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Influence of alpine plants growing on steep slopes on sediment trapping and transport by runoff
Catena 71 (2007) 330 – 339 www.elsevier.com/locate/catena
Influence of alpine plants growing on steep slopes on sediment trapping and transport by runoff Francis Isselin-Nondedeu ⁎, Alain Bédécarrats Cemagref, UR EM, 2 rue de la Papeterie, BP 76, 38402 Saint Martin d'Hères Cedex, France Received 1 June 2005; received in revised form 13 February 2007; accepted 23 February 2007
Abstract Understanding the interactions between soil and the organisms that are conducive to decreasing sediment runoff is a great concern on highelevation ski trails. Intense rainfalls on steep slopes combined with soil formed on gypsum result in recurrent erosion. This study was conducted in the northern French Alps to determine the abilities of species: (1) to make mounds and (2) to trap sediment and thereby to control erosion at the slope scale. We also investigated relevant above-ground plant characteristics related to those abilities. Sediment runoff or deposition was investigated at small and large spatial scales. We assessed whether hoof prints in soil reflect sediment runoff at the slope scale by trapping sediment. Populations of plants growing on two slope angles (25° and 35°) and three vegetation cover densities (15%, 35%, 60%) were surveyed. An experiment was also conducted to measure the sediment deposit upslope of target species and over three months during the autumn. Small mounds were found upslope of the plant and sediment deposit measurements showed that they resulted from a sedimentation process. Nevertheless the species differed in their capacity to make mounds. Sesleria caerulea and Festuca alpina had the highest amount of sediment deposition over the experimental period. Among the plant characteristics, plant length was positively correlated with mound area, while the roundness index of the canopy was negatively correlated with mound height. Mound formation was also positively related to the number of tillers or shoots. Sediment accumulation in cow hoof prints was linked to runoff that occurred at the slope scale. Low deposition in hoof prints means low sediment runoff or a large deposition on mounds, due to the increase in vegetation cover. All the findings stressed that understanding the processes in action at larger scales requires studying processes at smaller scales. © 2007 Elsevier B.V. All rights reserved. Keywords: Biogeomorphology; Phytogenic mound; Plant morphology; Gypsum; Sediment runoff; Geomorphic agent
1. Introduction Human pressure on the mountain environment has increased since the beginning of the 1970s, with ski run construction and topographical adjustments, altering both ecological and geomorphological processes. Assisted revegetation aims at stabilizing bare soil subjected to erosion, but this remains difficult on steep erodible slopes. Therefore, knowledge of interactions between soil and organisms that are conducive to controlling erosion is an important concern in the mountain environment. Biogeomorphology combines research in ecology and geomorphology exploring relationships between organisms and geo⁎ Corresponding author. Current address: Peatland Ecology Research Group, Département de phytologie Pavillon Paul-Comtois, Université Laval QC, G1K 7P4 Canada. Tel.: +418 656 2131 3595; fax: +418 656 7856. E-mail address: [email protected] (F. Isselin-Nondedeu). 0341-8162/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2007.02.001
morphological characteristics of the environment (Naylor et al., 2002). The field of biogeomorphology encompasses phytogeomorphology and zoogeomorphology. The research issues that are addressed in these disciplines are the effects of biological communities on geomorphological processes through hydrodynamic, biostabilization, and sediment transport and production. Numerous studies have focused on soil protection using living organisms, searching for positive interactions with geomorphological processes (Naylor, 2005). The effects of vegetation on soil can be divided into two major related categories: bioprotection and bioconstruction (Naylor et al., 2002). Vegetation protects soil against erosion by reducing water erosion (Rey, 2003; Puigdefabregas, 2005) and increasing the infiltration rate in soil (Graeme and Dunkerley, 1993; Ziegler and Giambelluca, 1998; Wainwright et al., 2002). Plants shelter and fix the soil with their roots (Körner, 2002; Gyssels et al., 2005) or reduce the energy of raindrops with their
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Fig. 1. Location of the ski area of la Plagne in the northern French Alps.
canopy (Bochet et al., 1998). Also, vegetation can act as a physical barrier, altering sediment flow at the soil surface (Van Dijk et al., 1996; Lee et al., 2000). The way the vegetation is spatially distributed along the slopes is an important factor for decreasing the sediment runoff (Lavee et al., 1998; Calvo-Cases et al., 2003). This barrier effect can lead to the formation of structures called phytogenic mounds. Such structures were found on the upslope side of large strips of grass disposed perpendicular to the slope (Dabney et al., 1995; Meyer et al., 1995; Van Dijk et al., 1996; Abu-Zreig et al., 2004) or under plants (El-Bana et al., 2003). Several mechanisms are involved in mound formation: the deposition of soil particles beneath plant canopy after a decrease in blowing wind (El-Bana et al., 2003), the differential erosion rates in the close environment of the plant (Babaji, 1987; Rostagno and del Valle Puerto Madryn, 1988), or the deposition of sediment resulting from a decrease in overland water flow (Sanchez and Puigdefabregas, 1994; Bochet et al., 2000). In alpine environments, plants are often associated with convex-shaped soil as terraces or mounds resulting from solifluction processes (Bird, 1974; Caine, 1974). Few authors have taken an interest in the relationships between plant morphology and the effects on soil erosion. Nonetheless, some have shown that plant length and a complete canopy are important features for sediment trapping (Van Dijk et al., 1996). Animals also act as geomorphic agents at different spatial scales, structuring small patches of soil or landscape units (Butler, 1995, 2001), especially when they dig in soil or trample (Trimble and Mendel, 1995; Gutterman, 2003). Consequences can be positive or neutral for slope stabilization (Trimble and Mendel, 1995; Nash et al., 2003). In terms of bioconstruction, the beaver is an animal that is well known for its ability to build dams and therefore modify sediment fluxes in hydrologic systems (Butler and Malanson, 2005). All the above-mentioned studies on plant–mound interactions were conducted on shrubs or herbaceous plants in semi-arid or arid environments. Studies comparing the sediment accumulation among species are scarce, and to our knowledge no report has been made for alpine species, which leaves a gap in our understanding of the erosion responses of the mountain ecosystems.
In this study, we aimed at determining the ability of alpine plant species to retain sediment and decrease runoff at the slope scale and also the morphological attributes of plants which is involved in. Our hypothesis was that erosion control at the slope scale resulted from a sediment trapping process at the micro scale, i.e. the plant scale. Sediment deposition or runoff was measured over a long period by a survey of existing mounds and over a short period by measuring sediment accumulation over 3 months. In addition, we expected mound sizes to vary with differences in plant morphology. The study was done in a French subalpine area on steep ski slopes constructed on gypsum. 2. Material and methods 2.1. Study site description The study was carried out in the ski resort of la Plagne (45°33′ N, 06°40′ E) in the northern French Alps (Fig. 1), in August– November 2002. Mean annual precipitation and temperature at the ski resort are 1165 mm and 2.7 °C, respectively. Rainfall distribution between and within years shows two peaks, one in Table 1 Dates and total precipitation of the ten major events recorded in the study period at 1970 m, north face (la Plagne, Meteo France) Major events
Date
P-tot (mm)
1 2 3
07/08/2002 08/08/2002 19/08/2002 Total 02/09/2002 09/09/2002 Total 17/10/2002 21/10/2002 25/10/2002 Total 02/11/2002 03/11/2002 Total
30 17 15 165.4 10 13 71.8 43.5 30.5 33 156.7 29 35 158.8
4 5 6 7 8 9 10
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Fig. 2. Measurements of mound shape associated with individual plant and measurements of plant morphology (adapted from Bochet et al., 2000).
Spring, the other in late Summer to Autumn. Especially on the north face of the mountain, heavy showers are frequently observed, with intensity ranging from 2.5 to 30 mm/h (Rovéra, 1990). This represents geomorphic effective intensities that can remove and transport soil materials. Rainfall conditions at 1970 m north face are shown in Table 1. Field experiments were conducted on ski trails between 2200 and 2350 m. Vegetation on ski trails was sown during the restoration process between 1979 and 1981. The base of seed mixtures used for restoration contains the following species: Dactylis glomerata, Anthyllis vulneraria, Trifolium pratense, T. repens, cultivars of Festuca rubra, Poa alpina, Lotus corniculatus. The bedrock is made up of gypsum and associated weathering evaporitic stones. The initial uppermost soil was most often removed during trail construction. Nowadays, gypsum often outcrops and the soil layer is thin, ranging from 2 to 10 cm. Ski trail soil is characterized by a high proportion of sand and gypsum sediment (between 55% and 65%), and approximately 25% loam and 10% clay. Organic compost remains in some places, but the organic pool was below 3.3 kg O.M./m2. Ski trails are currently managed for ski during the winter season and grazing by dairy cows during the season of vegetation.
local scale. At the general scale, trail topography varied from 40° to 20°. The standing vegetation on the ski trails have a random distribution in space, with a slight tendency of grouping at many places, due to patches of grasses. A survey was first conducted to note the presence or absence of mounds resulting from a long period of plant–soil interaction above 30 individuals. Species were chosen among the most abundant that grown on ski slopes. They arose from natural colonization and seed mixtures used for slopes restoration. We grouped them into two a priori plant types: graminoid plants, i.e. grasses and sedges (Poaceae: Agrostis alpina, Dactylis glomerata, Deschampsia caespitosa, Festuca alpina, F. rubra cv., Poa alpina, Sesleria caerulea; Cyperaceae: Carex sempervirens) and non-graminoid plants, i.e. forbs, legumes and small shrubs (Rosaceae: Dryas octopetala; Fabaceae: Anthyllis vulneraria, Lotus corniculatus, L. alpinus, Trifolium hybridum; Salicaceae: Salix retusa). The shape of mounds located on the upslope side of the plant was measured above 10 individuals per species when mounds were present (Fig. 2). The maximum height, width and length of the mounds were determined in order to calculate indicators of sediment deposit: area (mA) and height (mH).
2.2. Mound survey and measurements
2.3. Plant morphology
We investigated 25° and 35° slopes with 15%, 35% and 60% vegetation cover. In each situation, three 100-m2 plots were selected. Slope angles were measured with clinometers at the
The selected species naturally differ in their life-forms and canopy architectures. Sesleria caerulea and Carex sempervirens are densely tufted, forming large clumps from rhizomes, with more
Fig. 3. Left: small mound (8 cm in height, 12 cm in length) above a native alpine tussock grass (Festuca rubra subsp. rubra) growing on east-facing 30° ski slope, 2350 m a.s.l. Rocks are fragmented gypsum and rauchwacke. Right: water stored in cow hoof prints after rainfall (Autumn 2003, la Plagne, Savoie, French Alps).
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runoff and deposition measurements at the slope scale, we investigated natural micro-catchments such as hoof prints. Twenty five neat, 1-year-old dairy-cow hoof prints were selected in the three lower quarters of slope. The hoof prints measured approximately 15 cm in length (heel–toe distance), 10 cm in width, 10–15 cm in depth. Sediment deposit was measured with graduated sticks placed in the centre. 2.5. Data analysis
Fig. 4. Mean proportion of plant individuals with mounds at the upslope side of their canopies for the species growing on the 25° and 35° slopes. Results of the two-way ANOVA; the species D. glomerata, D. caespitosa and T. hybridum were not included in the analysis, as they have been found only on the 25° slopes.
or less matted leaves and stems stiff. Deschampsia caespitosa and Dactylis glomerata are clumped grasses, often forming tussocks with matted dry leaves at base. The other grasses, Agrostis alpina, Festuca alpina, F. rubra cv., Poa alpina, are tufted, moderately compact, especially the cultivar of F. rubra, with slender stems usually arching. Non-graminoid species have various aboveground architectures. Both Dryas octopetala and Salix retusa are semi-shrubs, with woody stems trailing and freely rooting, often forming clumps or mats. Anthyllis vulneraria, Lotus corniculatus and Trifolium hybridum have stems erect, from a branched base, canopies often weakly clumped. L. alpinus is tufted, more compact, slender stems creeping. Canopy dimensions were measured on 10 individuals per species of approximately the same height (9–12 cm) associated to the mounds selected. Two dimensions were considered: D1 corresponded to the length of the standing canopy developed perpendicular to the slope (Fig. 2), D2 was the measurement of the canopy along the slope. We used the D1 to D2 ratio (pR) as a simple index of plant shape assumed to interact with sediment transport by runoff. This pR represents a roundness index greater than 0, a value of 1 indicating a circular form. Tillers or shoot numbers for graminoid and non-graminoid species, respectively, were counted by area (cm2).
The goal of the statistical analysis was (i) to determine the plant's ability to retain sediment at the micro-scale and to decrease runoff at the slope scale and (ii) to identify plant morphological characteristics that influence sediment deposit. ANOVAs were first performed on the data resulting from the survey of mounds, with the slope angle, the species and the plant a-priori groups as factors. Normality of the dependent variables was confirmed using Liliefor's modification of the Kolmogorov–Smirnov test. Statistical analyses were carried out with the statistical package SYSTAT 11.0. We also performed a logistic regression on the two dimensions of plant canopy (D1, pR) to test whether the presence of mounds was related to plant form. We then compared how effective the species were by calculating means of sediment accumulation. We tested both the ability of hoof prints to catch sediment and the effect of the vegetation cover in decreasing erosion. Finally, to test the relations between plant morphology and mounds, we performed a principal component analysis (PCA). 3. Results 3.1. Mounds survey Small mounds made up of eroded particles of gypsum and organic matter, were observed at the upslope side of the plant canopy (Fig. 3). However, the occurrence of mounds at the upslope varied among the surveyed plant species (Fig. 4). There were significant effects of the species on the occurrence of mounds (F = 38.6; P b 0.001) as well as species–slope interactions (F = 72.9; P b 0.001). Indeed, among species sampled on
2.4. Sediment trapping dynamics Sediment accumulation at the micro-scale was estimated from late Summer to mid-Autumn 2002 (August 7–November 4) on the plots of the 25° slope. Graduated sticks were placed on mounds above the following target species: Agrostis alpina, Festuca alpina, F. rubra cv., Sesleria caerulea, Carex sempervirens and Dryas octopetala. The experiment was conducted on five individuals by species for three replicates by vegetation cover density (15%, 35% and 60%). The heights of depositions were measured after substantial rainfall events in order to explore the dynamics of sediment in relation to precipitations. For sediment
Fig. 5. Mean proportion of plant individuals with mounds at the upslope side of their canopies for the two groups of plants growing on the 25° and 35° slopes. Results of the two-way ANOVA; the species D. glomerata, D. caespitosa and T. hybridum were not included in the analysis, as they have been found only on the 25° slopes.
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Fig. 6. Height of sediment deposit above the target species after 90 days of experimentation. Columns are mean (± SE) height of sediment accumulation upslope of the plant canopy, and letters indicate significant differences between species (ANOVA–Tukey's HSD test, α = 0.05). The slope is 25° — black column: 15% vegetation cover; white column: 35%; grey column: 60%.
the 25° slope, the five following species: Sesleria caerulea, Carex sempervirens, Agrostis alpina, Festuca alpina and F. rubra were more effective in forming mounds, as no individual was found without a mound. On the 35° slope, only S. caerulea did the same. The less effective species were Deschampsia caespitosa, Salix retusa and Trifolium hybridum on the 25° slope, Anthyllis vulneraria and Lotus corniculatus on the 35° slope. Moreover, the grouping of species in graminoid vs. nongraminoid has a significant effect on the occurrence of mounds (F = 124.2; P b 0.001; slope × plant group: F = 7.4; P b 0.01), the occurrence of mounds was higher at the upslope of graminoid species on both slope angles (Fig. 5). The presence of mounds was significantly related to the plant form, both for all graminoids (plant length: Chi2 = 19.5, P b 0.0001; plant area: Chi2 = 4.6, P b 0.05; plant roundness: Chi2 = 60.5, P b 0.0001) and for all non-graminoid species (plant length: Chi2 = 8.9, P b 0.01; plant area: Chi 2 = 4.4, P b 0.05; plant roundness: Chi2 = 69.3, P b 0.0001). The presence of mounds at the upslope of the different plants increased as plant length or area increased. The existence of mounds may reveal a past or present sediment trapping process. 3.2. Effectiveness of species in sediment trapping The dynamics of sediment trapping at the micro scale is expressed in total height of sediment accumulation upslope of the plant after 90 days (Fig. 6). Carex sempervirens, Festuca alpina and Sesleria caerulea had the highest sediment accumulation at the end of the experiment, between 1 cm and 2.5 cm. Agrostis alpina and Festuca rubra showed lower sediment deposits, whereas Dryas showed a gap in sediment accumulation in 35% vegetated plots. The Fig. 7 shows the
details of sediment accumulations measured before and after the major precipitation events (Table 1) for all the target species. Generally, higher accumulations were observed after rainfalls of 29, 30, and 30.5 mm. Sediment deposit values under zero indicate a loss of sediment from the mounds after the rainfall event. This was the case for most of the species after the rainfall events of 33, 35 and 43.5 mm. Higher losses at the micro scale were recorded in 15% vegetation plots, whereas in 60% vegetation cover, there was almost no loss of sediment. The species that showed the highest deposits after major rainfall events were Sesleria caerulea, Carex sempervirens and Festuca alpina. On the contrary, Festuca rubra and Dryas octopetala showed losses of sediment from their mounds and for lower precipitation events than the other species. After the experimentation period, hoof prints trapped sediment (Fig. 8), but as vegetation cover increased, the height of sediment deposits decreased. With 60% vegetation cover, the sediment deposit was approximately 2.4 times less than on slopes with 15% and 35% vegetation cover. The comparisons of measurements in relation with the major rainfall events show that sediment deposits increased as rainfall increased (Fig. 9). Sediment deposits upslope of plants, deposits in hoof prints and initial mound height on the 25° slope are plotted together in Fig. 8. With an increase in vegetation cover, subsequently the mound height was higher and the sediment deposit above mounds and in hoof prints was lower. 3.3. Plant morphology and mound shape relationships Two main correlations were found between plant form and mound shapes (Table 2). Concerning graminoid species, mound area was positively correlated with plant length (R = 0.74) and
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Fig. 7. Sediment deposits on phytogenic mounds measured after major rainfall events (see Table 1) in the plots with 15%, 35% and 60% vegetation cover (25° slope). Bars are means (± SE) of sediment deposit or removal.
mound height was negatively correlated with plant roundness (R = − 0.58). In addition, for the non-graminoid species, mound area was positively correlated with plant length (R = 0.87), whereas mound height was negatively related with plant roundness (R = − 0.46). The number of graminoid tillers was positively related both with the height and the area of mounds found upslope of plant canopy (R = 0.63 and 0.59, respectively). However, no close correlations were found between the number of shoots of other plant types and mound variables. The same type of correlation was found for the analyses of the dynamics of sediment accumulation (Table 3). For graminoid species, the mound area was also positively correlated with plant length
(R = 0.65). The initial mound height was negatively correlated with plant roundness (R = − 0.53), as was height after sediment accumulation (R = − 0.56). The results of sediment accumulation above Dryas were quite similar: positive correlation between mound area and plant length (R = 0.56), negative correlations between initial mound height and plant roundness (R = − 0.46), as well as between mound height after sediment accumulation and plant roundness (R = − 0.44). We also observed internal correlation between mound height and area. Detailed relations between mound area and species traits are listed in Appendices A and B. Depending on the species, sites and plant traits, the statistical significance of these relations varied.
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4. Discussion
Table 2 Correlation matrix of PCA based on plant morphology and mound parameters
4.1. Species, plant morphology and effectiveness in sediment trapping
Graminoid
D1
pR
mH
mA
Tn°
Plant length D1 Plant roundness Mound height Mound area Tillers number
1.00 − 0.30⁎⁎ 0.31⁎⁎ 0.74⁎⁎⁎ 0.75⁎⁎⁎
.
. . 1.00 0.57⁎⁎⁎ 0.63⁎⁎
. . . 1.00 0.59⁎⁎
. . . . 1.00
Non-graminoid Plant length D1 Plant roundness Mound height Mound area Shoot number
1.00 − 0.08 0.35⁎⁎ 0.87⁎⁎⁎ 0.57⁎⁎
.
. . 1.00 0.50⁎⁎ 0.12
. . . 1.00 0.09
. . . . 1.00
Other studies that have investigated phytogenic mounds on steep slopes with erodible bedrock such as gypsum conducted experiments in semi-arid and arid ecosystems (Sanchez and Puigdefabregas, 1994; Bergkamp, 1998; Bochet et al., 2000; ElBana et al., 2003). The striking result of the research herein is that alpine plant species growing on steep slopes with gypsum can have associated mounds. Sediment deposition caused by a local decrease in overland water flow is assumed to be involved in this type of bioconstruction (Babaji, 1987; Sanchez and Puigdefabregas, 1994; Bochet et al., 2000). The observations at the micro scale confirm that phytogenic mounds were formed by the accumulation of sediment upslope of plant canopy. Moreover, the comparison between the height of accumulations after 3 months and the sediment deposits after major rainfall events that occurred during this period indicates that sediment deposits occurred mainly during the rainfalls around 30–35 mm. However, the different results of sediment accumulations obtained at various time scales show that species differed in their ability to form mounds and to trap sediment. Results from the mounds survey showed that graminoids and non-graminoids have had different behaviours. The highest proportion of plants with mounds was found upslope of the graminoid. Thereafter, among the graminoid species, Carex sempervirens and Sesleria caerulea were the most effective species with the highest mounds in two steep slope conditions. They retained large amounts of sediment during the experiment. The two seeded cultivars of Festuca rubra and Poa alpina used for revegetation seem to be less efficient in forming large mounds. Among nongraminoid species, Dryas and Salix had the largest mound area in contrast with the other species. However, despite this large mound area, Dryas was ineffective in retaining sediment during the experimental period. The organization of the canopy might explain these differences, particularly tillers or stem density. For
Fig. 8. Comparisons of sediment accumulations at different space and time scales: mound formation upslope of plants after 90 days (mounds t90), height of sediment deposit in hoof prints and the initial height of mounds before deposition measurements (mounds t0). Points are total means of plots (25° slope).
1.00 − 0.58⁎⁎⁎ − 0.37⁎⁎⁎ 0.11⁎
1.00 − 0.44⁎⁎⁎ − 0.08 0.14
Abbreviations for column are detailed in the lines. D1 measures the length of the canopy perpendicular to the slope. Values in column are Pearson coefficient with significance level (when significant: ⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001). Analyses were performed on graminoid species n = 410 (PCA1: 61.2%; PCA2: 22.8%) and (b) non-graminoid species n = 330 (PCA1: 56.3%; PCA2: 28.9%). All data have been log-transformed.
example, Dryas canopy was diffuse and porous, with large spaces between the creeping branches. Similarly, Festuca rubra cultivars presented spaced tillers that crept over soil. These results are consistent with past research which highlighted that vegetation or canopy completeness is a key feature in trapping sediments (Meyer et al., 1995; Van Dijk et al., 1996; Bochet et al., 2000). These discrepancies in terms of species efficiency can also be attributed to the form of the canopy, especially length and roundness. Various authors demonstrated that grasses were much more efficient when they developed perpendicular to the slope (Pethick et al., 1990; Abu-Zreig, 2001; Abu-Zreig et al., 2001). Our results showed that plant roundness was inversely correlated with sediment trapping and mound formation. This is in agreement with laboratory work (Babaji, 1987) and field measurements on erosion control (Bochet et al., 2000; Casermeiro et al., 2004). Because of interactions between hydrodynamics at the soil surface and plant form, water flows erode the upslope side as well as the downslope side of a plant when its canopy is circular.
Fig. 9. Sediment deposits in hoof prints measured after major rainfall events (see Table 1) in the plots with 15%, 35% and 60% vegetation cover. Bars are means (±SE) of sediment deposit or removal.
F. Isselin-Nondedeu, A. Bédécarrats / Catena 71 (2007) 330–339 Table 3 Correlation matrix of PCA based on plant morphology and mound parameters before (t0) and after 90 days of sediment accumulation (t90) Graminoid Plant length D1 Plant roundness Mound height t0 Mound area Mound height t90 Tillers number
D1 1.00 − 0.36⁎⁎⁎ 0.32⁎ 0.65⁎⁎⁎ 0.04 0.55⁎⁎
pR . 1.00 − 0.53⁎⁎⁎ − 0.34⁎⁎ − 0.56⁎⁎⁎ 0.17
mHt0
mA
Ht90
Tn°
. . 1.00 0.64⁎⁎⁎ 0.22⁎⁎ 0.62⁎⁎⁎
. . . 1.00 0.22⁎⁎ 0.43⁎
. . . . 1.00 0.51⁎⁎
. . . . . 1.00
Dryas Plant length D1 Plant roundness Mound height t0 Mound area Mound height t90
1.00 . − 0.22 1.00 0.39⁎⁎ − 0.46⁎⁎⁎ 0.56⁎⁎⁎ − 0.31⁎ − 0.21 − 0.44⁎⁎⁎
. . . . 1.00 . 0.90⁎⁎⁎ 1.00 − 0.06 − 0.26
. . . . 1.00
Abbreviations for column are detailed in the lines. D1 measures the length of the canopy perpendicular to the slope. Values in column are Pearson coefficient with significance level (when significant: ⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001). Analyses were performed on graminoid species n = 270 (PCA1: 47.9%; PCA2: 27.7%) and Dryas octopetala n = 40 (PCA1: 38.2%; PCA2: 26.7%). All data have been log-transformed.
At the large scale, the findings support general knowledge on the positive effect of vegetation cover in controlling soil erosion. The decrease in sediment accumulation combined with a decrease in precipitation or with an increase in vegetation cover on the slope emphasizes that erosion is dependent on runoff. The same process was demonstrated on steep slopes at the same study site for seed runoff (Isselin-Nondedeu et al., 2006). The absence of accumulation on mounds means that there was no sediment transport by runoff at the slope scale. Moreover, similar patterns of sediment accumulation in the hoof prints confirm that the decrease in accumulation observed after rainfall events was related to the stabilizing effect of vegetation on soil (Körner, 2002; Naylor et al., 2002; Gyssels et al., 2005). This effect was particularly observed at the 60% vegetation cover, where there was almost no deposition of sediment in comparison with the other vegetation covers. Another explanation might be the decrease of the contributing areas to runoff i.e. bare patches between plants. This one is a parameter often more important than the vegetation cover itself (Lavee et al., 1998; Calvo-Cases et al., 2003). In the 60% vegetation cover, this contributing area was probably too small to generate highly erosive runoff volumes able to detach soil particles and transport sediment downslope. The discussion in the previous section suggests that the relationships between plant morphology and effectiveness in sediment trapping is a parameter either important than the contributing areas when one considers the organization of the vegetation. In our experiments, hoof prints measured part of the runoff at the slope scale and showed the same trend as that which occurred at the plant–mound scale: low deposition in hoof prints means low runoff and large deposition on mounds (Figs. 7, 8 and 9). The height of mounds before the 90-day measurements showed no direct relation with the other trends (Fig. 9). Nevertheless, as they result from plant–soil interactions on a longer time scale, mound height is
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related to vegetation cover; when the vegetation cover increases the input of sediment decreases and also their detachment from the mounds. This emphasized that understanding large-scale erosion requires studying processes occurring at smaller scales. 4.2. Plant morphology and alpine geomorphic processes The phytogenic mounds we observed upslope of plants might result in part from the solifluction process. Solifluction, i.e. when soil mass creeps downslope, is a frequent geomorphic process, long observed in alpine and arctic environments, which reworks soil and leads to terrace formation (Bird, 1974; Caine, 1974). These structures result from the interplay between pedogenic processes and climatic constraints such as wetting–drying and freezing–heating cycles. The effect of vegetation on soil material deposition highlights the potential interaction between vegetation, plant morphology, erosion and solifluction processes. Körner (1999) made the point that terrace formation could counteract surface erosion. The short period during which we measured sediment accumulation is a limitation that does not allow us to categorically conclude on the process of mound formation or on the persistence of these mounds. This is particularly relevant because all the plots are covered every winter season by snowgrooming machines. The negative effects of the machines have been demonstrated in other studies (Wipf et al., 2005), as they compact snow as well as soil and limit plant growth. However, our surveys on the different plots showed the existence of old mounds associated with plants. The mounds were small and might be bigger in absence of snow grooming; therefore further experiments are necessary to evaluate the impacts of snow cover, human activities and animal disturbances. 4.3. Interactions between hoof prints and sediment transport by runoff During experiments on our study site, we used cow hoof prints in soil as indicators of sediment runoff at the slope scale. They behave like micro-catchments for sediment. This stresses the role of animals as geomorphic agents (Trimble and Mendel, 1995) and their potential role with regard to erosion. In addition, we have noted accumulation of water in hoof prints during rainfall and 1 day after, forming temporary micro-ponds (see Fig. 3). A similar process has been observed at larger scales when beavers create ponds and zones for sediment accumulation by constructing dams across river channels (Butler and Malanson, 2005). Results on the role of structures made by animals in relation to soil erosion are often contradictory, however, in desert environments, holes created by porcupines can accumulate rainfall water, seeds and organic matter (Gutterman, 2003). In addition, according to Littlemore and Barker (2001), trampling reduces vegetation cover, diversity, and biomass and damages roots, which may counteract the positive effects of plant–mound interactions. Further experiments are required to better understand the effects of grazing on phytogenic mounds as well as their duration in time. It is interesting to note that ski trails function as corridors, in agreement to the view of Butler (2001), channeling water flows, sediment, debris and cattle.
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Acknowledgements
Appendix B
We extend our thanks to Freddy Rey, Thierry Dutoit, and the reviewers for helpful comments and to Benjamin Ollier and Pascal Tardif for technical support in the field. We are also grateful to the ski resorts workers of la Plagne for allowing field experiments on ski trails.
Plant morphology and mound relationships among species (35° slope). Mean values of mound area, plant length and significance of correlation (⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001) are listed below.
Appendix A
Site
Species
Mound area cm2 (± SE)
Plant length cm (±SE)
F
R2
15%
Carex sempervirens Sesleria caerulea Agrostis alpina Festuca alpina Poa alpina Festuca rubra Carex sempervirens Sesleria caerulea Agrostis alpina Festuca alpina Festuca rubra Poa alpina Carex sempervirens Sesleria caerulea Festuca alpina Agrostis alpina Festuca rubra Poa alpina
117.4 (± 3.8)
17.7 (±1.1)
12.2⁎⁎
0.44
72.7 (±3.0) 34.5 (±2.1) 30.9 (±4.6) 18.6 (±1.3) 18.4 (±1.5) 88.3 (±3.5) 80.2 (±3.5) 39.9 (±7.1) 35.9 (±2.1) 21.3 (±1.6) 10.1 (±1.2) 167.5 (±4.9) 161.1 (±4.6) 57.9 (±2.7) 47.7 (±2.1) 30.5 (±1.7) 17.4 (±1.4)
15 (±1.2) 9.9 (±0.8) 8.3 (±0.7) 6.6 (±0.5) 8.3 (±0.6) 20.6 (±1.2) 21 (±1.2) 9.9 (±0.6) 9.4 (±0.6) 9.3 (±0.7) 6.9 (±0.8) 18.7 (±1.4) 18.6 (±1.4) 11.1 (±0.9) 10.9 (±0.9) 10.7 (±0.7) 8 (±0.7)
7.5⁎ 7.3⁎⁎ 14.4⁎⁎ 1.7NS 7.3⁎ 3.4NS 24.7⁎⁎⁎ 1.5NS 4.8⁎ 3.9NS 1.9NS 5.8⁎ 9.6⁎⁎ 18.5⁎⁎⁎ 1.1NS 7.4⁎ 5.1⁎⁎
0.48 0.52 0.64 0.36 0.52 0.34 0.76 0.16 0.32 0.44 0.19 0.4 0.55 0.7 0.14 0.46 0.54
Plant morphology and mound relationships among species (25° slope). Mean values of mound area, plant length and significance of correlation (⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001) are listed below. 35% Site
Species
15% Sesleria caerulea Carex sempervirens Deschampsia caespitosa Festuca alpina Agrostis alpina Dactylis glomerata Festuca rubra Poa alpina 35% Sesleria caerulea Carex sempervirens Agrostis alpina Festuca alpina Deschampsia caespitosa Dactylis glomerata Festuca rubra Poa alpina 60% Carex sempervirens Sesleria caerulea Festuca alpina Agrostis alpina Deschampsia caespitosa Festuca rubra Poa alpina Dactylis glomerata
Mound area cm2 (±SE)
Plant length F cm (±SE)
130 (± 4.3) 107.6 (±3.6) 73.2 (± 3.3) 71 (± 2.8) 54.5 (± 2.3) 42.4 (± 1.9) 34.3 (± 1.6) 24.3 (± 1.4) 134.5 (±3.8) 107.6 (±3.6) 66.2 (± 2.8) 65.6 (± 2.9) 55 (± 2.6) 42.4 (± 1.9) 32.4 (± 7.7) 19.7 (± 1.7) 187 (± 4.7) 153.5 (±3.5) 73.7 (± 3.0) 45.8 (± 2.5) 33 (± 1.9) 30.5 (± 2.2) 24.6 (± 1.2) 22.2 (± 1.8)
17 (±1.1) 16.8 (±1.3) 9.2 (±0.7) 9.5 (±0.8) 8.7 (±0.6) 15.2 (±1.2) 12.1 (±1) 6.7 (±1.4) 16.8 (±1.3) 7.7 (±0.7) 8.9 (±0.8) 9.7 (±0.9) 17 (±1.1) 14 (±0.8) 13.2 (±0.8) 9.3 (±0.7) 18.6 (±1) 9 (± 0.8) 10.8 (±0.7) 19.4 (±0.9) 10.4 (±0.8) 12.4 (±0.8) 10.3 (±0.5) 8.8 (±0.5)
22.3⁎⁎⁎ 10.2⁎ 18.5⁎⁎ 7.8⁎⁎ 5.2⁎⁎ 5.8⁎ 55.7⁎⁎⁎ 7.9⁎ 27.7⁎⁎⁎ 8.1⁎ 6.8⁎ 0.7NS 6.5NS 0.4NS 41.6⁎⁎⁎ 4.2NS 13.3⁎⁎ 9.1⁎ 1.8NS 6.6⁎ 2.5NS 6.2⁎⁎ 0.01NS 0.01NS
R2 0.73 0.38 0.82 0.48 0.4 0.49 0.87 0.66 0.77 0.2 0.27 0.16 0.77 0.12 0.84 0.51 0.62 0.21 0.19 0.26 0.46 0.44 0.1 0.08
Site
Species
Mound area cm2 (±SE)
Plant length cm (± SE)
F
R2
15%
Dryas octopetala Salix retusa Lotus alpinus Lotus corniculatus Anthyllis vulneraria Trifolium hybridum Dryas octopetala Salix retusa Lotus alpinus Trifolium hybridum Anthyllis vulneraria Lotus corniculatus Dryas octopetala Salix retusa Lotus alpinus Anthyllis vulneraria Lotus corniculatus Trifolium hybridum
385.2 (± 8.1) 301.3 (± 5.5) 75.1 (± 3.1) 41.7 (± 1.8) 41.2 (± 2.2) 29 (±2.1) 375.1 (± 6.8) 194.2 (± 4.1) 59.3 (± 3.1) 34 (±1.8) 31.9 (± 2.2) 24.3 (± 1.6) 313.8 (± 7.2) 245 (± 5.5) 37.5 (± 2.1) 32.6 (± 2.3) 29.3 (± 2.3) 26.4 (± 1.7)
73.7 (± 3.1) 63.7 (± 2.3) 13.7 (± 0.8) 13.8 (± 0.9) 11.2 (±0.8) 8 (±0.4) 73.5 (± 2.6) 66.7 (± 1.6) 14.6 (± 0.9) 10.2 (± 0.6) 9.9 (±0.6) 12.9 (± 0.9) 73.7 (± 3.1) 57 (±2.6) 13.3 (± 0.9) 10.7 (± 0.7) 10.7 (± 0.8) 8.6 (±0.6)
19.9⁎⁎ 3.8NS 3.6⁎ 8.4⁎ 2.4NS 16.1NS 18.7⁎ 0.02NS 1.1NS 1.2NS 3.7⁎ 0.2NS 11.8⁎ 0.2NS 1.8NS 1.5NS 176.6⁎⁎ 10.4⁎
0.83 0.66 0.37 0.67 0.17 0.88 0.73 0.05 0.11 0.23 0.38 0.06 0.75 0.05 0.31 0.33 0.99 0.77
35%
60%
60%
Site
Species
Mound area cm2 (±SE)
Plant length cm (±SE)
F
R2
15%
Dryas octopetala Salix retusa Lotus alpinus Lotus corniculatus Anthyllis vulneraria Salix retusa Dryas octopetala Lotus alpinus Anthyllis vulneraria Lotus corniculatus Dryas octopetala Salix retusa Lotus alpinus Anthyllis vulneraria Lotus corniculatus
316.7 (±4.9) 157.5 (±3.3) 62.9 (±4.0) 41.3 (±3.2) 33.8 (±2.5) 220.8 (±5.3) 121.4 (±2.9) 52.4 (±3.0) 30 (±2.0) 20 (±1.4) 132.9 (±4.1) 132 (±4.4) 69.2 (±2.9) 30 (±2.1) 25.2 (±6)
65.8 (± 1.7) 60 (±2.0) 15.5 (±1.0) 12.3 (±0.9) 12.3 (±0.8) 73.3 (±2.5) 50 (±2.2) 15.4 (±1.0) 12.1 (±0.8) 13 (±1.1) 42.1 (±2.1) 41.5 (±2.5) 14 (±1.2) 11.3 (±0.7) 12 (±0.7)
1.2NS 8.8⁎ 5.8⁎⁎ 3.1NS 1.4NS 4.8NS 10.1⁎ 4.2NS 7.5⁎ 0.2NS 9.4⁎ 28.4⁎⁎ 16.5⁎ 24.2⁎⁎ 0.7NS
0.23 0.2 0.49 0.81 0.26 0.55 0.67 0.21 0.32 0.21 0.53 0.78 0.83 0.84 0.09
35%
60%
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Influence of alpine plants growing on steep slopes on sediment trapping and transport by runoff Francis Isselin-Nondedeu ⁎, Alain Bédécarrats Cemagref, UR EM, 2 rue de la Papeterie, BP 76, 38402 Saint Martin d'Hères Cedex, France Received 1 June 2005; received in revised form 13 February 2007; accepted 23 February 2007
Abstract Understanding the interactions between soil and the organisms that are conducive to decreasing sediment runoff is a great concern on highelevation ski trails. Intense rainfalls on steep slopes combined with soil formed on gypsum result in recurrent erosion. This study was conducted in the northern French Alps to determine the abilities of species: (1) to make mounds and (2) to trap sediment and thereby to control erosion at the slope scale. We also investigated relevant above-ground plant characteristics related to those abilities. Sediment runoff or deposition was investigated at small and large spatial scales. We assessed whether hoof prints in soil reflect sediment runoff at the slope scale by trapping sediment. Populations of plants growing on two slope angles (25° and 35°) and three vegetation cover densities (15%, 35%, 60%) were surveyed. An experiment was also conducted to measure the sediment deposit upslope of target species and over three months during the autumn. Small mounds were found upslope of the plant and sediment deposit measurements showed that they resulted from a sedimentation process. Nevertheless the species differed in their capacity to make mounds. Sesleria caerulea and Festuca alpina had the highest amount of sediment deposition over the experimental period. Among the plant characteristics, plant length was positively correlated with mound area, while the roundness index of the canopy was negatively correlated with mound height. Mound formation was also positively related to the number of tillers or shoots. Sediment accumulation in cow hoof prints was linked to runoff that occurred at the slope scale. Low deposition in hoof prints means low sediment runoff or a large deposition on mounds, due to the increase in vegetation cover. All the findings stressed that understanding the processes in action at larger scales requires studying processes at smaller scales. © 2007 Elsevier B.V. All rights reserved. Keywords: Biogeomorphology; Phytogenic mound; Plant morphology; Gypsum; Sediment runoff; Geomorphic agent
1. Introduction Human pressure on the mountain environment has increased since the beginning of the 1970s, with ski run construction and topographical adjustments, altering both ecological and geomorphological processes. Assisted revegetation aims at stabilizing bare soil subjected to erosion, but this remains difficult on steep erodible slopes. Therefore, knowledge of interactions between soil and organisms that are conducive to controlling erosion is an important concern in the mountain environment. Biogeomorphology combines research in ecology and geomorphology exploring relationships between organisms and geo⁎ Corresponding author. Current address: Peatland Ecology Research Group, Département de phytologie Pavillon Paul-Comtois, Université Laval QC, G1K 7P4 Canada. Tel.: +418 656 2131 3595; fax: +418 656 7856. E-mail address: [email protected] (F. Isselin-Nondedeu). 0341-8162/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2007.02.001
morphological characteristics of the environment (Naylor et al., 2002). The field of biogeomorphology encompasses phytogeomorphology and zoogeomorphology. The research issues that are addressed in these disciplines are the effects of biological communities on geomorphological processes through hydrodynamic, biostabilization, and sediment transport and production. Numerous studies have focused on soil protection using living organisms, searching for positive interactions with geomorphological processes (Naylor, 2005). The effects of vegetation on soil can be divided into two major related categories: bioprotection and bioconstruction (Naylor et al., 2002). Vegetation protects soil against erosion by reducing water erosion (Rey, 2003; Puigdefabregas, 2005) and increasing the infiltration rate in soil (Graeme and Dunkerley, 1993; Ziegler and Giambelluca, 1998; Wainwright et al., 2002). Plants shelter and fix the soil with their roots (Körner, 2002; Gyssels et al., 2005) or reduce the energy of raindrops with their
F. Isselin-Nondedeu, A. Bédécarrats / Catena 71 (2007) 330–339
331
Fig. 1. Location of the ski area of la Plagne in the northern French Alps.
canopy (Bochet et al., 1998). Also, vegetation can act as a physical barrier, altering sediment flow at the soil surface (Van Dijk et al., 1996; Lee et al., 2000). The way the vegetation is spatially distributed along the slopes is an important factor for decreasing the sediment runoff (Lavee et al., 1998; Calvo-Cases et al., 2003). This barrier effect can lead to the formation of structures called phytogenic mounds. Such structures were found on the upslope side of large strips of grass disposed perpendicular to the slope (Dabney et al., 1995; Meyer et al., 1995; Van Dijk et al., 1996; Abu-Zreig et al., 2004) or under plants (El-Bana et al., 2003). Several mechanisms are involved in mound formation: the deposition of soil particles beneath plant canopy after a decrease in blowing wind (El-Bana et al., 2003), the differential erosion rates in the close environment of the plant (Babaji, 1987; Rostagno and del Valle Puerto Madryn, 1988), or the deposition of sediment resulting from a decrease in overland water flow (Sanchez and Puigdefabregas, 1994; Bochet et al., 2000). In alpine environments, plants are often associated with convex-shaped soil as terraces or mounds resulting from solifluction processes (Bird, 1974; Caine, 1974). Few authors have taken an interest in the relationships between plant morphology and the effects on soil erosion. Nonetheless, some have shown that plant length and a complete canopy are important features for sediment trapping (Van Dijk et al., 1996). Animals also act as geomorphic agents at different spatial scales, structuring small patches of soil or landscape units (Butler, 1995, 2001), especially when they dig in soil or trample (Trimble and Mendel, 1995; Gutterman, 2003). Consequences can be positive or neutral for slope stabilization (Trimble and Mendel, 1995; Nash et al., 2003). In terms of bioconstruction, the beaver is an animal that is well known for its ability to build dams and therefore modify sediment fluxes in hydrologic systems (Butler and Malanson, 2005). All the above-mentioned studies on plant–mound interactions were conducted on shrubs or herbaceous plants in semi-arid or arid environments. Studies comparing the sediment accumulation among species are scarce, and to our knowledge no report has been made for alpine species, which leaves a gap in our understanding of the erosion responses of the mountain ecosystems.
In this study, we aimed at determining the ability of alpine plant species to retain sediment and decrease runoff at the slope scale and also the morphological attributes of plants which is involved in. Our hypothesis was that erosion control at the slope scale resulted from a sediment trapping process at the micro scale, i.e. the plant scale. Sediment deposition or runoff was measured over a long period by a survey of existing mounds and over a short period by measuring sediment accumulation over 3 months. In addition, we expected mound sizes to vary with differences in plant morphology. The study was done in a French subalpine area on steep ski slopes constructed on gypsum. 2. Material and methods 2.1. Study site description The study was carried out in the ski resort of la Plagne (45°33′ N, 06°40′ E) in the northern French Alps (Fig. 1), in August– November 2002. Mean annual precipitation and temperature at the ski resort are 1165 mm and 2.7 °C, respectively. Rainfall distribution between and within years shows two peaks, one in Table 1 Dates and total precipitation of the ten major events recorded in the study period at 1970 m, north face (la Plagne, Meteo France) Major events
Date
P-tot (mm)
1 2 3
07/08/2002 08/08/2002 19/08/2002 Total 02/09/2002 09/09/2002 Total 17/10/2002 21/10/2002 25/10/2002 Total 02/11/2002 03/11/2002 Total
30 17 15 165.4 10 13 71.8 43.5 30.5 33 156.7 29 35 158.8
4 5 6 7 8 9 10
332
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Fig. 2. Measurements of mound shape associated with individual plant and measurements of plant morphology (adapted from Bochet et al., 2000).
Spring, the other in late Summer to Autumn. Especially on the north face of the mountain, heavy showers are frequently observed, with intensity ranging from 2.5 to 30 mm/h (Rovéra, 1990). This represents geomorphic effective intensities that can remove and transport soil materials. Rainfall conditions at 1970 m north face are shown in Table 1. Field experiments were conducted on ski trails between 2200 and 2350 m. Vegetation on ski trails was sown during the restoration process between 1979 and 1981. The base of seed mixtures used for restoration contains the following species: Dactylis glomerata, Anthyllis vulneraria, Trifolium pratense, T. repens, cultivars of Festuca rubra, Poa alpina, Lotus corniculatus. The bedrock is made up of gypsum and associated weathering evaporitic stones. The initial uppermost soil was most often removed during trail construction. Nowadays, gypsum often outcrops and the soil layer is thin, ranging from 2 to 10 cm. Ski trail soil is characterized by a high proportion of sand and gypsum sediment (between 55% and 65%), and approximately 25% loam and 10% clay. Organic compost remains in some places, but the organic pool was below 3.3 kg O.M./m2. Ski trails are currently managed for ski during the winter season and grazing by dairy cows during the season of vegetation.
local scale. At the general scale, trail topography varied from 40° to 20°. The standing vegetation on the ski trails have a random distribution in space, with a slight tendency of grouping at many places, due to patches of grasses. A survey was first conducted to note the presence or absence of mounds resulting from a long period of plant–soil interaction above 30 individuals. Species were chosen among the most abundant that grown on ski slopes. They arose from natural colonization and seed mixtures used for slopes restoration. We grouped them into two a priori plant types: graminoid plants, i.e. grasses and sedges (Poaceae: Agrostis alpina, Dactylis glomerata, Deschampsia caespitosa, Festuca alpina, F. rubra cv., Poa alpina, Sesleria caerulea; Cyperaceae: Carex sempervirens) and non-graminoid plants, i.e. forbs, legumes and small shrubs (Rosaceae: Dryas octopetala; Fabaceae: Anthyllis vulneraria, Lotus corniculatus, L. alpinus, Trifolium hybridum; Salicaceae: Salix retusa). The shape of mounds located on the upslope side of the plant was measured above 10 individuals per species when mounds were present (Fig. 2). The maximum height, width and length of the mounds were determined in order to calculate indicators of sediment deposit: area (mA) and height (mH).
2.2. Mound survey and measurements
2.3. Plant morphology
We investigated 25° and 35° slopes with 15%, 35% and 60% vegetation cover. In each situation, three 100-m2 plots were selected. Slope angles were measured with clinometers at the
The selected species naturally differ in their life-forms and canopy architectures. Sesleria caerulea and Carex sempervirens are densely tufted, forming large clumps from rhizomes, with more
Fig. 3. Left: small mound (8 cm in height, 12 cm in length) above a native alpine tussock grass (Festuca rubra subsp. rubra) growing on east-facing 30° ski slope, 2350 m a.s.l. Rocks are fragmented gypsum and rauchwacke. Right: water stored in cow hoof prints after rainfall (Autumn 2003, la Plagne, Savoie, French Alps).
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runoff and deposition measurements at the slope scale, we investigated natural micro-catchments such as hoof prints. Twenty five neat, 1-year-old dairy-cow hoof prints were selected in the three lower quarters of slope. The hoof prints measured approximately 15 cm in length (heel–toe distance), 10 cm in width, 10–15 cm in depth. Sediment deposit was measured with graduated sticks placed in the centre. 2.5. Data analysis
Fig. 4. Mean proportion of plant individuals with mounds at the upslope side of their canopies for the species growing on the 25° and 35° slopes. Results of the two-way ANOVA; the species D. glomerata, D. caespitosa and T. hybridum were not included in the analysis, as they have been found only on the 25° slopes.
or less matted leaves and stems stiff. Deschampsia caespitosa and Dactylis glomerata are clumped grasses, often forming tussocks with matted dry leaves at base. The other grasses, Agrostis alpina, Festuca alpina, F. rubra cv., Poa alpina, are tufted, moderately compact, especially the cultivar of F. rubra, with slender stems usually arching. Non-graminoid species have various aboveground architectures. Both Dryas octopetala and Salix retusa are semi-shrubs, with woody stems trailing and freely rooting, often forming clumps or mats. Anthyllis vulneraria, Lotus corniculatus and Trifolium hybridum have stems erect, from a branched base, canopies often weakly clumped. L. alpinus is tufted, more compact, slender stems creeping. Canopy dimensions were measured on 10 individuals per species of approximately the same height (9–12 cm) associated to the mounds selected. Two dimensions were considered: D1 corresponded to the length of the standing canopy developed perpendicular to the slope (Fig. 2), D2 was the measurement of the canopy along the slope. We used the D1 to D2 ratio (pR) as a simple index of plant shape assumed to interact with sediment transport by runoff. This pR represents a roundness index greater than 0, a value of 1 indicating a circular form. Tillers or shoot numbers for graminoid and non-graminoid species, respectively, were counted by area (cm2).
The goal of the statistical analysis was (i) to determine the plant's ability to retain sediment at the micro-scale and to decrease runoff at the slope scale and (ii) to identify plant morphological characteristics that influence sediment deposit. ANOVAs were first performed on the data resulting from the survey of mounds, with the slope angle, the species and the plant a-priori groups as factors. Normality of the dependent variables was confirmed using Liliefor's modification of the Kolmogorov–Smirnov test. Statistical analyses were carried out with the statistical package SYSTAT 11.0. We also performed a logistic regression on the two dimensions of plant canopy (D1, pR) to test whether the presence of mounds was related to plant form. We then compared how effective the species were by calculating means of sediment accumulation. We tested both the ability of hoof prints to catch sediment and the effect of the vegetation cover in decreasing erosion. Finally, to test the relations between plant morphology and mounds, we performed a principal component analysis (PCA). 3. Results 3.1. Mounds survey Small mounds made up of eroded particles of gypsum and organic matter, were observed at the upslope side of the plant canopy (Fig. 3). However, the occurrence of mounds at the upslope varied among the surveyed plant species (Fig. 4). There were significant effects of the species on the occurrence of mounds (F = 38.6; P b 0.001) as well as species–slope interactions (F = 72.9; P b 0.001). Indeed, among species sampled on
2.4. Sediment trapping dynamics Sediment accumulation at the micro-scale was estimated from late Summer to mid-Autumn 2002 (August 7–November 4) on the plots of the 25° slope. Graduated sticks were placed on mounds above the following target species: Agrostis alpina, Festuca alpina, F. rubra cv., Sesleria caerulea, Carex sempervirens and Dryas octopetala. The experiment was conducted on five individuals by species for three replicates by vegetation cover density (15%, 35% and 60%). The heights of depositions were measured after substantial rainfall events in order to explore the dynamics of sediment in relation to precipitations. For sediment
Fig. 5. Mean proportion of plant individuals with mounds at the upslope side of their canopies for the two groups of plants growing on the 25° and 35° slopes. Results of the two-way ANOVA; the species D. glomerata, D. caespitosa and T. hybridum were not included in the analysis, as they have been found only on the 25° slopes.
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Fig. 6. Height of sediment deposit above the target species after 90 days of experimentation. Columns are mean (± SE) height of sediment accumulation upslope of the plant canopy, and letters indicate significant differences between species (ANOVA–Tukey's HSD test, α = 0.05). The slope is 25° — black column: 15% vegetation cover; white column: 35%; grey column: 60%.
the 25° slope, the five following species: Sesleria caerulea, Carex sempervirens, Agrostis alpina, Festuca alpina and F. rubra were more effective in forming mounds, as no individual was found without a mound. On the 35° slope, only S. caerulea did the same. The less effective species were Deschampsia caespitosa, Salix retusa and Trifolium hybridum on the 25° slope, Anthyllis vulneraria and Lotus corniculatus on the 35° slope. Moreover, the grouping of species in graminoid vs. nongraminoid has a significant effect on the occurrence of mounds (F = 124.2; P b 0.001; slope × plant group: F = 7.4; P b 0.01), the occurrence of mounds was higher at the upslope of graminoid species on both slope angles (Fig. 5). The presence of mounds was significantly related to the plant form, both for all graminoids (plant length: Chi2 = 19.5, P b 0.0001; plant area: Chi2 = 4.6, P b 0.05; plant roundness: Chi2 = 60.5, P b 0.0001) and for all non-graminoid species (plant length: Chi2 = 8.9, P b 0.01; plant area: Chi 2 = 4.4, P b 0.05; plant roundness: Chi2 = 69.3, P b 0.0001). The presence of mounds at the upslope of the different plants increased as plant length or area increased. The existence of mounds may reveal a past or present sediment trapping process. 3.2. Effectiveness of species in sediment trapping The dynamics of sediment trapping at the micro scale is expressed in total height of sediment accumulation upslope of the plant after 90 days (Fig. 6). Carex sempervirens, Festuca alpina and Sesleria caerulea had the highest sediment accumulation at the end of the experiment, between 1 cm and 2.5 cm. Agrostis alpina and Festuca rubra showed lower sediment deposits, whereas Dryas showed a gap in sediment accumulation in 35% vegetated plots. The Fig. 7 shows the
details of sediment accumulations measured before and after the major precipitation events (Table 1) for all the target species. Generally, higher accumulations were observed after rainfalls of 29, 30, and 30.5 mm. Sediment deposit values under zero indicate a loss of sediment from the mounds after the rainfall event. This was the case for most of the species after the rainfall events of 33, 35 and 43.5 mm. Higher losses at the micro scale were recorded in 15% vegetation plots, whereas in 60% vegetation cover, there was almost no loss of sediment. The species that showed the highest deposits after major rainfall events were Sesleria caerulea, Carex sempervirens and Festuca alpina. On the contrary, Festuca rubra and Dryas octopetala showed losses of sediment from their mounds and for lower precipitation events than the other species. After the experimentation period, hoof prints trapped sediment (Fig. 8), but as vegetation cover increased, the height of sediment deposits decreased. With 60% vegetation cover, the sediment deposit was approximately 2.4 times less than on slopes with 15% and 35% vegetation cover. The comparisons of measurements in relation with the major rainfall events show that sediment deposits increased as rainfall increased (Fig. 9). Sediment deposits upslope of plants, deposits in hoof prints and initial mound height on the 25° slope are plotted together in Fig. 8. With an increase in vegetation cover, subsequently the mound height was higher and the sediment deposit above mounds and in hoof prints was lower. 3.3. Plant morphology and mound shape relationships Two main correlations were found between plant form and mound shapes (Table 2). Concerning graminoid species, mound area was positively correlated with plant length (R = 0.74) and
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Fig. 7. Sediment deposits on phytogenic mounds measured after major rainfall events (see Table 1) in the plots with 15%, 35% and 60% vegetation cover (25° slope). Bars are means (± SE) of sediment deposit or removal.
mound height was negatively correlated with plant roundness (R = − 0.58). In addition, for the non-graminoid species, mound area was positively correlated with plant length (R = 0.87), whereas mound height was negatively related with plant roundness (R = − 0.46). The number of graminoid tillers was positively related both with the height and the area of mounds found upslope of plant canopy (R = 0.63 and 0.59, respectively). However, no close correlations were found between the number of shoots of other plant types and mound variables. The same type of correlation was found for the analyses of the dynamics of sediment accumulation (Table 3). For graminoid species, the mound area was also positively correlated with plant length
(R = 0.65). The initial mound height was negatively correlated with plant roundness (R = − 0.53), as was height after sediment accumulation (R = − 0.56). The results of sediment accumulation above Dryas were quite similar: positive correlation between mound area and plant length (R = 0.56), negative correlations between initial mound height and plant roundness (R = − 0.46), as well as between mound height after sediment accumulation and plant roundness (R = − 0.44). We also observed internal correlation between mound height and area. Detailed relations between mound area and species traits are listed in Appendices A and B. Depending on the species, sites and plant traits, the statistical significance of these relations varied.
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4. Discussion
Table 2 Correlation matrix of PCA based on plant morphology and mound parameters
4.1. Species, plant morphology and effectiveness in sediment trapping
Graminoid
D1
pR
mH
mA
Tn°
Plant length D1 Plant roundness Mound height Mound area Tillers number
1.00 − 0.30⁎⁎ 0.31⁎⁎ 0.74⁎⁎⁎ 0.75⁎⁎⁎
.
. . 1.00 0.57⁎⁎⁎ 0.63⁎⁎
. . . 1.00 0.59⁎⁎
. . . . 1.00
Non-graminoid Plant length D1 Plant roundness Mound height Mound area Shoot number
1.00 − 0.08 0.35⁎⁎ 0.87⁎⁎⁎ 0.57⁎⁎
.
. . 1.00 0.50⁎⁎ 0.12
. . . 1.00 0.09
. . . . 1.00
Other studies that have investigated phytogenic mounds on steep slopes with erodible bedrock such as gypsum conducted experiments in semi-arid and arid ecosystems (Sanchez and Puigdefabregas, 1994; Bergkamp, 1998; Bochet et al., 2000; ElBana et al., 2003). The striking result of the research herein is that alpine plant species growing on steep slopes with gypsum can have associated mounds. Sediment deposition caused by a local decrease in overland water flow is assumed to be involved in this type of bioconstruction (Babaji, 1987; Sanchez and Puigdefabregas, 1994; Bochet et al., 2000). The observations at the micro scale confirm that phytogenic mounds were formed by the accumulation of sediment upslope of plant canopy. Moreover, the comparison between the height of accumulations after 3 months and the sediment deposits after major rainfall events that occurred during this period indicates that sediment deposits occurred mainly during the rainfalls around 30–35 mm. However, the different results of sediment accumulations obtained at various time scales show that species differed in their ability to form mounds and to trap sediment. Results from the mounds survey showed that graminoids and non-graminoids have had different behaviours. The highest proportion of plants with mounds was found upslope of the graminoid. Thereafter, among the graminoid species, Carex sempervirens and Sesleria caerulea were the most effective species with the highest mounds in two steep slope conditions. They retained large amounts of sediment during the experiment. The two seeded cultivars of Festuca rubra and Poa alpina used for revegetation seem to be less efficient in forming large mounds. Among nongraminoid species, Dryas and Salix had the largest mound area in contrast with the other species. However, despite this large mound area, Dryas was ineffective in retaining sediment during the experimental period. The organization of the canopy might explain these differences, particularly tillers or stem density. For
Fig. 8. Comparisons of sediment accumulations at different space and time scales: mound formation upslope of plants after 90 days (mounds t90), height of sediment deposit in hoof prints and the initial height of mounds before deposition measurements (mounds t0). Points are total means of plots (25° slope).
1.00 − 0.58⁎⁎⁎ − 0.37⁎⁎⁎ 0.11⁎
1.00 − 0.44⁎⁎⁎ − 0.08 0.14
Abbreviations for column are detailed in the lines. D1 measures the length of the canopy perpendicular to the slope. Values in column are Pearson coefficient with significance level (when significant: ⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001). Analyses were performed on graminoid species n = 410 (PCA1: 61.2%; PCA2: 22.8%) and (b) non-graminoid species n = 330 (PCA1: 56.3%; PCA2: 28.9%). All data have been log-transformed.
example, Dryas canopy was diffuse and porous, with large spaces between the creeping branches. Similarly, Festuca rubra cultivars presented spaced tillers that crept over soil. These results are consistent with past research which highlighted that vegetation or canopy completeness is a key feature in trapping sediments (Meyer et al., 1995; Van Dijk et al., 1996; Bochet et al., 2000). These discrepancies in terms of species efficiency can also be attributed to the form of the canopy, especially length and roundness. Various authors demonstrated that grasses were much more efficient when they developed perpendicular to the slope (Pethick et al., 1990; Abu-Zreig, 2001; Abu-Zreig et al., 2001). Our results showed that plant roundness was inversely correlated with sediment trapping and mound formation. This is in agreement with laboratory work (Babaji, 1987) and field measurements on erosion control (Bochet et al., 2000; Casermeiro et al., 2004). Because of interactions between hydrodynamics at the soil surface and plant form, water flows erode the upslope side as well as the downslope side of a plant when its canopy is circular.
Fig. 9. Sediment deposits in hoof prints measured after major rainfall events (see Table 1) in the plots with 15%, 35% and 60% vegetation cover. Bars are means (±SE) of sediment deposit or removal.
F. Isselin-Nondedeu, A. Bédécarrats / Catena 71 (2007) 330–339 Table 3 Correlation matrix of PCA based on plant morphology and mound parameters before (t0) and after 90 days of sediment accumulation (t90) Graminoid Plant length D1 Plant roundness Mound height t0 Mound area Mound height t90 Tillers number
D1 1.00 − 0.36⁎⁎⁎ 0.32⁎ 0.65⁎⁎⁎ 0.04 0.55⁎⁎
pR . 1.00 − 0.53⁎⁎⁎ − 0.34⁎⁎ − 0.56⁎⁎⁎ 0.17
mHt0
mA
Ht90
Tn°
. . 1.00 0.64⁎⁎⁎ 0.22⁎⁎ 0.62⁎⁎⁎
. . . 1.00 0.22⁎⁎ 0.43⁎
. . . . 1.00 0.51⁎⁎
. . . . . 1.00
Dryas Plant length D1 Plant roundness Mound height t0 Mound area Mound height t90
1.00 . − 0.22 1.00 0.39⁎⁎ − 0.46⁎⁎⁎ 0.56⁎⁎⁎ − 0.31⁎ − 0.21 − 0.44⁎⁎⁎
. . . . 1.00 . 0.90⁎⁎⁎ 1.00 − 0.06 − 0.26
. . . . 1.00
Abbreviations for column are detailed in the lines. D1 measures the length of the canopy perpendicular to the slope. Values in column are Pearson coefficient with significance level (when significant: ⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001). Analyses were performed on graminoid species n = 270 (PCA1: 47.9%; PCA2: 27.7%) and Dryas octopetala n = 40 (PCA1: 38.2%; PCA2: 26.7%). All data have been log-transformed.
At the large scale, the findings support general knowledge on the positive effect of vegetation cover in controlling soil erosion. The decrease in sediment accumulation combined with a decrease in precipitation or with an increase in vegetation cover on the slope emphasizes that erosion is dependent on runoff. The same process was demonstrated on steep slopes at the same study site for seed runoff (Isselin-Nondedeu et al., 2006). The absence of accumulation on mounds means that there was no sediment transport by runoff at the slope scale. Moreover, similar patterns of sediment accumulation in the hoof prints confirm that the decrease in accumulation observed after rainfall events was related to the stabilizing effect of vegetation on soil (Körner, 2002; Naylor et al., 2002; Gyssels et al., 2005). This effect was particularly observed at the 60% vegetation cover, where there was almost no deposition of sediment in comparison with the other vegetation covers. Another explanation might be the decrease of the contributing areas to runoff i.e. bare patches between plants. This one is a parameter often more important than the vegetation cover itself (Lavee et al., 1998; Calvo-Cases et al., 2003). In the 60% vegetation cover, this contributing area was probably too small to generate highly erosive runoff volumes able to detach soil particles and transport sediment downslope. The discussion in the previous section suggests that the relationships between plant morphology and effectiveness in sediment trapping is a parameter either important than the contributing areas when one considers the organization of the vegetation. In our experiments, hoof prints measured part of the runoff at the slope scale and showed the same trend as that which occurred at the plant–mound scale: low deposition in hoof prints means low runoff and large deposition on mounds (Figs. 7, 8 and 9). The height of mounds before the 90-day measurements showed no direct relation with the other trends (Fig. 9). Nevertheless, as they result from plant–soil interactions on a longer time scale, mound height is
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related to vegetation cover; when the vegetation cover increases the input of sediment decreases and also their detachment from the mounds. This emphasized that understanding large-scale erosion requires studying processes occurring at smaller scales. 4.2. Plant morphology and alpine geomorphic processes The phytogenic mounds we observed upslope of plants might result in part from the solifluction process. Solifluction, i.e. when soil mass creeps downslope, is a frequent geomorphic process, long observed in alpine and arctic environments, which reworks soil and leads to terrace formation (Bird, 1974; Caine, 1974). These structures result from the interplay between pedogenic processes and climatic constraints such as wetting–drying and freezing–heating cycles. The effect of vegetation on soil material deposition highlights the potential interaction between vegetation, plant morphology, erosion and solifluction processes. Körner (1999) made the point that terrace formation could counteract surface erosion. The short period during which we measured sediment accumulation is a limitation that does not allow us to categorically conclude on the process of mound formation or on the persistence of these mounds. This is particularly relevant because all the plots are covered every winter season by snowgrooming machines. The negative effects of the machines have been demonstrated in other studies (Wipf et al., 2005), as they compact snow as well as soil and limit plant growth. However, our surveys on the different plots showed the existence of old mounds associated with plants. The mounds were small and might be bigger in absence of snow grooming; therefore further experiments are necessary to evaluate the impacts of snow cover, human activities and animal disturbances. 4.3. Interactions between hoof prints and sediment transport by runoff During experiments on our study site, we used cow hoof prints in soil as indicators of sediment runoff at the slope scale. They behave like micro-catchments for sediment. This stresses the role of animals as geomorphic agents (Trimble and Mendel, 1995) and their potential role with regard to erosion. In addition, we have noted accumulation of water in hoof prints during rainfall and 1 day after, forming temporary micro-ponds (see Fig. 3). A similar process has been observed at larger scales when beavers create ponds and zones for sediment accumulation by constructing dams across river channels (Butler and Malanson, 2005). Results on the role of structures made by animals in relation to soil erosion are often contradictory, however, in desert environments, holes created by porcupines can accumulate rainfall water, seeds and organic matter (Gutterman, 2003). In addition, according to Littlemore and Barker (2001), trampling reduces vegetation cover, diversity, and biomass and damages roots, which may counteract the positive effects of plant–mound interactions. Further experiments are required to better understand the effects of grazing on phytogenic mounds as well as their duration in time. It is interesting to note that ski trails function as corridors, in agreement to the view of Butler (2001), channeling water flows, sediment, debris and cattle.
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Acknowledgements
Appendix B
We extend our thanks to Freddy Rey, Thierry Dutoit, and the reviewers for helpful comments and to Benjamin Ollier and Pascal Tardif for technical support in the field. We are also grateful to the ski resorts workers of la Plagne for allowing field experiments on ski trails.
Plant morphology and mound relationships among species (35° slope). Mean values of mound area, plant length and significance of correlation (⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001) are listed below.
Appendix A
Site
Species
Mound area cm2 (± SE)
Plant length cm (±SE)
F
R2
15%
Carex sempervirens Sesleria caerulea Agrostis alpina Festuca alpina Poa alpina Festuca rubra Carex sempervirens Sesleria caerulea Agrostis alpina Festuca alpina Festuca rubra Poa alpina Carex sempervirens Sesleria caerulea Festuca alpina Agrostis alpina Festuca rubra Poa alpina
117.4 (± 3.8)
17.7 (±1.1)
12.2⁎⁎
0.44
72.7 (±3.0) 34.5 (±2.1) 30.9 (±4.6) 18.6 (±1.3) 18.4 (±1.5) 88.3 (±3.5) 80.2 (±3.5) 39.9 (±7.1) 35.9 (±2.1) 21.3 (±1.6) 10.1 (±1.2) 167.5 (±4.9) 161.1 (±4.6) 57.9 (±2.7) 47.7 (±2.1) 30.5 (±1.7) 17.4 (±1.4)
15 (±1.2) 9.9 (±0.8) 8.3 (±0.7) 6.6 (±0.5) 8.3 (±0.6) 20.6 (±1.2) 21 (±1.2) 9.9 (±0.6) 9.4 (±0.6) 9.3 (±0.7) 6.9 (±0.8) 18.7 (±1.4) 18.6 (±1.4) 11.1 (±0.9) 10.9 (±0.9) 10.7 (±0.7) 8 (±0.7)
7.5⁎ 7.3⁎⁎ 14.4⁎⁎ 1.7NS 7.3⁎ 3.4NS 24.7⁎⁎⁎ 1.5NS 4.8⁎ 3.9NS 1.9NS 5.8⁎ 9.6⁎⁎ 18.5⁎⁎⁎ 1.1NS 7.4⁎ 5.1⁎⁎
0.48 0.52 0.64 0.36 0.52 0.34 0.76 0.16 0.32 0.44 0.19 0.4 0.55 0.7 0.14 0.46 0.54
Plant morphology and mound relationships among species (25° slope). Mean values of mound area, plant length and significance of correlation (⁎P b 0.05, ⁎⁎P b 0.01, ⁎⁎⁎P b 0.001) are listed below. 35% Site
Species
15% Sesleria caerulea Carex sempervirens Deschampsia caespitosa Festuca alpina Agrostis alpina Dactylis glomerata Festuca rubra Poa alpina 35% Sesleria caerulea Carex sempervirens Agrostis alpina Festuca alpina Deschampsia caespitosa Dactylis glomerata Festuca rubra Poa alpina 60% Carex sempervirens Sesleria caerulea Festuca alpina Agrostis alpina Deschampsia caespitosa Festuca rubra Poa alpina Dactylis glomerata
Mound area cm2 (±SE)
Plant length F cm (±SE)
130 (± 4.3) 107.6 (±3.6) 73.2 (± 3.3) 71 (± 2.8) 54.5 (± 2.3) 42.4 (± 1.9) 34.3 (± 1.6) 24.3 (± 1.4) 134.5 (±3.8) 107.6 (±3.6) 66.2 (± 2.8) 65.6 (± 2.9) 55 (± 2.6) 42.4 (± 1.9) 32.4 (± 7.7) 19.7 (± 1.7) 187 (± 4.7) 153.5 (±3.5) 73.7 (± 3.0) 45.8 (± 2.5) 33 (± 1.9) 30.5 (± 2.2) 24.6 (± 1.2) 22.2 (± 1.8)
17 (±1.1) 16.8 (±1.3) 9.2 (±0.7) 9.5 (±0.8) 8.7 (±0.6) 15.2 (±1.2) 12.1 (±1) 6.7 (±1.4) 16.8 (±1.3) 7.7 (±0.7) 8.9 (±0.8) 9.7 (±0.9) 17 (±1.1) 14 (±0.8) 13.2 (±0.8) 9.3 (±0.7) 18.6 (±1) 9 (± 0.8) 10.8 (±0.7) 19.4 (±0.9) 10.4 (±0.8) 12.4 (±0.8) 10.3 (±0.5) 8.8 (±0.5)
22.3⁎⁎⁎ 10.2⁎ 18.5⁎⁎ 7.8⁎⁎ 5.2⁎⁎ 5.8⁎ 55.7⁎⁎⁎ 7.9⁎ 27.7⁎⁎⁎ 8.1⁎ 6.8⁎ 0.7NS 6.5NS 0.4NS 41.6⁎⁎⁎ 4.2NS 13.3⁎⁎ 9.1⁎ 1.8NS 6.6⁎ 2.5NS 6.2⁎⁎ 0.01NS 0.01NS
R2 0.73 0.38 0.82 0.48 0.4 0.49 0.87 0.66 0.77 0.2 0.27 0.16 0.77 0.12 0.84 0.51 0.62 0.21 0.19 0.26 0.46 0.44 0.1 0.08
Site
Species
Mound area cm2 (±SE)
Plant length cm (± SE)
F
R2
15%
Dryas octopetala Salix retusa Lotus alpinus Lotus corniculatus Anthyllis vulneraria Trifolium hybridum Dryas octopetala Salix retusa Lotus alpinus Trifolium hybridum Anthyllis vulneraria Lotus corniculatus Dryas octopetala Salix retusa Lotus alpinus Anthyllis vulneraria Lotus corniculatus Trifolium hybridum
385.2 (± 8.1) 301.3 (± 5.5) 75.1 (± 3.1) 41.7 (± 1.8) 41.2 (± 2.2) 29 (±2.1) 375.1 (± 6.8) 194.2 (± 4.1) 59.3 (± 3.1) 34 (±1.8) 31.9 (± 2.2) 24.3 (± 1.6) 313.8 (± 7.2) 245 (± 5.5) 37.5 (± 2.1) 32.6 (± 2.3) 29.3 (± 2.3) 26.4 (± 1.7)
73.7 (± 3.1) 63.7 (± 2.3) 13.7 (± 0.8) 13.8 (± 0.9) 11.2 (±0.8) 8 (±0.4) 73.5 (± 2.6) 66.7 (± 1.6) 14.6 (± 0.9) 10.2 (± 0.6) 9.9 (±0.6) 12.9 (± 0.9) 73.7 (± 3.1) 57 (±2.6) 13.3 (± 0.9) 10.7 (± 0.7) 10.7 (± 0.8) 8.6 (±0.6)
19.9⁎⁎ 3.8NS 3.6⁎ 8.4⁎ 2.4NS 16.1NS 18.7⁎ 0.02NS 1.1NS 1.2NS 3.7⁎ 0.2NS 11.8⁎ 0.2NS 1.8NS 1.5NS 176.6⁎⁎ 10.4⁎
0.83 0.66 0.37 0.67 0.17 0.88 0.73 0.05 0.11 0.23 0.38 0.06 0.75 0.05 0.31 0.33 0.99 0.77
35%
60%
60%
Site
Species
Mound area cm2 (±SE)
Plant length cm (±SE)
F
R2
15%
Dryas octopetala Salix retusa Lotus alpinus Lotus corniculatus Anthyllis vulneraria Salix retusa Dryas octopetala Lotus alpinus Anthyllis vulneraria Lotus corniculatus Dryas octopetala Salix retusa Lotus alpinus Anthyllis vulneraria Lotus corniculatus
316.7 (±4.9) 157.5 (±3.3) 62.9 (±4.0) 41.3 (±3.2) 33.8 (±2.5) 220.8 (±5.3) 121.4 (±2.9) 52.4 (±3.0) 30 (±2.0) 20 (±1.4) 132.9 (±4.1) 132 (±4.4) 69.2 (±2.9) 30 (±2.1) 25.2 (±6)
65.8 (± 1.7) 60 (±2.0) 15.5 (±1.0) 12.3 (±0.9) 12.3 (±0.8) 73.3 (±2.5) 50 (±2.2) 15.4 (±1.0) 12.1 (±0.8) 13 (±1.1) 42.1 (±2.1) 41.5 (±2.5) 14 (±1.2) 11.3 (±0.7) 12 (±0.7)
1.2NS 8.8⁎ 5.8⁎⁎ 3.1NS 1.4NS 4.8NS 10.1⁎ 4.2NS 7.5⁎ 0.2NS 9.4⁎ 28.4⁎⁎ 16.5⁎ 24.2⁎⁎ 0.7NS
0.23 0.2 0.49 0.81 0.26 0.55 0.67 0.21 0.32 0.21 0.53 0.78 0.83 0.84 0.09
35%
60%
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