; Bormann and Likens 1979; Egnell and Valinger 2003; Shammas et al. 2003). Changes in the hydrological cycle also play an important role, since NO3– ions are very mobile and leaching losses are controlled by the soil water

doi: 10.1139/X04-185

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flux (Stevenson 1986). Nitrate concentrations in percolating soil water can be measured relatively reliably, while soil water fluxes show a high spatial variation (Manderscheid 1996; Bárdossy and Lehmann 1996; Netto et al. 1999), and a reliable measurement of soil water fluxes in unsaturated soil is practically not possible. Computer simulation models are often used to provide an estimate of the soil water fluxes needed to calculate leaching losses of NO3– from forest ecosystems (e.g., Ring 1995; Kennel 1996; Beier 1997). There is ample evidence that the water balance of forest ecosystems changes and that losses of NO3– with seepage water increase after clear-felling (Likens et al. 1970; Krause 1982; Adamson et al. 1987; Nohrstedt et al. 1994; Piirainen et al. 2002). It has been suggested that N losses can be controlled by using harvesting practices that only cause smallscale disturbance rather than harvesting large areas of forest, as is the case with conventional clear-felling (Knight et al. 1991; Prescott 1997). In this context, formation of gaps whereby only small groups of trees within a forest stand are removed has been seen as an appropriate alternative. Gap size has been found to be a crucial factor affecting many ecological factors and processes in gaps, including regeneration, species composition, microclimate, and nutrient turnover (Williamson 1975; Runkle 1982; Mladenoff 1987; Whitmore 1989; Parsons et al. 1994), and also small-scale disturbances have been shown to cause N leaching (Hobara et al. 2001). However, studies in this area are still scarce, and more knowledge about the consequences of gap formation on NO3– leaching is necessary for a successful application to forest management. The objectives of this study were to investigate soil water NO3– concentrations and NO3– leaching fluxes from gaps of different diameters in homogeneous, even-aged managed European beech (Fagus sylvatica L.) forest stands in the first years after gap formation. For this purpose, soil moisture content, nitrate-nitrogen (NO3-N) concentrations in soil solution, and N concentrations in precipitation were measured. Nitrate leaching fluxes were calculated as the product of the measured soil solution NO3-N concentrations and modelled soil water drainage fluxes. It was hypothesized that soil solution NO3-N concentrations, soil water drainage fluxes, and leaching losses would be greater in the gaps than under the closed canopy of the surrounding forest, and that the losses would increase with gap size.

2 weeks. The long-term annual mean in this area is 40 d with snow cover (DMI 1999). The study stand in Ravnsholte (RH stand), occupying an area of 3.7 ha, consisted of 75-year-old beech trees having an average canopy height of about 27 m. Stem density was 184 trees·ha–1, basal area was 20.1 m2·ha–1, and diameter at breast height (dbh) was on average 37.9 cm. Advanced but scattered regeneration of sycamore maple (Acer pseudoplatanus L.) already occurred in the understorey layer prior to gap formation. The 80-year-old beech stand in Hejede Overdrev (HO stand) was 3.9 ha with an average canopy height of about 28 m. There was no regeneration or understorey layer at the beginning of the study, but the grass Deschampsia flexuosa (L.) Trin., and various sedge species (Carex sp.) were present at the site of the large gap. In the small gap in the HO stand, Rubus idaeus developed a dense ground vegetation cover soon after gap formation. This HO stand was more open (stem density 112 trees·ha–1, basal area 18.2 m2·ha–1, dbh 46 cm) than the RH stand and more exposed to the prevailing westerly winds. The nutrient-rich soil, developed from glacial till, was classified as coarse–loamy Ultic Hapludalf at the RH stand and coarse–loamy Typic Hapludult at the HO stand (Soil Survey Staff 1998). Stone content (volumetric) in the upper 90 cm soil was 0%–15% at RH and 2% at HO. Selected soil properties from a soil pit dug in the two stands are given in Table 1. In January 2001 two, almost circular, gaps of different diameters were established by group felling in each of the stands. The diameters of the two gaps, measured from the canopy drip line, were about 27 m and 19 m in the RH stand, corresponding with 12 and 4 felled canopy trees, respectively. The diameters of the gaps in the HO stand were about 33 m and 20 m, corresponding with 9 and 3 felled canopy trees, respectively. According to Runkle (1982) and Gray et al. (2002), gap size was defined by the ratio of the gap diameter to the mean height of the surrounding stand (d/h ratio). In the present study, the gaps are classified as large gaps when d/h = 1.0 and as small gaps when d/h < 1.0 (Table 2). The trees were felled by hand and then pulled from outside the gaps by using a winch to avoid soil disturbance in the gap area. The treetops were left close to the gap edges in the surrounding forest, while the stems were removed. Both sites were fenced in February 2001 to avoid disturbances by visitors of the forest and deer browse.

Materials and methods

Field measurements Soil moisture content was measured, and soil solution and precipitation were collected along transects running through the approximate centre of each gap. In the RH stand, a north–south and a west–east transect were established in the two gaps. In the gaps of the HO stand, only a north–south transect was established. The transects extended for about 20–35 m from the gap centre into the surrounding forest. Measurement plots were established at approximately 4-m intervals along the transects, each measurement plot covering an area of approximately 1–2 m2. The number of measurement plots was 28 and 22 at the large and small gap site in the RH stand, respectively, and 16 and 12 at the large and small gap site in the HO stand, respectively. Stations for the measurement of soil moisture content and sampling of soil solution and precipitation were located close to each other at

Site description Two comparable, homogeneous European beech stands in the managed Ravnsholte (approx. 200 ha) and Hejede Overdrev (approx. 275 ha) forests, which form part of the Skjoldenæsholm forest area, were chosen for the study. The approximate distance between the two study sites was 1500 m. Located in Central Zealand in Denmark (55°31′N, 11°54′E, 85 m above sea level), the area is gently rolling. However, the maximum difference in height above sea level across the area of each of the two study sites was less than 2 m. The climate is cool–temperate with a mean annual air temperature of 8 °C (Danish Meteorological Institute (DMI) 1999). Annual precipitation in the area averages about 600 mm. Snow cover occurs occasionally, but generally never for longer than 1 or

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1.6 3.7 3.3 1.9 2.3 0.22 0.08 0.03 0.16 0.06 0.41 0.10 0.03 1.50 0.11 a

Note: Particle size: clay, 95 mm) were estimated for January 2002, in both the gaps and the forest, when there was little evapotranspiration and additional inputs of water to the soil from snowmelt. The average total drainage flux over the study period was 435, 393, and 318 mm for the small gaps, large gaps, and surrounding forests, respectively. Drainage fluxes (on a monthly basis) were somewhat higher in the small gaps than in the large gaps in the dormant season 2001–2002 (p = 0.04). For both gap sizes, drainage fluxes were slightly less in the gaps than in their surrounding forests (large gap sites: p = 0.04, small gap sites: p = 0.05). © 2005 NRC Canada

Ritter et al. Fig. 4. Monthly average NO3-N concentrations in soil solution collected at 90-cm-depth in (a) the two small gaps, (b) the two large gaps, and (c) the average of all forest sites (error bars are 1 SE). For calculation of the monthly values of NO3-N concentrations, resulting in one average value for gap and closed forest, respectively, see text. Black bars indicate the period when trees are in full leaf. RH, Ravnsholte stand; HO, Hejede Overdrev stand.

No effect of gap size or forest phase was found during the following 2002–2003 dormant season (size: p = 0.08, forest phase: p = 0.4). Calculated for the whole study period, drainage fluxes from the gaps were, on average, 23% (or 96 mm) higher than under closed canopy. Nitrate concentrations and leaching fluxes Gap size had no effect on soil solution NO3-N concentrations (p = 0.6). Monthly mean NO3-N concentrations were significantly higher in the gaps (7.2 mg NO3-N·L–1) than in the surrounding forest (2.2 mg NO3-N·L–1) (p = 0.01) and they gradually increased with time (Fig. 4). Although NO3-N concentrations were generally lower in the large gap of the HO stand than in the three other gaps, there was no significant difference between stands (p > 0.2). In the large gap of the HO stand, monthly mean NO3-N concentrations never exceeded 6 mg NO3-N·L–1. Monthly mean NO3-N concentrations in the other three gaps increased to a level higher than in the surrounding forest by the end of the first growing

315 Fig. 5. Input of (a) NO3-N and (b) NH4-N to the forest floor as precipitation in the gaps and as throughfall in the surrounding forest. Circles illustrate averages from sampling plots under closed canopy, along gap edges, and in and around the centre of the gap. Error bars for edges are 1 SE, calculated from the four samples of the four edge directions. Black bars indicate the period when trees are in full leaf.

season after gap formation, and maximum values varied between 11 and 16 mg NO3-N·L–1. In the surrounding forests, concentrations never exceeded 9 mg NO3-N·L–1 at all four sites. Total inorganic nitrogen (NO3-N plus NH4-N) measured in the throughfall of the surrounding forest in 2002 was 16 kg·ha–1, while the leaching loss of NO3-N was about half this value. In contrast, leaching losses of NO3-N from gaps were about double the input of inorganic N to the forest floor, 12 kg·ha–1, measured in the precipitation in the gap. Only in the large gap in the HO stand was the amount of input and output similar. With concentrations below detection limit, the leaching of NH4-N was negligible in both the forest and gaps. Neither NO3-N nor NH4-N monthly input was significantly different among gap, forest, and the four gap edges throughout the whole measurement period (NO3-N: p = 0.2; NH4-N: p = 0.8) (Fig. 5). Monthly leaching fluxes of NO3-N in the gaps were not affected by gap size in either season of the year (p ≥ 0.4). Leaching losses in the gaps (average 2.2 kg NO3-N·ha–1) were almost significantly higher than in the forest (average 0.7 kg NO3-N·ha–1) in the first dormant season after gap formation (p = 0.06). When trees were in full leaf, NO3-N fluxes occurred only in the gap (average 1.1 kg NO3-N·ha–1) © 2005 NRC Canada

316 Fig. 6. Monthly leaching fluxes of NO3-N from the rooting zone as estimated by the WATBAL model in (a) the two small gaps, (b) the two large gaps, and (c) the average of the four forest sites (error bars are 1 SE). For calculation of the monthly leaching fluxes, resulting in one average value for gap and closed forest, respectively, see text. Black bars indicate the period with trees in full leaf. RH, Ravnsholte stand; HO, Hejede Overdrev stand.

(p < 0.0001). During the dormant period 2002–2003, NO3-N fluxes were significantly higher in the gaps (average 1.4 kg NO3-N·ha–1) than under closed canopy (average 0.3 NO3N kg·ha–1) (p = 0.02). There was no significant difference between the two stands (p ≥ 0.1), even though losses of NO3-N from the large gap in the HO stand were, on average, 2.5 times lower than in the three other gaps (Fig. 6). Over the study period, the total loss of NO3-N from large gaps was about 5-fold and 13-fold higher than from the surrounding forest in the RH and OH stands, respectively. For the small gaps, the difference was 5-fold (RH stand) and 3-fold (OH stand). The total sum of NO3-N leaching from the gaps was highest for both gaps at the RH stand (40 and 41 kg·ha–1) and lowest for the large gap in stand HO (15 kg·ha–1), that of the small gap in the HO stand being intermediate (36 kg·ha–1). The highest losses occurred in the first months of 2002. The total loss from the surrounding forests averaged 7 kg·ha–1

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(range 1–11 kg·ha–1) for the whole study period. Calculated for the specific area of a gap, and not per hectare, total NO3-N losses of the whole study period from gaps were within the range of 1–3 kg·(gap area)–1.

Discussion Soil moisture and drainage water fluxes Reduced transpiration and canopy interception in the gaps explain the higher soil moisture and drainage levels observed in the gaps compared with the surrounding forest, where soil moisture levels declined during the growing season, and drainage did not occur before November. Reduced transpiration and canopy interception in gaps have been shown in other temperate beech forests (Bauhus and Bartsch 1995; Bartsch et al. 1999), mixed species stands (Zirlewagen and von Wilpert 2001), and in coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests (Gray et al. 2002). In a semi-natural, heterogeneously structured forest, the effect was found to diminish gradually with time after gap formation, which was attributed to root expansion into the gap, increased interception of bordering trees, and a vigorous regeneration in the gap as it aged (Ritter et al. 2005). The extension of roots into gaps has been reported and discussed by several authors (e.g., Parsons et al. 1994; Bauhus and Bartsch 1996; Brockway and Outcalt 1998). Taskinen et al. (2003) found that living roots from edge trees extended up to 5 m into a gap in a boreal, Norway spruce (Picea abies (L.) Karst.) forest within the first two growing seasons after gap formation. A more rapid extension of Norway spruce roots into gaps was observed by Müller and Wagner (2003). The pattern of peripheral low and central high soil moisture content observed in the large gaps in our study, particularly at the HO stand, may have been due to the presence of roots connected to trees bordering the gap. The size of aboveground and belowground gaps may therefore differ and affect the distribution of soil moisture contents. The drainage flux was unexpectedly higher in small than in large gaps and less in gaps than under closed canopy in the dormant season 2001–2002. This result is not clearly explainable, but presumably because of variation in microclimatic conditions, development of vegetation, or other factors beyond that, we were able to define with the high and low soil moisture regimes in the large gaps (Bosch and Hewlett 1982; von Wilpert and Mies 1995; Lesch and Scott 1997). The smaller field capacity values estimated for the small gap sites indicate that the soil in the small gaps would reach field capacity and thus generate drainage fluxes earlier than in the large gaps. Precipitation was higher in the dormant season of 2001–2002 than in the following dormant season (DMI, grid data), in which no effect of gap size was found. Even though soil moisture content was overall higher in the gaps than in the surrounding forest, simulated soil water drainage fluxes were zero during most of the growing season in both the forest and gaps. This indicates that evapotranspiration by ground vegetation and regeneration in our gaps was sufficient that gravitational drainage water was not generated. However, the greater amounts of water infiltrating the soil in the gap combined with lower evapotranspiration towards the end of the growing season resulted in drainage being initiated a month or more earlier in the gaps than in the forest. © 2005 NRC Canada

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Soil solution nitrate concentrations and leaching fluxes The increase in NO3-N concentrations in soil solution after gap formation, particularly in the large gap (d/h ratio = 1.0) of the RH stand, is consistent with results of studies on gaps and other small-scale disturbances in beech stands (Bartsch et al. 1999; Hobara et al. 2001). Similar high concentrations as in the gaps of the present study were reported by Bartsch (2000) in an old beech stand developed from natural regeneration in the first 4 years after gap formation and by von Wilpert et al. (1996) in mixed and pure stands of beech, Norway spruce, and silver fir (Abies alba Mill.). Such distinct changes in soil water chemistry in the first years after tree felling can be attributed to a disruption of the normally closed N cycle and enhanced mineralization and nitrification rates (Frazer et al. 1990; Smethurst and Nambiar 1990; Dahlgren and Driscoll 1994) and to reduced uptake of nutrients by roots (Foster et al. 1989; Gundersen 1998). High levels of NO3-N in soil solution also depend on deposition levels of N, tree species composition, and soil properties (Gundersen et al. 2003). In the present study, neither input of N to the forest floor by throughfall or precipitiation nor soil nitrification appear to have significantly contributed to elevated soil solution NO3-N levels in the forest or in the gaps. Input of N to the forest floor as bulk deposition in the gaps and as throughfall in the surrounding forest was similar. However, at least in the RH stand, where an in situ N mineralization study has been carried out (Ritter 2005), there was a nonsignificant increase in soil nitrification rates between the gaps and forest. This might indicate a stimulation of mineralization processes in the canopy opening, as also found by other authors in gaps and clearcuts (e.g., Bauhus and Barthel 1995; Bauhus 1996; Redding et al. 2004) Possibly, reduced plant uptake after removal of the large trees might partly be responsible for the elevated NO3-N concentrations in soil solution from the gaps. Dense ground vegetation cover and rapid regeneration after tree felling can later result in NO3– retention and less leaching losses (Bartsch et al. 1999; von Wilpert and Mies 1995). The lower NO3-N concentrations in soil solution from the large gap with sparse ground vegetation at the HO stand than the other gaps, however, cannot be so explained by the examinations that were part of the study. We assume that the more exposed location of the large gap in stand HO might have inhibited N turnover processes, or microbial immobilization may have been greater. Increased NO3-N concentrations in soil solution in the gaps together with greater drainage fluxes resulted in the observed higher leaching losses of NO3-N from the gaps compared with the forest. The range of gap size (d/h ratio) included in the present study was found not to have an effect on NO3-N leaching losses. This implies that even large gaps of more than 10 felled trees (at the given stand density), presumably more attractive in forest management, do not have a more negative effect than small gaps. However, it is still not known if the leaching losses from small gaps will decline earlier than losses from large gaps, as small gaps close earlier than large ones (Gysel 1951; Valverde and Silverton 1997). Studies have shown that the initially high NO3-N concentrations observed in young gaps decreased within a few years of gap formation or stand thinning operations (Knight et al. 1991). Von Wilpert and Mies (1995) reported a

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drop in NO3-N concentrations in the third year after gap formation concurrent with the emergence of nitrophile ground vegetation. The increased leaching losses of NO3– may therefore be expected to diminish in the near future, which would support the application of gap regeneration in forest management over conventional clearcutting practices.

Conclusion We found both concentrations of NO3-N in soil water and drainage water fluxes increased after gap formation, resulting in increased NO3-N leaching in our study. Leaching loss differences between gaps and closed forest were 3- to 13fold. Gap size had no effect on soil water NO3-N concentrations, and the drainage fluxes were only slightly higher in the small gaps during one dormant season. However, the effect of gap size on NO3– leaching may be on the duration of elevated NO3– leaching. Because gap closure can be expected to occur earlier in smaller gaps, a decline in NO3– leaching can be expected to also take place earlier than in larger gaps.

Acknowledgement We acknowledge the technical assistance of A. BorkenHagen, X. Haliti, A. Harder, P. Frederiksen, M.M. Krag, A. Overgaard Nielsen, and L. Thomassen of the Danish Centre for Forest, Landscape, and Planning during fieldwork and for doing the laboratory analyses. Thanks to I. Callesen for carrying out the soil classification of the two soil pits. The study was supported by the EU Fifth Framework Program (grant QLKS-CT-1999-01349).

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