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Journal of South American Earth Sciences

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Relationships between landform properties and vegetation patterns in theCerro Zonda Mt., Central Precordillera of San Juan. Argentina

Daniel Floresa,b,∗, Emmanuel Ocañaa,b, Aixa Inés Rodrígueza,c

a CONICET, Centro de Investigaciones de la Geósfera y la Biósfera, Av. Ignacio de la Rosa y Meglioli, J5400, San Juan, ArgentinabGabinete de Geología Ambiental, Instituto de Geología, Universidad Nacional de San Juan, Av. Ignacio de la Rosa y Meglioli, J5400, San Juan, Argentinac Instituto Geofísico y Sismológico Volponi, Universidad Nacional de San Juan, Av. Ignacio de la Rosa y Meglioli, J5400, San Juan, Argentina

A R T I C L E I N F O

Keywords:GeomorphologyVegetationDEMMonte Desert

A B S T R A C T

Arid environments are dynamic systems due, in part, to the action of hydrological processes. However, studies onthe relationship between the abiotic factors and the properties of arid vegetation communities of Argentina arescarce. In this paper, the connection between these two parameters is researched using DEM analysis and ex-haustive field work in order to model vegetation changes and landform evolution. The aim of this paper is todetermine whether geomorphological properties influence vegetation patterns along the Cerro Zonda Mt. Thismountain range is located in an arid central sector of Monte Desert, a phytogeographic region of Argentina. Theclassification of geomorphological units was made by combining interpretation of images provided by GoogleEarth and the analysis of Alos Palsar DEM using SAGA GIS. The geomorphic units were sorted into categoriesbased on slope, elevation, hillslope aspect, terrain roughness and topographic wetness. In addition, surfacecharacteristics of units were evaluated considering the properties of coverage of rock fragments, fine sedimentand mulch. Afterwards, community patterns were explored using PCA analysis and linear correlations. Ourresults showed five geomorphological units: a) Pediment of mountain upland; b) Active landforms; c) Inactivelandforms; d) Inactive landforms raised by neotectonics and e) Lacustrine landforms. The vascular flora is re-presented by 35 species distributed in 15 families. The landforms' surface property of rock fragments is corre-lated both directly with species richness and inversely with vegetation coverage. Furthermore, roughness ex-plains the higher vegetation diversity while slope promotes an increase in the vegetation cover, but this does notsuggest more species richness. In addition, our results indicate that topographic wetness, as well as hillslopeaspect and elevation, are not determining variables of vegetation patterns across the landforms. The vegetationpatterns of Cerro Zonda Mt. are associated with topographic properties as slope and roughness. In turn, rockfragment surface coverage also affects these vegetation patterns. The landforms' properties not only exposespatial changes in the development of the plant communities but also show the direct implications of geo-morphological changes over such communities. In particular, there is a need to integrate a geomorphologicalview for the analysis of vegetation in arid zones.

1. Introduction

The interdependencies between landforms and the vegetation thatgrows on them have long been of interest to both geomorphologists andgeobotanists (Kozłowska and Rączkowska, 2002). Arid landscapes arehighly dynamic due, in part, to the action of geomorphic processes; thisdynamic aspect maintains a direct relationship with the distributionand abundance of plants (Michaud et al., 2013; El-Keblawy et al.,2015). For example, the relationship between rainfall and wateravailability for plant growth in deserts is modified by topography whichaffects surface redistribution, infiltration and retention of water

(Tongway and Ludwig, 2001; Saco et al., 2007; Duniway et al., 2010).Vegetation cover and distribution in arid ecosystems are not continuousand are directly related to the availability of resources such as water(Aguiar and Sala, 1999). From a wide variety of vegetation-relatedstudies in arid regions, most focus on vegetation and plant communities(Simpson and Solbrig, 1977: Rossi and Villagra, 2003; Álvarez et al.,2006; Hadad et al., 2014; Morello, 1958; Abraham, 1979; Flores andSuvires, 2012). These studies are generally linked to rainfall, tem-perature (Rundel et al., 2007; Labraga and Villalba, 2009; Villagraet al., 2009), plant distribution, cover and grazing disturbance (Bisigatoet al., 2009), among others.

https://doi.org/10.1016/j.jsames.2019.102359Received 25 March 2019; Received in revised form 9 September 2019; Accepted 9 September 2019

∗ Corresponding author. CONICET, Centro de Investigaciones de la Geósfera y la Biósfera, Av. Ignacio de la Rosa y Meglioli, J5400, San Juan, Argentina.E-mail addresses: dflores@unsj-cuim.edu.ar, danielgermanflores@gmail.com (D. Flores).

Journal of South American Earth Sciences 96 (2019) 102359

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Despite the fact that vegetation cover in drylands is low (Aguiar andSala, 1999), geomorphological activity can be responsible for coverage,distribution and abundance of occurring plant species as well as pre-cipitation (Merritt, 2013) and water distribution (Rietkerk et al., 2002).

The Monte Desert is a dryland located in the arid west sector ofArgentina (Morello, 1958; Fernández and Busso, 1999; Arana et al.,2017). It is a natural and complex environment composed of spatialunits with different parental material, soil and vegetation across theentire area (460,000 km2) (Abraham et al., 2009), whose relationshipsvary from regional to microsite scales (Bisigato et al., 2009).

In Monte landscapes, the vegetation is subordinate to the abioticelements of the system as soil aggregation, infiltration rates and por-osity (Abril et al., 2009), while the topography and precipitation in-fluence the distribution and diversity of such vegetation (Abril and Noe,2007). More specifically, the topography directly affects the vegetationvia processes such as debris flow, rock fall occurrence, soil humidityand water redistribution; resulting in segregated vegetation distributionassociated with the water availability of each geomorphological unit(Flores et al., 2017).

Although the factors that affect vegetation cover and distributionare various, management of biotic variables does not suffice to achievecomprehension of vegetation distribution at regional or local level(Flores et al., 2015). Therefore, the research on process dynamics isimportant due to the relationship between theory of landscape ecologyand geomorphology, which has revealed a strong impact of landformdynamics on local diversity and structure of vegetation communities(Lawton and Kinne, 2000; Dauber et al., 2003).

The general purpose of this study is to determine whether geo-morphological characteristics of landforms influence on vegetationpatterns along the Cerro Zonda Mt. Our hypothesis entails that in thismountain range (Fig. 1), where the species are under high hydric stress,vegetation patterns will be affected by terrain properties that, in turn,could help estimate the availability and distribution of water and theaction of hydrological processes. Particularly, we expect that differenttopographic properties as slope, hillslope aspect, elevation, roughnessand wetness (as a basic stage in the relief analysis) will be determinantin vegetation diversity, coverage and richness. Our study is an attemptto expand the traditional method of examining the physical variables oftopography and vegetation separately; taking into account that varia-tions in hydrological events and erosion explain, to a great extent, thedistribution patterns of vegetation (Wharton et al., 1982; Mitsch andGosselink, 2000; Sharitz and Mitsch, 1993). Regarding this, we evi-dence the consequence of landform heterogeneity (due to the differentactions of dynamic processes) on vegetation parameters (cover, di-versity, distribution).

1.1. Study area

The Cerro Zonda Mt. is located in the Zonda Valley in San Juanprovince, in the central-western part of Argentina, between 31°33′ Sand 68°48′ W (Fig. 1E). This valley is an intermountain depression,situated between Central and Eastern Precordillera. Regional socio-economic development in this area is mainly based on agricultural andtouristic activities.

Geomorphologically, this region is associated with a neotectonicand hydric activity that correlates with the major active morphody-namic systems of Central Precordillera. Altitudes range from 870m asl(at the lowest sector of piedmont) to 1200m asl (at the knickpoint) and2219m asl at the highest point of the mountain range. The thrust faulthas affected 3 levels of alluvial fans that are arranged on the easternpiedmont of Cerro Zonda Mt. (Perucca et al., 2012).

Dry desert climate predominates in the area (BW) (Koppen, 1923),more specifically a sub-variety is distinguished (BWwk) involving rainysummers and dry and cold winters (Pereyra, 2000). The extreme ab-solute temperature reached 45 °C. The annual mean temperature rangesfrom 14 °C to 19 °C. The annual rainfall values range within

100–124mm (Minetti et al., 2004). Over 67% of the average annualprecipitation (around 76.7mm) falls from December to March insummer. Precipitation is heavy and intense (between 5 and 20mm)during a short space of time (less than 30min).

1.1.1. Geological settingsThe Precordillera comprises a fold and thrust belt located east of the

Andes Cordillera and is composed mainly of Paleozoic sedimentaryrocks. The region is divided into tree sub regions: 1) Occidental, 2)Central, and 3) Oriental. The lithology, structure, age and tectonicdifferences are the basis for the classification (Ortiz and Zambrano,1981). The Cerro Zonda Mt. is part of Central Precordillera and sup-ports two main geologic units. (I) Rock outcrop composed by Cambrian-Ordovician sedimentary rocks, mainly consisting of limestone, dolomiteand lutite and (II) the eastern piedmont relief consists of Quaternarycolluvial-alluvial deposits containing sand, silt, clay, and block ofgreywackes rocks limited to temporary channels (Ocaña et al., 2016).These units are subject to frequent neotectonic activity, which impactson the fluvial and alluvial systems. The north–south trending CerroZonda Sur Fault Zone is 10 km long and is characterized by east-vergingreverse faults with relatively smooth east-facing scarps, where Devo-nian sandstones overlay Pleistocene alluvial fan deposits (Fig. 1 E). Tothe north, the Cerro Zonda Norte Fault becomes northwest-trending andis characterized by west-verging reverse faults with several southwest-facing scarps where Miocene strata overlay Pleistocene alluvial fandeposits (Fig. 2 F) (Perucca et al., 2012).

1.1.2. Soil settingsThe piedmont soil studies in arid zones are scarce, however, in this

region, evidence exists of Quaternary inactive deposits that promote thedevelopment of soil horizons and the presence of vesicular horizons(Av), both associated with soils of Entisol and Aridisol Orders. Thesehorizons are composed of a high concentration of clays (> 20%) and alow rate of water infiltration (Ocaña et al., 2019). Furthermore, theseauthors mention that the Av development is related to ancient geo-morphological surfaces located topographically in high sectors ofpiedmont and also to the presence of desert pavement and varnish. Inthis sense, this Av horizon could be a limiting factor for the establish-ment and development of plant species due to the scarce water per-meability and infiltration.

1.1.3. Geomorphological settingsAt semi-detailed scale, Cerro Zonda Mt. presents a diversity of

landform units. A brief description of these landforms is presentedbelow.

1.1.3.1. Mountain upland unit

a) Pediment of mountain upland (PMU)

These landforms are located at high elevation, near the summit andcorrespond to quaternary debris coverage. They are present on siteswith low gradient (< 3°) and variable area (between 7300m2 and1200m2); and they are isolated from one another (Fig. 1E). They arethe most elevated landforms and occupy high-altitude depressions,originating small peneplains. The landforms consist primarily of a coverof unselected angular rocks, from between 5 and 25 cm in size, alongwith coarse sand and silt fragments in lower proportions. These unitshave irregular and elongated concave shapes. In one hand, the finesediments are predominantly light-colored (7.5 YR 6/3; light brown)due to their sand and silt composition. On the other hand, dark-coloredsediments (10 YR 3/2; very dark grayish brown) correspond to grey-wackes rocks, which contrast strongly with sand-silt sediments(Fig. 2A). The dominant geomorphological activity is sediment accu-mulation, as well as thermoclastism, sheetflow erosion and formation ofsmall shallow gullies (< 50 cm).

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1.1.3.2. Piedmont units

b) Fluvial-alluvial landformsb.1) Active landforms (AL)

These units correspond to current temporary ephemeral channelsthat transport and store sediment in debris flows resulting from the

torrential rains occurring in the region (Fig. 2 B and C). Blocks ex-ceeding 40 cm in diameter show evidence of temporary fluvial activitysince they exhibit no development of desert varnish. From the dynamicperspective, the continuous modification by fluvial processes allows forthe identification of the landforms' mass movement which is, in turn,linked to modern debris flow events composed of chaotic and un-selected sediments. Besides, parts of these units show evidence of

Fig. 1. A) Location of the Cerro Zonda Mt. on the Monte Desert phytogeographic province of Argentina, B) the Terrain Roughness Index map shows the intermediateroughness values of inactive landforms raised by neotectonics, C) the Topographic Wetness Index map indicates the moderate to high wetness values of the activelandforms, D) the slope map depicts the high value in degrees of a piedmont unit of inactive landforms raised by neotectonics, E) the Geomorphological map of CerroZonda Mt. evidences the spatial arrangement of landforms, the location of sampling sites and the Cerro Zonda fault in piedmont itself, F) the aspect map indicates thatthe piedmont units are facing east, north-east and south-east while PMU units are slightly facing to the west, G) the elevation map signals the variation in altitude ofthe Cerro Zonda Mt.'s main geomorphological units (piedmont 800–1550m asl and pediment of mountain upland starting at 1751m asl).

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Fig. 2. Different landforms occurring in the study area: A: Pediment of mountain upland (PMU): these landforms are on the summit of Cerro Zonda above 1950 m aslelevation, B: Active landforms (AL) in the piedmont proximal part. Note the tall shrub vegetation among landforms of chaotic blocks; C: Active landforms (AL) in themiddle sector of piedmont. Remains of dry trees are observed, dragged by debris flows events; D: Homogeneous distribution of vegetation in Inactive landforms (IL);E: Inactive landforms (IL): they show smaller-sized vegetation and blocks are covered with desert varnish; F: Inactive landforms Raised by Neotectonics (RIL) theselandforms, by being raised, have remained exposed Miocene deposits; G: Lacustrine landforms (LD) in the piedmont distal part cross-cut by current active channels;H: Contact between piedmont deposits and lacustrine landforms (LL), note the absence of vegetation on the landforms.

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rockslides as well as of concentrated and lateral channel erosion(Fig. 1E). In the distal sector of piedmont, these units present evidenceof sediment accumulation processes.

b.2) Inactive landforms (IL)

These units correspond to old alluvial fans and undifferentiatedglacis levels (Fig. 2 D and E). Topographically, they are arranged in asegmented manner and are at a higher elevation level than activelandforms (AL). They correspond to the oldest alluvial fan levels re-sulting from the deposition of dense debris flows that have apparentlyremained “unaffected” by active channels and hydrogeomorphologicalprocesses. Thus, they contribute to the formation of desert varnish, tothe occurrence of pedogenic processes and to the development of an Avsurface horizon, typical of drylands. The spatial arrangement of un-selected clasts is random; their sizes vary from blocks to angular andsubangular gravels (from between 10 and 25 cm long). Sheet-flow de-posits are also observed in these units.

1.1.3.2.1. Poligenetic landform

c) Inactive landforms raised by neotectonics (RIL)

These landforms correspond to reliefs showing typical character-istics of such an arid and seismic region. They are erosive surfaces,moderately inclined to the east. They appear on sandstones of theMiocene covered by quaternary fluvial sediments of Devonian grey-wackes. These units are more elevated than the other piedmont land-forms as a result of the neotectonic activity (Perucca et al., 2012) (Fig. 2F). They are located in the middle sector of the piedmont and arecomposed of clasts of heterogeneous granulometry, which range from70 cm boulders to 5 cm cobbles. In turn, the clasts are made up of over98% of devonian rocks (greywackes) and the remaining percentagebeing Miocene rocks (sandstones) within a sandy matrix. The land-forms' geomorphological activity is circumscribed by small channelsthat transport great amounts of clay to the active landforms (AL) si-tuated on the sides of these units. Fluvial-alluvial lateral erosion, thedevelopment of desert varnish and desert pavement, and the surfacerun-off at the highest sites are some of the processes associated withthese landforms.

d) Lacustrine landforms (LL)

These units correspond to lacustrine-palustrine relict deposits thatextend for about 6.65 km in a N-S direction. These landforms belong tothe Valentín Formation (Suvires and Gamboa, 2011) and are depictedas fine sandy and sandy loam, being of a yellowish-grey and greenishcolour (Fig. 2 G and H). Their formation age fluctuates from latePleistocene (16.7–15.2 ka BP) to middle Holocene age (9475–7685 yrBP) (Blanc and Perucca, 2017). Their spatial design is irregular andintermittent, located and aligned in contact with the distal part of thepiedmont. These units are eroded by temporary channels that descendfrom the highest units. Geomorphological activity is dominated to agreat extent by laminar erosion as well as by concentrated erosion onthe borders of the landforms. Another occurring process is sedimentaccumulation, evidenced by the analysis of the debris coverage thatoverlies sectors of lacustrine landforms composed of greywacke clasts.Aeolian erosion, signaled by deflation, affects the surface layers of la-custrine sediments. It has been observed that human activities such astrekking, quadricycles races, weekend-house constructions and thegrowth of agriculture -especially vineyard plantation-have had a ne-gative impact on the vegetation diversity since they have reduced thesurface availability and accelerated deposit erosion processes which, inturn, affect the landform.

2. Materials and methods

2.1. Geomorphology

2.1.1. Identification, classification of landformsThe implemented methodology involved a three steps plan in order

to acquire a detailed approximation of the landforms; first: the identi-fication and classification of geomorphological units at semi-detailedscale (1:25000) based on an exhaustive interpretation and analysis ofGoogle Earth images, in order to identify areas evidencing the action ofgeomorphological processes.

The geotechnical characteristics of landforms are a key element inthe classification process (Ocaña et al., 2017). Therefore, in order toachieve an integrated classification of the landforms, morphogenesis,morphodynamic and morphotectonic information was taken into ac-count in the analysis of the geomorphological units. On this basis, thefollowing classification were made: Lacustrine landforms (Morpho-genesis); Inactive landforms raised by Neotectonics (Morphodynamicplus Morphotectonic information).

2.1.2. Sampling sites (SS)In a second step, the placement of the sampling sites (SS) within the

landform's circumscribed in the preceding step. Each SS consisted of a50-m-long line where, in field work, the surface rock fragments, finesediments, mulch and vegetation coverage was recorded every 0.50m(100 points in total) using the modified Point Quadrat method (Passeraet al., 1983). In addition, one more meter was recorded on both sides ofthe mainline to include plant species that were not comprised in theprevious method, these were given a coverage value of 0.1%. Thus, a SSarea of 100m2 was charted. These sampling sites were distributed asfollows: 5 on PMU, 23 on AL, 23 on IL, 8 on RIL and 8 on LL, making atotal of 67 SS.

2.1.3. Topographic properties of landformsAs a third step, for each SS and landforms we defined: slope, aspect,

elevation, terrain roughness index (TRI) and topographic wetness index(TWI) obtained by the processing of an Alos Palsar DEM acquired fromthe Alaska Satellite Facility (Dataset: © JAXA/METI ALOS PALSAR L1.02007. Accessed through ASF DAAC 11 MAY 2018) at 12.5 m pixel re-solution, using terrain analysis tools of SAGA GIS software v2.2 (Conradet al., 2015).

The slope (calculated here using the average maximum technique)is a primary topographic attribute index which has been used directly inmodeling processes for the estimation of energy budgets and of debrisflow frequency (Ellen and Mark, 1993). Similarly, slope has been as-sociated with some vegetation characteristics such as species compo-sition (Austin et al., 1983). In addition, this index has been taken as thebasis for identifying sheet and rill erosion by water in the Universal SoilLoss Equation (Renard et al., 2011). The slope gradient, from a hy-drologic point of view, provides us with information about overlandand subsurface flow, and velocity and runoff rate (Moore et al., 1991).

Terrain wetness index has been used as a surrogate for very complexhydrological processes (for instance spatial patterns of soil moisture,spatial distribution and surface saturation of water (Western et al.,1999; Bárdossy and Lehmann, 1998)), for geomorphological processeslike erosion rates, catchment area calculation, run-off generation me-chanism (Gómez-Plaza et al., 2001) and for the prediction of biologicalprocesses such as plant species richness as well as spatial distribution ofspecies (Zinko et al., 2005; Sörensen et al., 2006). It is defined as ln (α/tanβ) where α is the local upslope area draining through a certain pointper unit contour length and tanβ is the local slope (Beven and Kirkby,1979).

The hillslope aspect is a significant variable in the study of the ef-fects of topography on vegetation development (Carson and Kirby,1972), hillslope morphology (McMahon and thesis, 1998) along withthe sediment transport (Gutiérrez-Jurado et al., 2007). At a given point

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on a surface z= f(x, y), the aspect (A) is defined as a function of thegradient at X and Y. The formula for the calculation of aspect data is:A= 270° + arctan (fy/fx)-90° fx/|fx|’ (Zhou and Liu, 2004).

Topographic Roughness can be defined as a characteristic related tothe irregularity of the territory. It has been used to provide a rapid andobjective measure of a terrain heterogeneity. Furthermore, this prop-erty has become a key variable in explaining the choice of habitats byterrestrial species as well as some questions related to geology and plantdiversity (Hobson, 1972). Riley et al. (1999) propose a model whichcomputes TRI values for each grid cell of a DEM. This model helpscalculate the sum change in elevation between a grid cell and its eighthneighbouring grid cell.

2.1.4. Data extraction from DEMThe basic data extraction from gridded elevation model (slope, as-

pect, elevation, TRI and TWI) was carried out by means of the “ex-traction by point location” method, entailing the recollection of specificcell data from the raster file through latitude and longitude locationpoint (in this case, the starting point of a sampling unit in the field). Atthe beginning, the Alos Palsar DEM was preprocessed by using theSAGA GIS terrain analysis tool “Fill sinks”. Afterward, the basic terrainanalysis was used for obtaining the parameters: slope grid (value indegrees), aspect, elevation, TWI grid and TRI grid. Finally, the vectorshape (points) was superimposed over the raster and the specific tool“extract values to points” from the software ESRI ArcGIS 9.2 spatialanalysis tool was implemented.

2.2. Fieldwork

The fieldwork involved an in situ analysis of the morphogenetic,morphodynamic and morphostructural aspects of the geomorphologicalunits. The indicators of the processes that the different geomorpholo-gical units underwent were supported by means of a description of thelandforms accompanied by evidence of their geomorphological activity,development and properties. The purpose of including this descriptionwas to provide information about landscape properties that have had arelevant impact both on the control of surface flow (Rango et al., 2006)and on vegetation distribution.

2.2.1. Evaluation of superficial parameters of the landformsSurface rock coverage was considered a parameter because of its

close link to surface hydrological processes such as water flow, in-filtration rate, superficial erosion and run-off (Musick, 1975; Woodet al., 2005; Hlaváčiková et al., 2015; Zhang et al., 2016). Due to this,the proportion of rock fragments was determined as a tool to explainthe properties of plant communities. Besides, the fine sediment (particlesize < 2mm, FAO, 1990) present between rock fragments was takeninto account as well, in order to provide us with information aboutsurface water dynamics (flow velocity, spatial distribution), aeolianprocesses (associated with the development of desert pavement andvarnish) and vegetation controls on run-off (reduction in flow velocityand, as a consequence, accumulation of fine sediments). In addition, thein situ determination of soil colour by using a Münsell (1975) chart wasperformed in order to identify, characterize and differentiate the su-perficial parameters of the landforms.

Although mulch coverage is closely related to vegetation coverage,it was included in this paper with the purpose, as Adams (1966)claimed, of estimating its effect both on the conservation and dynamicsof soil moisture and on the intensity of superficial hydrological pro-cesses conditions at patch scale.

2.2.2. Vegetation propertiesDespite there being many variables for assessing vegetation in

drylands, in this case, we focused on the proportion of plant coverage,diversity and species richness.

The relative vegetation coverage was calculated taking into account

the 100 data points for each sampling unit. For comparison betweenunits, species coverage values were organized in a matrix of species bysites.

The Shannon-Wiener diversity index (H’= – ∑ pi ln pi) was used,where pi was the relative proportion (area) of each community in thegeomorphological units.

The specific species richness (S) is the simplest way to measurebiodiversity, since it is based solely on the number of species present,without taking into account their importance value (Moreno, 2001). Weresearched “S” to have a complete inventory that allowed us to knowthe total number of species obtained by performing a census of thecommunity.

Aggregate numbers of herbaceous, woody and total species sampledper site were recollected during the spring–summer season in coin-cidence with the time of major precipitations.

We identified vascular plants species using the Flora del Cono Surcatalogue (Zuloaga et al., 2008).

2.3. Data analysis

To compare several independent samples of vegetation and topo-graphic properties, we performed a series of statistical analyses usingthe R software (R Core Team, 2017). First, we compared relative plantcoverage, richness and diversity between landforms using the non-parametric Kruskal-Wallis test, which also helped to determine thechanges in topographic properties (Slope, elevation, aspect, TRI andTWI) between geomorphological sites.

When significant differences were detected, we resorted to theMann-Whitney test as a post-hoc comparison tool between all land-forms. To assess which combinations of topographic properties andplant variables explained the variation between all the geomorpholo-gical units, we evaluated the relationship linking community attributesand site scores on the Principal Components Analysis (PCA) axes 1 and2 by means of Pearson's correlation coefficients. The variables assessedincluded plant coverage, diversity, species richness, mulch coverage,fine sediment coverage, rock fragment coverage, slope, aspect, eleva-tion TRI and TWI.

3. Results

3.1. Vegetation patterns across the Cerro Zonda

The vascular flora was represented by 35 species distributed in 15families, the best representatives being: Poaceae, Fabaceae, Cactaceaeand Zygophyllaceae. As regards the vegetation layers across the CerroZonda Mt., the shrub layer was dominant (49.03%) (F (3,18) = 7,1;p= 0,001), followed by the layer of succulents (cactus) with a 36.45%coverage (F (4, 34)= 11,5; p= 0,001) and herbs with an 11.81% cov-erage (F(2, 63)= 9,5; p= 0,001). Tree coverage (2.71%) did not showsignificant differences between geomorphological units (F (5, 87)= 3,7;p= 0,57) (Table 1).

Despite vegetation coverage not exceeding the 40%, the analysisbetween all geomorphological units showed significant differences (F(4,72) = 12,3; p= 0,001).

3.2. Superficial deposits characteristics and vegetation patterns

Only rock fragments coverage evidenced a significant direct corre-lation with species richness (r2=0.70, p=0.01) and a moderate butinverse correlation with vegetation coverage (r2=− 0.61, p=0.02).Sediment and mulch coverage did not have significant effects on ve-getation community patterns (Fig. 3). Richness was higher in active andpediment of mountain upland landforms (p=0.001, F (4; 62)= 30.98)meanwhile diversity exhibited the same trend (p= 0.001, F (4;

62)= 34.63), (Table 1). Furthermore, relative vegetation coverageshowed the opposite trend and it was higher in inactive and raised

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landforms (p=0.001, F (4; 62) = 5.98) (Table 2).Table 1: Individual analysis of the relative cover of the species oc-

curring in the different geomorphological units of Cerro Zonda. PMU:Pediment of Mountain upland, AL: Active landforms, IL: Inactivelandforms, RIL: Inactive landforms raised by neotectonics, LL: Lacus-trine landforms. The asterisk indicates the highest value.

3.2.1. Mountain upland units

a) Pediment of mountain upland (PMU)

These sites have an intermediate plant coverage (22%) in

comparison with all geomorphological units (max. 33.9%) (Table 2).The shrubs are dominant, with the species showing the highest cov-erage being Larrea cuneifolia (17.5% of total unit vegetation coverage).Other species like Senecio subulatus var. subulatus (1%), Gochnatia glu-tinosa (9.17%), Acantholippia seriphioides (5%) as well as Buddlejamendozensis (5%) are present only in these landforms. The scarce pre-sence of fine sediment (7.1%) is observed in small endorheic basins athigher altitudes, where water accumulates during summer rains, con-tributing to the development of herbs and cacti. In reference to surfacerock coverage, these units have the highest value (61.9%, Table 2) andthermoclastism is the dominant process of rock fracture, in additionsome herbaceous species such as Acantholippia seriphioides evidence thepotential of these new sites as a resource for establishment.

3.2.2. Piedmont units

b) Fluvial - alluvial landformsb.1) Active landforms (AL)

Plant coverage is estimated to be 24% (Table 2) and is representedby the shrubs Bulnesia retama (29.2%) and Larrea cuneifolia (14.5%),accompanied by species such as Cercidium praecox (6.5%) and Gra-bowskia obtusa (9.2%). We consider it noteworthy to mention that theheight of tree species in active landforms is greater than in otherlandforms, for instance, some trees exceed the 3m. Meanwhile, watererosion processes and transport of materials limit plant growth andreduce vegetation coverage as well as the development of small-sizedspecies.

Shrub distribution in these landforms is usually circumscribed tosmall lateral terraces or small central bars elevated as a result of theaccumulation of fine sediments (13.3%). More specifically, these sedi-ments accumulate at the base of larger rock fragments (> 30 cm) whosemain coverage rises to 58.4%. Thus, the high proportion of rocks servesas a natural barrier for fine earth and seeds.

b.2) Inactive landforms (IL)

33% percent of the landform area is covered with vegetation(Table 2), where the shrub layer is dominant (cover higher than 51%),although they share 8 species with active landforms, here individualsare smaller, both in height and crown size. Cactus and herb speciesevidence similar relative coverage (close to 23%). In general, the spatialdistribution of plant species is homogeneous in these landforms. Deu-terocohnia longipetala, the dominant herb species (relative cover 21.5%)forms dense populations preferably located in the proximal part of thepiedmont, diminishing toward the distal part where only isolated in-dividuals or small clumps of 4 or 5 individuals can be observed. Finesediments are usually observed in the center of these populations,which present a circular growth. Tephrocactus aoracanthus individuals(16.6%) are equally distributed and usually settled in dense popula-tions; although, in these landforms, coverage of rock fragments is closeto 46% (Fig. 2 D).

c) Poligenetic landforms: Inactive landforms raised by neotectonics(RIL)

Vegetation covers 33% percent of the landform area (Table 2),where the shrub layer is dominant (coverage higher than 51%). Al-though inactive landforms share 8 species with active landforms, hereindividuals are smaller, both in height and crown size. Cactus and herbspecies show similar relative coverage (close to 23%). In general, thespatial distribution of plant species in these landforms is homogeneous.Deuterocohnia longipetala, the dominant herb species (relative cover21.5%) forms dense populations predominantly located in the proximalpart of the piedmont, which in turn diminish toward the distal partwhere only isolated individuals or small clumps of 4 or 5 individuals

Table 1The analysis of relative coverage per species and vegetation layers that occur inthe different geomorphological units of Cerro Zonda Mt. is shown.

Relative vegetation species cover (%) for each deposit

MountainUnit

Piedmont units

PMU AL IL RIL LL

Occurring species (n= 5) (n= 23) (n= 23) (n= 8) (n=8)Portulaca aff

echinosperma0.2

Bouteloua barbata 1.6 0.2 0.6Deuterocohnia

longipetala19.1 2.2 21.2 23.2

Setaria cordobensis 0.4 1.8 0.5 0.7Aristida spegazzinii 1.9 1.3Aristida mendocina 0.8 1Pappophorum

caespitosum11.1 0.5

Allionia incarnata 0.3 0.3 1.1Euphorbia

catamarcensis2.1

Heliotropiumcurassavicum varcurasavicum

6.1

Mentzelia parvifolia 0.1 1.4Schismus barbatus 0.4 0.5Total herbs *40.4 8.5 23.2 28.5 0Bulnesia retama 29.3 13.4 15.9 12.2Jatropha excisa 0.4 1.9 1.6Larrea cuneifolia 15.5 14.5 26 31.9 9.1Zuccagnia punctata 2.1Cercidium praecox 6.4 0.6Grabowskia obtusa 9.8 3.5Larrea divaricata 5.8 9.1 5.4Lycium chilense 0.7 1.3 0.4Atriplex lampa 0.5 0.2 0.3Ligaria cuneifolia 0.2Gochnatia glutinosa 4.2Acantholippia

seriphioides5

Senecio subulatus var.subulatus

1

Buddleja mendozensis 5Senna aphylla 0.6Total shrubs 37.7 *73.3 51.5 50 21.3Tephrocactus

aoracanthus15.6 6.3 16.6 8.9 64.2

Trichocereuscandicans

0.7 2 1.9

Opuntia sulphurea 0.8 4 6 14.5Echinopsis leucanta 6.3 1.8 0.4 1.2Denmonza

rhodacantha1.1

Pyrrhocactuscatamarcensis

2.4

Total cactus 21.9 9.6 23 21.5 *78.7Prosopis flexuosa 2.5Prosopis chilensis 6.1 2.3Total trees 0 *8.6 2.3 0 0

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can be observed. Fine sediments are usually observed in the center ofthese circular growth populations. Tephrocactus aoracanthus individuals(16.6%) are equally distributed and usually settle in dense populations,although in these inactive landforms, coverage of rock fragments isclose to 46% (Fig. 2 D).

d) Lacustrine landforms (LL)

Cactus total coverage in these landforms is higher than in otherunits (78.7%) (Table 1). This proportion of cacti is divided solely intotwo species, Tephrocactus aoracanthus (64.2%) and Opuntia sulphurea(14.5%). Moreover, the shrub layer has the lowest coverage of all units

(21.3%, Table 1); in addition, the height of individuals of these speciesdoes not exceed 1m. The area of lacustrine landforms in contact withpiedmont debris coverage seems to be favorable for the development ofscarce shrub species like Bulnesia retama, since lacustrine sedimentsbecome a mix between fine sediments (coverage > 70%) and scarcefragments of greywackes rocks (coverage < 6.1%). Herb and treelayers are absent in these landforms.

3.3. Vegetation and topographic characteristics

Overall, community attributes and topographic characteristics werecorrelated with different geomorphic surfaces in Cerro Zonda Mt.

Fig. 3. Principal components analysis (PCA) of the 67 vegetation sampling units. The percentage of the variance explained by each axis is indicated in parenthesis.TWI: Terrain Wetness Index, TRI: Terrain Roughness Index.

Table 2Characteristics of the vegetation community. Superficial attributes and topographic features of each landform. Significant differences (p < 0.05) are indicated withletters in italic and bold. The asterisk indicates that the sum of the values is equal to 100%. The Ϭ represent mean value and SD standard deviation. SS area: Samplingsite area. Total area: the entire superficial extension of the landform.

Landform

Pediment of MU Active Inactive Raised by neotectonics Lacustrine

(PMU) (AL) (IL) (RIL) (LL)

n= 5 n=23 n=23 n=8 n=8

Vegetation Mean coverage*(%)

23.8 a 24.4 a 33 b 29 b 16.7 c

Community Diversity (H') 2.2 a 2.4 a 2 b 2 b 1 cProperties Richness 19 ab 23 a 17 b 17 b 4 cSup. deposit Rock fragments* 62.4 d 58.4 a 46.3 b 52.3 ab 6.1 cproperties Sediments* 7.6 a 13.3 a 17.4 a 14.7 a 73.5 bMean coverage

(%)Mulch* 6.2 3.9 3.3 4 3.7

Total unitarea

TotalSS area

Total unitarea

TotalSS area

Total unitarea

TotalSS area

Total unitarea

TotalSS area

Total unitarea

TotalSS area

Slope (degrees) (7.1/2.4) (7.9/4.5) (6.5/2.2) (7.3/3.5) (10.3/2.7) (10.8/3.7) (11.8/4.4) (11.9/3.7) (5.6/0.9) (2.5/2.1)Topographic Wetness index (10.4/1.4) (8.6/0.1) (10.9/2.1) (12.3/0.2) (10.4/1.6) (8.7/0.1) (8.7/1.4) (7.7/0.1) (10.3/0.9) (11.6/0.1)properties Roughness index (0.9/0.3) (1.7/0.2) (1/0.4) (1.6/0.1) (1.6/0.5) (1.8/0.2) (1.9/0.7) (1.7/0.2) (0.9/0.1) (1.5/0.1)(x/SD) Elevation (m asl) (1950/1.5) (1728.8/

58.3)(898/12.7)

(931.7/85.4)

(1031.2/26.3)

(945.4/63.9)

(1011.6/33.4)

(943.3/33) (835/0.8) (832.7/7.4)

Aspect (degrees) (259.3/67.7)

(263.9/122.1)

(87.3/38.8)

(70.6/31.1)

(83/33.9) (87.4/26) (92.1/54.7) (97.7/43.6) (85.4/17.2)

(85.7/26.5)

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Terrain roughness was the most significant variable to explain whichconsists of the areas with a higher diversity index (r2=0.98,p=0.001) (Fig. 3). The highest values of roughness were found onactive landforms and those landforms of mountain upland (PMU)(Table 2).

Bulnesia retama dominated the community associated with activelandforms and Tephrocactus aoracanthus prevailed in lacustrine land-forms, both reporting high wetness values. These landforms exhibited ahigher shrub and total coverage compared to the community dominatedby L. cuneifolia which was found on inactive, raised and pediment ofmountain unit landforms.

Slope values exert a positive influence over the vegetation cover,(r2= 0.65, p=0.001) but this in turn suggests less species richness,(r2= - 0.66, p= 0.01) (Fig. 3). Both landforms, inactive and inactiveraised by neotectonics have the highest slope values and share 17species (Table 2) in a 53% (Table 1); what is more, there is no differ-ence in vegetation coverage between them.

As regards the correlation of terrain wetness index, our results in-dicate that it is not a control variable of species richness across thedifferent landforms (r2= 0.01, p=0.17) (Fig. 3).

In this study, the 48% of the sampling units in piedmont hillslopeswas facing E (67°-112°), the 30% NE (22°-67°), 17% SE (112°-157°) and1% was facing N (337°-22°) (Fig. 1 F). The rest of the proportion (4%)corresponds to DRO, in which 4 SU were facing W (247°-292°) and 1NW (292°-337°).

We found that vegetation coverage was not significantly different ondifferently facing hillslopes along the landforms of Cerro Zonda Mt.(F(5, 61)= 1,06, p=0,38); the same trend was observed for diversity(F(5, 61)= 1,26, p=0,29) and richness (F(5, 61) = 0,49, p= 0,78).

The elevation of the piedmont fluctuated from 822 to 1100m asl,while the pediment of mountain upland (PMU) ranged from 625 to1780m asl. This different altitude did not have a limiting effect oncoverage, diversity or richness of vegetation community (r2= 0.001;0.11 and 0.008 respectively; p > 0.05) (Fig. 3).

The vegetation of all five landform types was represented graphi-cally along a toposequence beginning in the pediment of mountainupland, down through the piedmont and its landforms as inactive, risenby neotectonics, lacustrine and active. Along this toposequence, typicaldistribution and coverage were symbolized. Here, we combined DEMdata with vegetation coverage data from field work. This graphic re-presentation aims to provide a didactic idea for a native ecosystem andto show the dynamic interaction of landform variables, distribution andcoverage patterns of the vegetation community.

4. Discussion

Numerous articles have described the pattern of vegetation in var-ious areas of the Monte Desert phytogeographic province at differentscales and then related it to different factors such as geomorphologicalconditions and vegetation patterns, among others (e.g. Morello, 1958;Bisigato and Bertiller, 1997; Ares et al., 2003; Rossi and Villagra, 2003).Our results particularly support the hypothesis that landform propertiescondition the vegetation distribution and coverage, as well as the spe-cies richness of the geomorphological units of Cerro Zonda Mt.

Landforms on rock outcrops, located at a higher altitude, aredominated by the herbaceous species Deuterochonia longipetala. Both,inactive and raised by neotectonics landforms are covered by Larreacuneifolia while in lacustrine landforms, Tephrocactus aoracanthus wasidentified in the poorest and less diverse community in this study.Active landforms have the richest and most diverse community; theseare led by Bulnesia retama, (Table 1). These results suggest that at alandscape scale in Cerro Zonda Mt., topography determines hydro-logical processes, and these are an important factor for the distributionof plant communities as previously established in several studies re-garding Monte ecosystems (Rossi and Villagra, 2003; Flores et al.,2017). Nevertheless, in such ecosystems, plant distribution is also

associated with fertility islands and the edaphic factor at fine-scales(Bisigato et al., 2009).

In this study, the mophodynamics of units are well represented byroughness and slope values obtained from the DEM. These landscapeproperties would be related to the distribution and water availabilityinvolved in many hydrologic processes such as headward erosion inlacustrine landforms, debris flow and lateral erosion in active landformsand surface runoff in inactive landforms. In fact, both are importantvariables for predicting which habitat will be used by species and whichspecies will occur across a variety of environments (Kutiel, 1992; Nevoand Garcia, 1996). According to van der Meij et al. (2018), geomorphicprocesses change surface morphology by removing, redistributing anddepositing sediments. These changes create indirectly “new” sites forthe establishment of annual herbaceous and cacti species increasing thesite richness and diversity.

Topographic wetness index represents the tendency of the site toreceive water from upland area and the tendency to evacuate water(Gruber and Peckham, 2008) and reflects soil moisture as the mostimportant determinants of vegetation composition (Kopecký andČížková, 2010). Nevertheless, in this sector of Monte Desert, thisparameter does not show a positive correlation with vegetation di-versity. This could be because this index (and its algorithm) is based onterrain characteristics but does not take into account the intrinsicproperties of the deposits as rock fragments and fine sediments cov-erage.

The DEM analysis allowed us to determine sites with the highesttopographic wetness value such as active landforms that had collectedprecipitations during the summer season. However, these landforms areformed by aggradation from highly episodic water run-off and massiveflow events (de Haas et al., 2014), producing highly permeable depositsunable to retain surface moisture. The large trees and shrubs thatdominate these landforms (such as Bulnesia retama, Prosopis flexuosaand Prosopis chilensis) have deep roots that allow them to benefit fromthe water table.

The inactive and inactive raised landforms (IL, RIL), with high ve-getation coverage, are not the richest in species (Table 2). These resultsindicate that in these stable landforms, a few species adapted to ex-treme aridity dominate, such as Larrea cuneifolia and Tephrocactusaoracanthus (Table 1). In addition, these landforms, characterized by agreater varnish coverage, are prone to suffer upper temperatures due tohigh solar irradiation and heat capacity (Wood et al., 2005), thus pre-senting a limiting factor for the settling and development of species.The highest topographic position in the piedmont (Fig. 1 D) is placed inthe inactive landforms raised by neotectonics. This topographic char-acteristic appears to increase the aridity of the landforms through a risein the water run-off and wind exposure, favoring the establishment anddevelopment of cactus species (6) (Table 1). Hutchins et al. (1976)found that slopes receiving greater amounts of solar radiation hadhigher temperatures and a greater evaporative demand which was re-flected in less dense tree stands and less well-developed vegetation. Inthe Cerro Zonda Mt. these properties seem to affect the number ofspecies but no the biomass production reflected in the highest vegeta-tion coverage.

Surface characteristic of landforms such as rock fragments coveragehave a complex influence on soil hydrological processes (Zhang et al.,2016), furthermore we found that they collaborate with the control ofthe species richness and could be an important parameter of soil erosionvulnerability (Imeson and Prinsen, 2004). On one hand, rock fragmentsresting on the soil surface or partly incorporated in the top soil affectrainfall interception (moisture barrier), runoff generated by the rocksurface, infiltration as well as evaporation (Poesen and Lavee, 1994).On the other hand, the superficial rock fragment cover acts as a naturalbarrier for seeds and fine earth transported by superficial runoff events,in consequence, the combination between superficial processes andseed dispersal would benefit species richness. In addition, Wood et al.(2005), mention that the depth of soil water movement and solute

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transport are strongly tied to surface clast characteristics across alandscape in the Mojave Desert. These authors suggest that sites withclast coverage of rocks under 65% prevent the accumulation of solublesalts; furthermore, Suvires (2004) has found clay sediments with highvalues of Na+, Ca2+ and Mg2+ on piedmont landforms associated withthe lithology of Precordillera. However, in this research, we did not findhalophyte species associated with higher rock fragments nor sedimentcoverage.

These results suggest that the number of species present in the dif-ferent landforms is determined by variables that act at a microsite scale,such as the coverage of rock fragments. In active landforms, where thehighest values of topographic humidity were found, 65% of the totalspecies were small size herbaceous and cactus individuals (Fig. 4).

Cactus and herb individuals occurring on active landforms (AL) arerelated to large-sized trees like Prosopis chilensis and Prosopis flexuosa(mesquite) that act as containment barriers against heavy rainfallevents in the area, which could reflect the “Nurse Effect Syndrome”proposed by Niering et al. (1963). Our results suggest that this is acommon interaction in this sector of the Monte Desert, coinciding withFlores and Jurado's (2003) view on arid and semi-arid environments.

As regards the creation of new sites for plants in landforms withhigh rock coverage, Pye (1995) mentions that at finer scales, landformproperties influence the distribution of dust such as mountaintop andbench which contain less dust than mountain flank or mountain pied-mont landforms. Hydrological active landforms are characterized byrough surfaces with poorly sorted clasts combined with steep slopes,which this increases the potential for dust deposition and retentionwhen heavy rains are absent. As a result, microsites between surfaceclasts trap and protect the dust from entrainment by the wind (Cookeet al., 1993). These properties have allowed the creation of suitablesites for the establishment and development of cactus and herbaceousspecies and could also explain the highest richness values found both inthe pediment of mountain upland (PMU) and active landforms (AL)(Table 1).

Most of the hillslopes of Cerro Zonda Mt. are found to be east-,northeast- and southeast-facing, in which dissimilar values of vegeta-tion coverage are obtained. On the one hand, LL shows a lower vege-tation coverage and species richness than all the other piedmont de-posits. On the other hand, IL has the highest coverage (Table 2) and ALpresents the highest richness, even though, all are facing east. The PMUare subtly facing west, nevertheless these have middle coverage and ahigher richness value than the piedmont units. These results coincidewith Kutiel and Lavee's (1999) claim about the control effect of the

hillslope aspect, stating it does not play an important role in de-termining biotic features when the arid conditions are severe, due to thehigh evaporation potential as it is common in extreme arid zones.

Although altitudinal variation does not have a control effect onspecies coverage, it does have control over vegetation layer dominance.In piedmont landforms, located from 800 to 1030m asl, shrub speciesas Larrea cuneifolia and Bulnesia retama are dominant. However, in PMUlandforms, situated above 1700m asl, the herbaceous vegetation hasgreater coverage with Deuterocohnia longipetala being the prevailingspecies. Furthermore, 6 of 19 species are found only in these landforms(two herbaceous and four shrub species). Similar results have beenpreviously shown by Dalmasso and Marquez (2004) in the Pedernalmountain range, situated in the Eastern Precordillera, 51 km from theCerro Zonda Mt. in a southern direction. They state that in mid-altitudelandscapes (up to 1500m asl) as the piedmont, Larrea cuneifolia isdominant and cacti species are abundant too. But in high-altitude units(above 1900 m asl), the vegetation consists of grasslands with isolatedshrub individuals. In addition, the same authors indicate that treespecies such as Prosopis (mesquite) only are found in low-altitudes (upto 900m asl) like valleys and temporary channels.

5. Conclusions

At a landscape scale, the vegetation pattern communities in aridzones could be established through the analysis of different landformsand its topographic properties.

Vegetation is typically dominated by shrubs, herbs and cacti, whosedistribution differences are mainly determined by water availabilitywhich is dependent of topographic characteristic of the landscape aswell as surface properties. However, in each of these, the interspecificinteractions of the community could be playing a very important role incompetition and recruitment of individuals.

Within the study area, slope values and roughness determine thevegetation coverage at a regional scale. In addition, the interpretationof the topographic wetness index value could lead to confusion due, inpart, to false speculations about the wet sites and their relationship witha higher plant coverage. Although this index is usually interpreted as arelative measure of the availability of soil moisture in the long term,this claim would not apply in areas with a moderate slope and rough-ness, because they (active landforms in our research) usually evacuateheavy precipitation water in the form of debris flow, diminishing thecapacity of water retention while increasing permeability.

At a local scale, the high coverage of rock fragments in active

Fig. 4. Topobiosequence in eastern piedmont sector of Cerro Zonda. The topographic data was obtained from the DEM, whereas vegetation was drawn using adrawing program. Rhc: Relative herbs coverage; Rsc: Relative shrubs coverage; Rcc: Relative cactus coverage; Rtc: Relative tree coverage. Vegetation is not to scale.

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landforms (AL) contributes to the formation of new areas due to thecapture and retainment of fine sediments, carried in turn by the actionof heavy summer rainfall, thus increasing the richness and diversity ofthe vegetation. For the remainder of the year, these landforms (AL) canbehave as inactive landforms (IL) allowing for the colonization of an-nual herbaceous species. In inactive landforms, both the high desertvarnish and pavement coverage have negative effects on the vegetationsince they constrain species richness. However, they show a positiveeffect on the coverage of a dominant species such as Larrea cuneifoliaand Deuterocohnia longipetala.

Finally, relationships between landforms and vegetation in theCerro Zonda Mt. can be summarized by combining a terrain analysis ofDEM with evidence of the dynamic processes in the field. These twomethodologies should be complementary and should not be analyzedseparately.

Acknowledgements

We are grateful to an anonymous reviewer and Prof. JimenaOlivares for comments on this paper. This research was supported byPICT 4343-2016, Agencia Nacional de Promoción Científica yTecnológica, Argentina.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsames.2019.102359.

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