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Interspecific variation in soil compaction sensitivity among forest floor species
BIOLOGICAL CONSERVATION
Biological Conservation 119 (2004) 207–217 www.elsevier.com/locate/biocon
Interspecific variation in soil compaction sensitivity among forest floor species S. Godefroid *, N. Koedam Laboratory of General Botany and Nature Management (APNA), Vrije Universiteit Brussel – Pleinlaan 2-1050 Brussels, Belgium Received 11 March 2003; received in revised form 24 October 2003; accepted 17 November 2003
Abstract The present study aimed at exploring the response of herbaceous plant species to soil compaction in forest soils. The research was conducted in central Belgium, in a 4383 ha beech forest. Of the 107 taxa studied, the cover of 65 species (61%) was significantly related to soil compaction. Twenty four forest species (58% of all forest species tested) showed significant growth responses to soil compaction. A few, such as Carex strigosa, Epilobium montanum and Mycelis muralis, showed monotonic reduction in growth with increasing compaction, but about half showed a bell-like response with maximum growth at 200 N (Hyacinthoides non-scripta) or 400 N (e.g., Carex pilulifera, Melica uniflora) or even 600 N (Oreopteris limbosperma). Only four species (Carex remota, C. sylvatica, Rumex sanguineus, Veronica montana) showed positive growth responses with increasing compaction, up to 1200 N. This contrasts with non-forest species where 18 out of 41 showed high tolerance to heavily compacted soils (e.g., Geum urbanum, Glechoma hederacea, Impatiens parviflora, Polygonum hydropiper, Veronica serpyllifolia). The sensitivity of forest species has obvious implications for both conservation and management. The practical measures for long-term conservation of forest herbs are discussed. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Soil degradation; Soil penetration resistance (SPR); Species response; GAM; Forest management
1. Introduction Compaction is perceived as one of the leading causes of soil degradation resulting from forest operations (Brais, 2001). Forest soil sustainability is at risk if mechanised harvesting operations cause soil damage (Hutchings et al., 2002). Therefore, soil compaction is a major concern whenever forest management activities involve large machines. Soil compaction during timber harvesting typically alters soil structure and hydrology by increasing bulk density, breaking down aggregates, decreasing porosity, aeration and infiltration capacity and by increasing soil strength, water runoff, erosion and waterlogging (Kozlowski, 1999; Grigal, 2000; Startsev and McNabb, 2000). Soil compaction is a problem which may rise in the future as a result of increasing weight of machinery (Langmaack et al., 2002).
*
Corresponding author. Tel.: +32-2-629-34-11; fax: +32-2-629-34-13. E-mail address: [email protected] (S. Godefroid).
0006-3207/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2003.11.009
Although soil compaction may benefit the growth of some plants, the harmful effects are much more common (Kozlowski, 1999). Appreciable compaction of soil leads to physiological dysfunctions in plants. Often, reduced water absorption and leaf water deficits develop. Soil compaction also induces changes in the amounts and balances of stress hormones in plants, especially increases in abscisic acid and ethylene (Kozlowski, 1999). Most of the studies related to compaction deal with cultivated plants (e.g., Montagu et al., 2001; Bayhan et al., 2002; Grzesiak et al., 2002), overstory trees (Jansson and Wasterlund, 1999; Brais, 2001; Balbuena et al., 2002; Gomez et al., 2002) and arable soils (Parackova and Zaujec, 2001; Langmaack et al., 2002; Rosolem et al., 2002). Forest soils, however, differ in many ways from cultivated soils (Fisher and Binkley, 2000) and knowledge is lacking about the relationship between soil compaction and the distribution of most woodland plant species (Lipiec and Hakansson, 2000; Startsev and McNabb, 2000; Brais, 2001; McNabb et al., 2001; Miller et al., 2001). Unlike agricultural systems,
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forest soils generally receive fewer mechanical treatments and chemical inputs (Marshall, 2000). In view of the increasing wheel loads of forest vehicles, the question arises as to the extent soil compaction affects the presence and development of understory forest species. In this paper, the main question dealt with is: to what extent does soil compaction influence understory species in forested areas? This question has long-term implications for biological conservation in forest ecosystems, as many forest species from the ground floor are vulnerable because of poor dispersal abilities (Brunet and von Oheimb, 1998; Ehrlen and Eriksson, 2000) forest fragmentation (e.g., Grashof-Bokdam, 1997; Godefroid and Koedam, 2003) and because they are part of ecosystems that are very difficult to recreate (Peterken, 1977). Assessing the role of soil compaction in the development of the herb layer in forest biotopes is therefore an important task for the protection and enhancement of forest biological diversity.
2. Study area The research was conducted in the Sonian Forest, south of Brussels (50°470 N; 4°260 E). This area has been proposed as a Site of Community Importance (Natura 2000 area, in fulfillment of the EC-Habitat Directive 92/ 43/EEC). It is a remnant of the huge forest that is supposed to have covered much of Western Europe after the last Ice Age. The forest actually covers an area of 4383 ha, 1654 ha of which are situated within the administrative limits of the Brussels Capital Region, this constituting a management unit and being the area considered in the present study. Some 20,000 years ago, sandstone and flintstone formed the upper layer in the area of the Sonian Forest. After the last Ice Age, this layer was covered with loess. Today, almost the whole surface of the forest (95%) is composed of a 3–4 m thick silt layer (pHH2 O around 4.0 in the upper 10 cm), which corresponds to the loess deposition. The forest ranges in altitude from 65 to 130 m a.s.l. The climate of the area is temperate and humid, with a growing season of 7 months (April–October). Mean annual temperature is 9.9 °C, annual precipitation is 835 mm. The natural vegetation is a deciduous forest in which oaks (Quercus robur and Quercus petraea) and beech (Fagus sylvatica) are the main species (Herbauts et al., 1996). Since the plantation work of the Austrian administration at the end of the 18th century, it is now composed of 74% beech (Fagus sylvatica) with only a few other woody species. Sixteen percent is occupied by oak stands (Quercus robur) and 8% by introduced conifers (Pinus sylvestris, Larix decidua, Picea abies). Beech is subjected to frequent uprooting by wind in the Sonian Forest. Compaction occurs naturally between 40 and 120 cm depth (fragipan) as a result of an intense postglacial
drying, but is also in the upper layers induced by heavy machinery, pedestrian traffic, scouting and mountain biking. Thinning is carried out on a rotation of 8 years with ‘‘Timberjacks’’ (9 tons, 130 HP), ‘‘Igland’’ (8 tons, 100 HP) or ‘‘Agrip’’ (8 tons, 100 HP) being the main equipment (Herbauts et al., 1998). Results of Herbauts et al. (1996, 1998) already provide evidence that on these loess materials soil compaction due to logging operations leads to rapid soil degradation through active hydromorphic processes.
3. Methods Compaction is strongly related to the original bulk density, forest type and soil parent material (Seixas and McDonald, 1997; Fisher and Binkley, 2000; Williamson and Neilsen, 2000) and therefore sampling areas had to fulfill certain prerequisites: (1) same soil type; the prevailing soil type with an ABC profile was chosen (USDA: Hapludalf and Glossudalf; FAO: Luvisols and Podzoluvisols; French classification: Sols lessives, Sols lessives hydromorphes and Sols lessives a pseudogley); (2) same topography (i.e., level ground); and (3) same overstory species (i.e., beech stands), as tree species can also change the distribution of pore sizes in soils (Nihlgard, 1971). In order to have a broad range of compaction values, stands were chosen to give as wide a range of ages as possible (planted between 1815 and 1969). The 50 stands were classified into eight groups with ages of about 40, 70, 120, 130, 140, 160, 165 and 185 years old. A map of the areas meeting these conditions was drawn by overlaying the soil map and the stand map of the whole forest in G.I.S. Arc View (ESRI, 1996). Within these pre-defined areas, 499 rectangular quadrats (10 1 m2 ) were randomly laid out for vegetation sampling and soil compaction measurements. The species composition was characterized by classical phytosociological plots (e.g., Westhoff and Van der Maarel, 1973), which means that total coverage for each species (vertical projection onto the ground) was estimated visually and recorded within seven cover classes: r: 1 or 2 individuals; +: few individuals (<20) with cover <5%; 1: many individuals (20–100) with cover <5%; 2: 5%–25% cover; 3: 25%–50% cover; 4: 50%–75% cover; 5: 75%–100% cover. Since Braun-Blanquet cover-abundance values are not suitable for mathematical treatment, raw data were transformed by the corresponding cover percentage values (median of each scale interval): 0.2, 0.5, 2.5, 15, 37.5, 62.5 and 87.5 accounting, respectively, for r, + and cover classes 1–5 (arbitrary values were taken for r, + and 1). In each of the 499 sample plots, three measurements of soil compaction were recorded using a cone-penetrometer (Eijkelkamp Agrisearch Equipment, The Netherlands), a device forced into the soil to measure its resistance to
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vertical penetration as frequently used to measure soil resistance (e.g., Seixas and McDonald, 1997; Jansson and Wasterlund, 1999). The average value was taken for statistical analyses. The penetrometer has a 60° cone-shaped tip. According to the soil strength (resisting force), three tips were used (1–2 cm diameter) with a conversion coefficient which allows the comparison of the readings. When the tip is pushed into the soil, the penetrometer provides a continuous plotting of soil strength and soil depth, unless impenetrable stone or wood is encountered (Miller et al., 2001). The readings are expressed in Newtons (N), the SI unit of force (a force of 1 N will accelerate a mass of 1 kg at the rate of 1 m per second). Measurements were performed down to 20 cm depth, as a 0–20 depth range is usually used in the literature (e.g., Servadio et al., 2001; Brevik et al., 2002) and because it avoided the natural compaction beginning at 40 cm depth. Because of the relative ease of conducting in situ compaction tests, this method was chosen for field compaction studies. Furthermore, Dawidowski et al. (2001) highlighted the fact that measuring soil compaction in situ with a portable penetrometer was statistically similar to traditional laboratory compression tests of soil cores. Because penetrometer readings can be strongly dependent on soil moisture content (Miller et al., 2001; Vaz et al., 2001), field samplings were carried out in the spring when soils are near field capacity so that penetrometer readings are least influenced by differences in soil moisture (Miller et al., 2001). The Canoco 4.5 statistical package (ter Braak and Smilauer, 2002) was used to summarise relationships between variables using a regression model fitted to soil compaction predictor. A Generalised Additive Modeling regression (GAM; Hastie and Tibshirani, 1990) was used with a cubic smooth spline function. GAM regression has been used in numerous studies of speciesenvironment relationships (e.g., Bio et al., 1998; Austin, 1999; Guisan and Zimmermann, 2000; Vetaas, 2002) and was chosen because it does not assume any general shape of the response prior to the estimation (Austin and Meyers, 1996). The model was tested with different degrees of smoothing. The optimum degree of smoothing (i.e., giving the best P -value for the deviance-based test) for each species was found to be 2. The response data are relative covers (subjectively estimated percentages) and therefore a Poison distribution was assumed with a logistic link function. An index of light intensity was attributed to the plots using the species indicator values of Hill et al. (1999). The mean indicator value for each plot was calculated by averaging the indicator value of each species for light intensity (L), weighted by their respective abundance percentage, using the following equation: mL ¼
ðx1 L1 þ x2 L2 þ þ xn Ln Þ ; ðx1 þ x2 þ þ xn Þ
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where x1 ; x2 , . . .,xn are the abundance percentages of those species present in the plot and L1 ; L2 ; . . . ; Ln represent the light indicator values of the species according to Hill et al. (1999). Correlations between soil compaction and environmental ecological parameters (i.e., light intensity, stand age) were measured using the Spearman rank correlation coefficient rs . In the framework of this study, we focused particularly on the behaviour of true forest species and ancient forest species. Due to their poor colonising ability (Honnay et al., 1998), these species are limited to primary or ÔancientÕ woodland areas, i.e., ‘‘woodland sites which have been continuously wooded since about the year 1775, the approximate date of the publication of the Ferraris maps’’ (Hermy and Stieperaere, 1981), the earliest reliable mapping of this area. In this paper, Ôtrue forest speciesÕ and Ôancient forest speciesÕ, as defined by Honnay et al. (1998) for Belgium, were classified as forest species. All the other species were considered as non-forest species. Nomenclature follows Lambinon et al. (1998). The highly variable and taxonomically disputed Rubus fruticosus agg. was considered a single species.
4. Results Of the 107 taxa studied, the cover of 65 species (61%) was significantly related to soil compaction (Figs. 1 and 2). Out of the 41 forest species, 24 (58%) showed bellshaped or monotonic responses to soil compaction. About half of them showed a bell-like response with maximum growth at 200 N (Hyacinthoides non-scripta) or 400 N (Carex pilulifera, Deschampsia flexuosa, Dryopteris dilatata, Melica uniflora and Teucrium scorodonia) or even 600 N (Oreopteris limbosperma). A few, such as Carex strigosa, Epilobium montanum, Mycelis muralis, showed monotonic reduction in growth with increasing compaction. Four species (Carex remota, C. sylvatica, Rumex sanguineus, Veronica montana) had positive growth responses with increasing compaction, at least up to 1200 N. Species found on disturbed ground (e.g., Arctium minus, Sonchus oleraceus, Stellaria media), as well as on woodland edges and clear-cuttings (e.g., Alliaria petiolata, Epilobium angustifolium, Rubus fruticosus agg.) also showed significant responses to soil compaction. Among these non-forest species, 18 can tolerate very high compaction levels (e.g., Geum urbanum, Glechoma hederacea, Impatiens parviflora, Polygonum hydropiper, Veronica serpyllifolia), having a better growth at sites with penetration forces of or above 1000 N. The reverse was observed for Galium aparine and Urtica dioica, which showed a progressively decreasing trend as soil compaction increased.
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Fig. 1. The response (estimated percentage cover values) of forest species significantly related to soil compaction (expressed in Newtons). Species which had no significant response to compaction are: Adoxa moschatellina, Anemone nemorosa, Athyrium filix-femina, Cardamine flexuosa, Dryopteris carthusiana, Dryopteris filix-mas, Festuca gigantea, Holcus mollis, Hypericum pulchrum, Impatiens noli-tangere, Lamium galeobdolon, Lonicera periclymenum, Luzula pilosa, Lysimachia nemorum, Moehringia trinervia, Ranunculus ficaria, Scrophularia nodosa, Stachys sylvatica, Stellaria alsine, Viola reichenbachiana.
These species survive compaction values exceeding 400 N poorly. For most other taxa, bell-shaped curves were found, with optimum responses (best growth) varying from 150 N (Galeopsis tetrahit) up to 700 N
(Cirsium oleraceum, Juncus tenuis). Among these species, some were characterised by a narrow amplitude (e.g., Cirsium oleraceum), while others showed a broader tolerance range (e.g., Juncus effusus).
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Fig. 1. ðcontinuedÞ
Data from Fig. 3 showed that there was a weak but significant positive correlation between soil compaction (measured range: 68–793 N; average: 365 N) and stand age. The relationship is, however, weak as it gives an average difference of only 100 N between 50 year old stands and 200 year old stands. Our results therefore show that stand age has only a marginal influence on the soil compaction status. We also found a significant positive correlation between soil compaction and light intensity (Fig. 4).
5. Discussion 5.1. Species response to soil compaction The most interesting results of the present contribution concern the forest species as, to our knowledge, no other work has examined statistically the response of European forest understory herbs to soil compaction. The only study known to us dealing with forest herbs gave similar conclusions for six species from the Nearctic zone (Small and McCarthy, 2002). The avoidance by some species of highly compacted sites may be due to oxygen stress as a result of puddling (Kozlowski, 1999; Grigal, 2000; Startsev and McNabb, 2000), which re-
duces air and water movement through soil pores. Soils with stagnant water are indeed low in oxygen (Fisher and Binkley, 2000) and respiration of roots shifts toward an anaerobic state (Kozlowski, 1999). Soil compaction can also severely reduce plant growth by restricting root growth (Rosolem et al., 2002) and lower the percentage of water and air space in the soil (Holshouser, 2001). This may disadvantage species developing superficial roots as soil compaction normally affects the upper soil layer rather than the deeper layers; in arable soils, compaction of subsoil is more often a problem as a result of agricultural practices (Parackova and Zaujec, 2001). For the Sonian Forest, Herbauts et al. (1996) found a macroporosity between 5.7% and 9.5% in rutted soils, while De Bruycker (1984), cited by Herbauts et al. (1996), considered for the same area that a soil macroporosity of 10% of the total soil volume is the lowest value to allow a sufficient diffusion of air for aerobic microbial activity and viability of roots. A reduced absorption of the major nutrients by compaction of both surface soils and subsoils (Kozlowski, 1999) may account for differences between species as some have higher nutrient requirements than others. However, this point can not be confirmed for nitrogen from our dataset, as the mean nitrogen indicator value (Hill et al., 1999) of compaction-tolerant
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Fig. 2. The response (estimated percentage cover values) of non-forest species significantly related to soil compaction (expressed in Newtons). Species which had no significant response to compaction are: Alliaria petiolata, Anthoxanthum odoratum, Carex hirta, Carex ovalis, Carex pallescens, Cerastium fontanum, Cerastium glomeratum, Cirsium arvense, Cirsium vulgare, Epilobium angustifolium, Epilobium parviflorum, Geranium robertianum, Hypericum humifusum, Lotus corniculatus, Luzula multiflora, Myosotis arvensis, Poa trivialis, Rubus idaeus, Sagina procumbens, Taraxacum officinale, Veronica arvensis, Veronica officinalis.
species (arbitrarily defined as having an optimum <400 N on a measured range 0–800 N; n ¼ 13) is not significantly lower than that of plants which do not tolerate high compaction (arbitrarily defined as having an opti-
mum P 400 N; n ¼ 11) (mean nitrogen index ¼ 4.69 and 4.91, respectively; Z ¼ 0:33; P ¼ 0:74, Mann–Whitney test). This may be due to the fact that compaction might play a more important role for other nutrients, such as
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Fig. 2. ðcontinuedÞ
S, B, Mn or K that may have leached out of the surface into the subsoil (Holshouser, 2001). Herbauts et al. (1996) have also demonstrated for the same study area that soil compaction generates reduced forms of irons and induces a strong leaching of iron hydroxides from the eluvial (Eg ) to the illuvial (Btg ) horizon.
Kozlowski (1999) mentioned a reduced total photosynthesis when soils become increasingly compacted, as a result of smaller leaf areas. This may favour species that can physiologically better compensate for this reduction in leaf area. The fact that formation and function of mycorrhizal relationships are affected by soil
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soil compaction (N)
Fig. 2. ðcontinuedÞ
compaction (Entry et al., 2002) can also contribute to a differential impact of soil compaction according to the response of each species to destruction of mycorrhiza.
900 800 700 600 500 400 300 200 100 0
5.2. Conservation and management implications
0
50
100
150
200
stand age (yr) Fig. 3. Relationship between soil compaction and stand age (rs ¼ 0:13; P ¼ 0:0205; n ¼ 320; y ¼ 0:7306x þ 249:11).
In the present study, a substantial number of understory species showed a significant response to soil compaction in a forested area. Mechanised harvesting operations are now widely recognised as the major source of soil compaction (e.g., Jansson and Wasterlund, 1999; Servadio et al., 2001; Langmaack et al., 2002). In the study area, forest harvesting of beech stands developed on acid and loamy soils in the loessic
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800
soil compaction (N)
700 600 500 400 300 200 100 0 4,00
4,50
5,00
5,50
6,00
6,50
7,00
light indicator values Fig. 4. Relationship between soil compaction and light intensity (rs ¼ 0:21; P < 0:0001; n ¼ 499; y ¼ 54:28x þ 67:896).
belt of middle Belgium is known to lead to strong physical degradation due to compaction under repeated logging traffic (Herbauts et al., 1996, 1998). An increased light flux to the forest floor is known to be a direct consequence of regular tree cutting (Brunet et al., 1996; Boncina, 2000). Although this is primarily true in the case of recent cutting which could produce both increased light and compaction, it is also valid in our study area as the regrowth of brushwood (which could decrease the light flux) after cutting is systematically eliminated by foresters. This is consistent with the positive relationship we found between compaction measurements and light intensity index. It confirms that the measured soil compaction is, to a certain extent, induced by logging traffic. Performing fewer mechanised interventions seems therefore highly desirable. It appears, however, that 80% of the compaction can occur on the first pass (Holshouser, 2001), so that reducing the intervention frequency alone may not be sufficient to avoid soil compaction. Another possible factor contributing to the soil compaction could be stand aging due to: (1) history of increasing mechanisation of forestry; (2) increasing weight of stand. As forest exploitation has begun at the same moment over the whole study area, increasing mechanisation can not be regarded as responsible for the phenomenon in this case, but the weight of the trees themselves might slightly enhance the compaction phenomenon. Other recommendations to prevent soil compaction may consist of the following modifications in silvicultural practices: (1) using lighter harvesting machines (Jansson and Wasterlund, 1999); (2) reducing tyre pressures (Canillas and Salokhe, 2001); (3) placing harvesting residues over areas where machinery traffic is used (Hutchings et al., 2002); (4) winter-harvesting on frozen ground (Alban et al., 1994); (5) confining machinery to a series of parallel access routes spaced at
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intervals of about 20 m and winching material to the extraction machine (Savill, pers. comm.). In the Sonian Forest, many species with high conservation values (e.g., Carex strigosa, Hyacinthoides non-scripta, Primula elatior) occur on fresh or damp soils. It is well known that damp soils are more sensitive to plastic deformation than dry soils (e.g., McNabb et al., 2001). Therefore, machinery should not be taken onto sites when the soil is wet. The fact that the optimum growth response of most of the forest herbs studied lies below the average compaction value found in the study area (365 N), implies that they currently survive in sub-optimal conditions. The most sensitive species could disappear with further compaction and may be replaced by non-forest species that show greater tolerance to soil compaction. The increased light afforded by logging or thinning could also favour such ruderal or alien species. Forest managers should be aware of the possible extension of particularly competitive species, such as Impatiens glandulifera. This problem requires attention and efforts should be made to limit compaction in order to minimise the chances of development and spread of these undesirable species representing a possible threat to the optimal development of typical forest vegetation.
Acknowledgements This study was financially supported by the Brussels Institute for Environmental Management (BIM-IBGE) in the framework of the vegetation monitoring in the Sonian Forest. We also acknowledge the thorough reviews by Brian Davis, Peter Savill and two anonymous reviewers whose critical comments were crucial in the development of the revised manuscript.
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Holshouser, D.L. (Ed.) 2001. Soybean Production Guide. Tidewater Agricultural Research and Extension Center, Information Series No. 408. Honnay, O., Degroote, B., Hermy, M., 1998. Ancient-forest plant species in western Belgium: a species list and possible ecological mechanisms. Belgian Journal of Botany 130, 139–154. Hutchings, T.R., Moffat, A.J., French, C.J., 2002. Soil compaction under timber harvesting machinery: a preliminary report on the role of brash mats in its prevention. Soil Use and Management 18, 34–38. Jansson, K.J., Wasterlund, I., 1999. Effect of traffic by lightweight forest machinery on the growth of young Picea abies trees. Scandinavian Journal of Forest Research 14, 581– 588. Kozlowski, T.T., 1999. Soil compaction and growth of woody plants. Scandinavian Journal of Forest Research 14, 596–619. Lambinon, J., De Langhe, J.E., Delvosalle, L., Duvigneaud, J., 1998. Flora van Belgi€e, het Groothertogdom Luxemburg, Noord-Frankrijk en de aangrenzende gebieden. Nationale Plantentuin van Belgi€e, Meise. Langmaack, M., Schrader, S., Rapp-Bernhardt, U., Kotzke, K., 2002. Soil structure rehabilitation of arable soil degraded by compaction. Geoderma 105, 141–152. Lipiec, J., Hakansson, I., 2000. Influences of degree of compactness and matric water tension on some important plant growth factors. Soil and Tillage Research 53, 87–94. Marshall, V.G., 2000. Impacts of forest harvesting on biological processes in northern forest soils. Forest Ecology and Management 133, 43–60. McNabb, D.H., Startsev, A.D., Nguyen, H., 2001. Soil wetness and traffic level effects on bulk density and air-filled porosity of compacted boreal forest soils. Soil Science Society of America Journal 65, 1238–1247. Miller, R.E., Hazard, J., Howes, S., 2001. Precision, accuracy and efficiency of four tools for measuring soil bulk density strength Introduction. USDA Forest Service, Pacific Northwest Research Station, Research Paper No. 532. Montagu, K.D., Conroy, J.P., Atwell, B.J., 2001. The position of localized soil compaction determines root and subsequent shoot growth responses. Journal of Experimental Botany 52, 2127–2133. Nihlgard, B., 1971. Pedological influences of spruce planted on former beech forest soils in Scania, south Sweden. Oikos 22, 302–314. Parackova, A., Zaujec, A., 2001. Evaluation of human impacts on soils on the Borska lowland. Ekologia Bratislava 20 (Suppl. 3), 299–304. Peterken, G.F., 1977. Habitat conservation priorities in British and European woodlands. Biological Conservation 11, 223–236. Rosolem, C.A., Foloni, J.S.S., Tiritan, C.S., 2002. Root growth and nutrient accumulation in cover crops as affected by soil compaction. Soil and Tillage Research 65, 109–115. Seixas, F., McDonald, T., 1997. Soil compaction effects of forwarding and its relationship with 6- and 8-wheel drive machines. Forest Products Journal 47, 46–52. Servadio, P., Marsili, A., Pagliali, M., Pellegrini, S., Vignozzi, N., 2001. Effects of some clay soil qualities following the passage of rubber-tracked and wheeled tractors in central Italy. Soil and Tillage Research 61, 143–155. Small, C.J., McCarthy, B.C., 2002. Effects of simulated post-harvest light availability and soil compaction on deciduous forest herbs. Canadian Journal of Forest Research 32, 1753–1762. Startsev, A.D., McNabb, D.H., 2000. Effects of skidding on forest soil infiltration in west-central Alberta. Canadian Journal of Soil Science 80, 617–624. ter Braak, C.J.F., Smilauer, P., 2002. CANOCO reference manual and CanoDraw for Windows. UserÕs guide: software for
S. Godefroid, N. Koedam / Biological Conservation 119 (2004) 207–217 Canonical Ordination (version 4.5). Microcomputer power, Ithaca, NY. Vaz, C.M.P., Bassoi, L.H., Hopmans, J.W., 2001. Contribution of water and bulk density to field soil penetration resistance as measured by a combined cone penetrometer-TDR probe. Soil and Tillage Research 60, 35–42. Vetaas, O.R., 2002. Realized and potential climate niches: a comparison of four rhododendron tree species. Journal of Biogeography 29, 545–554.
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Biological Conservation 119 (2004) 207–217 www.elsevier.com/locate/biocon
Interspecific variation in soil compaction sensitivity among forest floor species S. Godefroid *, N. Koedam Laboratory of General Botany and Nature Management (APNA), Vrije Universiteit Brussel – Pleinlaan 2-1050 Brussels, Belgium Received 11 March 2003; received in revised form 24 October 2003; accepted 17 November 2003
Abstract The present study aimed at exploring the response of herbaceous plant species to soil compaction in forest soils. The research was conducted in central Belgium, in a 4383 ha beech forest. Of the 107 taxa studied, the cover of 65 species (61%) was significantly related to soil compaction. Twenty four forest species (58% of all forest species tested) showed significant growth responses to soil compaction. A few, such as Carex strigosa, Epilobium montanum and Mycelis muralis, showed monotonic reduction in growth with increasing compaction, but about half showed a bell-like response with maximum growth at 200 N (Hyacinthoides non-scripta) or 400 N (e.g., Carex pilulifera, Melica uniflora) or even 600 N (Oreopteris limbosperma). Only four species (Carex remota, C. sylvatica, Rumex sanguineus, Veronica montana) showed positive growth responses with increasing compaction, up to 1200 N. This contrasts with non-forest species where 18 out of 41 showed high tolerance to heavily compacted soils (e.g., Geum urbanum, Glechoma hederacea, Impatiens parviflora, Polygonum hydropiper, Veronica serpyllifolia). The sensitivity of forest species has obvious implications for both conservation and management. The practical measures for long-term conservation of forest herbs are discussed. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Soil degradation; Soil penetration resistance (SPR); Species response; GAM; Forest management
1. Introduction Compaction is perceived as one of the leading causes of soil degradation resulting from forest operations (Brais, 2001). Forest soil sustainability is at risk if mechanised harvesting operations cause soil damage (Hutchings et al., 2002). Therefore, soil compaction is a major concern whenever forest management activities involve large machines. Soil compaction during timber harvesting typically alters soil structure and hydrology by increasing bulk density, breaking down aggregates, decreasing porosity, aeration and infiltration capacity and by increasing soil strength, water runoff, erosion and waterlogging (Kozlowski, 1999; Grigal, 2000; Startsev and McNabb, 2000). Soil compaction is a problem which may rise in the future as a result of increasing weight of machinery (Langmaack et al., 2002).
*
Corresponding author. Tel.: +32-2-629-34-11; fax: +32-2-629-34-13. E-mail address: [email protected] (S. Godefroid).
0006-3207/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2003.11.009
Although soil compaction may benefit the growth of some plants, the harmful effects are much more common (Kozlowski, 1999). Appreciable compaction of soil leads to physiological dysfunctions in plants. Often, reduced water absorption and leaf water deficits develop. Soil compaction also induces changes in the amounts and balances of stress hormones in plants, especially increases in abscisic acid and ethylene (Kozlowski, 1999). Most of the studies related to compaction deal with cultivated plants (e.g., Montagu et al., 2001; Bayhan et al., 2002; Grzesiak et al., 2002), overstory trees (Jansson and Wasterlund, 1999; Brais, 2001; Balbuena et al., 2002; Gomez et al., 2002) and arable soils (Parackova and Zaujec, 2001; Langmaack et al., 2002; Rosolem et al., 2002). Forest soils, however, differ in many ways from cultivated soils (Fisher and Binkley, 2000) and knowledge is lacking about the relationship between soil compaction and the distribution of most woodland plant species (Lipiec and Hakansson, 2000; Startsev and McNabb, 2000; Brais, 2001; McNabb et al., 2001; Miller et al., 2001). Unlike agricultural systems,
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forest soils generally receive fewer mechanical treatments and chemical inputs (Marshall, 2000). In view of the increasing wheel loads of forest vehicles, the question arises as to the extent soil compaction affects the presence and development of understory forest species. In this paper, the main question dealt with is: to what extent does soil compaction influence understory species in forested areas? This question has long-term implications for biological conservation in forest ecosystems, as many forest species from the ground floor are vulnerable because of poor dispersal abilities (Brunet and von Oheimb, 1998; Ehrlen and Eriksson, 2000) forest fragmentation (e.g., Grashof-Bokdam, 1997; Godefroid and Koedam, 2003) and because they are part of ecosystems that are very difficult to recreate (Peterken, 1977). Assessing the role of soil compaction in the development of the herb layer in forest biotopes is therefore an important task for the protection and enhancement of forest biological diversity.
2. Study area The research was conducted in the Sonian Forest, south of Brussels (50°470 N; 4°260 E). This area has been proposed as a Site of Community Importance (Natura 2000 area, in fulfillment of the EC-Habitat Directive 92/ 43/EEC). It is a remnant of the huge forest that is supposed to have covered much of Western Europe after the last Ice Age. The forest actually covers an area of 4383 ha, 1654 ha of which are situated within the administrative limits of the Brussels Capital Region, this constituting a management unit and being the area considered in the present study. Some 20,000 years ago, sandstone and flintstone formed the upper layer in the area of the Sonian Forest. After the last Ice Age, this layer was covered with loess. Today, almost the whole surface of the forest (95%) is composed of a 3–4 m thick silt layer (pHH2 O around 4.0 in the upper 10 cm), which corresponds to the loess deposition. The forest ranges in altitude from 65 to 130 m a.s.l. The climate of the area is temperate and humid, with a growing season of 7 months (April–October). Mean annual temperature is 9.9 °C, annual precipitation is 835 mm. The natural vegetation is a deciduous forest in which oaks (Quercus robur and Quercus petraea) and beech (Fagus sylvatica) are the main species (Herbauts et al., 1996). Since the plantation work of the Austrian administration at the end of the 18th century, it is now composed of 74% beech (Fagus sylvatica) with only a few other woody species. Sixteen percent is occupied by oak stands (Quercus robur) and 8% by introduced conifers (Pinus sylvestris, Larix decidua, Picea abies). Beech is subjected to frequent uprooting by wind in the Sonian Forest. Compaction occurs naturally between 40 and 120 cm depth (fragipan) as a result of an intense postglacial
drying, but is also in the upper layers induced by heavy machinery, pedestrian traffic, scouting and mountain biking. Thinning is carried out on a rotation of 8 years with ‘‘Timberjacks’’ (9 tons, 130 HP), ‘‘Igland’’ (8 tons, 100 HP) or ‘‘Agrip’’ (8 tons, 100 HP) being the main equipment (Herbauts et al., 1998). Results of Herbauts et al. (1996, 1998) already provide evidence that on these loess materials soil compaction due to logging operations leads to rapid soil degradation through active hydromorphic processes.
3. Methods Compaction is strongly related to the original bulk density, forest type and soil parent material (Seixas and McDonald, 1997; Fisher and Binkley, 2000; Williamson and Neilsen, 2000) and therefore sampling areas had to fulfill certain prerequisites: (1) same soil type; the prevailing soil type with an ABC profile was chosen (USDA: Hapludalf and Glossudalf; FAO: Luvisols and Podzoluvisols; French classification: Sols lessives, Sols lessives hydromorphes and Sols lessives a pseudogley); (2) same topography (i.e., level ground); and (3) same overstory species (i.e., beech stands), as tree species can also change the distribution of pore sizes in soils (Nihlgard, 1971). In order to have a broad range of compaction values, stands were chosen to give as wide a range of ages as possible (planted between 1815 and 1969). The 50 stands were classified into eight groups with ages of about 40, 70, 120, 130, 140, 160, 165 and 185 years old. A map of the areas meeting these conditions was drawn by overlaying the soil map and the stand map of the whole forest in G.I.S. Arc View (ESRI, 1996). Within these pre-defined areas, 499 rectangular quadrats (10 1 m2 ) were randomly laid out for vegetation sampling and soil compaction measurements. The species composition was characterized by classical phytosociological plots (e.g., Westhoff and Van der Maarel, 1973), which means that total coverage for each species (vertical projection onto the ground) was estimated visually and recorded within seven cover classes: r: 1 or 2 individuals; +: few individuals (<20) with cover <5%; 1: many individuals (20–100) with cover <5%; 2: 5%–25% cover; 3: 25%–50% cover; 4: 50%–75% cover; 5: 75%–100% cover. Since Braun-Blanquet cover-abundance values are not suitable for mathematical treatment, raw data were transformed by the corresponding cover percentage values (median of each scale interval): 0.2, 0.5, 2.5, 15, 37.5, 62.5 and 87.5 accounting, respectively, for r, + and cover classes 1–5 (arbitrary values were taken for r, + and 1). In each of the 499 sample plots, three measurements of soil compaction were recorded using a cone-penetrometer (Eijkelkamp Agrisearch Equipment, The Netherlands), a device forced into the soil to measure its resistance to
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vertical penetration as frequently used to measure soil resistance (e.g., Seixas and McDonald, 1997; Jansson and Wasterlund, 1999). The average value was taken for statistical analyses. The penetrometer has a 60° cone-shaped tip. According to the soil strength (resisting force), three tips were used (1–2 cm diameter) with a conversion coefficient which allows the comparison of the readings. When the tip is pushed into the soil, the penetrometer provides a continuous plotting of soil strength and soil depth, unless impenetrable stone or wood is encountered (Miller et al., 2001). The readings are expressed in Newtons (N), the SI unit of force (a force of 1 N will accelerate a mass of 1 kg at the rate of 1 m per second). Measurements were performed down to 20 cm depth, as a 0–20 depth range is usually used in the literature (e.g., Servadio et al., 2001; Brevik et al., 2002) and because it avoided the natural compaction beginning at 40 cm depth. Because of the relative ease of conducting in situ compaction tests, this method was chosen for field compaction studies. Furthermore, Dawidowski et al. (2001) highlighted the fact that measuring soil compaction in situ with a portable penetrometer was statistically similar to traditional laboratory compression tests of soil cores. Because penetrometer readings can be strongly dependent on soil moisture content (Miller et al., 2001; Vaz et al., 2001), field samplings were carried out in the spring when soils are near field capacity so that penetrometer readings are least influenced by differences in soil moisture (Miller et al., 2001). The Canoco 4.5 statistical package (ter Braak and Smilauer, 2002) was used to summarise relationships between variables using a regression model fitted to soil compaction predictor. A Generalised Additive Modeling regression (GAM; Hastie and Tibshirani, 1990) was used with a cubic smooth spline function. GAM regression has been used in numerous studies of speciesenvironment relationships (e.g., Bio et al., 1998; Austin, 1999; Guisan and Zimmermann, 2000; Vetaas, 2002) and was chosen because it does not assume any general shape of the response prior to the estimation (Austin and Meyers, 1996). The model was tested with different degrees of smoothing. The optimum degree of smoothing (i.e., giving the best P -value for the deviance-based test) for each species was found to be 2. The response data are relative covers (subjectively estimated percentages) and therefore a Poison distribution was assumed with a logistic link function. An index of light intensity was attributed to the plots using the species indicator values of Hill et al. (1999). The mean indicator value for each plot was calculated by averaging the indicator value of each species for light intensity (L), weighted by their respective abundance percentage, using the following equation: mL ¼
ðx1 L1 þ x2 L2 þ þ xn Ln Þ ; ðx1 þ x2 þ þ xn Þ
209
where x1 ; x2 , . . .,xn are the abundance percentages of those species present in the plot and L1 ; L2 ; . . . ; Ln represent the light indicator values of the species according to Hill et al. (1999). Correlations between soil compaction and environmental ecological parameters (i.e., light intensity, stand age) were measured using the Spearman rank correlation coefficient rs . In the framework of this study, we focused particularly on the behaviour of true forest species and ancient forest species. Due to their poor colonising ability (Honnay et al., 1998), these species are limited to primary or ÔancientÕ woodland areas, i.e., ‘‘woodland sites which have been continuously wooded since about the year 1775, the approximate date of the publication of the Ferraris maps’’ (Hermy and Stieperaere, 1981), the earliest reliable mapping of this area. In this paper, Ôtrue forest speciesÕ and Ôancient forest speciesÕ, as defined by Honnay et al. (1998) for Belgium, were classified as forest species. All the other species were considered as non-forest species. Nomenclature follows Lambinon et al. (1998). The highly variable and taxonomically disputed Rubus fruticosus agg. was considered a single species.
4. Results Of the 107 taxa studied, the cover of 65 species (61%) was significantly related to soil compaction (Figs. 1 and 2). Out of the 41 forest species, 24 (58%) showed bellshaped or monotonic responses to soil compaction. About half of them showed a bell-like response with maximum growth at 200 N (Hyacinthoides non-scripta) or 400 N (Carex pilulifera, Deschampsia flexuosa, Dryopteris dilatata, Melica uniflora and Teucrium scorodonia) or even 600 N (Oreopteris limbosperma). A few, such as Carex strigosa, Epilobium montanum, Mycelis muralis, showed monotonic reduction in growth with increasing compaction. Four species (Carex remota, C. sylvatica, Rumex sanguineus, Veronica montana) had positive growth responses with increasing compaction, at least up to 1200 N. Species found on disturbed ground (e.g., Arctium minus, Sonchus oleraceus, Stellaria media), as well as on woodland edges and clear-cuttings (e.g., Alliaria petiolata, Epilobium angustifolium, Rubus fruticosus agg.) also showed significant responses to soil compaction. Among these non-forest species, 18 can tolerate very high compaction levels (e.g., Geum urbanum, Glechoma hederacea, Impatiens parviflora, Polygonum hydropiper, Veronica serpyllifolia), having a better growth at sites with penetration forces of or above 1000 N. The reverse was observed for Galium aparine and Urtica dioica, which showed a progressively decreasing trend as soil compaction increased.
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Fig. 1. The response (estimated percentage cover values) of forest species significantly related to soil compaction (expressed in Newtons). Species which had no significant response to compaction are: Adoxa moschatellina, Anemone nemorosa, Athyrium filix-femina, Cardamine flexuosa, Dryopteris carthusiana, Dryopteris filix-mas, Festuca gigantea, Holcus mollis, Hypericum pulchrum, Impatiens noli-tangere, Lamium galeobdolon, Lonicera periclymenum, Luzula pilosa, Lysimachia nemorum, Moehringia trinervia, Ranunculus ficaria, Scrophularia nodosa, Stachys sylvatica, Stellaria alsine, Viola reichenbachiana.
These species survive compaction values exceeding 400 N poorly. For most other taxa, bell-shaped curves were found, with optimum responses (best growth) varying from 150 N (Galeopsis tetrahit) up to 700 N
(Cirsium oleraceum, Juncus tenuis). Among these species, some were characterised by a narrow amplitude (e.g., Cirsium oleraceum), while others showed a broader tolerance range (e.g., Juncus effusus).
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Fig. 1. ðcontinuedÞ
Data from Fig. 3 showed that there was a weak but significant positive correlation between soil compaction (measured range: 68–793 N; average: 365 N) and stand age. The relationship is, however, weak as it gives an average difference of only 100 N between 50 year old stands and 200 year old stands. Our results therefore show that stand age has only a marginal influence on the soil compaction status. We also found a significant positive correlation between soil compaction and light intensity (Fig. 4).
5. Discussion 5.1. Species response to soil compaction The most interesting results of the present contribution concern the forest species as, to our knowledge, no other work has examined statistically the response of European forest understory herbs to soil compaction. The only study known to us dealing with forest herbs gave similar conclusions for six species from the Nearctic zone (Small and McCarthy, 2002). The avoidance by some species of highly compacted sites may be due to oxygen stress as a result of puddling (Kozlowski, 1999; Grigal, 2000; Startsev and McNabb, 2000), which re-
duces air and water movement through soil pores. Soils with stagnant water are indeed low in oxygen (Fisher and Binkley, 2000) and respiration of roots shifts toward an anaerobic state (Kozlowski, 1999). Soil compaction can also severely reduce plant growth by restricting root growth (Rosolem et al., 2002) and lower the percentage of water and air space in the soil (Holshouser, 2001). This may disadvantage species developing superficial roots as soil compaction normally affects the upper soil layer rather than the deeper layers; in arable soils, compaction of subsoil is more often a problem as a result of agricultural practices (Parackova and Zaujec, 2001). For the Sonian Forest, Herbauts et al. (1996) found a macroporosity between 5.7% and 9.5% in rutted soils, while De Bruycker (1984), cited by Herbauts et al. (1996), considered for the same area that a soil macroporosity of 10% of the total soil volume is the lowest value to allow a sufficient diffusion of air for aerobic microbial activity and viability of roots. A reduced absorption of the major nutrients by compaction of both surface soils and subsoils (Kozlowski, 1999) may account for differences between species as some have higher nutrient requirements than others. However, this point can not be confirmed for nitrogen from our dataset, as the mean nitrogen indicator value (Hill et al., 1999) of compaction-tolerant
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Fig. 2. The response (estimated percentage cover values) of non-forest species significantly related to soil compaction (expressed in Newtons). Species which had no significant response to compaction are: Alliaria petiolata, Anthoxanthum odoratum, Carex hirta, Carex ovalis, Carex pallescens, Cerastium fontanum, Cerastium glomeratum, Cirsium arvense, Cirsium vulgare, Epilobium angustifolium, Epilobium parviflorum, Geranium robertianum, Hypericum humifusum, Lotus corniculatus, Luzula multiflora, Myosotis arvensis, Poa trivialis, Rubus idaeus, Sagina procumbens, Taraxacum officinale, Veronica arvensis, Veronica officinalis.
species (arbitrarily defined as having an optimum <400 N on a measured range 0–800 N; n ¼ 13) is not significantly lower than that of plants which do not tolerate high compaction (arbitrarily defined as having an opti-
mum P 400 N; n ¼ 11) (mean nitrogen index ¼ 4.69 and 4.91, respectively; Z ¼ 0:33; P ¼ 0:74, Mann–Whitney test). This may be due to the fact that compaction might play a more important role for other nutrients, such as
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Fig. 2. ðcontinuedÞ
S, B, Mn or K that may have leached out of the surface into the subsoil (Holshouser, 2001). Herbauts et al. (1996) have also demonstrated for the same study area that soil compaction generates reduced forms of irons and induces a strong leaching of iron hydroxides from the eluvial (Eg ) to the illuvial (Btg ) horizon.
Kozlowski (1999) mentioned a reduced total photosynthesis when soils become increasingly compacted, as a result of smaller leaf areas. This may favour species that can physiologically better compensate for this reduction in leaf area. The fact that formation and function of mycorrhizal relationships are affected by soil
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soil compaction (N)
Fig. 2. ðcontinuedÞ
compaction (Entry et al., 2002) can also contribute to a differential impact of soil compaction according to the response of each species to destruction of mycorrhiza.
900 800 700 600 500 400 300 200 100 0
5.2. Conservation and management implications
0
50
100
150
200
stand age (yr) Fig. 3. Relationship between soil compaction and stand age (rs ¼ 0:13; P ¼ 0:0205; n ¼ 320; y ¼ 0:7306x þ 249:11).
In the present study, a substantial number of understory species showed a significant response to soil compaction in a forested area. Mechanised harvesting operations are now widely recognised as the major source of soil compaction (e.g., Jansson and Wasterlund, 1999; Servadio et al., 2001; Langmaack et al., 2002). In the study area, forest harvesting of beech stands developed on acid and loamy soils in the loessic
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800
soil compaction (N)
700 600 500 400 300 200 100 0 4,00
4,50
5,00
5,50
6,00
6,50
7,00
light indicator values Fig. 4. Relationship between soil compaction and light intensity (rs ¼ 0:21; P < 0:0001; n ¼ 499; y ¼ 54:28x þ 67:896).
belt of middle Belgium is known to lead to strong physical degradation due to compaction under repeated logging traffic (Herbauts et al., 1996, 1998). An increased light flux to the forest floor is known to be a direct consequence of regular tree cutting (Brunet et al., 1996; Boncina, 2000). Although this is primarily true in the case of recent cutting which could produce both increased light and compaction, it is also valid in our study area as the regrowth of brushwood (which could decrease the light flux) after cutting is systematically eliminated by foresters. This is consistent with the positive relationship we found between compaction measurements and light intensity index. It confirms that the measured soil compaction is, to a certain extent, induced by logging traffic. Performing fewer mechanised interventions seems therefore highly desirable. It appears, however, that 80% of the compaction can occur on the first pass (Holshouser, 2001), so that reducing the intervention frequency alone may not be sufficient to avoid soil compaction. Another possible factor contributing to the soil compaction could be stand aging due to: (1) history of increasing mechanisation of forestry; (2) increasing weight of stand. As forest exploitation has begun at the same moment over the whole study area, increasing mechanisation can not be regarded as responsible for the phenomenon in this case, but the weight of the trees themselves might slightly enhance the compaction phenomenon. Other recommendations to prevent soil compaction may consist of the following modifications in silvicultural practices: (1) using lighter harvesting machines (Jansson and Wasterlund, 1999); (2) reducing tyre pressures (Canillas and Salokhe, 2001); (3) placing harvesting residues over areas where machinery traffic is used (Hutchings et al., 2002); (4) winter-harvesting on frozen ground (Alban et al., 1994); (5) confining machinery to a series of parallel access routes spaced at
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intervals of about 20 m and winching material to the extraction machine (Savill, pers. comm.). In the Sonian Forest, many species with high conservation values (e.g., Carex strigosa, Hyacinthoides non-scripta, Primula elatior) occur on fresh or damp soils. It is well known that damp soils are more sensitive to plastic deformation than dry soils (e.g., McNabb et al., 2001). Therefore, machinery should not be taken onto sites when the soil is wet. The fact that the optimum growth response of most of the forest herbs studied lies below the average compaction value found in the study area (365 N), implies that they currently survive in sub-optimal conditions. The most sensitive species could disappear with further compaction and may be replaced by non-forest species that show greater tolerance to soil compaction. The increased light afforded by logging or thinning could also favour such ruderal or alien species. Forest managers should be aware of the possible extension of particularly competitive species, such as Impatiens glandulifera. This problem requires attention and efforts should be made to limit compaction in order to minimise the chances of development and spread of these undesirable species representing a possible threat to the optimal development of typical forest vegetation.
Acknowledgements This study was financially supported by the Brussels Institute for Environmental Management (BIM-IBGE) in the framework of the vegetation monitoring in the Sonian Forest. We also acknowledge the thorough reviews by Brian Davis, Peter Savill and two anonymous reviewers whose critical comments were crucial in the development of the revised manuscript.
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