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Quad bike impacts on vegetation and soil physicochemical properties in an arid ecosystem
Acta Oecologica 97 (2019) 14–22
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Quad bike impacts on vegetation and soil physicochemical properties in an arid ecosystem
T
Ana L. Navas Romeroa,∗, Mario A. Herrera Morattaa, Antonio D. Dalmassoa, Agustina Barrosb a Instituto Argentino de Investigación en Zonas Áridas (IADIZA), Centro Científico Tecnológico (CCT) CONICET, Mendoza. Av. A. Ruiz Leal s/n Parque General San Martín, Mendoza, CP, 5500, C. C. 503, Argentina b Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), Centro Científico Tecnológico (CCT) CONICET, Mendoza. Av. A. Ruiz Leal s/n Parque General San Martín, Mendoza, CP, 5500, C. C. 503, Argentina
A R T I C LE I N FO
A B S T R A C T
Keywords: All-terrain Recreational ecology Trampling Off-road vehicles
The increase use of Off-Road Vehicles (ORVs) coupled with the absence of control strategies have led to extensive use of natural areas for the creation of recreational trails. We assessed the effects of quad bikes on vegetation and soils in an arid ecosystem subjected to intensive quad bike use. We selected randomly eight traffic sites (disturbed sites) and eight adjacent control sites (undisturbed sites). Plant cover and composition were assessed with the point-intercept method and phytosociological census. In each site, we collected soil samples to assess soil physicochemical properties, including apparent specific weight (ASG), actual specific weight (RSG), porosity, texture, electric conductivity (EC) and pH. Soil compaction was measured at 30 spaces points per site. Plant cover and richness was significant lower in disturbed sites, with only four species present in areas subjected to disturbance. There were also changes in species dominance, with native perennial shrubs and grasses characterizing the undisturbed sites while the disturbed sites were mainly dominated by the invasive exotic herb Salsola kali. ORVs traffic also affected soil physicochemical properties including soil compaction, ASG, EC and soil pH. Soil compaction was more than double in disturbed sites and ASG tended to be higher under this condition. The EC was significantly higher and soil pH was significantly lower in the disturbed sites. Reduced vegetation cover and changes to soils physicochemical properties on quadbike trails highlights the impacts of ORVs in the landscape and the need to develop management strategies to minimize disturbance from ORVs on vegetation and soils.
1. Introduction
et al., 2010; Sun and Walsh, 1998). Common impacts on vegetation cited in the literature includes reduced vegetation cover, loss of species diversity and changes in species composition (Buckley, 2004; Hosier and Eaton, 1980). Impacts on soils includes compaction, muddiness, water runoff, and erosion (Hammitt et al., 2015; Leung and Marion, 2000; Webb and Wilshire, 1983). Impacts on chemical conditions includes a decrease in soil pH that can affect the bioavailability of nutrients for plants because there is less cation exchange capacity (Kissling et al., 2009). This can result in greater electric conductivity on soils which can lead to higher salinity. Soil salinization can cause desertification, which is considered one of the major ecological threats for arid ecosystems (Acosta et al., 2011; Ardahanlioglu et al., 2003; Rezapour, 2014; Zheng et al., 2009). Most of the studies of ORVs impacts are concentrated in the Northern Hemisphere, including USA (e.g. Anders and Leatherman, 1987; Belnap, 2002; Lei, 2004; Kelly, 2014) and different countries in
The promotion and demand for leisure activities, has resulted in an increase of outdoor sports in natural environments around the world. This includes motor-based activities, such as off-road vehicles (ORVs), including quad bikes, trail bikes, four-wheel drives that use natural environments to conduct these activities. ORVs are currently one of the world's most popular recreational and sports practices, and it continues to grow (Schlacher et al., 2008; Schlacher and Thompson, 2008; Smith and Burr, 2011). Off road activities can have social impacts, including social conflicts, safety concerns; as well as environmental impacts, such as impact on wildlife, damage to vegetation and trail degradation (Havlick, 2002; Newsome et al., 2013; Taylor, 2006). Although the impacts from ORVs are known to be highly degrading (Belnap, 2002), research on the physical and environmental impacts caused by different ORVs activities remains limited (Cole, 1993; Monz
∗
Corresponding author. E-mail address: [email protected] (A.L. Navas Romero).
https://doi.org/10.1016/j.actao.2019.04.007 Received 15 February 2018; Received in revised form 21 February 2019; Accepted 25 April 2019 Available online 02 May 2019 1146-609X/ © 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. Location of the study area to assess the impacts from quad bikes on vegetation and soils in the Precordillera region in Mendoza, Argentina. The maps include information about the area surveyed as well as the recreational trails used by ORVs vehicles and other activities in the region. This information has been obtained through wikiloc and runstatic volunteered geographic information platforms.
to the traits of the species present in the community, as traits can reflect plant ecological strategies to tolerate disturbance (BernhardtRömermann et al., 2011; Díaz et al., 2007). For example, slow-growing plant species may be more susceptible to trampling or vehicular damage compared to annual fast-growing plants (Bernhardt-Römermann et al., 2011; Garnier et al., 2004). Some life forms, such as phanerophytes and chamaephytes, which have its buds above the soil surface, are very vulnerable to disturbance as they are directly exposed to trampling (Bernhardt-Römermann et al., 2011). Taller plants with erect form, such as many shrubs, can be more sensitive to disturbance because stems can be easily broken compared to prostrate herbs or grasses with a flexible stem (Liddle, 1997). Differences on plants’ susceptibility to damage can result in changes in species composition after disturbance. These include the loss of native resident species less resistant to physical disturbance due to its morphological characteristics (i.e. erect form, phanerophytes, slow growing) and the facilitation of trampling resistant species, such as many ruderal & alien plants (Barros et al., 2015; Burden and Randerson, 1972; Kuss, 1986; Liddle and Grieg-Smith, 1975). The shift in species dominance from native residents to ruderal and alien species, is also related to the competitive ability of the new species to suppress residents, including producing large number of seeds and their adaptation
Europe (e.g. Selva et al., 2011; Iglesias-Merchán et al., 2016; Psaralexi et al., 2017). Less research has been conducted in the Southern Hemisphere and only concentrated on the east coast of Australia (Schlacher and Thompson, 2008; Sheppard et al., 2009; Lucrezi and Schlacher, 2010). In South America, to the best of our knowledge, there are no studies assessing ORVs environmental impacts. The lack of the studies in this region is a conservation concern, particularly for arid ecosystems due to the high susceptibility to anthropogenic disturbance and the increased popularity of the activity in the region (Jones et al., 2016). Arid ecosystems can be more severely affected and damaged by ORVs compared to humid ecosystems due to inherent differences in abiotic site conditions and biotic factors. In contrast to humid environments, arid ecosystems are characterized by stressful abiotic conditions including short water availability, high thermal amplitude and shallow loose soils as well as low plant cover and diversity, which can limit their capacity to recover from anthropogenic disturbance (Gamoun et al., 2018; Odum, 1969; Verstraete and Schwartz, 1991). Because in arid ecosystems plants are often slow growing, ecological impacts can be exacerbated, particularly when the type of disturbance, such as ORVs, is frequent & intense (Webb and Wilshire, 1983; Buckley, 2004; Hammitt et al., 2015). The severity of anthropogenic disturbance from ORVs is also related 15
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2.2. Field design
to meet the changing environmental conditions (Kuss, 1986; Levine et al., 2003). The multiple problems caused by this activity in continuous growth, the lack of control strategies and the scarcity of management actions, has led to extensive natural areas being used as trails for ORVs with the consequent impact to the natural environment as described above (Lathrop and Rowlands, 1983; Webb and Wilshire, 1983; Monz et al., 2010; Kelly, 2014). In addition, the popularity of the activity has led to the creation of unauthorized trails on conservation areas in public and commercial land tenures, as well as the proliferation of high profile events where competitors transit through natural plant communities not previously disturbed (Jones et al., 2016). For example, large scale events such as the Dakar Rally conducted in the Andes in South America (Jones et al., 2013), reflects the popularity of the activity for the region and the need to assess and minimize the environmental impacts associated to ORVs. The objective of this study was to evaluate the effects of ORVs, specifically quad bikes, in an arid ecosystem in western Argentina. We assessed the impacts of quad bikes on vegetation and soils physicochemical parameters. We hypothesize that the cumulative impacts of quad bikes have resulted in 1) changes in vegetation cover, richness and composition, with only few species tolerant to ORVs disturbance 2) changes on soils physicochemical properties, with some parameters highly sensitive to ORVs disturbance. The study was conducted in an area of high conservation value in a private land where the unauthorized trails constructed by ORVs are very popular and it continues to grow.
2.2.1. Vegetation sampling A block design was used to assess the effect of ORVs on vegetation and soils. We conducted the field assessment between June to August 2016. We selected at random a total of 8 zones, including 8 disturbed sites (considered trail circuits, disturbed) with similar degree of use and 8 control paired sites with no apparent disturbance (undisturbed) (Fig. 2). The distance between each zone was 100 m. In each trail circuit and paired site, we placed a 30 m long transect in a diagonal direction, where we assessed vegetation cover per species with the Modified Point-Quadrat method (Passera et al., 1983), with interceptions every 0.3 m. We selected a diagonal direction to be able to cover the total area impacted by each circuit trail, including the edges and center (Fig. 2). In each site, we conducted a phytosociological census recording the total number of species present including those not captured through the point intercept method. Within each site, we identified all vascular species present (including all grasses, herbs and shrubs), and we estimated vegetation cover and bare soil. We also calculated species richness and diversity per transect. Species diversity was estimated using the Shannon index (Hveg) with the following equation: s
H´ =
∑ pi. log2pi i=1
where S is the number of species; pi is the relative abundance of each n plant species ( Ni ); ni is the number species for I; and N is number total the individuals for all species detected.
2. Materials & methods
2.2.2. Physicochemical soil properties To analyze soils physico-chemical properties, we collected 16 soil samples per zone. Soil porosity, apparent specific gravity (ASG) and relative specific gravity (RSG) were determined from samples obtained using a metallic tube of known volume (165.5 cm3). Soil texture, electric conductivity (EC), and potential hydrogenation (pH) were obtained by composite samples collected with a punch of 5 cm3. Each sample (500 g) was collected from the topsoil surface (15 cm). Samples were stored in plastic bags for transfer and subsequent laboratory analysis. We measured soil EC and pH with distilled water using a conductivity meter and a pH meter, respectively. We performed texture analysis using the Guitián Ojea and Carballas (1976)method. To determine soil compaction, we divided each trail circuit into five locations: two controls outside each trail circuit (undisturbed, T1 and T2), two locations on the traffic path (M1 and M2) and one location at the middle of the trail circuit outside the traffic path (C) (Fig. 2). We measure compaction with a point geotester penetrometer (Mark: Part # 59035 Geotester Penetrometer) and according to the soil particle size, we modified the typified following Bucchi (1972). We performed 30 measurements per location in each trail circuit (Fig. 2).
2.1. Study area The study area is located in the district of Capdeville, adjacent to Villavicencio Natural Conservation Area in Las Heras Department, province of Mendoza, in Western Argentina (latitude 32°46′39.69″S, longitude 68°49′54.29″O) (Fig. 1). It is located in the morphostructural area of the Precordillera of the Andes. The local relief is relatively low, standing out the Cerro La Cal with 1090 a.m.s.l. The area belongs to the arid bioclimatic region (Martinez Carretero, 1985), characterized by an annual rainfall of 223 mm and an average annual temperature of 17 °C, 72% of which occurs in summer (October–March) (Rubí Bianchi et al., 2010). It is a high conservation value area, corresponding to the Monte phytogeographyc region, characterized by large levels of endemisms. Vegetation is dominated by native shrubs such as Larrea cuneifolia below 1100 m of altitude, & L. divaricata, from 1200 m to 1700–1800 a.m.s.l (Roig, 1976). Other dominant growth forms are native grasses including the species Pappophorum caespitosum, P. phillippianum (bitter grass), Sporobolus cryptandrus (sporobolo), Nassella tenuis (coiron) and Leptochloa dubia (Passera, 1983). Many of the species are nanophanerophytes, chamaephytes and hemicryptophytes, with few geophyte and terophyte species (Table 1). The topography of the area is characterized by a structure of folds & faults, with winding axes, stretching mostly in a NNE-SSW direction. The soils are classified as Entisols or Liticos on rocky outcrops, and Torripsamment on the shores river (Masotta and Berra, 1994). Off-road recreation is one of the most popular activities in the area with multiple trail circuits created for quad bikes & other recreation activities such as mountain biking, hiking and running. Based on popular volunteered geographic information platforms used in the region, such as wikiloc (https://www.wikiloc.com/) and runstatic (https:// www.runtastic.com/es/), an area of 7 km2 in the Precordillera of Mendoza is used for off-road recreation with 182 recreational trails. The average width of trails varies from 1 to 3 m and areas have been used for over 10 years. These trails are unauthorized trails created by users, not professionally designed.
2.3. Statistical analysis To assess the effects of ORVs on vegetation and soils we performed a series of statistical analyses using Infostat statistical package and Sigmaplot program (version 11) for graphics. First we compared plant cover, richness and diversity between disturbed and undisturbed sites using the non-parametric Mann Whitney test. To compare species composition (richness and abundance) between the disturbed and undisturbed sites we used the range abundance curve. This method allows to visually assess the distribution patterns of species abundance in plant communities, with the abundance of each species represented in a logarithmic scale (Whittaker, 1972; Magurran and McGill, 2011). To determine changes in soil physico-chemical properties (pH, EC, ASG, RSG and porosity) between disturbed and undisturbed sites, we 16
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Table 1 Floristic composition in disturbed and undisturbed sites. The hypsometric levels for both locations was 700 m above sea level. The soil type was loamy clay. Species
Growth habit
Life form Raunkiaer
Origin
Disturbed
Undisturbed
Atriplex argentina Pappophorum phillippianum Leptochloa crinita Portulaca confertifolia Echinopsis leucantha Salsola kali Bulnesia retama Larrea cuneifolia Pterocactus meglioli Suaeda divaricata Tribulus terrestres Disakisperma dubium Allionia incarnata Ehretia cortesía Tephrocactus aoracantha Cover %
Subshrub Perennial Grass Perennial Grass Perennial Herb Subshrub Succulent Annual Herb Shrub Shrub Herb Succulent Shrub Annual Herb Perennial Herb Perennial Herb Shrub Subshrub Succulent
chamaephyte hemicryptophyte hemicryptophyte hemicryptophyte succulent therophyte nanophanerophyte nanophanerophyte succulent nanophanerophyte therophyte hemicryptophyte therophyte geophyte succulent
South America America America South America South America Europe South America South America South America South America Eurasia America America South America South America
. . . . . + . . . + + . + . . 20
2.2 1.1 + + + + + + + + + + + + + 40
Fig. 2. Methodology used to examine the effects of the OVRs on vegetation and soils in trails along 5 km. Control transects were located off the trail, in T1 (Monte natural undisturbed right), and T2 (Monte natural undisturbed left). On the disturbed transect, measures were conducted in M1 (disturbed zone right), C (disturbed zone center), and M2 (disturbed zone left).
Whitney test, n = 8, W = 36, p < 0.001). Species diversity was 11 times lower on disturbed compared to undisturbed sites (H’ = 0.11 vs. H′ 1.29) (Table 1, Fig. 4). When visually comparing the range-abundance curves between the two conditions, we found that the shape of the curves differed, both in richness and abundance (Fig. 3). In the undisturbed sites there were in total 18 species, including shrubs, subshrubs, perennial and annual herbs and succulents (Table 1, Fig. 3). In the disturbed sites only four species were present, of which two were non-native ruderal herbs (Table 1, Fig. 3). There were clear differences in species abundance, with higher evenness among species in the undisturbed compared to the disturbed sites (Fig. 3). In the undisturbed sites the most common species was the native shrub Atriplex argentina, accompanied by shrubs (i.e. Suaeda divaricata, Ehretia cortesia), the non-native herb Salsola kali, and a dense herbaceous and grass layer (i.e. Pappophorum pillipianum, Portulaca sp.). In the disturbed sites, there was a high dominance of Salsola kali, considered a ruderal species and typical of removed soils. The three other species present were much less abundant (Fig. 3).
used the non-parametric Mann Whitney test. For soil compaction, we applied the Kruskal-Wallis non-parametric test to compare several independent samples. We conducted post hoc comparisons when significant differences were detected. To assess what combinations of soil and plant variables explained the variation between the disturbed and the undisturbed sites, we performed a Principal Component Analysis (CPA). Variables assessed included plant cover, bare soil and soil physico-chemical properties (pH, RSG, ASG, EC, Compaction, and porosity). All variables were standardized and an eigenvector value of 0.7 was considered. 3. Results 3.1. Vegetation Traffic from ORVs resulted in significant reductions in vegetation cover (Mann-Whitney test, n = 8, W = 63.0, p < 0.001) with cover three times lower on disturbed compared to undisturbed sites (20% vs. 55.9%) (Fig. 3). As a result of a decrease in plant cover, plant richness and diversity was also significantly reduced by ORVs passage. While the average number of species was 1 ± 0.72 in the disturbed sites, on the undisturbed areas the average number was 5.13 ± 1.55 (Mann-
3.2. Soils We detected changes in the structural properties of the soil. Soil 17
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these two variables (Table 2 and Fig. 5). We also detected changes on the chemical properties of soils. The pH was significantly lower on the disturbed areas compared to the undisturbed (Mann-Whitney test, n = 8, W = 88, p = 0.035) (Table 2). Soil EC tended to be significantly different between conditions, with higher values of EC on the altered areas (Mann-Whitney test, n = 8, W = 50, p = 0.060) (Table 2 and Fig. 5). The Principal Component Analysis showed that PC1 explained 57% of the variation and was mostly defined by plant vegetation cover, soil pH, compaction and apparent specific gravity (Fig. 7). PCA 2 accounted for 14% and was defined by soil relative specific gravity and porosity. The cophenetic correlation factor was 0.93 and the two axes explained 71% of the samples (Fig. 7). The undisturbed sites had higher plant cover, higher soil porosity and a basic pH in contrast to disturbed sites that had lower plant cover, compacted soils, high electric conductivity and soil density and a lower pH. 4. Discussion The use of quad bikes for recreation in this arid ecosystem in the Mendoza Precordillera resulted in a high loss of vegetation, changes in species composition, increase of exposed rock and loss of soil. It also resulted in changes in soils physicochemical properties, including on soil compaction and pH. The negative effects on vegetation and soils highlight the severity of environmental impacts from ORVs. It also highlights the need to regulate ORVs given the nature of this activity where motorized users can travel larger distances than on foot, and hence affecting extensive areas with natural vegetation (Hammitt et al., 2015; Monz et al., 2010). For the study region, this is demonstrated by the large number of off-road recreational trails that have been created by multiple users (Fig. 1). Although the impacts on vegetation and soils from quadbikes were similar to those found in natural plant communities subjected to trampling disturbance by hikers (Cole, 2004; Gomez-Limon and de Lucio, 1995; Kutiel et al., 2000; Lucas-Borja et al., 2011; McDougall and Wright, 2004; Pickering et al., 2010; Scherrer and Pickering, 2006; Yaşar Korkanç, 2014), the severity of impacts were greater as found in
Fig. 3. Relative abundance (diversity dominance, range-abundance). Species from each sample are plotted from highest to lowest abundance (from highest to lowest Pi) within that sample. The points show the position of each species. The size of the bubble represents the abundance of each species. The black color represents the species that were present in the undisturbed and disturbed sites.
compaction was significantly affected by ORVs passage (Kruskal-Wallis test; H = 66.73, p < 0.0001). Values were more than double on the traffic path (M1, M2) compared to undisturbed areas (T1, T2) (Fig. 6). As expected, soil compaction values were similar outside the traffic path on the disturbed area (C) compared to the undisturbed (Fig. 6). The ASG tended to be significantly different between conditions (Kruskal-Wallis test; H = 66.73, p = 0.052) (Table 2), with values higher in the undisturbed areas (Fig. 5). The RSG tended to be higher on the disturbed areas and soil porosity tended to be lower (up to 15%) under this condition. However, we did not find statistical differences for
Fig. 4. Box-plot corresponding to bare soil cover (BS), mulch (M), and rock (R) and vegetation (V), for the disturbed and undisturbed sites. Box: interquartile range containing 100% of data, line across box: median; circle dot: mean. 18
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Table 2 Physicochemical soil properties in disturbed and undisturbed areas. All data are expressed as the mean, standard error (SE) and coefficient of variation (CV). Note: RSG (relative specific gravity), ASG (apparent specific gravity), EC (electric conductivity), pH (potential of hydrogenation). Significant differences are indicated in bold. Variable
ESG (g · cm-3) ASG (g · cm-3) Porosity (%) EC (μS/cm to 25 °C) pH
Disturbed area
Undisturbed area
Kruskal-Wallis test
n
Mean
S.E.
c.v.
n
Mean
S.E.
c.v.
g.l
W
p
8 8 8 8 8
2.38 1.49 37.25 11117.5 7.91
0.04 0.03 1.92 2715.78 0.22
4.68 6.64 14.54 69.09 2.84
8 8 8 8 8
2.31 1.3 43.96 5917.38 8.15
0.07 0.23 9.57 3617.08 0.07
2.95 17.64 21.64 61.13 2.3
1 1 1 1 1
58.5 49.5 82.0 50.0 88.0
0.3409 0.0522 0.1529 0.0608 0.0353
Fig. 5. Box-plot (mean X̅ and the standard deviation S. D) corresponding to a) RSG (apparent specific density), b) ASG (relative specific density), c) porosity, d) EC (electric conductivity) and e) pH for each treatment (disturbed, undisturbed). Box: interquartile range containing 100% of data, line across box: median; circle dot: mean.
Bernhardt-Römermann et al., 2011; Kuss, 1986). Other traits such as the shallow root system of many of the native herbs and the upright position of shrubs, which can be easily broken through trampling damage, contributed to species loss in ORVs disturbed areas (Rowe et al., 2018; Whinam and Chilcott, 2003; Wilshire, 1983). The only few species present in disturbed areas were mainly nonnative plants that are facilitated by disturbance, with the dominance of Salsola kali. This species can be favored through reduced competition from native resident species and its ability to colonize highly disturbed sites. In arid and semi-arid regions of Argentina, for example, has become widespread (Tolaba, 2006), and is often the dominant species in many early successional communities (Allen, 1982). It often colonizes native communities after drought, intense grazing and on compacted soils (Hyder et al., 1975). Its success has been attributed to many factors, including its rapid ability to establish under drought conditions, the ability to exhibit allopathic effects against other species, and the ability to absorb water more quickly than some native species (Allen, 1982). S. kali also germinates well under low water potentials of the soil and has a wide temperature range for germination (Allen, 1982). The altered abiotic conditions on ORVs sites, including changes in soil physical and chemical properties, could have favored the local extinction of species poorly adapted to these conditions. Results from
previous studies assessing motorized vehicles impacts in coastal dunes and other ecosystems (Hosier and Eaton, 1980; Knisley et al., 2018; Kutiel et al., 2000; Liddle and Grieg-Smith, 1975). This is due to the high ground contact pressure from ORVs (Hammitt et al., 2015; Monz et al., 2010). For example, experimental studies have found that just after 50 one-time passes of a motorcycle, vegetation cover can decrease by half with many types of plants and microfloral crust affected (Bowles and Maun, 1982). In our study, we found that quad bikes resulted in decrease of over 67% on plant cover, with important impacts on species richness and composition. The low plant cover in the disturbed sites resulted in a dramatic drop on the number of species present compared to undisturbed sites (17 vs. 3) and an important shift in species dominance. While the undisturbed areas were characterized by several perennial shrubs and grasses and some non-native herbs, only three species were able to withstand ORVs disturbance, with a high dominance of the ruderal nonnative herb Salsola kali. The low tolerance of native plants to ORVs disturbance can be partly attributed to the suite of morphological traits of these arid plant species. This includes species life forms in the natural plant community, characterized by chamaephytes and hemicryptophyte species which have its buds at a distance or close to the ground, and hence more directly exposed to vehicular damage (Andersen, 1995; 19
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soil pH and increasing electric conductivity. Similar results were found by Yaşar Korkanç (2014), in tourist locations in Aladag Natural Park in Turkey, where increases in trampling resulted in soil compaction and increases in electric conductivity. High values of electric conductivity can increase soil solution osmotic potential (Bernstein, 1961), making water less available for plant uptake (Carter, 1982; Demiral, 2017). In contrast to our study, Yaşar Korkanç (2014), Lei (2004) and Kissling et al. (2009), found no difference in soil pH in areas subjected to trampling disturbance; however, Kutiel et al. (2000), and Sarah and Zhevelev (2007) found and effect in soil pH, but with contrasting results. Unlike our study tourist trampling increased soil pH. The reduction in soil pH from vehicular damage could be related to the secondary effects of the high activity of the hydrogen ion in a heterogeneous soil environment. A lower soil pH value is usually associated with high levels of toxic substances such as Al and Mn in the soil solution (McLean, 1976); affects the availability of nutrients for plants (Couto, 1982). The physical and biological effects of ORVs on the ecosystem, are alarming. Lathrop and Rowlands (1983) stated that “the desert is not as” hard “as some enthusiasts have put it so lugubriously, and restoration as a management goal is for all practical purposes unattainable as long as ORVs usage is not regulated.
Fig. 6. Box-plot (mean X̅ and the standard deviation S. D) corresponding to soil compaction for each subzone. Box: interquartile range containing 100% of data, line across box: median; circle dot: mean.
5. Conclusions The use of the natural areas of the Precordillera of Mendoza by ORVs combined with a lack of regulations and management of resources resulted in damage to vegetation and impacts on soil quality, including plant cover and reduced diversity, soil compaction and changes in soil EC and pH. Because reduced vegetation cover could slowed plant succession, it is critical to maintain adequate cover to preserve soil health and fertility. Management actions to minimize impacts includes reducing the number of informal tracks used by ORVs, designating tracks professionally designed and maintained by land managers, as well as monitoring soil and vegetation variables on popular areas for motorized activities. Some of the variables assessed in this study, such as soil compaction and non-native plant cover or occurrence, are important indicators to be considered for monitoring programs. In particular, the early detection of non-native plants in ORVs sites, such as Salsola kali, could prevent its potential invasion into natural vegetation. Understanding the severity and types of impacts of common activities such as ORVs and the ways in which these impacts can be diminished, are important to meet conservation goals and at the same time provide recreational opportunities in arid and other types of ecosystems. Fig. 7. Results from Principal Component Analysis (PCA). The biplot represent the response and explanatory variables, respectively. The circles indicate the eight sites on disturbed areas, and the triangles the eight sites on the undisturbed areas. RSG = apparent specific density, ASG = relative specific density, EC = electric conductivity, BS = bare soil, pH = potential of hidrogenation, C = compaction.
Acknowledgments We thank Ricardo Mauricio for his collaboration in the fieldwork. This work was supported by Project LafargeHolcim-CONICET and The Scientific and Technological Research Fund (FONCYT, PICT 20151455).
this study showed that ORVs sites affected soil physical properties, including an increase in bulk density, and actual density, a reduction in porosity, and an increase in compaction, with the latter one the most salient feature. The effects from ORVs on soils support previous research studies, including in sand dunes in Wales and England, where they found an increase in apparent bulk density and greater soil compaction with higher number of vehicles passes, which affected species diversity (Liddle and Moore, 1974; Liddle and Grieg-Smith, 1975). These adverse effects were also observed by another study (Raghavan and McKyes, 1978) that found that soil compaction limited roots penetration and caused increased runoff. The combined effects of compaction and vegetation removal can also affect microclimate by increasing diurnal fluctuations in soil and air temperature in trampled and vehicle use areas (Lembrechts et al., 2017). Off road vehicles also affected soil chemical properties, decreasing
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Quad bike impacts on vegetation and soil physicochemical properties in an arid ecosystem
T
Ana L. Navas Romeroa,∗, Mario A. Herrera Morattaa, Antonio D. Dalmassoa, Agustina Barrosb a Instituto Argentino de Investigación en Zonas Áridas (IADIZA), Centro Científico Tecnológico (CCT) CONICET, Mendoza. Av. A. Ruiz Leal s/n Parque General San Martín, Mendoza, CP, 5500, C. C. 503, Argentina b Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), Centro Científico Tecnológico (CCT) CONICET, Mendoza. Av. A. Ruiz Leal s/n Parque General San Martín, Mendoza, CP, 5500, C. C. 503, Argentina
A R T I C LE I N FO
A B S T R A C T
Keywords: All-terrain Recreational ecology Trampling Off-road vehicles
The increase use of Off-Road Vehicles (ORVs) coupled with the absence of control strategies have led to extensive use of natural areas for the creation of recreational trails. We assessed the effects of quad bikes on vegetation and soils in an arid ecosystem subjected to intensive quad bike use. We selected randomly eight traffic sites (disturbed sites) and eight adjacent control sites (undisturbed sites). Plant cover and composition were assessed with the point-intercept method and phytosociological census. In each site, we collected soil samples to assess soil physicochemical properties, including apparent specific weight (ASG), actual specific weight (RSG), porosity, texture, electric conductivity (EC) and pH. Soil compaction was measured at 30 spaces points per site. Plant cover and richness was significant lower in disturbed sites, with only four species present in areas subjected to disturbance. There were also changes in species dominance, with native perennial shrubs and grasses characterizing the undisturbed sites while the disturbed sites were mainly dominated by the invasive exotic herb Salsola kali. ORVs traffic also affected soil physicochemical properties including soil compaction, ASG, EC and soil pH. Soil compaction was more than double in disturbed sites and ASG tended to be higher under this condition. The EC was significantly higher and soil pH was significantly lower in the disturbed sites. Reduced vegetation cover and changes to soils physicochemical properties on quadbike trails highlights the impacts of ORVs in the landscape and the need to develop management strategies to minimize disturbance from ORVs on vegetation and soils.
1. Introduction
et al., 2010; Sun and Walsh, 1998). Common impacts on vegetation cited in the literature includes reduced vegetation cover, loss of species diversity and changes in species composition (Buckley, 2004; Hosier and Eaton, 1980). Impacts on soils includes compaction, muddiness, water runoff, and erosion (Hammitt et al., 2015; Leung and Marion, 2000; Webb and Wilshire, 1983). Impacts on chemical conditions includes a decrease in soil pH that can affect the bioavailability of nutrients for plants because there is less cation exchange capacity (Kissling et al., 2009). This can result in greater electric conductivity on soils which can lead to higher salinity. Soil salinization can cause desertification, which is considered one of the major ecological threats for arid ecosystems (Acosta et al., 2011; Ardahanlioglu et al., 2003; Rezapour, 2014; Zheng et al., 2009). Most of the studies of ORVs impacts are concentrated in the Northern Hemisphere, including USA (e.g. Anders and Leatherman, 1987; Belnap, 2002; Lei, 2004; Kelly, 2014) and different countries in
The promotion and demand for leisure activities, has resulted in an increase of outdoor sports in natural environments around the world. This includes motor-based activities, such as off-road vehicles (ORVs), including quad bikes, trail bikes, four-wheel drives that use natural environments to conduct these activities. ORVs are currently one of the world's most popular recreational and sports practices, and it continues to grow (Schlacher et al., 2008; Schlacher and Thompson, 2008; Smith and Burr, 2011). Off road activities can have social impacts, including social conflicts, safety concerns; as well as environmental impacts, such as impact on wildlife, damage to vegetation and trail degradation (Havlick, 2002; Newsome et al., 2013; Taylor, 2006). Although the impacts from ORVs are known to be highly degrading (Belnap, 2002), research on the physical and environmental impacts caused by different ORVs activities remains limited (Cole, 1993; Monz
∗
Corresponding author. E-mail address: [email protected] (A.L. Navas Romero).
https://doi.org/10.1016/j.actao.2019.04.007 Received 15 February 2018; Received in revised form 21 February 2019; Accepted 25 April 2019 Available online 02 May 2019 1146-609X/ © 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. Location of the study area to assess the impacts from quad bikes on vegetation and soils in the Precordillera region in Mendoza, Argentina. The maps include information about the area surveyed as well as the recreational trails used by ORVs vehicles and other activities in the region. This information has been obtained through wikiloc and runstatic volunteered geographic information platforms.
to the traits of the species present in the community, as traits can reflect plant ecological strategies to tolerate disturbance (BernhardtRömermann et al., 2011; Díaz et al., 2007). For example, slow-growing plant species may be more susceptible to trampling or vehicular damage compared to annual fast-growing plants (Bernhardt-Römermann et al., 2011; Garnier et al., 2004). Some life forms, such as phanerophytes and chamaephytes, which have its buds above the soil surface, are very vulnerable to disturbance as they are directly exposed to trampling (Bernhardt-Römermann et al., 2011). Taller plants with erect form, such as many shrubs, can be more sensitive to disturbance because stems can be easily broken compared to prostrate herbs or grasses with a flexible stem (Liddle, 1997). Differences on plants’ susceptibility to damage can result in changes in species composition after disturbance. These include the loss of native resident species less resistant to physical disturbance due to its morphological characteristics (i.e. erect form, phanerophytes, slow growing) and the facilitation of trampling resistant species, such as many ruderal & alien plants (Barros et al., 2015; Burden and Randerson, 1972; Kuss, 1986; Liddle and Grieg-Smith, 1975). The shift in species dominance from native residents to ruderal and alien species, is also related to the competitive ability of the new species to suppress residents, including producing large number of seeds and their adaptation
Europe (e.g. Selva et al., 2011; Iglesias-Merchán et al., 2016; Psaralexi et al., 2017). Less research has been conducted in the Southern Hemisphere and only concentrated on the east coast of Australia (Schlacher and Thompson, 2008; Sheppard et al., 2009; Lucrezi and Schlacher, 2010). In South America, to the best of our knowledge, there are no studies assessing ORVs environmental impacts. The lack of the studies in this region is a conservation concern, particularly for arid ecosystems due to the high susceptibility to anthropogenic disturbance and the increased popularity of the activity in the region (Jones et al., 2016). Arid ecosystems can be more severely affected and damaged by ORVs compared to humid ecosystems due to inherent differences in abiotic site conditions and biotic factors. In contrast to humid environments, arid ecosystems are characterized by stressful abiotic conditions including short water availability, high thermal amplitude and shallow loose soils as well as low plant cover and diversity, which can limit their capacity to recover from anthropogenic disturbance (Gamoun et al., 2018; Odum, 1969; Verstraete and Schwartz, 1991). Because in arid ecosystems plants are often slow growing, ecological impacts can be exacerbated, particularly when the type of disturbance, such as ORVs, is frequent & intense (Webb and Wilshire, 1983; Buckley, 2004; Hammitt et al., 2015). The severity of anthropogenic disturbance from ORVs is also related 15
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2.2. Field design
to meet the changing environmental conditions (Kuss, 1986; Levine et al., 2003). The multiple problems caused by this activity in continuous growth, the lack of control strategies and the scarcity of management actions, has led to extensive natural areas being used as trails for ORVs with the consequent impact to the natural environment as described above (Lathrop and Rowlands, 1983; Webb and Wilshire, 1983; Monz et al., 2010; Kelly, 2014). In addition, the popularity of the activity has led to the creation of unauthorized trails on conservation areas in public and commercial land tenures, as well as the proliferation of high profile events where competitors transit through natural plant communities not previously disturbed (Jones et al., 2016). For example, large scale events such as the Dakar Rally conducted in the Andes in South America (Jones et al., 2013), reflects the popularity of the activity for the region and the need to assess and minimize the environmental impacts associated to ORVs. The objective of this study was to evaluate the effects of ORVs, specifically quad bikes, in an arid ecosystem in western Argentina. We assessed the impacts of quad bikes on vegetation and soils physicochemical parameters. We hypothesize that the cumulative impacts of quad bikes have resulted in 1) changes in vegetation cover, richness and composition, with only few species tolerant to ORVs disturbance 2) changes on soils physicochemical properties, with some parameters highly sensitive to ORVs disturbance. The study was conducted in an area of high conservation value in a private land where the unauthorized trails constructed by ORVs are very popular and it continues to grow.
2.2.1. Vegetation sampling A block design was used to assess the effect of ORVs on vegetation and soils. We conducted the field assessment between June to August 2016. We selected at random a total of 8 zones, including 8 disturbed sites (considered trail circuits, disturbed) with similar degree of use and 8 control paired sites with no apparent disturbance (undisturbed) (Fig. 2). The distance between each zone was 100 m. In each trail circuit and paired site, we placed a 30 m long transect in a diagonal direction, where we assessed vegetation cover per species with the Modified Point-Quadrat method (Passera et al., 1983), with interceptions every 0.3 m. We selected a diagonal direction to be able to cover the total area impacted by each circuit trail, including the edges and center (Fig. 2). In each site, we conducted a phytosociological census recording the total number of species present including those not captured through the point intercept method. Within each site, we identified all vascular species present (including all grasses, herbs and shrubs), and we estimated vegetation cover and bare soil. We also calculated species richness and diversity per transect. Species diversity was estimated using the Shannon index (Hveg) with the following equation: s
H´ =
∑ pi. log2pi i=1
where S is the number of species; pi is the relative abundance of each n plant species ( Ni ); ni is the number species for I; and N is number total the individuals for all species detected.
2. Materials & methods
2.2.2. Physicochemical soil properties To analyze soils physico-chemical properties, we collected 16 soil samples per zone. Soil porosity, apparent specific gravity (ASG) and relative specific gravity (RSG) were determined from samples obtained using a metallic tube of known volume (165.5 cm3). Soil texture, electric conductivity (EC), and potential hydrogenation (pH) were obtained by composite samples collected with a punch of 5 cm3. Each sample (500 g) was collected from the topsoil surface (15 cm). Samples were stored in plastic bags for transfer and subsequent laboratory analysis. We measured soil EC and pH with distilled water using a conductivity meter and a pH meter, respectively. We performed texture analysis using the Guitián Ojea and Carballas (1976)method. To determine soil compaction, we divided each trail circuit into five locations: two controls outside each trail circuit (undisturbed, T1 and T2), two locations on the traffic path (M1 and M2) and one location at the middle of the trail circuit outside the traffic path (C) (Fig. 2). We measure compaction with a point geotester penetrometer (Mark: Part # 59035 Geotester Penetrometer) and according to the soil particle size, we modified the typified following Bucchi (1972). We performed 30 measurements per location in each trail circuit (Fig. 2).
2.1. Study area The study area is located in the district of Capdeville, adjacent to Villavicencio Natural Conservation Area in Las Heras Department, province of Mendoza, in Western Argentina (latitude 32°46′39.69″S, longitude 68°49′54.29″O) (Fig. 1). It is located in the morphostructural area of the Precordillera of the Andes. The local relief is relatively low, standing out the Cerro La Cal with 1090 a.m.s.l. The area belongs to the arid bioclimatic region (Martinez Carretero, 1985), characterized by an annual rainfall of 223 mm and an average annual temperature of 17 °C, 72% of which occurs in summer (October–March) (Rubí Bianchi et al., 2010). It is a high conservation value area, corresponding to the Monte phytogeographyc region, characterized by large levels of endemisms. Vegetation is dominated by native shrubs such as Larrea cuneifolia below 1100 m of altitude, & L. divaricata, from 1200 m to 1700–1800 a.m.s.l (Roig, 1976). Other dominant growth forms are native grasses including the species Pappophorum caespitosum, P. phillippianum (bitter grass), Sporobolus cryptandrus (sporobolo), Nassella tenuis (coiron) and Leptochloa dubia (Passera, 1983). Many of the species are nanophanerophytes, chamaephytes and hemicryptophytes, with few geophyte and terophyte species (Table 1). The topography of the area is characterized by a structure of folds & faults, with winding axes, stretching mostly in a NNE-SSW direction. The soils are classified as Entisols or Liticos on rocky outcrops, and Torripsamment on the shores river (Masotta and Berra, 1994). Off-road recreation is one of the most popular activities in the area with multiple trail circuits created for quad bikes & other recreation activities such as mountain biking, hiking and running. Based on popular volunteered geographic information platforms used in the region, such as wikiloc (https://www.wikiloc.com/) and runstatic (https:// www.runtastic.com/es/), an area of 7 km2 in the Precordillera of Mendoza is used for off-road recreation with 182 recreational trails. The average width of trails varies from 1 to 3 m and areas have been used for over 10 years. These trails are unauthorized trails created by users, not professionally designed.
2.3. Statistical analysis To assess the effects of ORVs on vegetation and soils we performed a series of statistical analyses using Infostat statistical package and Sigmaplot program (version 11) for graphics. First we compared plant cover, richness and diversity between disturbed and undisturbed sites using the non-parametric Mann Whitney test. To compare species composition (richness and abundance) between the disturbed and undisturbed sites we used the range abundance curve. This method allows to visually assess the distribution patterns of species abundance in plant communities, with the abundance of each species represented in a logarithmic scale (Whittaker, 1972; Magurran and McGill, 2011). To determine changes in soil physico-chemical properties (pH, EC, ASG, RSG and porosity) between disturbed and undisturbed sites, we 16
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Table 1 Floristic composition in disturbed and undisturbed sites. The hypsometric levels for both locations was 700 m above sea level. The soil type was loamy clay. Species
Growth habit
Life form Raunkiaer
Origin
Disturbed
Undisturbed
Atriplex argentina Pappophorum phillippianum Leptochloa crinita Portulaca confertifolia Echinopsis leucantha Salsola kali Bulnesia retama Larrea cuneifolia Pterocactus meglioli Suaeda divaricata Tribulus terrestres Disakisperma dubium Allionia incarnata Ehretia cortesía Tephrocactus aoracantha Cover %
Subshrub Perennial Grass Perennial Grass Perennial Herb Subshrub Succulent Annual Herb Shrub Shrub Herb Succulent Shrub Annual Herb Perennial Herb Perennial Herb Shrub Subshrub Succulent
chamaephyte hemicryptophyte hemicryptophyte hemicryptophyte succulent therophyte nanophanerophyte nanophanerophyte succulent nanophanerophyte therophyte hemicryptophyte therophyte geophyte succulent
South America America America South America South America Europe South America South America South America South America Eurasia America America South America South America
. . . . . + . . . + + . + . . 20
2.2 1.1 + + + + + + + + + + + + + 40
Fig. 2. Methodology used to examine the effects of the OVRs on vegetation and soils in trails along 5 km. Control transects were located off the trail, in T1 (Monte natural undisturbed right), and T2 (Monte natural undisturbed left). On the disturbed transect, measures were conducted in M1 (disturbed zone right), C (disturbed zone center), and M2 (disturbed zone left).
Whitney test, n = 8, W = 36, p < 0.001). Species diversity was 11 times lower on disturbed compared to undisturbed sites (H’ = 0.11 vs. H′ 1.29) (Table 1, Fig. 4). When visually comparing the range-abundance curves between the two conditions, we found that the shape of the curves differed, both in richness and abundance (Fig. 3). In the undisturbed sites there were in total 18 species, including shrubs, subshrubs, perennial and annual herbs and succulents (Table 1, Fig. 3). In the disturbed sites only four species were present, of which two were non-native ruderal herbs (Table 1, Fig. 3). There were clear differences in species abundance, with higher evenness among species in the undisturbed compared to the disturbed sites (Fig. 3). In the undisturbed sites the most common species was the native shrub Atriplex argentina, accompanied by shrubs (i.e. Suaeda divaricata, Ehretia cortesia), the non-native herb Salsola kali, and a dense herbaceous and grass layer (i.e. Pappophorum pillipianum, Portulaca sp.). In the disturbed sites, there was a high dominance of Salsola kali, considered a ruderal species and typical of removed soils. The three other species present were much less abundant (Fig. 3).
used the non-parametric Mann Whitney test. For soil compaction, we applied the Kruskal-Wallis non-parametric test to compare several independent samples. We conducted post hoc comparisons when significant differences were detected. To assess what combinations of soil and plant variables explained the variation between the disturbed and the undisturbed sites, we performed a Principal Component Analysis (CPA). Variables assessed included plant cover, bare soil and soil physico-chemical properties (pH, RSG, ASG, EC, Compaction, and porosity). All variables were standardized and an eigenvector value of 0.7 was considered. 3. Results 3.1. Vegetation Traffic from ORVs resulted in significant reductions in vegetation cover (Mann-Whitney test, n = 8, W = 63.0, p < 0.001) with cover three times lower on disturbed compared to undisturbed sites (20% vs. 55.9%) (Fig. 3). As a result of a decrease in plant cover, plant richness and diversity was also significantly reduced by ORVs passage. While the average number of species was 1 ± 0.72 in the disturbed sites, on the undisturbed areas the average number was 5.13 ± 1.55 (Mann-
3.2. Soils We detected changes in the structural properties of the soil. Soil 17
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these two variables (Table 2 and Fig. 5). We also detected changes on the chemical properties of soils. The pH was significantly lower on the disturbed areas compared to the undisturbed (Mann-Whitney test, n = 8, W = 88, p = 0.035) (Table 2). Soil EC tended to be significantly different between conditions, with higher values of EC on the altered areas (Mann-Whitney test, n = 8, W = 50, p = 0.060) (Table 2 and Fig. 5). The Principal Component Analysis showed that PC1 explained 57% of the variation and was mostly defined by plant vegetation cover, soil pH, compaction and apparent specific gravity (Fig. 7). PCA 2 accounted for 14% and was defined by soil relative specific gravity and porosity. The cophenetic correlation factor was 0.93 and the two axes explained 71% of the samples (Fig. 7). The undisturbed sites had higher plant cover, higher soil porosity and a basic pH in contrast to disturbed sites that had lower plant cover, compacted soils, high electric conductivity and soil density and a lower pH. 4. Discussion The use of quad bikes for recreation in this arid ecosystem in the Mendoza Precordillera resulted in a high loss of vegetation, changes in species composition, increase of exposed rock and loss of soil. It also resulted in changes in soils physicochemical properties, including on soil compaction and pH. The negative effects on vegetation and soils highlight the severity of environmental impacts from ORVs. It also highlights the need to regulate ORVs given the nature of this activity where motorized users can travel larger distances than on foot, and hence affecting extensive areas with natural vegetation (Hammitt et al., 2015; Monz et al., 2010). For the study region, this is demonstrated by the large number of off-road recreational trails that have been created by multiple users (Fig. 1). Although the impacts on vegetation and soils from quadbikes were similar to those found in natural plant communities subjected to trampling disturbance by hikers (Cole, 2004; Gomez-Limon and de Lucio, 1995; Kutiel et al., 2000; Lucas-Borja et al., 2011; McDougall and Wright, 2004; Pickering et al., 2010; Scherrer and Pickering, 2006; Yaşar Korkanç, 2014), the severity of impacts were greater as found in
Fig. 3. Relative abundance (diversity dominance, range-abundance). Species from each sample are plotted from highest to lowest abundance (from highest to lowest Pi) within that sample. The points show the position of each species. The size of the bubble represents the abundance of each species. The black color represents the species that were present in the undisturbed and disturbed sites.
compaction was significantly affected by ORVs passage (Kruskal-Wallis test; H = 66.73, p < 0.0001). Values were more than double on the traffic path (M1, M2) compared to undisturbed areas (T1, T2) (Fig. 6). As expected, soil compaction values were similar outside the traffic path on the disturbed area (C) compared to the undisturbed (Fig. 6). The ASG tended to be significantly different between conditions (Kruskal-Wallis test; H = 66.73, p = 0.052) (Table 2), with values higher in the undisturbed areas (Fig. 5). The RSG tended to be higher on the disturbed areas and soil porosity tended to be lower (up to 15%) under this condition. However, we did not find statistical differences for
Fig. 4. Box-plot corresponding to bare soil cover (BS), mulch (M), and rock (R) and vegetation (V), for the disturbed and undisturbed sites. Box: interquartile range containing 100% of data, line across box: median; circle dot: mean. 18
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Table 2 Physicochemical soil properties in disturbed and undisturbed areas. All data are expressed as the mean, standard error (SE) and coefficient of variation (CV). Note: RSG (relative specific gravity), ASG (apparent specific gravity), EC (electric conductivity), pH (potential of hydrogenation). Significant differences are indicated in bold. Variable
ESG (g · cm-3) ASG (g · cm-3) Porosity (%) EC (μS/cm to 25 °C) pH
Disturbed area
Undisturbed area
Kruskal-Wallis test
n
Mean
S.E.
c.v.
n
Mean
S.E.
c.v.
g.l
W
p
8 8 8 8 8
2.38 1.49 37.25 11117.5 7.91
0.04 0.03 1.92 2715.78 0.22
4.68 6.64 14.54 69.09 2.84
8 8 8 8 8
2.31 1.3 43.96 5917.38 8.15
0.07 0.23 9.57 3617.08 0.07
2.95 17.64 21.64 61.13 2.3
1 1 1 1 1
58.5 49.5 82.0 50.0 88.0
0.3409 0.0522 0.1529 0.0608 0.0353
Fig. 5. Box-plot (mean X̅ and the standard deviation S. D) corresponding to a) RSG (apparent specific density), b) ASG (relative specific density), c) porosity, d) EC (electric conductivity) and e) pH for each treatment (disturbed, undisturbed). Box: interquartile range containing 100% of data, line across box: median; circle dot: mean.
Bernhardt-Römermann et al., 2011; Kuss, 1986). Other traits such as the shallow root system of many of the native herbs and the upright position of shrubs, which can be easily broken through trampling damage, contributed to species loss in ORVs disturbed areas (Rowe et al., 2018; Whinam and Chilcott, 2003; Wilshire, 1983). The only few species present in disturbed areas were mainly nonnative plants that are facilitated by disturbance, with the dominance of Salsola kali. This species can be favored through reduced competition from native resident species and its ability to colonize highly disturbed sites. In arid and semi-arid regions of Argentina, for example, has become widespread (Tolaba, 2006), and is often the dominant species in many early successional communities (Allen, 1982). It often colonizes native communities after drought, intense grazing and on compacted soils (Hyder et al., 1975). Its success has been attributed to many factors, including its rapid ability to establish under drought conditions, the ability to exhibit allopathic effects against other species, and the ability to absorb water more quickly than some native species (Allen, 1982). S. kali also germinates well under low water potentials of the soil and has a wide temperature range for germination (Allen, 1982). The altered abiotic conditions on ORVs sites, including changes in soil physical and chemical properties, could have favored the local extinction of species poorly adapted to these conditions. Results from
previous studies assessing motorized vehicles impacts in coastal dunes and other ecosystems (Hosier and Eaton, 1980; Knisley et al., 2018; Kutiel et al., 2000; Liddle and Grieg-Smith, 1975). This is due to the high ground contact pressure from ORVs (Hammitt et al., 2015; Monz et al., 2010). For example, experimental studies have found that just after 50 one-time passes of a motorcycle, vegetation cover can decrease by half with many types of plants and microfloral crust affected (Bowles and Maun, 1982). In our study, we found that quad bikes resulted in decrease of over 67% on plant cover, with important impacts on species richness and composition. The low plant cover in the disturbed sites resulted in a dramatic drop on the number of species present compared to undisturbed sites (17 vs. 3) and an important shift in species dominance. While the undisturbed areas were characterized by several perennial shrubs and grasses and some non-native herbs, only three species were able to withstand ORVs disturbance, with a high dominance of the ruderal nonnative herb Salsola kali. The low tolerance of native plants to ORVs disturbance can be partly attributed to the suite of morphological traits of these arid plant species. This includes species life forms in the natural plant community, characterized by chamaephytes and hemicryptophyte species which have its buds at a distance or close to the ground, and hence more directly exposed to vehicular damage (Andersen, 1995; 19
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soil pH and increasing electric conductivity. Similar results were found by Yaşar Korkanç (2014), in tourist locations in Aladag Natural Park in Turkey, where increases in trampling resulted in soil compaction and increases in electric conductivity. High values of electric conductivity can increase soil solution osmotic potential (Bernstein, 1961), making water less available for plant uptake (Carter, 1982; Demiral, 2017). In contrast to our study, Yaşar Korkanç (2014), Lei (2004) and Kissling et al. (2009), found no difference in soil pH in areas subjected to trampling disturbance; however, Kutiel et al. (2000), and Sarah and Zhevelev (2007) found and effect in soil pH, but with contrasting results. Unlike our study tourist trampling increased soil pH. The reduction in soil pH from vehicular damage could be related to the secondary effects of the high activity of the hydrogen ion in a heterogeneous soil environment. A lower soil pH value is usually associated with high levels of toxic substances such as Al and Mn in the soil solution (McLean, 1976); affects the availability of nutrients for plants (Couto, 1982). The physical and biological effects of ORVs on the ecosystem, are alarming. Lathrop and Rowlands (1983) stated that “the desert is not as” hard “as some enthusiasts have put it so lugubriously, and restoration as a management goal is for all practical purposes unattainable as long as ORVs usage is not regulated.
Fig. 6. Box-plot (mean X̅ and the standard deviation S. D) corresponding to soil compaction for each subzone. Box: interquartile range containing 100% of data, line across box: median; circle dot: mean.
5. Conclusions The use of the natural areas of the Precordillera of Mendoza by ORVs combined with a lack of regulations and management of resources resulted in damage to vegetation and impacts on soil quality, including plant cover and reduced diversity, soil compaction and changes in soil EC and pH. Because reduced vegetation cover could slowed plant succession, it is critical to maintain adequate cover to preserve soil health and fertility. Management actions to minimize impacts includes reducing the number of informal tracks used by ORVs, designating tracks professionally designed and maintained by land managers, as well as monitoring soil and vegetation variables on popular areas for motorized activities. Some of the variables assessed in this study, such as soil compaction and non-native plant cover or occurrence, are important indicators to be considered for monitoring programs. In particular, the early detection of non-native plants in ORVs sites, such as Salsola kali, could prevent its potential invasion into natural vegetation. Understanding the severity and types of impacts of common activities such as ORVs and the ways in which these impacts can be diminished, are important to meet conservation goals and at the same time provide recreational opportunities in arid and other types of ecosystems. Fig. 7. Results from Principal Component Analysis (PCA). The biplot represent the response and explanatory variables, respectively. The circles indicate the eight sites on disturbed areas, and the triangles the eight sites on the undisturbed areas. RSG = apparent specific density, ASG = relative specific density, EC = electric conductivity, BS = bare soil, pH = potential of hidrogenation, C = compaction.
Acknowledgments We thank Ricardo Mauricio for his collaboration in the fieldwork. This work was supported by Project LafargeHolcim-CONICET and The Scientific and Technological Research Fund (FONCYT, PICT 20151455).
this study showed that ORVs sites affected soil physical properties, including an increase in bulk density, and actual density, a reduction in porosity, and an increase in compaction, with the latter one the most salient feature. The effects from ORVs on soils support previous research studies, including in sand dunes in Wales and England, where they found an increase in apparent bulk density and greater soil compaction with higher number of vehicles passes, which affected species diversity (Liddle and Moore, 1974; Liddle and Grieg-Smith, 1975). These adverse effects were also observed by another study (Raghavan and McKyes, 1978) that found that soil compaction limited roots penetration and caused increased runoff. The combined effects of compaction and vegetation removal can also affect microclimate by increasing diurnal fluctuations in soil and air temperature in trampled and vehicle use areas (Lembrechts et al., 2017). Off road vehicles also affected soil chemical properties, decreasing
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