Greater abundance of Fagus sylvatica in coniferous flood protection forests due to climate change: impact of modified root densities on infiltration
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Climate change is expected to modify the spatial distributions of zonal forest communities and thus, their species compositions. The aim of this paper was to study the impact of higher abundance of beech on water storage capacity in current coniferous flood protection forests due to varying root densities of the main tree species. Two forest communities in the northern pre-Alps in Switzerland with similar soil properties but varying in species composition were investigated (space-for-time substitution). It was assumed that the Vaccinio myrtillii-Abieti-Piceetum (site A) will be replaced by a Luzulo-Abieti-Fagetum (site B). We irrigated 16 hydromorphic soils (1 m2, 70 mm/h, three consecutive irrigations) at site A and 10 at site B and recorded water-content variations with time domain—and frequency domain reflectometry. Roots were extracted from soil cores taken from the positions where the water-content probes were inserted, and digitally measured. Infiltration capacity ωI was mainly limited to the upper soil at site A but was approximately constant down to 0.7 m depth at site B. Between 0.3 and 1.0 m soil depth, root densities at site B exceeded those at site A. Root density was the main predictor for ωI (R2 = 0.57) at site A as shown by a multiple linear regression analysis. Assuming that the root density in the current coniferous forest (A) will increase to that of the beech stand (B) due to the greater abundance of beech, the water storage capacity will increase by 9.2 mm in consequence of the expected forest transformation.
KeywordsClimate change Flood protection forests Water storage capacity Beech Spruce Forest transformation
Climate change is expected to increase the annual mean temperature in the northern part of Switzerland by approximately 2 °C by 2050 compared with 1990 (OcCC/ProClim-(ed.) 2007). As a consequence, the spatial distribution of zonal forest communities and thus the species compositions of these forests will be modified by temperature increase.
Approximately 36 % of the total forested area in Switzerland has an important protection function (Brändli 2010). Flood protection forests are likely to become even more necessary since floods are expected to occur more frequently due to climate change (OcCC/ProClim-(ed.) 2007). Such forests are in the catchment areas of the large Swiss rivers and are located in the montane and subalpine belts. Simulations indicate that the currently dominant Piceo-Abietion community in the northern pre-Alps in Switzerland will be replaced by Abieti-Fagion (Brzeziecki et al. 1995). Thus, the abundance of Fagus sylvatica will increase at the expense of Picea abies and, to a lesser extent, of Abies alba. This forest transformation may influence the efficiency of these flood protection forests.
Species change in forests impacts their hydrological properties directly by modifying their transpiration and interception rates. The interception rates of deciduous trees vary during the course of the year, while those of evergreen conifers remain nearly constant throughout all seasons (Christiansen et al. 2006). Furthermore, the transpiration rates of conifers and deciduous trees differ. Peck and Mayer (1996) analysed several studies about transpiration rates of trees and concluded that beech transpires on average 27 % more than Norway spruce, even though the transpiration of beech is mainly limited to the growing season. Hence, tree species significantly influence the soil water content and, consequently, how much water the soil can potentially store.
The transformation of forest composition also affects soil hydrological characteristics indirectly by modifying soil properties. The thickness of the forest floor layer declined during a Pinus sylvestris to F.sylvatica transformation in Germany, but the humus topsoil increased (Bens et al. 2007). Organic layers have high water storage capacities (Guevara-Escobar et al. 2007), but may also reduce infiltration capacity as they are water repellent (e.g. Doerr et al. 2000). Moreover, changes in tree species impact the rhizosphere since different species have different root systems and root densities, resulting in varying spatial patterns of water uptake, and thus drying out of the soils (Schwärzel et al. 2009).
Roots are important generators of preferential flow paths, which may improve the forest soil’s infiltration capacity substantially (Beven and Germann 1982). Noguchi et al. (1999) demonstrated that 70 % of the macropores in the topsoil and 55 % in the subsoil in a Japanese forest were related to roots. Lange et al. (2009, 2010) compared the root distributions in hydromorphic soils with the porosity that effectively carried preferential flow, and concluded that root density was related to infiltration capacity. Pores generated by woody roots can persist for decades, as has been shown in Denmark and Switzerland (Jorgensen et al. 2002; Hagedorn and Bundt 2002). Furthermore, saturated hydraulic conductivity ks can be improved by roots since Li and Ghodrati (1994) found that ks was five to six times higher in samples with root channels than in those without roots. Thus, roots seem to be one of the most important generators of well-connected pores in forest soils. Periodical anaerobic conditions in soils limit the root growth of certain species. Lehnardt and Brechtel (1980) found the roots of adult beech stands penetrated down to 0.6 m, whereas the maximum root depth of spruce was only 0.4 m in comparable hydromorphic soils, indicating that spruce is more liable to oxygen deficiency in the rhizosphere.
compare the infiltration characteristics of two forest sites with comparable soil properties but varying abundances of beech and spruce;
evaluate the relevance of roots and other soil properties for infiltration characteristics and
model the modification of water storage capacity resulting from the greater abundance of beech in coniferous flood protection forests.
Materials and methods
Site and soil description
The experiments were conducted in the Gantrisch region in the Swiss pre-Alps approximately 30 km south of Berne at altitudes between 880 and 1,000 m a.s.l. Annual precipitation averages 1,700 mm and the mean annual temperature is 5.9 °C at 1,160 m a.s.l. (Zimmermann et al. 2006). The study sites are located in the Flysch zone and features mainly marled clays interlaced with stony or sandy layers. A space-for-time substitution was applied to evaluate the potential effects of greater abundance of beech in the flood protection forest, currently composed of spruce and fir. Two adult forest sites with comparable soil properties but varying in forest composition and altitude were investigated. The distance between the sites is approximately 700 m and the difference in the altitude ≈120 m. Site A defines the current forest stand and is classified as a Vaccinio myrtillii-Abieti-Piceetum (Ellenberg and Klötzli 1972), with Norway spruce (P. abies (L.) Karst.) as the most abundant species. Silver fir (A. alba Miller) and European beech (F. sylvatica L.) are secondary.
The prevailing soil types are Gleys, cambic Gleys, gleyic and stagnic Cambisols and Cambisols, according to FAO-UNESCO (1988). Beneath depths of 0.03–0.25 m, hydromorphic attributes such as iron and manganese concretions and mottles are found. The physical and chemical properties of the soil were determined from dried samples (48 h, 105 °C for bulk density and 60 °C for pH and texture). Three cylinders per horizon, with a volume of 1,000 cm3 and a height of 0.1 m, were collected to determine bulk density. pH was determined in a CaCl2 solution and the pipette method was applied for particle-size distribution separation. Considering all horizons investigated at site A, the percentage of sand was 18.6–71.6 %, silt content was 12.7–43.5 % and clay varied between 9.9 and 42.0 %. Bulk density was between 0.19 and 1.19 Mg m−3 in topsoils and 0.75 and 1.60 Mg m−3 in subsoils. The slopes of the investigated areas did not exceed 15°.
Characteristics of the study sites
Altitude (m) a.s.l.
Gleys, cambic Gleys, gleyic or stagnic Cambisols, Cambisols
Main tree species
Norway spruce, silver fir
Silver fir, European beech
Number of irrigated plots
We conducted sprinkling experiments to measure variations in the volumetric water content θ(Z, t) as a function of time (t) at various depths (Z). At site A, the volumetric water content was measured with TDR equipment (time domain reflectometry). The wave guides consisted of two paired stainless steel rods, 0.15 m long, 30 mm apart and 5 mm in diameter. A 50-Ω coax cable connected the rods to an SDMX 50 coaxial multiplexer controlled by a CR10X micro-logger. A Campbell TDR100 device (Campbell Scientific, Logan, USA) generated the electrical pulses and received the signals. The transfer function of Roth et al. (1990) was applied to calculate the volumetric water content. At site B, soil moisture was recorded with FDR equipment (frequency domain reflectometry) from Decagon Device (Pullman, USA). We used 10HS soil moisture sensors (0.1 m long) and collected data with an Em50 data logger. Dielectric permittivity was converted to volumetric water content by applying the transfer function of Topp et al. (1980). Variations in water content were recorded at 60 s intervals at both sites. The wave guides were installed horizontally from a trench into each soil genetic horizon according to the soil profile descriptions. Thus, depth of water-content probes varied between the investigated plots. Each soil profile was equipped with five probes, where a soil was composed of less than five horizons, the thickest layer was equipped with two wave guides one below the other.
The sprinkler device consisted of an aluminium plate (1 m × 1 m) perforated with 100 holes in a 0.1 m × 0.1 m square pattern. The holes were attached to PVC tubes (inner diameters 2 mm) connected to a water reservoir with a constant water table. The plate was moved by a motor ±50 mm back and forth in both horizontal dimensions during the irrigation experiments so that it took approximately 1,800 s until a tube returned to exactly the same position. The intensity of irrigation was 70 mm h−1 and lasted 1 h. No visible surface run-off occurred during the experiments. A waterproof tent (3 m × 3 m) covered the set-up and the soil profiles during the experiments.
At each plot, we conducted three consecutive irrigation experiments in 23 h intervals. The first and second irrigations primarily served to saturate the finer pores in the soil where capillarity may be dominant. As shown by Lange et al. (2011) at the same location, water flow released by the third artificial rainfall event was mainly restricted to larger, fast-draining pores, where capillarity can be neglected. Thus, the principal focus of our interpretation was on the results of the third irrigation, where the pore-size spectra of pores carrying mobile water were comparable in all plots and the influence of the varying antecedent soil moisture level was negligible.
Overall, we recorded 371 time series of water-content variations from 16 plots at site A and 10 plots at site B. The plots were distributed over an area of approximately 10,000 m2 at site A and 2,000 m2 at site B and the distances of the irrigated areas to the closest tree was about 1 m (with the exceptions of three plots at site A where the distance to the stem was about 3.5 m). The depths of the probes were between 0.04 and 0.97 m. We conducted the infiltration experiments in fall 2006 and summer and fall 2007 (site A), and in summer and fall 2010 (site B).
Soil cores for the analyses of the root length distribution were taken subsequent to irrigation with a HUMAX soil corer (diameter 0.1 m) to depths between 0.5 and 1 m, at the same positions as those where the TDR and FDR probes were installed. The cores consisted of 0.25 m long segments in plastic tubes in which the soil was left undisturbed. The roots were sorted out and washed in a 1-mm sieve with tap water. All root fragments (woody and herbal roots) were collected and stored at 4 °C for no more than 12 weeks. Root length (cm) was measured with winRHIZO (V4.1c; Regent Instruments Inc., Quebec, Canada) for each morphological horizon separately.
To test distributions for different means, a rank-based test was applied (Mann–Whitney U test) since some data were not normally distributed. Statistical analyses were conducted with SYSTAT (V10; Cranes Software International, Bangalore, India).
Analyses of wcws
The key points of a wcw, initial (θin) and maximal (θmax) water content were derived from the mean of ten water-content recordings between the beginning of the irrigation t0 and t0 − 540 [s] (θin) and between the arrival time of the drainage front tD and tD − 540 [s] (θmax). The final water content θF refers to the interval between 74,460 and 75,000 s (see Fig. 5a), where water content was constant during data recording, θmax was averaged over the last 540 s of the irrigation (3,060–3,600 s). A wcw is defined with its amplitudes of infiltration ωI (θmax − θin), which corresponds to the mobile water content (see Eqs. 3 and 6), and drainage ωD (θmax − θF) and the amount of non-draining water ωF (θF − θin, Fig. 5a).
Since F and L are calculated according to the maximal velocity of the wetting front (Eq. 5) between the soil surface and depth Z and on measured volumetric water content at Z (Eq. 6), L and F are also controlled by the wetting front’s advance and the water-carrying porosity above Z, which has to be considered when interpreting the water-flow geometry.
Number of recorded wcws and arithmetic means of the water-content waves’ amplitudes
Soil properties influencing the amplitude of infiltration
Multiple linear regression analyses with ωI as the explanatory variable
RL, u, θin
Predicted amplitude of infiltration with greater abundance of beech
Discussion and conclusions
Averaged ωI at the beech site (B) exceeded those at the spruce site (A) by 0.005, 0.025 and 0.011 m3 m−3 for the first, second and third irrigation, respectively, due to the proportion of wcws with no increase in water content being lower at site B and larger ωI for water-content measurements with ωI ≥ 0.01 m3 m−3. This was probably because the initial soil water contents were significantly lower at site B than at site A (about 0.1 m3 m−3 for all irrigations). The total porosities of comparable hydromorphic soils measured in the Flysch zone were between 0.42 and 0.57 m3 m−3 (Richard and Lüscher 1987). Thus, averaged θin at site B was presumably at least 0.08 m3 m−3 below saturation since the mean of antecedent soil water content was at most 0.34 m3 m−3, even for the third irrigation. Nevertheless, the volumes of non-draining water within approximately 20 h, ωF, were close to zero for the third irrigation and did not differ significantly between sites A and B (Table 2). This observation suggests that mainly larger pores were involved in infiltration and the water-carrying pore-size spectra were comparable at both sites investigated for the third irrigation.
The bulk density and soil texture, and thus the total porosity, did not vary significantly between sites A and B. The fact that θin was lower for the third irrigation, although still with complete drainage, but ωI was larger, indicates that there were proportionally more larger, fast-draining pores at site B than at site A. This interpretation is supported by the contact lengths since at site B, L was larger at a depth range of 0.1–0.7 m. Consequently, potential infiltration was mainly restricted to topsoils at site A, while ωI was approximately constant down to a depth of 0.7 m at site B.
The interpretation of contact lengths and film thicknesses for the third irrigation gives additional information about water-flow geometry for wcws with ωI ≥ 0.01 m3 m−3 since the amplitude of infiltration is given by the product of L and F (Eq. 3). At site A, F, and accordingly the velocity of the wetting front (Eq. 4), was approximately constant between 0.1 and 1 m depth. Consequently, variations in ωI were due to differing L, as Hincapie and Germann (2009) also found. At site B, film thicknesses increased between 0.5 and 0.7 m and, according to Eq. (4), wetting fronts accelerated, while L decreased. This was possibly due to water films flowing together when porosity was reduced in deeper soil, resulting in thicker films and increasing vW (Eq. 4). This funnel effect requires well-connected pores. Since we did not detect such an acceleration of the wetting front at site A, we assume that the connectivity of vertical pores was less distinct at site A.
Root densities were higher at site B, also in hydromorphic horizons. The forest sites investigated differed mainly in their abundances of beech and spruce since fir occurs in both forest communities. Accordingly, beech most likely had larger root densities, especially below depths of approximately 0.3 m. These findings are in accordance with those of Schmid and Kazda (2001), who observed that the relative number of spruce roots in mixed beech–spruce stands was lower than that of beech, and mainly concentrated in the upper soil. Furthermore, beech is less liable to temporarily anaerobic conditions in the rhizosphere than spruce (Lehnardt and Brechtel 1980). Soil acidity can substantially influence root growth and root distribution. Jentschke et al. (2001), for example, showed that fine root biomass in the mineral soil of Norway spruce forests was reduced in heavily acidified stands, but increased in organic layers. On the other hand, Finér et al. (2007) found no correlation between pH and root biomass in spruce and beech stands but demonstrated that root biomass in beech stands exceed that of spruce stands markedly. In this study, the root density was higher at the more acid site B. Consequently, the difference in root densities between the two investigated sites could have been even stronger if soil acidity would have been comparable. Thus, infiltration capacity at the current coniferous site could possibly increase more strongly than we suggested when beech will be more abundant due to climate change.
Soil properties influencing the amplitude of infiltration
At site A, the root length density was the most significant predictor of ωI, as many other studies that focused on the influence of roots on soil water flow have also found (e.g. Angers and Caron 1998; Perillo et al. 1999). It is then most likely that roots generated the porosity accessible to water. According to the Stokes-flow approach, the amplitude of infiltration ωI is given by the product of L and F (Eq. 3). Thus, for wcws with ωI ≥ 0.01 m3 m−3, where L and F were calculable, the root densities may be related to film thickness, contact length or both parameters. Since L was clearly related to RL (R2 = 0.43), but the coefficient of regression between F and RL was only 0.03, we draw the conclusion that the porosity carrying mobile water was indeed mainly generated by roots. Our investigations were conducted on a plot scale. This raises the question whether the impact of roots on soil hydrology can be assessed on larger scales in a similar way to the approach used in this study. The key issue is if the root densities measured at particular soil depths are the representative of the entire stand’s root densities. According to Brunner et al. (2004), the number of fine root segments is independent of their distance from the nearest tree in a forest community very similar to site A. Fine roots accounted for approximately 90 % of total root length density at site A, where ωI was determined by root densities. Thus, we can assume that the observed root densities based on 16 (site A) and 10 (site B) soil profiles represent the stand’s root density well enough.
The predictors applied did not enable ωI to be predicted well at site B. Larger pores, where water flow is gravity driven, can also be generated by physical processes such as thawing and freezing (Beven and Germann 1982) or shrinking and swelling (Romkens and Prasad 2006), or by biological factors such as earthworms (e.g. Lamande et al. 2003). Earthworms were not found during soil excavation at site B, possibly because of the soils low pH (Potthoff et al. 2008). Since bedrock, soil genesis, soil properties such as bulk density and texture, as well as climatic conditions, are similar at both sites, no significant differences in pore-generating processes besides roots are expected. The current root density does not necessarily represent total porosity generated by roots since decomposed roots are also able to generate pores accessible to water (Jorgensen et al. 2002). Thus, it may be possible that, even at site B, pores carrying mobile water were mainly generated by roots, but with the method of root measurement we used, decayed roots could not be detected.
Schwarzel et al. (2009) showed that the spatial distribution of soil–water variability in spruce and beech stands was related to not only soil properties but also to root distributions due to the water uptake of trees. The daily transpiration rates of beech stands can exceed 5 mm when water availability is sufficient (Marc and Robinson 2004), while spruce forests transpire at most 1.5–2 mm day−1 (Clausnitzer et al. 2011). Since the mobile water content was significantly influenced by the initial soil water content at site B and, to a lesser extent, also at site A, roots indirectly affect ωI by modifying the antecedent soil moisture due to water uptake. Thus, beech will reduce θin during the growing season more than spruce and will make additional water storage capacity available, which will decrease during winter and early spring when beech hardly transpires.
In this study, we focused on the significance of different species’ roots for gravity-driven water flow in soils, but the predicted transformation of forests can also affect the soil’s hydrological properties via other processes. These processes include the varying characteristics of organic layers (Greiffenhagen 2005) and the trees’ interception efficiency (Schume et al. 2003). The porosity generated by roots may also increase lateral water flow (Redding and Devito 2010) and therefore promote run-off generation (Uchida et al. 2005).
Change in water storage capacity
The greater abundance of beech in coniferous flood protection forests leads to increase in root densities and thus to an additional water storage capacity of 9–10 mm for soil depths between 0.3 and 1.0 m. Thus, the soil water storage capacity will be increased by approximately 15 % of a 1-h heavy precipitation with a recurrence interval of 100 years. Moreover, the initial water content will be reduced during the growing season because of the higher transpiration rates of beech, resulting in even more water storage capacity.
During forest transformation, beech and spruce will presumably coexist during a certain time period in the current coniferous stand. As shown by Schmid and Kazda (2002), fine root density in mixed beech–spruce forests growing on stagnic Cambisols was almost twice that of monospecific spruce stands. Thus, within a certain time period during forest transformation when beech initially grows in current coniferous forests, water storage capacity would possibly increase more than suggested by this study.
It is assumed, however, that climate change will lead to more frequent and intense heavy rainfall events (OcCC/ProClim-(ed.) 2007). Hence, even with greater infiltration capacity, the flood protection provided by a forest would not necessarily become more effective with the higher root densities associated with the predicted species shift, but the negative effects of more intense rainfall may be mitigated.
Hydrological models normally include vegetation by considering the transpiration rate which can be determined, for example, by root depth, leaf area index and potential evapotranspiration (e.g. Abbaspour et al. 2007). The results presented in this paper indicate that also modifications in porosity generated by roots substantially influence infiltration properties. Considering root distribution versus soil depth in water-flow models will not only enhance the assessment of transpiration rates but also the estimation of the significance of a modified pore system due to land use change or forest transformation. Thus, more detailed information about root distribution in different forest stands may increase the validity of such models considerably.
This study has begun to investigate the impact of a forest transformation on infiltration due to the different root densities of the main tree species. To assess the significance of more abundant beech in coniferous flood protection forests, future investigations should be more comprehensive. They should, for example, include further factors that affect infiltration such as transpiration, interception and organic layer, as well as the impacts of species transformation on the lateral subsurface flow, which may result in more peak flow generation.
We are grateful to Philipp Mösch and Dieter Müller for allowing us access to the sites. We thank Roger Köchli and Marco Walser for the help in the field and Silvia Dingwall for proofreading this manuscript. Two reviewers are acknowledged for their helpful and constructive comments which substantially improved the manuscript. This study was supported by the COST Action E38 (Woody Root Processes) and the research programme Forests and Climate Change (Swiss Federal Institute for Forest, Snow and Landscape Research WSL and the Federal Office for the Environment FOEN).
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