Andean rolling mosses gather on stone pavements: Geoecology of Grimmia longirostris Hook. in a high periglacial páramo

Catena 187 (2020) 104389

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Andean rolling mosses gather on stone pavements: Geoecology of Grimmia longirostris Hook. in a high periglacial páramo

T

Francisco L. Pérez Department of Geography and the Environment, University of Texas, Austin, TX 78712-1098, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Andean páramo Geoecology Globular mosses Moss balls Needle ice Periglacial Rolling mosses Stone pavements

Rolling mosses of Grimmia longirostris Hook. were studied in Páramo Piedras Blancas, a high Andean area in Venezuela, using a geoecological research framework focused on interactions between geomorphic, ecological, and soil processes. Spatial patterns of mosses were analyzed downslope from granitic outcrops on three slopes at 4445–4480 m elevation. Fragments from epilithic mosses on the apex outcrops were the source of other mosses. Distance of moss transport below rocks was measured at all sites; mosses on the gentlest (≤4°), 34 m-long, slope were censused along a belt transect on 17 contiguous quadrats; density of moss growth forms, stone size, percent of bare-soil cover, and surface soil properties were determined. Surface clast morphology and profile characteristics of a periglacial stone pavement at the slope base were also examined. Mosses on this site included four growth forms: epilithic mosses on outcrops; mobile, rolling moss balls; epilithic mosses on loose clasts; and unattached elongated mosses. Moss balls (density: ≤0.87 mosses/m2) lay on upper-transect plots over bare soils (≤92% cover) disturbed by needle ice. Soils here contained ~43–51% fine grains (≤0.063 mm) and 2.9–5.9% organic matter; this made them exceptionally prone to ice segregation and needle-ice formation. Lower-transect plots showed 78–100% stone cover—cobbles and blocks—but little exposed soil; mosses attached to loose clasts were exceedingly abundant (≤31/m2) on this section. Unattached elongated mosses, often resting along boundaries between adjacent stones, were also common (≤5.5/m2). The basal stone pavement showed a shallow (≤7 cm) rock veneer over a 24 cm-thick profile with exceptionally fine soil—up to 83.2% fine grains, ≤8.6% organic matter. A conceptual developmental model emphasizing the close linkages among the spatial distribution of mosses, patterns of surface clasts, and characteristics of soils and stone pavements is proposed: (i) Stones and mosses are transported downhill by needle ice and other frost agents; moss fragments gradually assume a globular shape, forming rolling moss balls. (ii) When these reach the stones and pavements on the basal slope, they cannot be transported any further, and remain immobile. (iii) Some mosses come to rest along boundaries between stones and develop into unattached elongated mosses or, (iv) they may become gradually attached to cobbles on the stone pavement; clasts with epilithic mosses can continue moving toward the basin floor. (v) Some mosses become fragmented and provide a source of elongated mosses, or remain partially attached to clasts. Stone pavements develop by a combination of rock accumulation following needle-ice transport, rock upheaving and sorting by ground frost, slope runoff, and downward soil illuviation.

“Rolling stones can gather moss!” (Jón Eythórsson, 1951) 1. Introduction In a lucid review of interactions of organisms with their environment, Reinhardt et al. (2010: 94) suggest that “Life and landscape are often so intimately connected that they cannot be separately treated.” Following such premise, this study aims at bringing together ecological and geomorphological research on rolling mosses of Grimmia longirostris

Hook., and associated substrates in Páramo Piedras Blancas, Venezuela; páramos are the high elevation areas above timberline in the tropical Andes. 1.1. Development of rolling mosses Rolling mosses (Dixon, 1899; Benninghoff, 1955), also known as moss balls (Lid, 1938; Glime, 2017) or globular mosses (Beck et al., 1986; Mägdefrau, 1987) are terrestrial, normally saxicolous, mosses that become mobile and grow free of any attachment after being

E-mail address: [email protected]. https://doi.org/10.1016/j.catena.2019.104389 Received 10 May 2019; Received in revised form 19 October 2019; Accepted 24 November 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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side up and are fitted together like a mosaic.” (Washburn, 1980: 173). Stones on pavements collect by a combination of processes. Upwards migration of clasts through the soil to its surface may be caused by freezing and thawing, wetting and drying, or bioturbation (Goudie, 2004). Ground saturation allows rotation and sifting of stones, and fine materials may also be removed or redistributed by meltwater (Washburn, 1980). Small, flat tabular stones are often rotated during their ascent through the soil profile, and appear at the surface vertically packed ‘on edge’ (Troll, 1958; Tricart, 1963). Due to their peculiar arrangement, periglacial pavements are also known as ‘stone packings’ (Hastenrath, 1971; Schubert, 1975), and nidos de piedras (Corte, 1955). Stone pavements are common on high tropical mountains, both in the Andes and Africa (Troll, 1958; Hastenrath and Wilkinson, 1971; Hastenrath, 1974; Schubert, 1975).

dislodged from their original rock substratum. Moss detachment and/or dispersal can be caused by various geomorphic agents, including recurrent frost activity, extreme drought and desiccation, running water, rainsplash, or heavy winds; disturbance or trampling by animals may also be important in some areas (Hedberg, 1964; Mägdefrau, 1987; Pérez, 2010). As a detached moss polster is recurrently shifted around during downhill transport, all its sides are eventually exposed to sunlight and the moss tends to expand uniformly in all directions, thus a spheroidal ball-like form gradually develops. Ultimately, a rolling moss may become too large to be turned over regularly, increasingly remaining on the same side, thus grows flattened and breaks apart following damage by erosion, or loss of stem tensile strength (Hedberg, 1964; Shacklette, 1966; Beck et al., 1986). The first moss balls collected (Grimmia kidderi) in the Kerguelen Islands (Indian Ocean), were described (Kidder, 1876: 25) as ‘…growing in small (detached) globular masses on hill-sides…”. Subsequently, rolling mosses have been found in ~36 localities in all continents except Australia, and in several oceanic islands (Pérez, 2010). The vast majority of globular mosses occur in periglacial, high-latitude and mountain, areas where freeze-thaw events and/or wind often disrupt and fragment moss cushions; further frost or aeolian activity then transports the loosened mosses. Moss balls most often form in species of cladocarpous or acrocarpous genera of leafy mosses that produce dense hummocky cushions. Over 40 moss species develop globular specimens; the genus Grimmia, with ~11–12 vagrant species worldwide, has more representatives than any other taxa, particularly in tropical mountains (Table 1). Local spatial patterns of globular mosses have been examined only at a few sites. Moss balls normally result from the breakup of large epilithic moss polsters; as these are carried downhill by frost or other transport agent, moss balls typically travel a modest distance (≤5-8 m) below the outcrops that provide the source of fragments, and show a simple elongated pattern of concentration downslope from the rocks (Beck et al., 1986; Pérez, 1991, 2010). In Kerguelen, Kidder (1876) found wind-dispersed mosses were distributed in a fan shape, radiating from a central point; wind transport also frequently concentrates moss balls in shallow soil depressions (Hébrard, 1970; Imshaug, 1971).

1.3. Research objectives The initial goal of this study is to analyze the spatial arrangement of rolling mosses on three field sites in the Páramo Piedras Blancas. Two sites, located downslope from small outcrops at ~4445 and ~4480 m, showed relatively simple distribution patterns of moss balls; the third site, found below an extensive outcrop at ~4460 m, displayed a more complex pattern of moss dispersion, which was compared to those observed on the other two locations. This third site constitutes the main focus of this geoecological study, which will primarily: (i) contrast the different moss growth forms and their shapes along a gentle, 34 m-long slope, based on the presence and density of mosses on 17 individual sampling quadrats (plots), arranged along a continuous belt transect; (ii) discuss the spatial distribution of these moss forms in association with the inferred activity and frequency of needle ice formation on the individual plots; (iii) examine surface soil properties along this slope, and evaluate their suitability to support ice segregation and needle ice development; (iv) investigate the horizon features of a soil profile excavated down to bedrock in the periglacial stone pavement, on the slope base; (v) determine the surface sedimentological characteristics of this stone pavement, in order to identify the processes presumably involved in pavement generation; (vi) assess the influence of the pavement on moss patterns along the slope through its likely inhibition of needle ice formation; and (vii) propose a developmental sequence for the moss forms censused, and for the processes leading to pavement formation.

1.2. Periglacial stone pavements Periglacial stone pavements are “…accumulations of rock fragments, especially cobbles and boulders, in which the surface stones lie with a flat

2. Study area: Páramo Piedras Blancas

Table 1 Grimmia species previously reported to form moss balls in high tropical mountains, all on areas with frequent needle-ice formation; locality and altitude indicated. Moss taxonomy for Grimmia follows Muñoz and Pando (2000).

Páramo Piedras Blancas is in the NW Venezuelan Andes, SE of Lake Maracaibo, at 8°52′N 70°55′W. This high alpine area reaches 4740 m at Pico Piedras Blancas, and is part of the extensive (2004 km2) Sierra La Culata National Park (SLCNP) (Fig. 1). Precipitation above 4100 m averages 796–814 mm/yr (Sarmiento, 1986), but rainfall at the study sites is ~700 mm/yr (Silva, 2001). Rain is unimodal: the May-Aug. wet season receives ≥100 mm/mo, whereas ≤20 mm/mo fall during the Dec.-March dry period; snowfall contributes 41–50% precipitation on areas ≥4200 m (Silva (2001). Owing to its high elevation and nearequatorial position, this páramo experiences a periglacial climate with broad daily fluctuations. Mean annual temperature is ~2.8 °C; air temperature may drop during the night to −12 °C, and rise next day to 23 °C, but persistent cloud cover greatly reduces diurnal amplitude in the rainy season; soils are subjected to ~350 freeze/thaw cycles each year (Pérez, 1987a). High frost recurrence causes widespread formation of needle ice, a frost type that severely disturbs the soil surface at high elevations. Among the most common soil patterns are nubbins and buds (Washburn, 1980, Pérez, 1996), small round to elongate earth lumps a few cm in diameter, which may cover the ground. Striated ground (Schubert, 1973; Pérez, 1984) develops as nubbins become aligned with the direction of solar rays, causing the soil surface to develop a raked appearance;

1. Bryum argenteum Hedw. Sierra Nevada del Cocuy, Colombian Andes, ~4400 m (Cleef, 1981) 2. Grimmia laevigata (Brid.) Brid. (=G. campestris Burchell ex. Hook). Mount Elgon, Kenya- Uganda, 4300 m (Potier de La Varde, 1955; Hedberg, 1964) 3. Grimmia longirostris Hook. Sierra La Culata, Venezuelan Andes, 4290–4510 m (Pérez, 1991); Bale Mtns, Ethiopiaa (altitude not reported) (Glime, 2017) 4. Grimmia obtusolinealis C. Müll. Bale Mtns, Ethiopia, 4377 m (Greven, 2014) 5. Grimmia ovalis (Hedw.) Lindb. (=G. ovata W. et M.). Mount Kenya, Kenya, 4200–4500 m (Hedberg, 1964; Beck et al., 1986; Mägdefrau, 1987); Mount Kilimanjaro, Tanzania, 4450 m (Potier de La Varde, 1955; Hedberg, 1964) 6. Grimmia torquata Drumm. Haleakalā. Maui, Hawai’i, 2175–2725 m (Pérez, 2010) 7. Grimmia trichophylla Grev. Haleakalā. Maui, Hawai’i, 2175–2725 m (Pérez, 2010) 8. Grimmia spp. Sierra Nevada del Cocuy, Colombian Andes, ~4400 m (Cleef, 1981) 9. Grimmia spp.b Guagua Pichincha, Ecuadorian Andes, ~4620 m (Pérez, 2010) 10. Racomitrium crispulum (Hook. f. et Wils.) Dix. Sierra Nevada del Cocuy, Colombian Andes, ~4400 m (Cleef, 1981) a

Reported presence of this species in Ethiopia may be in error, as Greven (2014) refers to mosses shown on the same photograph in Glime (2017: Fig. 32) as ‘G. obtusolinealis’. b This Grimmia species in Ecuador probably is G. longirostris, originally collected by Humboldt and Bonpland in Chimborazo in 1802. 2

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Fig. 1. (A) Location of the Venezuelan Andes cordillera (stippled section). (B) Generalized topographic map of the NE Venezuelan Andes; contour interval is 500 m. Sierra La Culata National Park (SLCNP) is the dark stippled area SE of Lake Maracaibo; Páramo Piedras Blancas, the study area, is shown by a dark triangle. Rivers are shown by thin dash/dot lines; dark dots indicate major cities. Scale is in km. Source: Cartografía Nacional, Caracas.

uniform, soils of coarse, gravelly sand, lacking any distinctive horizons, (Pérez, 1987b, 2002). Longitudinal profiles show gentle slopes with a well-developed convexity throughout their upper and middle segments (Pérez, 1987a), suggesting that widespread periglacial soil creep is a significant factor controlling their development (Ritter et al., 1995). Movement data for 6 consecutive years (Pérez, 1987a, 1992a) ascertained high rates (6–19 cm/yr) of substrate surface creep, both for loose, isolated stones and cobbles on sorted stripes. Site 3 is also on the upper reaches of Mifafí valley, in an extensive area of gently undulating topography occupied by several shallow glacial tarns—the Apersogadas lakes at ~4310–4470 m—and by numerous small rock outcrops with roche moutonnée features (Fig. 2). Site 3 extends below one such roche moutonnée and is characterized by a gently concave slope that forms part of a small, shallow basin; this site will be examined in greater detail below (see Section 4.2). Bedrock at the study sites was relatively uniform, and varied from granite and quartzite (site 1), to pegmatitic granite (site 2), and granitic gneiss with mica-schist intrusions (Pérez, 1987a, 1992a). Soils are derived from igneous and metamorphic rocks; Lithic Haplocryepts and Cryorthents occupy the highest slope areas; soils along creeks are Humic Cryaquepts and Typic Cryosaprists, and basal slopes are associated with Typic Cryorthents and Entic Cryumbrepts (Pérez, 1992a, 1996; Soil Survey Staff, 2014). Soils are commonly shallow, rocky, and contain low amounts of organic matter (SOM). Much of the mineral fraction—92–96%—is sand; the fine fraction is largely silt, with little clay (Pérez, 1984). Vascular páramo vegetation is dominated by species of tall caulescent rosettes and several nanoshrubs, grasses, and herbs; plant nomenclature follows Luteyn (1999). Coespeletia timotensis rosettes reach their high limit at ≥4500 m. At higher elevations, the periglacial desert is characterized by a sparse but diverse epedaphic cryptogamic plant cover on ground areas affected by intense frost activity; in Piedras Blancas, vagrant forms include the lichens Thamnolia vermicularis, Xanthoparmelia vagans, and Catapyrenium lachneum, and the moss Grimmia longirostris Hook., the focus of study here (Pérez, 1991, 1994).

pebble-sorted stripes are another frequent pattern (Schubert, 1975; Schubert and Vivas, 1993; Pérez, 1992a). Needle ice can also induce significant frost heave and creep of surficial materials, including large stones (Pérez, 1987a; Ponti et al., 2018). Subjacent rocks in this mountainous region are Precambrian metamorphics (Iglesias Group), with granitic intrusions of Upper Paleozoic age (Kovisars, 1972); the Piedras Blancas area is underlain mainly by granites, gneiss, mica-schist, pegmatite, quartzite, and amphibolite (Schubert and Vivas, 1993). The central Venezuelan Andes were affected by extensive alpine glaciation during the Late Pleistocene; this regional episode is named the Mérida Glaciation (Schubert, 1974; Schubert and Valastro, 1974). The Mifafí creek valley—where all study sites are located—was covered by sizable glaciers, which extended for ~19 km length and reached thicknesses of ≤250 m, until at least ~12650 ± 130 yrs BP (Schubert and Vivas, 1993). Glaciers left a profusion of erosional and depositional landforms, including extensive talus slopes, rock glaciers, and glacial tarns, at high elevations (Schubert, 1975; Pérez, 1985, 1988) (Fig. 2). During late Holocene, starting around 1290 ± 80 AD, the Little Ice Age (LIA) of Medieval times brought important temperature fluctuations to the area (Rull et al., 1987). This cold period lasted until the mid-19th century, and apparently triggered again the development of sizable taluses, as well as a valley-flank rock glacier ≤1.5 km away from the study sites (Schubert, 1975, 1980; Pérez, 1988). Lichenometric growth curves developed for Rhizocarpon sect. Rhizocarpon in the Venezuelan Andes (Vareschi, 1970) indicate this rock glacier may have started accumulating just a short time before ~1400 CE (Pérez, 1988). Slope morphology, as well as various surficial geomorphic features, show all study sites have been considerably affected by recent frost activity. Sites 1 and 2 are located on the gentle (11-13°) upper valleyflank slopes at the edge of a small plateau extending at ~4440–4500 m near Pico Los Nevados (4685 m) (Fig. 2.1). Glacial geology maps (Schubert, 1975; Schubert and Vivas, 1993) indicate these slopes have developed on the walls of a glacial cirque at the highest elevations of Mifafí creek valley, and are now covered by deep (~30–130 + cm), 3

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Fig. 2. Satellite view of the upper Mifafí valley, showing location of the three study sites, indicated by dark numbered dots. The access dirt road to Piedras Blancas appears as a winding white line following the valley axis, from the lower left to the upper right of the photo. Sizable talus slopes cover much of the SE-facing flank of Mifafí valley, north of the creek. Key: Mc: Mifafí creek. PLN: Pico Los Nevados (4685 m); the light-gray area at the base of Pico Los Nevados shows a rock glacier; the main frontal lobe of the rock glacier, and several large blocks on it, are clearly visible. Ap: Apersogadas glacial tarns; the largest one on the eastern edge of the photo appears to be partially covered by ice. AM: the Alto de Micanón (4650 m) is ≤1 km away south of this point. Black scale bar: about 0.5 km. See text for further details. Photo used with permission from Google Maps/Google Earth, following their Universal Terms of Service. Imagery date: Jan. 4, 2019. (2.1) Uppermost reaches of Mifafí valley. The photo shows the SE-facing flank at the valley head, which is occupied by a large glacial cirque; dense stands of tall columnar Andean rosettes (Coespeletia timotensis) cover the valley floor (~4370 m) and surrounding slopes. The light-colored tongue-shaped slope areas are occupied by deep, gravelly-sand, soils, transported downhill from a small plateau above them at ~4500–4525 m; Pico Los Nevados (4685 m) appears on the upper right. Study site 1 (~4480 m) is located right below the edge of the plateau and the peak, on the right side of the photograph. Photo: PPB.294, Aug. 1980. (2.2) General slope view at ~4460 m, next to study site 2, located downhill from this spot. Sandy soils are covered by nubbins and sorted pebble networks, produced by frequent frost activity. The sparse plant cover is restricted to isolated Hinterhubera imbricata specimens showing circular growth forms as they cluster around individual surface stones. Photo: PPB.747, Jan. 1992.

3. Fieldwork and laboratory methods

forms—rolling moss balls (rm), unattached elongated mosses (em), epilithic mosses attached to loose (non-embedded) clasts (ec), and epilithic mosses attached to outcrops (eo). In addition, some of the larger ec or rm mosses may be gradually eroded, and become fragmented (fm). Density (n/m2) of all moss forms was computed for each plot. For practical reasons, as well as due to conservation restrictions in this protected National Park, moss collection was limited to 25 representative rolling and elongated unattached mosses each, and 10 specimens of epilithic mosses on clasts; all were later measured in the lab, along with 15 additional epilithic mosses on clasts, evaluated in the field, for a total of 25 sampled individuals. Surface (0–10 cm) soil samples were taken on the center of four sampling plots with a 50-mm diameter, 125-cm3 iron cylinder, gently pressed on the soil surface to avoid disturbing bulk density. A soil profile was excavated down to bedrock on the stone pavement at the basal slope. Size—i.e., long (a) visible axis of clasts exposed on the ground surface (Pisarska et al., 2011)—and percentages of soil and stone cover were determined on each plot by photosieving (Caine, 1969; Ponti et al., 2018), a field technique frequently used to estimate particle size which provides substantial reduction of fieldwork time (Sime and Ferguson, 2003). Comparison of results with in situ stone measurements has shown minor to no statistical differences between techniques (Caine, 1969). A 200 cm-long scale was laid on the ground, and orthogonal photographs were taken on all plots; photos were

3.1. Field sampling and observations Elevation was assessed with Thommen altimeter—accuracy ± 10 m—and cross-checked on topographic maps (Cartografía Nacional, Mesa Redonda 6042-IV-SO, 1:25,000, 1974). Slope angles were determined with clinometer; aspect with Brunton compass. Moss balls were studied downslope from small outcrops at two sites (1, at ~4480 m, 2 at ~4445 m) with moderately steep gradients (≤13°). Distance from the base of the outcrops to each moss ball resting on the slope was measured with a metric tape stretched along an azimuth parallel to slope direction (Pérez, 2010); this is henceforth called distance of moss ball transport. Distance of moss ball transport was also determined at site 3 (~4460 m) along a continuous 34 m-long belt transect (MuellerDombois and Ellenberg, 2003) extending below tall, extensive outcrops on a gentle gradient (0–4°). This transect included 17 contiguous 2 × 2 m (4 m2) sampling quadrats but—due to sparse moss presence—plot size was doubled to 2 × 4 m (8 m2) on the upper 8 plots. Plot corners were ostensibly marked with color-coded flags, removed after sampling. A total of 914 individual moss specimens ≥25 mm were found on transect plots; all mosses resting on plots were censused, carefully examined, and assigned to one of four basic moss growth 4

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examined on a large screen with a superimposed 8x10 point grid, and 50 randomly generated grid points chosen for analysis without replacement (Dunkerley, 1996), thus each point represents 2% of a plot sample. The a axis of each censused clast was meticulously measured, and clasts were assigned to four size classes: blocks (B), with, a ≥ 10 cm; cobbles (C), 5–10 cm; pebbles (P), 2.5–5 cm; sample points with grains ≤2.5 cm were classified as bare soil (S). Clast size frequency-distribution was assessed from 250 stones measured for the stone pavement on eight (8–15) lower-transect plots. 3.2. Laboratory and analytical techniques Moss specimens were air-dried for 8 weeks in a stable (20 ± 2 °C, 60 ± 5% RH) room, then measured with vernier caliper—0.1 mm precision—along three axes (a: long, b: intermediate, c: short) used to determine shape parameters with standard sedimentological methods. Several moss shape indices were calculated: sphericity (c2/ [a × b])0.333, compactness (c/a), flatness ([a + b]/2c), and elongation (a/b) (Johansson, 1963; Goudie, 1981; Dackombe and Gardiner, 1983). Two additional indices, (c/b) and (b/a), were generated and used to project overall moss shape on modified Zingg diagrams (Dackombe and Gardiner, 1983; Pérez, 1991, 2010). Soil samples were air-dried for 8 weeks, oven-dried at 105 °C for ≥ 48 hrs, and weighed. Gravel (≥2 mm) fractions were sieved; true particle density was assessed by water displacement for several gravel specimens (Blake and Hartge, 1986). After subtracting gravel volume, bulk density of dry soil fractions was calculated using the formula in Curtis and Post (1964). Particle-size distribution was analyzed in dry samples by hand sifting through a 19-mesh sequence (19–0.09 mm); smaller soil grains were examined with ASTM-152H hydrometer after dispersing with sodium hexametaphosphate; all soil fractions are given as percentages by weight. Average grain size was calculated as the 50th percentile (D50, median) diameter (Folk, 1980). Texture types follow Soil Survey Staff (2014). SOM content was estimated by loss of weight on ignition at 375 °C, as this temperature does not affect fine-grain content (Ball, 1964). Dry-soil colors were determined with Munsell Soil Color Charts (1992). Chi-Square (χ2) goodness of fit tests were used to assess spatial uniformity in the observed distribution of moss forms along the belt transect (Bailey, 1995). Data sets for moss density and shape indices were tested for departure from normality with an omnibus test (d'Agostino and Pearson, 1973); kurtosis and skewness tables (Jones, 1969) also helped assess normality of data populations. As most data sets were small, they tested as not normally-distributed, with some positive skewness or moderate leptokurtosis (Sokal and Rohlf, 1995). Differences among data sets were compared with nonparametric Kolmogorov-Smirnov (K-S) or Mann-Whitney (U) tests, where condition of normality is not essential. Test results were evaluated with critical table values (Rohlf and Sokal, 1994).

Fig. 3. Moss ball site 1, ~4480 m, 11° slope gradient. Most of the vertical rock face is covered by G. longirostris cushions; short H. imbricata nanoshrubs grow around the outcrop base. About 14–15 dark moss balls are seen downslope from this 136 cm-wide, 37 cm-high outcrop; the two largest moss balls to the left of the circled 57 mm-diameter lens cap are 64 and 66 mm across. Several white, elongated thalli of vagrant Thamnolia vermicularis lichens are present on the site. Photo: PPB.979. Jan. 1992.

Site 2 (~4445 m) was also on a exposed ridge with a 13° slope and S135°E aspect (Fig. 2.2); mosses were downhill of a ~297 cm-wide, 55 cm-high granitic outcrop. The outcrop face had ~138 individual G. longirostris moss cushions, 15–105 mm diameter; H. imbricata was also common near the rock. The soil was covered by nubbins and pebblesorted stripes; 109 globular mosses were located 1–232 cm downslope from the outcrop, but most remained near it. Mean distance was 42.5 ± 45.1 cm, with a 25 cm median (Fig. 4B); moss ball diameter was 13–55 mm.

4. Results

4.1.1. Distribution of mosses and clasts below a large outcrop Mosses at site 3 (~4460 m, S146°E aspect) were found downslope from a large, ~3.7 m-tall, ~19 m-long outcrop—a roche moutonnée—of granitic gneiss with minor mica-schist intrusions (Figs. 5A, 6A). Slope gradient gradually diminished from 4° at the top to 0° on the basin floor, which was occupied by an extensive stone pavement. The slope presented a complex spatial distribution of mosses and surface clasts. Rolling moss balls, present on 15 of the 17 plots, were the most widespread moss form; χ2 goodness of fit tests showed rolling mosses were not uniformly distributed, but clustered significantly (χ2: 176.2, 16 DF, p < 0.001) on plots 2–4 and on plot 15, near the slope base (Figs. 5C, 6B). Epilithic mosses on outcrops were restricted to a few low (17–48 cm-high), widely spaced outcrops or boulders on the upper 9 plots. A χ2 test confirmed these mosses were significantly concentrated (χ2: 26.13, 8 DF, p < 0.001) on just a few plots, but this simply indicates that the outcrops themselves were more common near

4.1. Moss ball transport downslope of small outcrops Site 1 (~4480 m) was on a exposed ridge, devoid of rosette plants, with an 11° gradient and S157°SE orientation. Moss balls were found downslope of a small, 136 cm-wide, 37 cm-high granite outcrop with its downhill-facing side ~70% covered by G. longirostris moss polsters; short Hinterhubera imbricata nanoshrubs grew densely at the outcrop base. The ground around the rock was completely covered by small nubbins (Fig. 3); 119 rolling mosses were located below the outcrop. Distance of transport varied from 0 cm for a moss in contact with the outcrop to 412 cm; average distance was 140.8 ± 11.7 cm, median was 138 cm (Fig. 4A). The slope was carefully searched below the outcrop; this revealed several moss balls partially buried by soil. Diameter of moss balls or fragments was 11–82 mm. 5

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minute mud, or cake polygons (Hastenrath, 1973, 1974), where soil was raised on small polygonal sections, and pebbles were scarce. Gaps around stones (Washburn, 1980) were frequently seen around large rocks; these are produced as growing ice needles push soil away from clasts. Needle ice was observed on the ground surface on all occasions this site was visited. 4.1.3. Surface sedimentology and mosses on the lower slope Ground texture changed sharply after plot 6, as bare-soil cover dropped swiftly from ≥90% to 0% at plot 10, and then remained low (≤12–15%) across the gentle (2.5–0°) gradient toward the basin floor. Parallel to this, percentage ground cover by loose clasts increased considerably, with cobbles reaching 48–52% and blocks 30–36% (Fig. 6C). This transition in surface granulometry was also accompanied by a substantial shift of moss forms. Rolling mosses remained sparse (0–0.87/m2) on the lower slope, whereas a surge in epilithic mosses on loose clasts took place. These epilithic mosses abruptly made their appearance on plot 8, becoming ubiquitous on the basal transect, and also showed the greatest departure from a homogeneous distribution (χ2: 353.8, 9 DF, p < 0.001); they attained astonishingly high densities (12.25–31/m2) as they gathered along the upper edge of the stone pavement (Figs. 5C, 6B). This geomorphic surface exhibited classical characteristics of such periglacial features (Fig. 5D), and is described in detail below (Section 4.4). Elongated unattached mosses were also widespread and common on pavement plots 8–14, but reached only modest densities (2.5–5.5/m2) and presented a more uniform spatial distribution (χ2: 107.58, 9 DF, p < 0.001). Just as moss balls on the upper slope, these specimens were unattached to rocks, but had a distinctly non-spheroidal shape; instead, they were irregular and elongated (Fig. 7A) and were seen to often lay on the ground along the linear boundaries between larger stones, as if wedged there.

Fig. 4. Percentage frequency-distribution histograms for distances of transport between individual moss balls and source outcrops upslope. (A) Site 1, ~4480 m, n: 119 mosses; see Fig. 3. (B) Site 2, ~4445 m, n: 109 mosses. Class interval for both histograms is 0.5 m.

4.2. Shape of moss growth forms along the slope All numerical indices for the three sampled moss forms indicated several shape differences. As anticipated after visual assessment, rolling moss balls had the highest sphericity and compactness; both values were significantly higher (K-S: p < 0.001) than those of either elongated or epilithic mosses. Conversely, rolling mosses showed substantially lower flatness and elongation values than either elongated or epilithic forms on clasts (Table 2). In contrast, comparisons between elongated and epilithic mosses revealed no statistical shape differences; essentially, moss balls had a distinct morphology from the other growth forms (Fig. 7A). This assessment is validated by Fig. 7B, which shows the distinctive nature of moss balls—all clustered within the equant or spheroidal shape class—and the overall shape similarity of epilithic and elongated unattached mosses—mainly in the oblate and triaxial categories. K-S tests indicated that moss balls had a substantially different (p < 0.001) shape from both epilithic and elongated forms, but these two were not statistically distinct from each other.

the slope apex. The other two moss forms (epilithic on clasts, and elongated unattached) were restricted to plots 8–17, and are discussed below (Section 4.1.2.). The sparse site vegetation was examined in detail. The outcrop was covered by many G. longirostris polsters, and also provided a substrate for several tall C. timotensis rosettes and dense clusters of H. imbricata nanoshrubs. Widely dispersed patches of vascular plants on the slope included a nanograss (Agrostis breviculmis), a cushion (Arenaria musciformis), and a few herbs (Calandrinia acaulis, Geranium multiceps, Montia meridensis). Vagrant Thamnolia vermicularis lichens were common on bare-soil areas covered by nubbins (Fig. 5B).

4.1.2. Surface sedimentology and mosses on the upper slope The upper 14 m-long transect section (plots 1–7) was characterized by wide expanses of bare soil (80–92% cover) and a few small clasts, mostly cobbles (Figs. 5A, 6C). Only rolling mosses and epilithic mosses on outcrops were present along the upper transect (plots 1–7). Rolling moss balls were found on all plots, albeit at modest densities (0.62–4.87 mosses/m2) (Fig. 6B); moss polsters attached to outcrops had an even lower density (0.5–1.75/m2). Several moss balls had become fused with T. vermicularis lichens, and a few contained small inclusions of Marsupella spp., a liverwort common in páramo cryptogamic crusts (Pérez, 1996). Soil areas were covered by a myriad of raised fine-earth buds and nubbins, arranged in a quasi-polygonal pattern, separated by a network of linear depressions where pebbles accumulated (Fig. 5B). These miniature sorted stone nets (Troll, 1958; Hastenrath, 1971; Schubert, 1975), also known as cellular soils, Zellenböden, or Texturböden (Graf, 1976; Heine, 1977) are extremely common in high tropical mountains subject to needle-ice disturbance. The slope also displayed areas with

4.3. Characteristics of soils along the slope Surface soils showed only modest textural variation along the slope; gravel content was low (~13–38%), and most gravel particles were within the fine gravel (≤4.75 mm) fraction (Soil Survey Staff, 2014). All samples, either loam or sandy loam, had an unusually high percentage of fine grains (silt + clay, ≤0.063 mm), ranging from 43.1% (plot 4) to 51.1% (plot 6). Although content of clay-sized grains was high (3.7–6.8%) for this páramo, the bulk of the fines (88–92%) consist of silt (Table 3); this prominent silt content is thought to occur from the prevalent mechanical weathering produced by recurrent frost activity (Washburn, 1980; Pérez, 1984). Even though soils were fairly similar along the transect, there was a trend of diminishing grain size toward the slope base; this was associated with a downhill increase in percentage of fines, and a simultaneous drop in gravel content (Fig. 8A). 6

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Fig. 5. Belt transect at study site 3, ~4460 m; slope gradient ~0–4°. (A) General site view; transect extends toward the lower left, downhill of the ~3.7 m-high granite outcrop at upper right. Several tall (≤2.2 m) C. timotensis rosettes grow on the outcrop. The ground surface is completely covered by small nubbins and striated soils; note ~8 small outcrops or embedded blocks (lower right) with their downhill faces covered by dark mosses. On the upper left, and partially covered by snow, appear Micanón Peak (4650 m) and, below it, a rocky plateau (Alto de Micanón). Photo: PPB.1307, March. 2000. (B) View of quadrats 2–3, ~4–6 m downhill distance along transect; slope angle is ~3.5°, downslope is toward the bottom. This segment is covered by incipient sorted pebble nets; many white, elongated thalli of vagrant T. vermicularis lichens are present here. Larger stones on upper left are separated from the surrounding soil by ~5 mm-wide stone gaps. Horizontal scale is 15″ (~38 cm) long; vertical scale along the downslope direction is divided into 5 cm-long segments. Photo: PPB.1275. (C) View of quadrats 9–10, ~18–20 m downhill distance; slope angle is ~2°, downslope is toward the lower left. The photo covers an area ~2.2 m-wide on the foreground. The ground is covered by cobbles and blocks; note the numerous large, healthy mosses present on this slope segment, with mostly epilithic mosses attached to loose cobbles and elongated mosses. Photo: PPB.1290. (D) Orthogonal view of plots 12–13, ~24–25 m downhill distance; slope angle is ~1°. Photo shows the stone pavement; ~8 small saxicolous mosses grow on loose clasts, on the upper right. The largest stones in the photo are ~18 cm long; white scale is 1 m-long. See text and Table 4 for details. Photo: PPB.1315.

Bulk density (0.97–1.26 Mg m−3) was moderately low; this might be ascribed to frequent frost (needle ice) activity on this slope. Although the site has scarce vascular vegetation, SOM was fairly high (2.9–5.9%); this might simply indicate slow rates of organic decomposition in this high Andean area (Table 3).

also had a rather high (8.6%) SOM content. The excavated profile was observed to be underlain by seemingly unweathered granitic gneiss, similar to that found in the outcrops above the slope transect (Table 4).

4.4. Soil profile in the stone pavement

5.1. Patterns of moss transport downslope of small outcrops

The stone pavement on the basin floor presented a continuous surficial cover of angular to subangular clasts, neatly fitted together, as in a jigsaw puzzle; this Cjj horizon was just a shallow veneer one or two rocks thick, with a maximum depth of 7 cm. Individual clasts ranged widely in size (a axis), from ~2.7 to 23.8 cm; 21% were pebbles, 50% were cobble-sized, 27% were blocks ≤20 cm, and 2% exceeded 20 cm; average size (D50) was 8.9 cm (Table 4, Fig. 8B). Larger stones lay flat on the ground surface with their a axis horizontal, whereas smaller tabular pebbles and cobbles often appeared vertically packed, with their c axis exposed on edge at the surface (Fig. 5D); in some places, small clasts were also grouped into roughly circular areas, ~12–18 cm across, surrounded by larger stones (cf. Troll, 1958; Tricart, 1963). All soil horizons in the shallow, 24 cm-thick profile under the surface rocks had low contents of ≤30 mm fine gravel, from ≤11% in A11 and A13 to ~30% in A12h, which included just a few tiny pebbles. The percentage of fines was ample in all horizons, but reached an exceptionally high level—83.2%—in A12h (3–12 cm depth); this horizon

The spatial distribution of moss balls below two small outcrops on moderately steep (≤13°) gradients may be described as: (i) moss deposition occurred on a down-elongated area, roughly as wide across the slope as the outcrop (Fig. 3); (ii), most mosses were found near the outcrops; and (iii), there was a gradual reduction in moss deposition with further distance downhill (Fig. 4). This pattern may be further characterized as open-ended, with no discernible boundary of moss ball deposition at its lower end. This overall distribution of mosses has also been found on steep slopes (33°) in Haleakalā, Maui (Pérez, 2010), and at three additional steep sites (18–37°) examined in Piedras Blancas (Pérez, unpubl. data). Failure to find additional mosses further away from source outcrops in these sites may be ascribed to the following: (i) as moss balls continue rolling downhill on a steep incline, there are no sizable obstacles to stop them, thus become dispersed throughout the slope and/or hidden by ground irregularities; (ii) some rolling mosses are also buried and mixed with the soil, as both are disturbed by needle ice; (iii) many moss balls eventually become damaged, fragmented, and

5. Discussion and integration of results

7

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Fig. 6. (A) Longitudinal slope profile along belt transect, from NW (left) to SE (right). Topographic cross-section downslope of granite outcrops, where columnar rosette plants grow. Scale in meters; profile shows no vertical exaggeration. Small dark arrows indicate slope segments associated with different geomorphic features: a-b = upper slope, with exposed, bare soils; b-c = lower slope, with clast accumulation on a stone pavement; c marks the lowest point of this small basin; stone pavement extends beyond c, up the opposite basin slope. (B) Density (N/m2) of moss growth forms censused on individual plots. Key: em: elongated unattached mosses, resting on stone pavement; rm: unattached rolling moss balls, resting on bare soil; ec: epilithic mosses attached to loose clasts (cobbles or blocks); eo: epilithic mosses attached to outcrops or embedded blocks. (C) Percentages of ground cover by soil grains or clasts on individual plots. Key: s: exposed soil; p: pebbles; c: cobbles; b: blocks. Numbers over vertical bars refer to the sequence of sampling plots. Open circles below bars show locations of four surficial soil samples, dark circle indicates the location of a soil profile excavated in the stone pavement. (D) Proposed developmental sequence of moss growth forms down the basin slope along belt transect. 1: Epilithic mosses growing on granite outcrops (eo) at the slope apex may become detached by different agents, and fall on the ground below rocks. 2: Mosses are now transported downhill, mainly by needle-ice growth on the ground surface (vertical lines), and gradually assume a globular shape, forming rolling moss balls (rm); predominant moss ball shapes are spheroidal (left) or depressed-spheroidal (right). 3: Rolling mosses eventually reach the stone pavement on the lower slope, remaining on the clasts; they cannot be transported any further by needle ice, which only grows on bare soil patches. Some mosses lay in between stones, and develop into unattached elongated mosses (em). 4. Moss balls and/or elongated specimens may gradually become attached to rocks, mostly cobbles (ec), on the stone pavement; clasts covered by epilithic mosses can still slowly continue moving (dashed arrow), toward the basin floor. 5. Some mosses (dotted arrow) may become fragmented (fm) but remain partially attached to clasts (see Fig. 7C).

then break apart into smaller fragments (Shacklette, 1966), that are too small to locate (Fig. 7C).

5.2. Patterns of moss transport downslope of a large outcrop The spatial distribution of mosses along the gentle transect on site 3 was different from, and much more complex than, that just described for the other sites. Although spheroidal moss balls were located not far 8

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Fig. 7. (A) Moss photomosaic: sample specimens of different moss forms collected along the sampling transect; labels identify moss types on consecutive photo rows. rm: spheroidal and depressed-spheroidal rolling moss balls; em: elongated mosses; ec: upper-side view of epilithic mosses attached to loose cobbles; ec: lower-side view of the same epilithic specimens shown on the row above; they are displayed in the same sequence, rotated about their horizontal axis to show the underlying cobble. Photos for all other rows show the ‘upper’ moss surface; eo, mosses attached to outcrops, or fm, fragmented mosses, are not shown on this moss collection. Scale is 15 cm-long. (B) Shape classes for moss specimens sampled along the belt transect, based on axial indices (Dackombe and Gardiner, 1983). Shape categories: O: oblate, or discoid, E: equant, or spheroid, T: triaxial, or bladed, P: prolate, or roller. Key for moss forms: dark circles = loose rolling moss balls found on bare soil (rm); dark triangles = loose elongated mosses on stone pavement (em); open circles = epilithic mosses on loose clasts (ec). Sample N: 25 for each moss form. (C) Two large mosses on plots 16–17, ~32–34 m downhill distance along transect; slope angle is ~1.5°. Ground surface is covered by stones and some runoff-deposited sediments. These mosses, still attached to clasts, are eroded at the top and in process of gradual fragmentation (fm). The specimen on the left is ~88 mm diameter (hole: ~43 mm), the one on the right ~108 mm (central hole: ~50 mm); mosses might have been gnawed by rabbits, seemingly common in the basal slope. A healthy ~50-mm across moss, also attached to a cobble, is partially seen on the lower right corner. Scale in inches, total length: 15 in (~38 cm). Photo: PPB.1310. March 2000.

Spheroidal moss balls attain and maintain their shape due to continuous rolling movement, but lack of frost activity results in their scarcity over the basal pavement. Mosses, by now largely immobile, appear to evolve either into epilithic or elongated mosses (Fig. 7A). The extremely high density—12.25–31/m2—of epilithic mosses implies many specimens may become attached to stones shortly after stopping, although some could gradually continue descending toward the basin floor. Some epilithic specimens might also originate by spore colonization from nearby mosses, except most mosses in this environment

from the outcrops on bare soils disturbed by needle ice, the stone pavement on the near-flat basal slope greatly influenced the density, shape, and growth form of mosses along the transect. Rolling mosses seemed to suddenly stop en masse as they arrived to the upper stone pavement boundary, causing an abrupt increase in moss density and a conspicuous change in morphology (Figs. 5C, 6C). This significant accumulation of mosses resulted from the virtual cessation of needle-ice activity on the ground surface; because ice needles grow on exposed soils, a stone cover prevented this main agent of moss transport. 9

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Table 2 Average morphological indices for mosses on the sampling transect, 4460 m. Number of specimens examined: 25 N for each moss form. Differences among moss forms calculated with Kolmogorov-Smirnov (K-S) and Mann-Whitney tests. See text for additional details. Shape index

Rolling moss balls (rm)

Elongated mosses (em)

Epilithic mosses on loose clasts (ec)

Sphericity Compactness Flatness Elongation Size (a axis), mm

0.884 ± .025c 0.770 ± .041c 1.209 ± .051c 1.171 ± .087c 26.8 ± 6.7c

0.676 ± 0.091– 0.462 ± 0.094– 1.908 ± 0.416– 1.507 ± 0.379– 38.6 ± 11.4d

0.577 ± .087a 0.394 ± .105a 2.400 ± .517a 1.288 ± .213b 47.1 ± 12.9a

Key, significance levels for statistical comparisons, K-S: Rolling moss balls vs. elongated mosses: c: p < 0.001. Elongated mosses vs. epilithic mosses on clasts: d: p < 0.05. – : no significant difference. Epilithic mosses on clasts vs. rolling moss balls: a: p < 0.001; b: p < 0.05.

lack sporophytes, as constant disturbance arrests thriving of sporebearing capsules (Beck et al., 1986; Mägdefrau, 1987; Pérez, 1991, 2010). Epilithic mosses were 5–6 times more common than elongated mosses, but the process of development for these last is not readily apparent. Elongated unattached mosses might result after: (i) large spheroidal mosses break apart into elongated fragments (Fig. 7A, C), and some neatly fit on the linear junction area between stones, or (ii) small rolling mosses reach the pavement, then stop moving and grow along the slightly depressed, elongated zone between adjacent stones. Compared to other moss sites examined, moss distribution at site 3 may be considered close-ended: as mosses are unable to migrate beyond the basin boundary, they become concentrated on the basal slope and basin floor. Moss growth forms and transport processes along the slope may be combined into a proposed sequence of development (Fig. 6D); this conceptual model of moss evolution emphasizes the close linkages perceived among (i) the presumed modes of downslope movement for both mosses and stones, (ii) the field evidence for changes in moss morphology observed among different topographic positions on the transect, (iii) the distinct spatial distribution and densities of moss types along the slope and, (iv) the geomorphic and pedological processes involved on the gradual generation of a stone pavement on the basal slope. The proposed developmental model (Fig. 6D) also brings attention to the complex interactions in space and time exemplified by páramo topographic, geomorphic, pedological, and vegetation features. As Paine (1985) and Pickett (1989) point out, a significant problem for geomorphologists and ecologists is that, normally, there is not enough time to observe how landscapes actually evolve, thus field studies must assume that landform and vegetation features, only observed in the present landscape at various stages of development, are related to each other within a progressively evolving framework, and researchers must

Fig. 8. (A) Particle size-distribution (0.002–25 mm) for substrate samples—soil and gravel fractions included—along sampling transect. Numbers correspond to individual study plots; numerals with letters indicate soil horizons in stonepavement profile (plot 11); see Fig. 6C and Tables 3 and 4 for additional details. (B) Surface horizon, stone pavement (11a). Cumulative percentage frequencydistribution for stone size (a axis, 2.5–25 cm) on transect plots (9–14); this sample corresponds with surface Cjj horizon, Table 4.

make inferences about changes through time based on the variety of forms seen at present. This is the well-known ergodic hypothesis (Huggett, 1988; Fryirs et al., 2012) or space-for-time-substitution principle (Pickett, 1989; Damgaard, 2019), which allows in this páramo area to construct a site-specific sequence of landscape adjustment and change based on the geomorphic and plant features observed along the transect (Fryirs et al., 2012).

Table 3 Selected soil properties for surface soils (0–10 cm depth) along belt transect; plot number and distance along transect (+m) indicated; see Fig. 6C for sample locations. Texture type follows Soil Survey Staff (2014). All texture percentages given by dry weight; sand, silt, and clay content refer to the soil fraction (≤2 mm); D50 is the particle median size. Dry colors follow Munsell Soil Color Charts (1992). See Table 4 for description of soil profile excavated under stone pavement at plot 11 (+22 m). Consult text for additional details. Property

Plot 2 (+3 m)

Plot 4 (+7 m)

Plot 6 (+11 m)

Plot 8 (+15 m)

Texture type % gravel % sand % silt % clay-sized grains D50, mm Bulk density, Mg m−3 Color

Sandy loam 32.1 51.4 44.8 3.8 0.068 0.976 2.5Y 6/3 Light yellowish brown 5.91

Sandy loam 38.5 56.9 39.4 3.7 0.094 1.139 2.5Y 5/3 Light olive brown 5.66

Loam 12.7 48.9 44.3 6.8 0.061 1.163 2.5Y 7/4 Pale yellow 3.18

Loam 14.4 49.1 44.7 6.2 0.062 1.266 2.5Y 6/4 Light yellowish brown 2.93

% organic matter

10

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Table 4 Description of soil profile in stone pavement at lower transect segment, plot 11, +22 m; 4460 m, ≤1°slope angle, S146°E orientation. Lithic Haplocryept developed on granitic gneiss. Plot was devoid of vascular vegetation, but a few epilithic Grimmia mosses grew on loose clasts. Horizon descriptions follow Soil Survey Staff (2014) and Munsell Soil Color Charts (1992). Horizon

Depth, cm

Horizon description

Cjj

+7–0

Openwork layer of frost-sorted, sharply angular to subangular, clasts, fitted together like in a mosaic as a stone pavement. Ground totally covered by a layer 1–2 clasts thick; larger fragments lie flat on ground surface, but most smaller tabular stones are vertically packed on edge (Fig. 5D). Stones mostly white to light brownish gray (2.5Y 8/1–6/2) or pale yellow (2.5Y 7/4) granitic gneiss clasts, with smaller gray to dark gray (2.5Y 6/1–4/1) mica schist fragments; clasts include 21% pebbles ≤5 cm (a), 50% cobbles (5–10 cm), 27% blocks (10–20 cm), 2% boulders (≥20 cm). D50: mean, 89 mm, cobbles; (sample 11a, see Fig. 8B). Abrupt smooth boundary to:

A11

0–3

Sandy loam with 10.5% fine gravel, with no pebbles or larger clasts; soil fraction: 70.3% sand, 25.1% silt, 4.6% clay. D50: 0.329 mm; (sample 11b, Fig. 8A). Bulk density, soil fraction: 1.318 Mg m−3. Weak, single-grain structureless, loose to soft dry consistence. Dry color: 2.5Y5/3, light olive brown; 3.0% organic matter. Abrupt smooth boundary to:

A12h

3–12

Silt loam with 30.6% fine gravel, with very few ≤ 30 mm pebbles but no larger clasts; soil fraction: 16.8% sand, 77.9% silt, 5.3% clay. D50: 0.022 mm (sample 11c, Fig. 8A). Bulk density, soil fraction: 0.819 Mg m−3. Moderate fine granular structure, loose to soft dry consistence. Dry color: 2.5Y 6/3, light yellowish brown; 8.6% (illuvial?) organic matter. Clear smooth boundary to:

A13

12–24

R

24+

Sandy loam with 11.2% fine gravel, without pebbles or larger clasts; soil fraction: 57.9% sand, 38.9% silt, 3.2% clay. D50: 0.096 mm (sample 11d, Fig. 8A). Bulk density, soil fraction: 1.230 Mg m−3. Weak, single-grain structureless, loose to soft dry consistence. Dry color: 2.5Y 7/4, pale yellow; 3.9% organic matter. Soil horizon rests unconformably over bedrock; abrupt smooth boundary to: Granitic gneiss with weakly defined foliation and a tendency for biotite to become oriented in bands. Contained ± 15% biotite, ± 30% quartz, ± 55% feldspars (plagioclase and orthoclase)

5.3. Probable agents of moss and stone downslope transport

~4100 m altitude (Durant et al., 1994).

The main agent of downslope transport at all sites, both for mosses and stones, is believed to be needle ice, possibly helped by other types of ground frost, e.g., intergranular ice crystals (cryoreptation, Pérez, 1992a), often observed in Piedras Blancas’ soils. As noted, needle ice covered much of site 3 on multiple visits, and many characteristic soil structures associated with it were widespread at all sites (Figs. 2.2, 3, 5A, B). The unusually high content of fine grains in all surface soils at site 3 (Fig. 8A, Table 3) would render these ideally suited for ice segregation and needle ice formation, even at moisture levels below ~10–12%—(Meentemeyer and Zippin, 1981; Pérez, 1992a). Undoubtedly, ice needles are quite capable of upheaving light moss balls—no heavier than ~25–30 g—but even heavy stones can be easily uplifted by needle ice. In Piedras Blancas, the largest stone found to be heaved by needle ice, ≥5 cm height at 4385 m, was 26 × 19 cm (Pérez, 1987a). The literature shows even bigger stones can be transported by needle ice. In the Tatra Mtns of Poland, Gerlach (1959) measured a 30 × 7 × 5 cm boulder, raised 11 cm above the soil surface. In the French Pyrenees, Brunet (1956) reported stones, heaved by ~3 cm-high needle ice, as large as 40 × 25 × 15 cm and weighing ~10 kg. Páramo mosses and/or stones may also be transported by other geomorphic agents, including running water, rainsplash, and impact by hail. In Piedras Blancas, disturbance and trampling by several animals could be quite significant. The area is frequented by herds of semi-feral cattle; these are seen up to ~4340 m, and their trails criss-cross even steep slopes (Pérez, 1992b); in fact, cow excrement was common across site 3 and along the sampling transect. Small groups of wild horses are found up to ~4200 m, but were not observed near the study sites. Rabbits (Sylvilagus brasiliensis meridensis), with fairly large populations in most Venezuelan páramos (Díaz et al., 1997), are probably the most important agent of animal disturbance. Rabbits seemed abundant at site 3, where their dung pellets were widespread, especially near the basal slope, in areas with high density of epilithic mosses on clasts; animals sighted near the transect seemed rather large and heavy. Although the diet of rabbits contains mainly grasses, they also forage on mosses, which are used as materials for nest construction (Díaz et al., 1997). Rabbits may induce substantial damage and/or movement of mosses and rocks during their foraging activities; larger mosses may have been nibbled and fragmented by rabbits (Fig. 7C). Another small mammal found in the Mifafí páramo is the shrew (Cryptotis thomasi), which uses rosette leaves and grasses to build their nests, but also lines “… the tunnels between shelters and nests with lichens and mosses” (Díaz et al., 1997: 297). Shrew colonies have been reported in this páramo below

5.4. Formation of periglacial stone pavements in the high Páramo Piedras Blancas Stone pavements were previously reported from high Venezuelan páramos (Schubert, 1975, 1980; Schubert and Vivas, 1993) but had not been investigated in any detail. At Piedras Blancas, pavements are restricted to small shallow depressions next to gentle slopes, and to nearly level areas surrounding several shallow glacial tarns—the Apersogadas, at ~4310–4450 m (Pérez, pers. observ.) (Fig. 2). Schubert (1975) first proposed these pavements accumulated largely by gradual downhill migration of clasts from the surrounding slopes caused by frost activity, mainly that of needle ice. Field observations, as well as the numerous and widespread miniature ground features, strongly support the idea that needle ice is the most likely agent of transport. The size of the largest rocks (≤24 cm) on the pavement (Figs. 5D, 8B) indicates stones could have been readily transported to the basal slope by needle-ice activity, even under the present climatic conditions. Absence of any sizable stones in subsoil horizons of the pavement profile (Table 4), and the vertical ‘on edge’ arrangement of smaller surface clasts, also indicate these rocks have been upheaved and sorted out of lower horizons by frost processes (Goudie, 2004). Finally, the high silt and SOM contents in horizon A12h (Table 4) point to the strong possibility that these are of at least partially illuvial origin (Washburn, 1980; Soil Survey Staff, 2014), and imply that runoff may have contributed to the profile structure. In summary, the stone pavement is presumed to owe its origin to a combination of: (1) downslope migration of stones due to needle-ice activity; (2) frost-sorting processes of coarse substrate materials; and (3) intermittent runoff activity. 5.5. Probable age of stone pavements Stone pavements at Piedras Blancas could well still be developing under the current climatic regimes of frost activity, needle-ice formation, and stone transport; yet, these miniature geomorphic features—as well as many other prominent periglacial landforms of the páramo—in all likelihood mostly developed during the recent past; thus, they should have an older origin, and have at least a partially relict nature. In a sense, much of the high páramo landscape faithfully preserves the imprint of the recent past, and the area represents a nearly ideal example of landscape memory (Brierley, 2010; Fryirs et al., 2012); although Andean geomorphic and biotic systems continue responding to contemporary processes and sporadic disturbance events, the present11

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day geomorphic features are just a continuation of the past. Initial accumulation of stone pavements may have started immediately after the Late Pleistocene Mérida Glaciation (Schubert, 1974; Schubert and Valastro, 1974) > 12000 years ago and waned afterwards, but geomorphic processes leading to it should have experienced accelerated deposition rates during the Little Ice Age, between the late 13th and the mid-19th century (Rull et al., 1987). In fact, it may be argued that, in the high páramo, the present is not the key to the past; rather, the past should be considered the key to the present (Blundell and Scott, 1998). In conclusion, this research hopefully illustrates how the processes involved in the development of periglacial geomorphic features and soils, and in the ecological interactions of páramo vegetation, can be studied and integrated under a combined geoecological approach.

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Declaration of Competing Interest The author declared that there is no conflict of interest. Acknowledgements This research was supported by a leave of absence from the Univ. of Texas-Austin Research Institute, and by the Teresa Lozano Long Institute of Latin American Studies (LLILAS, UT), through grants from the Houston Endowment and the Andrew W. Mellon Foundation. At Instituto Nacional de Parques (INAP), Mérida, Arnoldo Durán, superintendent of SLCNP, and Lorena Vergara, perito forestal, kindly issued research and collection permits. I am especially grateful to Marciano and Felipe Aldana, rangers of Estación Biológica Juan Manuel Páez (Mifafí sector) for their help, hospitality, and candid conversations. H. Hoenicka B. and A. Lucchetti assisted with fieldwork. Bruce Allen and Robert E. Magill (Missouri Botanical Garden, St. Louis), Dana Griffin, III (Curator Emeritus, Univ. of Florida-Gainesville Herbarium), and Billie L. Turner (Dept. of Botany and Herbarium, Univ. of Texas-Austin) helped with moss identification. José L. Pérez kindly examined many rock samples. Detailed comments by Gary J. Brierley (Univ. of Auckland, New Zealand) and David R. Butler (Texas State Univ., San Marcos, Texas) greatly improved he original manuscript. My father, Francisco Pérez Conca (1918–2003) provided vital logistic assistance and much loving help. I am fortunate to have the constant unflagging support and encouragement of Martha Marie Kowalak-Pérez. I sincerely appreciate the kindness, guidance, and early mentorship of Carlos Schubert Paetow (1938-1994), pioneer of glacial and periglacial investigations in the páramos. The author declares no conflict of interest exists with this research. References Bailey, N.T.J., 1995. Statistical Methods in Biology, third ed. Cambridge Univ. Press, Cambridge. Ball, D.F., 1964. Loss-on-ignition as an estimate of organic matter and organic carbon in non-calcareous soils. J. Soil Sci. 15, 84–92. Beck, E., Mägdefrau, K., Senser, M., 1986. Globular mosses. Flora 178, 73–83. Benninghoff, W.S., 1955. Jökla mys. J. Glaciol. 2, 514–515. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods, Agronomy Monograph, second ed., vol. 9. American Society of Agronomy, Madison, Wisconsin, pp. 363–375. Blundell, D.J., Scott, A.C. (Eds.), 1998. Lyell: the Past is the Key to the Present. Geol. Soc. London, Spec. Publ, vol. 143. pp. 1–376. Brierley, G.J., 2010. Landscape memory: the imprint of the past on contemporary landscape forms and processes. Area 42, 76–85. Brunet, R., 1956. Deux processus d’erosion en haute montagne Pyrénéenne. Rev. Géomorph. Dynam. 7, 143–147. Caine, T.N., 1969. The analysis of surface fabrics by means of ground photography. Arct. Alp. Res. 1, 127–134. Cleef, A.M., 1981. The Vegetation of the Páramos of the Colombian Cordillera Oriental. J. Cramer, Vaduz. Corte, A., 1955. Contribución a la morfología periglacial especialmente criopedológica de la República Argentina. Acta Geogr. Fenniae, Helsinki 14, 83–102. Curtis, R.O., Post, B.W., 1964. Estimating bulk density from organic-matter content in some Vermont forest soils. Proc. Soil Sci. Soc. Am. 28, 285–286.

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