Microbiotic crusts in the high equatorial Andes, and their influence on paramo soils

CATEWA ELSEVIER

Catena 31 (1997) 173-198

Microbiotic crusts in the high equatorial Andes, and their influence on paramo soils Francisco L. Pkrez Department

*

ofGeography, Uniuersity ofTexas, Austin, TX 78712-1098, USA

Received 6 January 1997; revised 25 June 1997; accepted 2 July 1997

Abstract Although microbiotic crusts are known from many lowland deserts, they have apparently never been reported from alpine areas. This paper describes microbiotic crusts from an equatorial high Andean location (P&amo de Piedras Blancas) in Venezuela. The main objectives are to describe the basic characteristics of the crusts, to examine the properties of the soils on which they occur, and to discuss their possible geomorphic significance for paramo soils. Properties of crusts and of the soils beneath them were compared with those of adjacent bare soils at three sites, between 4360 and 4550 m altitude. Measurements on unconfined soil compressive strength and infiltration rates were taken in the field. Crust specimens were also gathered, and later examined and sectioned under a microscope to ascertain micromorphological details. Laboratory data included soil texture, organic matter content, color, pH, water-storage at field capacity, and stability of soil aggregates. Crusts formed discontinuous, garland-like patches parallel to contours, or covered small round soil buds, produced by frost sorting. Microbiotic crusts contained many plants, including an unclassified hepatic (genus Marsupella Dumort.), a moss (Grimmia longirostris Hook.), and several lichen species. Crusts consisted of a thin organic layer (5-35 mm) overlying mineral soil, and showed three parallel bands with diffuse boundaries. The outer layer had green-reddish brown, live plant shoots and stems with few fine mineral grains. The middle layer had anatomically recognizable dark-brown plant material and a greater content of mineral grains. The light-colored basal layer had little organic matter mixed with coarse mineral particles; it also contained many small orthovughs (vesicles) and some larger arched chambers, caused mainly by frost activity. Soils with a microbiotic crust were darker than bare soils. Organic matter content in crust areas was two to three times higher than in adjacent bare soil; pH dropped N 0.3 units below crusts. Soils with crust had finer texture than bare ground: the content of fines in crusts was 1.5 times higher than in bare soils, but gravel in these was four times higher than below cryptogams. Indices of particle concentration showed that grains I 0.5-l mm were more abundant in crusts. Finer soils in crusts may result from a combination of (a) prior random texture variation; (b)

* Corresponding author. Fax: + I-512-471-5049; e-mail: [email protected]. 0341-8162/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. F’II SO34 1-8 162(97)00040-4

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cryptogam colonization of line-soil patches sorted by frost; (cl interception and capture of aeolian dust by cryptogams, especially by mosses; and (d) reduction of fine-particle erosion by crusts; as coarse grains are lost, mean particle size within crusts drops. Geomorphic effects of crusts were also significant: (a) Infiltration rates were 70% greater in crust than in noncrust soils (67.0 vs. 40.4 mm/min); (b) Crust soils stored more water at field capacity (48.7-88.4%) than bare areas (2O.S42%); (c) Crust soils had a higher resistance to raindrop erosion than bare areas (47.5 drops/O.1 g vs. 10.2 drops/O.1 g). Undisturbed crust specimens were even more effective in resisting drop impact, surviving up to 1100 drops/O.1 g; (d) Cryptogams increased soil compressibility more than 200%, from 54 g/cm2 in bare soils, to 118.1 g/cm2 in crust areas. All these geomorphic changes should result in lower rates of soil erosion in paramo areas with dense crusts.

Resumen A pesar de que las costras microbi6ticas (criptogamicas) son comunes en muchos desiertos calidos de1 mundo, estas no han sido aparentemente reportadas para areas alpinas. En este trabajo se describen las costras microbioticas de una localidad ecuatorial andina de alta montana (Ptiamo de Piedras Blancas) en Venezuela. Los objetivos de1 estudio son describir las caracteristicas basicas de las costras microbibticas, examinar las propiedades de 10s suelos en 10s que estas se encuentran, y discutir la posible importancia geomorfol6gica de las costras para 10s suelos de1 p&ramo. Las propiedades de las costras fueron comparadas con las de suelos desnudos adyacentes en tres parcelas entre 4360 y 4550 m. Las mediciones de campo incluyen compresibilidad de1 suelo y tasas de infiltracibn. Especimenes de costras tambien fueron colectados, y mas tarde examinados y seccionados bajo el microscopio para determinar sus detalles micromorfol6gicos. Los datos de laboratorio incluyen textura de1 suelo, contenido de materia organica, color, pH, contenido de agua a capacidad de campo, y estabilidad de 10s agregados de1 suelo. En el p&ram0 andino, las delgadas (5 a 35 mm) costras microbi6ticas estan compuestas de diversas plantas, incluyendo una Hepkica no clasificada de1 genera Marsupella Dumort., un musgo (Grimmia Zongirostris Hook.) y varias especies de liquenes. Las costras microbi6ticas ejercen una influencia importante en varias propiedades pedologicas, que a su vez afectan diversos procesos geomorfolbgicos. Los suelos en las costras tienen un porcentaje de materia organica y particulas finas bastante mas alto que 10s suelos desnudos adyacentes. La materia organica es contribuida por el crecimiento de las plantas, mientras que la textura mas fina resulta de diferencias previas de1 substrato (incluyendo aquellas producidas por el escogimiento debido al frecuente congelamiento de1 suelo) y de la captura de material eolico fine (principalmente con di&metro menor de 0.5 mm) por las plantas de la costra criptogamica, especialmente por 10s musgos. Estas modificaciones pedoldgicas provocan: (1) tasas de infiltracidn mas altas en areas con costras; (2) una mayor capacidad de retencidn de agua en 10s suelos de las costras microbi6ticas; (3) mayor resistencia al desprendimiento y erosion causados por el impact0 de gotas de lluvia en las costras, comparadas con suelos desnudos adyacentes, y (4) una mayor agregacion de 10s suelos en las costras, expresada en forma de una alta cohesividad y resistencia a la compresi6n. Todos estos efectos debetian resultar en reducidas tasas de erosion en areas de1 paramo donde las costras microbidticas son densas y/o extensas. 0 1997 Elsevier Science B.V. Keywords: Andes; Bryophytes;

Soil geomorphology;

Soil micromorphology;

Soil moisture

content;

Soil

properties

Keywords: Andes; Briofitas; Propiedades

de suelos

Geomorfologia

de suelos;

Humedad

de1 suelo; Micromorfologia

de suelos;

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1. Introduction Biological crusts are important components of arid-ecosystem soils (Cameron and Blank, 1966; Friedman and Galun, 1974; West, 1990); they were apparently first reported by Fritsche (1907) and have since been found in warm deserts of many continents. These crusts consist of communities of tiny organisms that grow as a thin layer (5-50 mm) on, or immediately below, the soil surface. Crust assemblages may contain cyanobacteria (blue-green algae), green algae, diatoms, lichens, mosses, liverworts, and fungi; in some deserts where vascular plants are sparse or lacking, crusts may constitute the main biological ground-cover. Biotic crusts are known by a wide variety of names, including ‘microfloral’ (Loope and Gifford, 1972) ‘organogenic’ (Evenari, 1985) ‘cryptobiotic’ (Belnap, 1993) and (in Central Asia) fu@r crusts (Durrell and Shields, 1961; Danin, 1976). They have also been called ‘popcorn soil’ (Rushforth and Brotherson, 1982) ‘epedaphic’ organisms (Friedman and Galun, 1974) (algal) ‘clumps’ or ‘mats’ (Llano, 1962; Cameron et al., 1970) and ‘patena’ (Bliss, 1988). Over the past two decades, however, most authors have referred to these crusts as either ‘cryptogamic’ (Kleiner and Harper, 1972; Anderson et al., 1982a,b) or ‘microphytic’ (West, 1990; Danin and Ganor, 1991). The more accurate term of ‘microbiotic’ crusts has been recently proposed (St. Clair and Johansen, 1993; Danin, 1996) because cryptogams (plants without seeds) include ferns-which are absent from crusts-but exclude fungi and cyanobacteria, which are not plants yet are common crust components; the term ‘microphytic’ also excludes non-botanical components. In this report, the more precise name ‘microbiotic crust’ will be used. The first part of this report will discuss the available information on microbiotic crusts, emphasizing their geomorphic and pedological aspects, as well as the sparse existing data for crusts in cold regions. The second part of the paper will describe in some detail the microbiotic crusts found in the high equatorial Venezuelan Andes, as these seem to be the first instance of microbiotic crusts ever described for any high-altitude area in the world. 1.1. Microhiotic

crusts in warm deserts

Several studies have discussed the biological significance of microbiotic crusts, which can increase the content of organic matter and of various nutrients in desert soils (Fletcher and Martin, 1948; Shields et al., 1957; Kleiner and Harper, 1972; Anderson et al., 1982a; Yair, 1990); crusts tend to concentrate organic compounds in the upper centimeters of the soil (Cameron and Blank, 1966; Graetz and Tongway, 1986). Blue-green algae and associated crust organisms ( Azotobacter, Nostoc) can also fix atmospheric nitrogen (Snyder and Wullstein, 1973; Rychert and Skujins, 1974) thus enriching the desert soil. The high organic-matter content of crusts, along with their shading of the underlying soil, result in improved water-storage capacity (Durrell and Shields, 1961; Friedman and Galun, 1974 (p. 17 1); Ladyman and Muldavin, 1996). and allow soils to remain moist for a longer time (Cameron and Blank, 1966). Seedling establishment of vascular plants may be facilitated by an algal (Booth, 1941; Cameron and Blank, 1966) or a moss mat (Bliss and Svoboda, 1984; Bliss, 1988). Hydrological processes and soil responses to these are particularly affected by microbiotic crusts, which generally increase infiltration rates into the soil (Booth, 1941;

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Fletcher and Martin, 1948; Loope and Gifford, 1972). Consequently, areas with dense organic crusts also produce lower runoff amounts. Higher infiltration may occur due to the greater surface roughness of crust areas, where a hilly micro-topography creates more surface detention storage (Danin, 1976; Evenari et al., 1982; West, 1990; Ladyman and Muldavin, 1996). The type of organism present in the crust also affects infiltration. Brotherson and Rushforth (1983) reported infiltration rates 15 times greater in a moss crust than in adjacent bare soil, but, in contrast, algal and lichen crusts reduced infiltration rates by N 55%. Other studies have also reported lower infiltration rates in soils with algal (Yair, 1990) or lichen (Graetz and Tongway, 1986) crusts. Alternatively, Micher et al. (1988) noted that precipitation rates affect infiltration into the crust; light rainfalls ( < 5 mm) can be efficiently absorbed and retained by crusts, but heavier rainfalls may not. Greater infiltration in some soils with crust results in a substantial reduction of particle erosion. Booth (1941) obtained erosion rates nearly 17 times higher in bare ground than in adjacent crusted soils of Oklahoma. Comparable results have been found in arid areas of Spain (Alexander and Calvo, 1990) Australia (Miicher et al., 1988) and Arizona (Fletcher and Martin, 1948; Brotherson and Rushforth, 1983). Depth of water penetration can also be significantly increased under some crusts (Rushforth and Brotherson, 1982). The evaporative water loss is also reduced, because crusts act as a ‘mulch’ which conserves moisture (Booth, 1941; Brotherson and Rushforth, 1983; Bliss and Svoboda, 1984). In contrast, some studies (West, 1990 (p. 187)) found greater evaporation from dark-colored crusts, which absorb more heat-and therefore reach higher temperatures-than adjacent light-colored, bare soils. Crust organisms greatly affect soil stability and structure. Binding of mineral particles by sheaths of algal filaments, and by fungal mycelia and hyphae, facilitates strong soil aggregation (Cameron and Blank, 1966; Starks et al., 1981; Anderson et al., 1982a; Miicher et al., 1988). Mosses, liverworts and lichens also promote particle aggregation by covering the soil surface with their stems, shoots, thalli, and other vegetative bodies, and by permeating the soil with their root-like rhizoids, which firmly hold mineral particles within a tightly knit fabric (Schulten, 1985). Soils insulated by a dense crust are more resistant to both rainsplash erosion and disaggregation by raindrop impact (McCalla, 1947; Kleiner and Harper, 1972; Tchoupopnou, 1989). In addition, soils are shielded against aeolian entrainment of fine materials, as the irregular, undulating surface of the crust breaks up the flow micro-patterns of wind and slows it down (Brady, 1974; Anderson et al., 1982b). Actually, due to this, microbiotic crusts often intercept and capture airborne fines-primarily silt. As a result of this aeolian enrichment, soils in crust areas are commonly finer than those of contiguous bare sites (Kleiner and Harper, 1977; Danin and Yaalon, 1981; Eldridge and Greene, 1994). This trapping process is especially effective if the crust organisms include mosses (Moore and Scott, 1979; Danin and Ganor, 1991; Perez, 1991a). 1.2. Microbiotic

crusts in periglacial

environments

The literature for microbiotic soil crusts in deserts is extensive (see West, 1990). In contrast, knowledge about similar crusts in cold regions is fragmentary and inadequate,

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although microbiotic crusts are known from polar areas in both hemispheres. Antarctica’s crusts have been studied mainly by Roy E. Cameron. In this continent, soil crusts appear to be solely composed of blue-green algae; the lilamentous cyanobacteria in them are the most abundant terrestrial species in Antarctica, which are also common components of temperate desert crusts (Cameron and Blank, 1966). Several localities with algal ‘mats’ have been found in the dry valleys and coastal areas of Southern Victoria Land (Cameron and Devaney, 1970; Cameron et al., 1970). Cyanobacterial crusts are present where daily freeze/thaw cycles occur, and therefore liquid water is sporadically available. Llano (1962) (p. 216) provides details and a photo of blue-green algae crusts growing in McMurdo Sound ‘on sterile ground’ in a pattern of “...scattered...small, brittle, rosette-shaped clumps...each bushlike growth spaced neatly from its neighbors”. Information on Arctic crusts is even scarcer. Soils in the High Arctic tundra of North America are often covered by “ . ..a black crust of lichens and blue-green algae called patena.” (Bliss, 1988 (p. 18)). Arctic crusts are diverse, and contain many species of lichens, mosses, liverworts, and blue-green algae (Bliss and Svoboda, 1984). Washburn (1973) (p. 82) a 1so mentions that, in northeast Greenland, “...some (soil) nubbins were bare of vegetation, others were covered with black organic crust...“, and that some nubbins with organic crust were “ . ..disrupted as if by needle ice.” His description and photo (Washburn, 1973 (p. 85)) are very similar to those offered by Llano (1962) for Antarctica. Microbiotic crusts had apparently not been described for alpine areas until recently (Perez, 1994, 1996), after their discovery in the P&amo de Piedras Blancas (Venezuelan Andes). Anderson et al. (1982bXp. 359)) noted that “ . ..To the untrained eye ,... (desert) crusts are not recognized as a biological phenomenon and usually go unnoticed”. In the 1970s I started biopedological research in the plant/substrate relations of giant rosettes in the Venezuelan Andes (see Fig. 2) (Perez, 1991 b). At that time, my untrained and unskilled eye could not perceive much beyond these tall plants; but soon, I realized that the frost-disturbed ground of the high paramo was often covered by ‘black organic crusts’ similar to those found in low-altitude deserts. I suspect that microbiotic crusts must be present in other mountain areas, but an exhaustive literature search has failed, so far, to locate any relevant reports dealing with these crusts in alpine areas. A few studies from other tropical mountains suggest, however, that there may also be microbiotic crusts there. Hedberg (1964) (p. 69) reported on the existence of ‘loose-lying cakes’ of Nostoc commune (a blue-green alga) at 4200 m on Mt. Kenya, where rounded mats N 20 cm in diameter were found on moist soils affected by needle ice. Given that Nostoc is commonly represented in arid-zone crusts (Cameron and Blank, 1966 (p. 30); Friedman and Galun, 1974 (p. 16811, and since the species N. commune is found both in Antarctic and Arctic crusts (Cameron and Devaney, 1970; Bliss and Svoboda, 19841, it is likely that this organism also forms crusts on Mt. Kenya. Winkler (1976) (p. 802) studied the Hepaticae of the Sierra Nevada de Santa Marta (N. Colombia), where MarsupeEla trollii produces small cushions (“... Polsterbildungen charakterisieren Marsupella...“) in dry, high paramos. As noted below, MarsupeEZa is the most important crust component in the Venezuelan paramo, thus it is probable that crusts resembling those described here are also found in the nearby, climatically similar, mountains of Santa Marta.

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The remainder of this paper will focus on the characteristics of microbiotic crusts found in the high P&ram0 de Piedras Blancas (Venezuelan Andes). The goals of the research were: (1) to briefly describe the physical and biological characteristics of microbiotic crusts in this high tropical mountain area; (2) to examine in detail the properties of the soils on which the biotic crusts occur; (3) to discuss the possible geomorphic significance of these soil crusts, based on both field and laboratory measurements.

2. The study area Piedras Blancas is in the Sierra de La Culata (NW Venezuelan Andes), at 8”52’N and 70”54’W, and extends from about 3700 to 4700 m (Fig. 1). This high paramo is

‘13

Caribbean

Sea

VENEZUELA

“d Km

COLOMBIA

$.

/

BRAZIL

Fig. 1. (A) Location of the Venezuelan Andes (stippled area). (B) Generalized map of the northwestern Venezuelan Andes; contour interval is 1000 m. Dots indicate major towns and localities; rivers are shown by dashed lines. The P6ramo de Piedras Blancas is shown by a dark triangle.

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semi-arid (pcirumo dese’rtico) and receives < 800 mm yearly precipitation. Nearby stations record a strongly unimodal regime: > 100 mm fall monthly during the rainy period (May to August); the dry season (December to March) receives a mean of < 20 mm/month. Due to its equatorial position and high elevation, Piedras Blancas has a cold periglacial climate characterized by extreme diurnal fluctuations; air temperatures can drop to - 12°C and rise to 23°C in the dry season. This daily amplitude is

Fig. 2. View of a steep slope (about 3 1”) at 4340 m elevation. The ground is totally covered by small bare soil nubbins, caused by needle-ice activity. This site lacks a microbiotic crust, possibly because of rapid soil movement. The largest of the two rosettes of Coespeletia timotensis seen on the background is about 140.cm tall. July 1980, P&ram0 Piedras Blancas.

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31 (1997) 173-198

considerably reduced during the rainy period due to a persistent cloud cover. Approximately 325 to 350 daily freeze/thaw cycles occur annually in this area (Perez, 1987a); this high frost recurrence causes frequent and widespread formation of needle ice, a type of ground frost which severely disturbs the ground surface. Different types of patterned ground formed by needle ice activity are found in the higher elevations of the paramo (Schubert, 1975; Perez, 1984, 1987a, 1992; Schubert and Vivas, 1993) where microbiotic crusts are also more common. The most frequent soil patterns are soil ‘nubbins’ (Fig. 2) and ‘buds’ (Washburn, 1973 (p. 82)), small round to elongate earth lumps, a few centimeters in diameter, which can cover the ground surface. According to the American classification system (Soil Survey Staff, 1994) soils in this paramo are Entisols, Inceptisols, and Histosols. Small wet areas near glacial tams and creeks are occupied by Terric or Typic cryosaprists and by Humic cryaquents and cryaquepts. Steep, mobile talus slopes are associated with Lithic or Typic cryorthents. More gentle and stable slopes are covered by Lithic and Typic cryochrepts or by Entic cryumbrepts (Perez, 1991b,c, 1992). Paramo soils are commonly shallow and coarse, with a high percentage of gravel and sand, and a low content of organic matter. The fine fraction is mainly silt, with little clay (Perez, 1984, 1991d). The abundance of silt grains is caused mainly by the mechanical weathering of soil particles by repeated frost action. Vascular paramo vegetation is characterized by several species of caulescent rosettes (Espeletiinae), associated with many nanoshrubs and herbs (Vareschi, 1970). The most common rosette in the high paramo is Coespeletia timotensis Cuatr., which has a monopodial stem ending in a dense crown of long, pubescent leaves (Fig. 2). The non-vascular vegetation of the paramo has received much less attention, even though a diverse array of epilithic, epedaphic, and erratic cryptogams (lichens, mosses, liverworts) is found in the highest paramo areas (Griffin, 1979; Perez, 1991c, 1994, 1996, 1997a,b).

3. Field methods Microbiotic crusts were sampled at three localities (Table 1). The lower-altitude site (A) was at 4360 m on a flat area (2”) 180 m NE of the westernmost Apersogada lagoon, one in a group of six small glacial tarns. Vascular plant cover was low and consisted of a few Andean rosettes; erratic globular mosses (‘moss balls’, Perez, 1991a) were also common. Locality B was at 4415 m on a SE-facing 12” slope below the Los Caracoles ridge. Here, small clumps of Andean rosettes were interspersed with ground patches partially covered by microbiotic crusts. Locality C, at 4550 m, was on a gentle (10”) summit plateau below Pica Los Nevados (4685 m). This site was devoid of vascular plants, and its soil surface was totally covered by small nubbins and soil buds, which were associated with the microbiotic crusts. At each site, five pairs of samples were obtained with a small trowel from the upper 3 cm of the soil in areas with microbiotic crusts and from adjacent bare-soil areas (‘control’). All samples were taken from areas devoid of vascular plants and away from Andean rosettes, as these plants affect soil properties via addition of organic matter (Perez, 1991d). Control soils (sites A and B) were sampled a few meters away from

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Table 1 Selected characteristics of microbiotic crusts and adjacent soils at three paramo sites. Values indicate the averages (+S.D.) of five samples. Significance levels shown are for a Mann-Whitney (U) test comparison between crust and site soils Site, Altitude (m)

Sampling position

Percentage of organic matter

A, 4360

crust

8.19t0.77

Bare soil B, 4415

c, 4550

3.8+0.18”

Percentageof gravel ( > 2 mm) 9.5k5.9

Percentage of Fines ( < 0.05 mm)

Color’

27.6 f 4.2

IO YR 2.5/l (very dark gray to black) IO YR 6/3 (pale brown) 7.5 YR 3/4 (dark brown) 10 YR 6.5/2 (light brownish gray) lOYR3/1 (very dark gray) 10 YR 6/2 (light brownish gray)

29.3 + 3.2”

22.9 &

1.4’

crust

X.48+0.19

16.1 f2.0

39.5 +

I .6

Bare soil

3.44-LO.13”

26.7k2.2”

27.5 f

1.9”

CNSt

4.91 to.37

4.2 f 0.5

26.0 f

I .O

Bare soil

1.68+0.18”

51.1 k6.1”

1 I. I f 2.0”

“P < 0.01. hP < 0.1. ‘Color shown is the mode, i.e., the most common color (Munsell Color. 1992).

crust areas on terrain with comparable topography; controls at site C were only gathered lo-20 cm from their paired crust counterparts. In addition, 56 specimens of microbiotic crust were gathered from the three sites (15 from A, 21 from B, 20 from Cl. Crust fragments were about 10 to 35 mm thick and varied in maximum width from 25 to 82 mm; they were carefully wrapped in tissue paper and cotton, labelled, and stored in cylindrical tin soil-cans for transportation to the laboratory. Some soil properties were determined directly in the field at site C. On January 7, 1992, the unconfined soil compressive strength was measured in 10 crust-covered soils and 10 adjacent bare areas with a hand-held penetrometer provided with a foot-adaptor sensitive to low shearing strength (Davidson, 1965; Perez, 1991~). Shearing strength was determined perpendicularly to the ground surface; readings when the penetrometer tip encountered pebbles were discarded and repeated (Chorley, 1959). On January 9, 1992, infiltration rates were measured for 10 pairs of crust and control soils. A thin-walled stainless steel cylinder 15-cm long and 30.5-mm wide was gently turned into the crust or soil to a depth of 2 3 cm and 50 cc of water were introduced into the cylinder. Time of infiltration into the ground was measured with a stopwatch (Brotherson and Rushforth, 1983). Since water-intake rates are greatly affected by soil moisture (Bertrand, 19651, two paired samples of the surface soil (O-5 cm) were taken before the experiments, and kept in hermetic containers until processed in the laboratory for water content.

4. Laboratory

and analytical procedures

Soil samples were air-dried, then oven-dried for 48 h at 105°C were rubbed down by hand and sieved through a 19-mesh sequence

and weighed. Soils (30 to 0.09 mm) to

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determine particle-size distribution. Finer soil fractions were estimated with a hydrometer (ASTM 152H) using a Calgon’” solution as dispersing agent. The content of gravel (> 2 mm) and fines ( < 0.05 mm) is given as a percentage by weight. Soil color was assessed for all dry samples with the Munsell Color (1992). The organic matter content of the ‘soil fraction’ ( < 2 mm) was calculated by loss of weight on ignition at 375°C as this temperature does not affect readings of clay content (Ball, 1964). The pH was determined in a 2:l paste (water:soil) by electrode; three replicate measurements per sample were taken. Water-storage capacity was estimated for soils of sites B and C by saturating 15 g of material in covered, ribbed-glass funnels lined with filter paper. Samples were left to equilibrate in a room with stable temperature (21°C) and relative humidity (50-53%) until excess pore water had been removed by gravity (Smith and Atkinson, 1975). This was empirically determined to occur 8 to 9 h after saturation (Perez, 1991a); at this point, samples were considered to have attained their field capacity (Pitty, 1979). Water content of these soils-and of the two field samples-was gravimetrically determined by oven-drying them overnight (2 16 h). Water content was calculated with the formula: [(wet soil weight - dry soil weight) /( dry soil weight)] and is given as a percent by weight of the dry soil fraction (Brady, 1974). Stability of soil aggregates was determined by bombarding 10 pairs of samples (site C) with water drops and counting the drops required to disintegrate the sample (Griffiths, 1965); a procedure outlined by McCalla (1944, 1947) was used with slight modification. Samples were pretreated by saturating and air-drying them overnight at 105°C. This produced comparably cohesive samples for crust and control soils, which hardened in lumpy masses inside beakers. Small soil lumps (from 0.2 to 0.6 g) were carefully taken from pretreated samples and placed on a l-mm mesh copper sieve. Distilled-water drops 4.2 mm in diameter falling from a 30-cm height off a burette were allowed to strike the lumps; drop scale was controlled by the size of burettes used. Lumps were considered destroyed when totally washed through the screen. Drops fell at a constant rate of N 30/min; this resulted in reasonably brief experimental runs while also allowing enough time for accurate drop counting. Prior to lab analysis, the 56 crust samples were examined in detail under a stereoscopic microscope at low (lo-50 X 1 magnification. Crust specimens were sectioned, comminuted, and disaggregated under the microscope to ascertain internal details. Micromorphological terminology follows Brewer (1976). The small sample sizes obtained produced non-normally distributed data sets, thus properties of crusts and bare soils were compared with the non-parametric Mann-Whitney (U) test. The degree and strength of association observed between water content at field capacity and the percentage of organic matter or fines were assessed with linear and polynomial regressions (Salvatore, 1982). Particle-size distributions for sample populations were plotted as envelope diagrams, as these allow for visual comparison of contrasting soil positions (crust vs. control) (Perez, 1991b, 1992). Relative concentration of grain sizes was examined with an index previously utilized for patterned-ground sorting (Ballantyne and Matthews, 1983; Perez, 199lc, 1992). A concentration index

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(CI) is defined as CI = CC/CB, where CC is the percentage by weight of a size-class in the crust, and CB is the percentage of the same fraction in the bare soil. A CI = 1 shows no concentration of a particular size-class. A CI < 1 indicates that the size-class is deficient in the crust, compared with the paired bare soil. A CI > 1 shows that the fraction is deficient in the control soil, but is more abundant in the crust. A CI = 0 means that the size-class is lacking from the crust, and is only present in bare soil. Concentration indices for paired soil populations in all sampling sites were calculated for nine grain size-fractions, with a class interval of 1 phi (0) unit.

5. Results and discussion 5.1. Distribution

of microbiotic

crusts in Piedras Blancas

Microbiotic soil crusts are found in Piedras Blancas above 4000 m, and up to N 4600 m elevation. Their upper boundary is partially set by topography, as crusts are more common and dense on flat and gently-sloping areas, but are generally lacking from talus and other steep slopes (Fig. 21, where rapid soil movement (Perez, 1987a) would rupture and overturn any existing crusts (Weber, 1962; Halloy, 1991). Crusts are not well developed in areas with vascular-plant cover, where water interception and shading by plants, litter accumulation on the ground, or even allelopathic effects, may discourage

microbiotic crust Fig. 3. View of site B (Ptiamo Piedras Blancas, N 4415 m), showing a discontinuous (elongated darker areas) covering soil nubbins. Photo depicts an area 90-cm wide in the middleground; downslope is towards the lower left of the picture. Note the near complete lack of vascular plants, except for a small cluster (on the upper left) of Hinterhubera imbricata Cuatr. (January 1992).

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cryptogam growth (Rychert and Skujins, 1974; Anderson et al., 1982a; Perez, 1994). It seems likely that vascular vegetation helps set the lower altitudinal limit of microbiotic crusts in the paramo. Additional factors affecting crust distribution are soil texture, moisture, and needle-ice activity. As in other areas (Anderson et al., 1982b; Brotherson and Rushforth, 1983; Yair, 19901, paramo crusts are more common on silty soils which store much water. These soils are subjected to recurrent frost (needle ice) activity; this may cause some crust rupturing, but serves also as an important source of moisture. During cooling, capillarity allows water to migrate from lower soil horizons to the surface freezing front. When needle ice melts the next morning, a superficial saturated soil layer results (Perez, 1991a); this water can then be readily imbibed by crust organisms. All crust areas I

Fig. 4. P&ram0 Pi&as Blancas, site C (4550 m). The small dark domes are soil buds covered by a microbiotic crust, mainly of Marsupella spp. Lighter, lower inter-bud areas are devoid of a cryptogamic cover and have an abundance of white quartz pebbles. The lens cap on the lower left has a diameter of 57 mm; the photo covers an area about 70-cm wide in the foreground (January 1988).

F.L. Pkrez/Cafena

31 (1997) 173-198

Fig. 5. View of a gently inclined (4-10”) summit plateau at 4545 m elevation (site C). The ground surface is occupied by soil nubbins and buds, most of them covered by dark microbiotic crusts. Note the complete lack of vascular plants on the plateau. The granitic block on the left side is about 1.6 m tall; the photo covers an area approximately 10-m wide in the middleground (December 1985).

examined were heavily affected by needle ice, and their ground surface was covered by soil nubbins and/or buds. Crusts usually form small, discontinuous patches covering < 50% of the ground; these are garland-shaped and perpendicular to the slope, suggesting some influence of soil movement (Fig. 3). In higher paramo areas, crusts assume the shape of small round, evenly-spaced patches; these are associated with a pronounced soil microtopography, with crusts covering hemispheric soil buds, 3 to 12 cm across and up to 6 cm high (Fig. 4). The lower ground between buds is often covered by small pebbles, sorted out of the soil and concentrated at its surface by frost action (Perez, 1992, 1996). This crust morphology is similar to that found both in the Arctic (Washburn, 1973) and the Antarctic (Llano, 19621, where crusts are also associated with miniature patterned ground. Extensive paramo areas are occupied by crust-covered buds which impart a distinctive, peculiar hummocky appearance to the ground surface (Fig. 5). 5.2. Biological

components

and characteristics

of the crusts

All crusts investigated had a similar composition. The most common organism was l found an undetermined species of Marsupella Dumort. (Hepaticae, Gymnomitriaceae)

’ Magill, R.E., 1990. Personal

communication,

Missouri Botanical

Garden,

St. Louis, MCI 63166, UsA

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in 94.6% of the specimens. Marsupella is a genus with N 42 species, largely confined to the northern hemisphere but with localized extensions into tropical mountains. Most Marsupella species inhabit arctic and alpine areas, where they are pioneers in inhospitable sites. At high elevations, they are commonly found as compact turfy mats on bare soils of insolated, exposed sites. Several species are often found “...on patches where the poor soil has been turned bare, e.g., by frost heaving...” (Schuster, 1974 (p. 33)). In South America, Marsupella has been reported from alpine areas of Tierra de1 Fuego (Argentina) and from the Colombian paramos (Schuster, 1974; Winkler, 1976). A moss, Grimmia longirostris Hook., was also very important in the paramo crusts; 58.9% of the samples contained this moss. Grimmia is a cosmopolitan genus of acrocarpous mosses which form dense hummocky cushions; the genus is particularly common in tropical mountains and sub-antarctic islands (Hedberg, 1964; Perez, 1991a). Together, these two plant species made up > 95% of the visible cover in all crusts examined. In addition, there were a number of less important species. Small patches of a common epedaphic lichen of the high paramo, Stereocaulon cf. congestum Nyl., were found on 14.3% of the crusts examined. According to Vareschi (1970) (p. 75) this terricolous lichen is found up to the summits of the highest peaks (4760 m) in Piedras Blancas. Three more lichens and a liverwort were also identified. Three crust specimens contained Catapyrenium lachneum (Ach.) R. Santesson, a common squamulose lichen found in microbiotic crusts of many desert areas (Friedman and Galun, 1974). This species had not been previously found in South America (Breuss, 1993). Piedras Blancas seems to be its most equatorial location, ’ where it grows on soil lumps loosened by needle-ice disturbance (Perez, 1994, 1997b). Weber 3 found a piece of Riccia spp. (Hepaticae) on a crust specimen. Fragments of Xanthoparmelia uagans (Nylander) Hale and of Thamnolia uermicularis (SW.) Ach. ex Schaerer were also found on crust samples. Thamnolia is a common lichen of the Andean paramo (Vareschi, 1970) where it is normally found as an erratic form completely unattached from any substrate (Perez, 1991~). X. uagans is also a vagrant species, previously reported from Venezuela and Ecuador (Perez, 1994, 1997a). Loose thalli of these lichens can be incorporated and fused into growing shoots of Marsupella or Grimmia (Perez, 1991a), but they do not appear to be significant components of microbiotic paramo crusts. A further unidentified moss was also found on two crust specimens. Although only the macroscopic or ‘thallophytic’ (Box, 1981) components of crusts were identified, detailed inspection showed other unspecified microscopic organisms in them. Many specimens also had dense masses of (presumed) algal filaments and/or fungal hyphae, but time-consuming cultures of fresh material would be needed in order to arrive to a positive identification 4. It is biologically relevant to note that microbiotic crusts serve as sources of erratic life-forms in the paramo. Crusts are disturbed by

2

Brews, O., 1993. Personal communication, A-1014, Wien, Burgring 7, Austria. 3 Weber, W.A., 1992. Personal communication. 80302, USA. 4 Magill, R.E., 1990. Personal Communication,

Naturhistorisches Herbarium,

Museum

University

Missouri Botanical

Wien,

of Colorado

Botanische

Abteilung,

Museum. Boulder, CO

Garden, St. Louis, MO 63166, USA.

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several agents, including frost, desiccation, wind, trampling by cattle, nibbling by rabbits, and traffic of off-road vehicles (Ptrez, 199lc, 1993). All these result in detachment of crust fragments, which can be transported by various processes, thus producing different kinds of ‘vagrant’ organisms (PCrez, 1994; cf. Trotet, 1968). Some of these mobile organisms can, presumably, re-attach themselves to a fresh substrate after transport, thus initiating a new microbiotic-crust colony. In this view, the erratic forms of the cryptogams would be mainly an efficient means of asexual dispersal. 5.3. Physical structure and micromorphology

of crusts

A complex interweaving network of plant stems, shoots, rhizoids, protonema, and filaments caused a high degree of cohesiveness in crusts, which were lifted from the ground without any damage; most specimens could be sectioned with a razor without much crumbling. Crusts consisted of a thin organic layer overlying mineral soil; crust thickness was about 5-11 mm when Marsupella (M) was the leading cryptogam, but crusts with Grimmia (G) were up to 35 mm thick. Microscopic analysis showed that specimens had three parallel layers with gradual, diffuse boundaries (cf. Danin and Barbour, 1982; PCrez, 1991a); these layers were usually thicker in G than in M crusts, but were essentially comparable. The outer layer was 1.5-4 mm (M) or 3-8 mm (G) thick; it consisted of light to reddish brown or green, live liverwort and moss shoots and stems, with few mineral particles between them. Most grains had a diameter of 100 to 200 microns, but few were larger than N 500 microns, which seems to be the maximum particle size that can be physically wedged in the inter-shoot spaces of G. longirostris (PCrez, 199la). The middle layer was 1.5-2.5 mm (M) or 2-9 mm (G) thick; material here ranged from still recognizable, dark brown, dead shoots and rhizoids to small (40-100 microns) fragmented and partially decomposed plant pieces. This layer had a lower percentage of organic matter and a greater content of fine mineral particles-mainly 100-200 microns -than the crust surface. The bottom layer was 2-4.5 mm (M) or lo-18 mm (G) thick. This band was lighter in color than the others-owing to its lower organic content-and consisted of mainly fine mineral grains and larger (0.9-3.7 mm) quartz pebbles, mixed with minute organic-matter fragments. The basal layer was often permeated with thin, colorless or light brown (fungal and algal?) filaments 0.4-0.9 mm long and I lo-15 microns thick, which aggregated the soil tightly. These filaments were, for the most part, evenly distributed, but sometimes were concentrated in dense weft-like clumps, 600-800 microns in diameter. Grimmia crusts also had in this lower layer numerous small protonema and moss rhizoids which helped bind mineral grains. The basal layer also showed many orthovughs (irregular vesicular cavities) and a few mammillated metavughs, with smoother walls and rounded, spheroidal surfaces. Most vesicles were small (I 900 microns), but some reached 1.5 to 4 mm diameter. Several buds examined, especially those covered by Marsupella, also included some larger (2 25-30 mm diameter) vesicular arched chambers of roughly pyramidal shape (cf. Cameron and Blank, 1966; Washburn, 1973). Chamber walls were usually smooth and densely covered by masses of thin filaments, which prevented their collapse; several chambers also contained a pebble. One chamber included a dead specimen of what

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seemed to be a Bibio spp. fly larvae (Schaller, 1968 (p. 20)) and another one had the head of a small unidentified organism (presumably a springtail [Collembola]). Similar vesicular layers have been reported for soils below microbiotic crusts in Russia (Brewer, 1976) Israel (Danin, 1976) the Sinai (Evenari et al., 1974) and Australia (Graetz and Tongway, 1986; Mucher et al., 1988). Vesicles have been attributed to the action of entrapped air bubbles during wetting/drying or warming/cooling cycles (Springer, 1958; Evenari et al., 1974; Brewer, 1976) but experimental work (Van Vliet-Lanoe et al., 1984; Van Vliet-Lanoe, 1985) shows conclusively that vesicles can also be produced by soil freezing and thawing. It appears likely that paramo vesicles are caused mainly by frost, and perhaps secondarily by wet/dry cycles. 5.4. Pedological

characteristics

of the crusts

The presence of cryptogams was initially evident by a pronounced darkening of the soil surface. The color of bare soils varied little among sites, with all samples being light brownish gray to pale brown (10 YR 6/2 to 6/3). In contrast, crusted soils presented a very dark gray to black (10 YR 2.5/l) surface, or-in places where mosses were abundant-a dark brown (7.5 YR 3/4) modal color. Organic matter content was two to three times greater under microbiotic crusts than in adjacent bare areas (Table 1). Bare soils exhibited a trend of lower organic matter with higher elevation; thus, crusts at site C showed the highest percentage increment of organic material. This influence of organic-matter addition by the thin microbiotic crusts is significant; in comparison, tall columnar Andean rosettes are able to raise the organic content of soil beneath their crowns only 36 to 52% over bare-soil levels (Perez, 1991d). Soil pH was consistently depressed beneath cryptogams (from 0.13 to 0.54 units; average change: 0.31 units); this probably resulted also from the increased organic matter content. The influence of microbiotic crusts on soil acidity became weaker with greater altitude (Table 1). Soils within organic crusts were always finer than in adjacent bare areas. This was due to two trends: first, gravel content was considerably lower in crust areas (4.2 to 16.1%; mean: 9.9%) than in non-crusted soils (26.7 to 51.1%; mean: 35.7%); second, the percentage of fine particles in the soil fraction was higher in all crusts (26.0 to 39.5%; mean: 31.0%) than in control samples (11.1 to 27.5%; mean: 20.5%) (Table 1). These textural variations were statistically significant, and as a result, soils in crust and control areas showed strikingly different particle-size distributions. A pronounced intersite variation was also found; textural envelope diagrams indicate that differences between crust and bare soils become progressively greater with increasing altitude (Fig. 6). High particle-concentration indices also show that crust soils at site C were more dissimilar from their control counterparts than at the two other sites (Fig. 7). Vertical bars on Fig. 7 show confidence intervals (P < 0.01) for a t distribution; these can be used as statistical tests of comparison with an index of 1. Bars that do not overlap with the CI = 1 line indicate grain fractions significantly concentrated in the crust (if CI > 1) or in the control (bare soil) samples (when CI < 1). The pattern of concentration is the same in the three populations: fine particles are more abundant in the microbiotic crusts, and the size boundary at which this break occurs lies somewhere between 0.5 mm (at sites B and C) and 1 mm diameter (at site A).

189

F.L. Pkrez/ Catena 31 (1997) 173-198

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Fig. 6. Particle-size distributions (mm) for crust-covered and adjacent bare soils at three sites (A: 4360 m; B: 4415 m; C: 4550 m) in Piedras Blancas. Each graphic envelope shows the textural variation of five samples. Light stippling = soils with microbiotic crust; dark stippling = bare soils.

The reasons for the pedological differences in color, organic matter, and pH are obvious and need no further discussion. The variation in particle size between crust and bare-soil samples could have several independent causes: (I) cryptogams may preferentially colonize finer soils, due to their greater ability to store water (Anderson et al., 1982a,b; Brotherson and Rushforth, 1983; West, 19901. In this view, some of the textural differences are caused by prior, random soil variability. (2) Organic crusts may also be more successful in soil patches where fines have already been concentrated by frost-sorting processes. Reports delving into cryptogam ecology in polar regions (Cameron and Devaney, 1970; Cameron et al., 1970; Starks et al., 19811 indicate that moisture availability is a key factor controlling the distribution of cryptogamic mats;

F.L. P&e:/Catena

190

31 (1997) 173-198

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therefore, fine sorted soils in patterned-ground areas have denser cryptogamic colonies (Thomson, 1982). (3) A developing microbiotic crust would also intercept and capture fine material carried by wind, so that its soil would become progressively finer (Kleiner and Harper, 1977; Danin and Yaalon, 1981). (4) Finally, an established microbiotic crust may prevent the finer particles from being eroded; as coarser grains are preferentially

191

F. L. Pe’rez / Catena 31 (I 997) I73- 198

lost from crust areas (Eldridge and Greene, 19941, the average particle size within the crust soil should gradually decline. The textural differences observed in the Piedras Blancas crusts may result from a combination of these factors. Discontinuous and apparently randomly distributed crusts (e.g., at site B) are probably affected by prior texture differences. Crusts which display a clear spatial pattern (e.g., crust-covered buds at site C> are controlled by frost sorting of tines and pebbles. Trapping of aeolian dust by crust plants is also significant. Microtextural analyses show that fine-grain content increases from the crust base to the crust surface by N 66% (Perez, 1996). This upward fining (cf. Yair, 1990) indicates an efficient capture of fines by the tiny plants, which, judging from the indices on Fig. 7, are able to trap mainly particles < 0.5 to 1 mm diameter (Perez, 199la (p. 146)). Dust capture is important because it allows the crust to build up its own microhabitat (Danin and Ganor, 1991), as finer material increases the water-retention capacity of the substrate and allows in turn even better crust development. 5.5. Pedological

and geomorphic

effects of microbiotic

crusts

The pronounced pedological modifications produced by microbiotic crusts may also have significant implications for several geomorphic variables. These effects of crusts are discussed separately below. 5.5,1. Infiltration rates Infiltration rates in crust-covered soils were about 70% greater than in noncrust areas. It took an average of 113.9 s for 50 cc of water to soak into the ground in bare areas, but only 65 s in crusts (cf. Brotherson and Rushforth, 1983); these time periods correspond to infiltration rates of 40.4 and 67.0 mm/min, respectively (Table 2); differences were statistically significant (P < 0.005). Field soil-moisture content at the time of these measurements was equally low in crust (1.1%) and control soils (1.7%), thus rates reported evaluate the capacity of both soils to absorb water under comparably dry conditions. Although precise rate measurements were not taken, water soaked into crusts at steady pace, but, in bare soils, infiltration rates were not linear and dropped steadily

Table 2 Average values for selected soil properties of microbiotic crusts and adjacent bare soils. All values indicate the means (+ S.D.) of 10 measurements. Significance levels shown are for a Mann-Whitney (U) test comparison between crust and bare soils. ‘Factor’ refers to the number of times the average value of a specific property increases from bare soils to adjacent microbiotic crusts Sampling position

Infiltration rates (mm/min) Site C

Site B

crust

67.0+ 18.5 40.4+ 15.lb x 1.7

88.4 ~fr4.5 42.0 + 4.0a x2.1

Bare soil Factor

“P
hf < 0.005.

Water-storage capacity (% dry weight)

Soil stability (N drops/O. 1 g soil)

Compressive strength (g/cm’)

Site C

Site C

Site C

48.7 f 2.4 20.5 + 2.5” x2.4

47.5 + 6.1 10.2 k 3.6” x 4.7

118.1 k30.3 54.0* 1 l.9a x 2.2

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with time. This suggests that bare soils become saturated sooner than crust soils, and that runoff should occur with greater ease in bare areas, as their lower infiltration capacity further decreases with time. The probability of runoff generation can be assessed from this. When soils are totally dry (at the start of the rainy season), runoff seems unlikely even on bare soils. The height of the water column absorbed by soil during experiments was 68.4 mm, but this high water supply is not attained in the paramo even in the rainy season, when precipitation rates in severe storms may reach N 0.35 mm/min (Perez, 1991~). However, as bare soils gradually reach saturation following extended precipitation periods, they could produce runoff; in any case, this would occur there well before it happened on adjacent crust soils. 5.5.2. Water-storage capacity Microbiotic-crust soils were able to store more water than their bare counterparts in both sites investigated (Table 2). Water content of crusts at field capacity was 88.4% at site B, compared with only 42.0% in control soils. Crusts at site C stored much less water (48.7%) than at site B, but the difference with contiguous bare soils was even more significant, as these only reached a moisture content of 20.5%. Both data sets

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Fig. 8. Scatter diagrams showing relationship between water content of soils at field capacity and A = organic matter percentage; B = percentage of fines ( < 0.05 mm). Key: dark symbols = microbiotic-crust soils; light symbols = bare soils. Circles = site B (44 15 m); triangles = site C (4550 m).

F.L. Pe’rez/Catena31

(1997) 173-198

193

differed statistically at the P < 0.001 level. These data clearly indicate that, because substantially greater amounts of water can be stored in crust soils they are, indeed, less likely to reach saturation and produce runoff. Variation in water-holding capacity between sampling positions, as well as between sites, was mainly due to differences in the fine-particle and organic matter contents (Table 1, Fig. 8). Crusts and bare soils at the higher elevation site were coarser and had a lower organic fraction than those at site B, thus the differences observed. Organic matter seemed to play a greater role in increasing water retention than fine mineral grains, as small increments of organic matter were associated with large moisture increases (Perez, 1991d), while a higher rise in fines was needed to produce a comparable increase in moisture retention. It is relevant to note that a crust cover can partially offset the effects of altitude on soil properties. Fines and organic matter soils at the normally drop with greater elevation (Perez, 1987b) but microbiotic high-altitude site (C) closely resemble the bare soils found at site B, some 130 m lower down the mountain flank (Table 1). 5.5.3. Soil aggregate stability Presence of a microbiotic crust greatly increased the ability of soil to withstand (rain) drop impact. Soil clumps from crust sites were destroyed after an average of 47.5 drops/O.1 g hit them, while control samples were obliterated with only 10.2 drops/O.1 g; this difference is highly significant (P < 0.001). The only comparable data I could find was that of McCalla (19471, who also devised the laboratory method I used; McCalla obtained a similar difference in aggregate strength (24.4 drops/O.1 g for crusted soils vs. 9.6/0.1 g drops for paired bare soils) in Nebraska. Resistance to drop impact was due to the cohesiveness that both organic matter and tine particles confer to crust soils. Two linear regressions between these parameters (taken as independent variables) and the number of drops needed to destroy the sample indicate that organic matter accounts for 92.8% (P < 0.001, R* = 0.928) of the variation in aggregate stability, while fine grains can explain 88.5% (P < 0.001, R2 = 0.885) of such variation. In hindsight, I think that crust resistance to drop impact should be preferably measured in the field in undisturbed, in situ crusts and bare soils. This is because most of the resistance to raindrop impact is actually caused by insulation of the ground surface by a dense cover of cryptogams, and lab pretreatment destroys this. I repeated the drop test with untreated crusts. A few crust specimens weighing from 0.55 to 1.03 g resisted 500 drops each without any appreciable deterioration. I reduced the size of the crust samples, but obtained similar results; i.e., a small sample of only 0.1 g was bombarded with 1100 drops, dried and reweighed, and found to have lost no mass at all. Therefore, this sample survived, unscathed, more than 23 times the number of drops that sufficed to destroy a similar treated crust! Experiments were discontinued because of the absurdly high number of drops involved in the tests, and because undisturbed bare-soil samples-to which crusts could be compared-were not available. 5.5.4. Compressive strength Field measurements showed that crusts increase soil compressibility by - 220%, from a mean of 54 g/cm* in bare areas to 118.1 g/cm2 in crusts (Table 2); the

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difference between sample sets was statistically significant (P < 0.001). Greater compressive strength occurs because soil particles in the crust are strongly aggregated into cohesive masses by many live plant structures (Schulten, 1985) and by organic matter. The soil surface is also protected by a continuous plant cover, which efficiently increases resistance to detachment by various geomorphic agents like wind, needle-ice heaving, runoff, rainsplash, and even trampling by cattle-all important contributors to soil erosion in the high paramo.

6. Summary

and conclusions

Microbiotic crusts, which appear not to have been studied in other alpine areas of the world, are described here in detail for the equatorial Andean paramo. These thin cryptogamic communities are shown to exert an important influence on diverse pedological properties, which in turn affect several geomorphic processes. Soils in crusts have substantially greater percentages of organic matter and fine particles. The increased organic matter is provided by the growth of the plants, while finer soils may result from prior texture differences-including those associated with frost sorting-and from trapping of aeolian material (mainly < 500 microns in size) by plants in the microbiotic crusts, especially mosses. These pedological modifications result in: (1) greater infiltration rates in crust areas; (2) increased water-storage capacity of the soils under microbiotic crusts; (3) significantly higher resistance to raindrop detachment and erosion in crusts, as compared to adjacent bare soils, and (4) stronger aggregation in crust soils, expressed in terms of greater soil cohesiveness. Clearly, more field and lab research should follow this brief pilot study, but all the pronounced changes observed ought to result in lower rates of soil erosion in paramo areas where crusts are dense and/or widespread. The author would kindly appreciate any communications from interested readers about further occurrences of microbiotic crusts in mountain and/or alpine areas.

Acknowledgements Funds were contributed by the Andrew W. Mellon Foundation, through the Institute of Latin American Studies, and the Research Institute (Univ. of Texas, Austin). I am indebted to Drs. Bruce Allen and Robert E. Magi11 (Missouri Botanical Garden), Othmar Breuss (Naturhistorisches Museum, Vienna), Dana Griffin, III (Univ. of Florida), Billie L. Turner (Univ. of Texas), and William A. Weber (Museum, Univ. of Colorado) for their valuable help with plant identification. I thank H. Hoenicka B., A. Lucchetti, and 0. Vera for their assistance in the field. My father, Francisco Perez Conca, helped with travel logistics. Dr. Karl W. Butzer generously allowed the use of his Soils Laboratory (Univ. of Texas). Drs. Avinoam Danin (Hebrew Univ. of Jerusalem) and William A. Weber kindly shared data and publications. Drs. Dietrich Barsch (Universitat Heidelberg), In& L. Bergquist (Univ.of Texas), H. J. Miicher (CSIRO, Division of Soils, Canberra), Olav Slaymaker (Univ. of British Columbia, Vancouver), Jean Tricart

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(Universitk Louis Pasteur, Strasbourg), and Aaron Yair (Hebrew Univ. of Jerusalem) critically evaluated the manuscript.

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