Conservation of soil moisture by different stone covers on alpine talus slopes (Lassen, California)

Catena 33 Ž1998. 155–177

Conservation of soil moisture by different stone covers on alpine talus slopes žLassen, California / Francisco L. Perez ´

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Department of Geography, UniÕersity of Texas, Austin, TX 78712-1098, USA Received 27 October 1997; revised 23 June 1998; accepted 17 August 1998

Abstract This study reports on the influence of stone covers with different clast sizes on the soil moisture of alpine talus slopes in Lassen ŽCalifornia.. Fifteen four-plot sets were sampled in the dry season ŽJuly 1990. in sandy areas and in talus covered with pebbles, cobbles, or blocks between 2740 and 2775 m. Three depths Ž0–5, 5–10, 10–15 cm. were sampled. Field moisture content increased gradually with depth in all soil profiles, and also in plots covered by increasingly larger rocks. Surface soils in sand areas were very dry, but under rocks had water contents 6 to 14 times greater. Differences among plots decreased with depth, but subsoil samples in sand were still drier than those beneath any stone cover at similar depths. Blocks were most effective in conserving moisture; water content below them was higher than even in deep Ž10–15 cm. sand soils. Soil temperatures were recorded in sand and under blocks for an 11-day period. Minima were not significantly different, but average maxima were 5.68C lower under blocks than in sand, which reached highs ; 4.48C lower than the air. Differences in soil moisture among talus types are ascribed to lower evaporation losses under stones, due to both disruption of capillarity by the coarse particles, which prevented water flow to the talus surface, and to their efficient reduction of maximum temperatures. An irrigation experiment was conducted at 2110 m on a steep talus on the Chaos Crags from July 18 to Aug. 2, 1993. Four 100 = 75 cm plots with the same surface types than at Lassen received 22.5 mm water; moisture content was then periodically sampled. Watering produced similar water distributions among soil depths and talus types to those in Lassen. Evaporation occurred quickly in bare soils due to high air and soil temperatures. The sand surface was already dry 2 days after watering, but stone-covered plots remained moist until day 15, when soils under blocks still retained 77–97% of the water content Žpercent by weight. at the start of the test. q 1998 Elsevier Science B.V. All rights reserved. Keywords: California; Cascade range; Clasts; Soil moisture; Soil properties; Talus slopes

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Fax: q1-512-471-5049; E-mail: [email protected]

0341-8162r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 0 9 1 - 5

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1. Introduction Alpine talus slopes, commonly formed by accumulation of coarse debris, remain dry at the surface as most of the water quickly percolates through the rubble or evaporates ŽTansley, 1911; Fisher, 1952; Yair and Klein, 1973; McCune, 1977.. Yet, many studies show that the fine-grained material beneath surface stones often has a high moisture content ŽWeaver, 1919; Jenny, 1930; Cox, 1933; Touring Club Italiano, 1958; Wardle, 1991; Perez, 1995.. This is ascribed to the insulation provided by a cover of stones and ´ the air spaces between them, which reduce evaporation ŽHarshberger, 1929; Jenny, 1930; Fisher, 1952; Tranquillini, 1964.. A layer of loose rock fragments reduces evaporative losses because of its extremely low unsaturated hydraulic conductivity at low suctions; capillary movement of water to the talus surface is thus prevented, and upward water transport through the clasts occurs only by vapor diffusion ŽPoesen and Lavee, 1994.. The effect of a stone layer on soil moisture is clearly relevant for vegetation on talus slopes, where plant cover is usually much higher on blocky areas ŽCooper, 1916; Harshberger, 1929; Tansley, 1939; Whitehead, 1951; Perez, 1991.. ´ In spite of the apparent importance of stone covers for water conservation, only a few studies have investigated their effect on talus soil moisture. As early as 1919, Weaver conducted tests in the Rockies of Colorado, where bare-soil talus at ; 2500 m lost by evaporation, over a 30-h period, eight times more water than soil covered by a one-inch layer of gravel. Perez ´ Ž1991. studied soil moisture content during two dry seasons on a high-altitude equatorial Andean talus, and found that surface water content below openwork stone layers was 12 to 20 times greater than in adjacent bare sandy talus. Although not dealing with alpine talus, relevant field experiments were conducted on low-elevation talus mantled slopes in the Sinai Peninsula by Yair and Klein Ž1973. and Yair and Lavee Ž1974, 1976.. They simulated rainfall events on steep taluses covered by stones, where infiltration and runoff rates were studied in detail. In the Sinai, runoff was correlated with stone size, as the impervious surfaces of boulders shed water—‘rockflow’ ŽPoesen and Lavee, 1994. —which became concentrated in small patches of fine-grained material between blocks. Debris slopes in the Sinai have a compacted fine-grain layer under the stones with a lower infiltration rate than surface clasts. As rockflow supply exceeds the infiltration rate of this fine material, runoff is quickly generated along the boundary between the two horizons. Wilcox et al. Ž1988. Žp. 203. investigated infiltration on steep ŽF 358. low-elevation debris slopes in the semiarid Guadalupe Mtns ŽNew Mexico.. Results were opposite to those in the Sinai: infiltration was negatively associated with size of stones in surface covers. Wilcox et al. thought this discrepancy occurred because the small-stone plots were mostly ‘erosion pavements’, where raindrop impact had caused strong soil crusting and compaction, while larger rock fragments had ‘‘ . . . mostly originated from weathered limestone cliff faces which commonly protrude from the Guadalupe Rim.’’ 2. Water relations of stone covers in desert areas In contrast to the scanty references on talus slopes, the literature dealing with stone covers in deserts is simply staggering. Farmers in arid areas are familiar with the

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beneficial effects of surficial ‘gravel mulches’, which can reduce soil water losses, increase runoff, influence ground temperatures, and greatly increase crop yields ŽLamb and Chapman, 1943; Adams, 1967; Corey and Kemper, 1968; Unger, 1971; Fairbourn, 1973; Arana 1974; Kemper et al., 1994; Kosmas et al., 1994.. The idea of ˜ and Lopez, ´ using gravel covers to conserve soil water or to control and harvest rainwater is very old, as ancient Nabateans appear to have employed them in the Sinai desert by the 4th century BC ŽCorey and Kemper, 1968; Evenari et al., 1982; Poesen and Lavee, 1997.. Many studies have identified several variables of a stone cover that are relevant to water conservation. A recent, thorough review by Poesen and Lavee Ž1994. discussed the overall significance of rock-fragment layers in top soils. The relatiÕe position and degree of penetration of clasts appear to affect soil water relations. Unger Ž1971. experimented with gravel layers placed on the surface and at 5, 15, and 25 cm depth, and determined that only the first two increased soil moisture. This was related to the dual role of gravel horizons, which can act as barriers both for upward and downward water movement. Water percolates through gravel into the soil beneath only when the soil above the coarse horizon approaches saturation; thus, depth of infiltration was greater with surficial gravel layers. At the same time, the gravel reduced evaporation by causing a capillary discontinuity in soil which prevented water flow to the surface. For this last reason, roots of shrubs in the Negev desert ŽIsrael. are able to exploit the greater moisture Žup to 250% more than in the surrounding soil. of ‘water pockets’ below stones, as deep as 80 cm within the soil profile ŽEvenari et al., 1982, p. 261.. The position of rock fragments on the soil surface also affects infiltration processes, as stones resting on the ground protect structural soil pores from sealing due to drop impact ŽValentin, 1994., while embedded fragments reduce the total area of exposed macropores ŽPoesen, 1986; Poesen and Ingelmo-Sanchez, 1992.. As these ´ authors mention, such findings contradict the general view that stone mulches should always be considered beneficial, since their effects depend on the degree of rock incorporation into the soil. The size of the particles covering the soil has a strong influence on its water budget. Corey and Kemper Ž1968. noted that the grain size of the mulch layer should be significant mainly in relation to the texture of the underlying soil, and that evaporation would be reduced only if gravel particles are bigger than the grains of the soil beneath. Actually, the key issue is not the size of the gravel grains, but of the pores between them. A gravel mulch will be a poor conductor for upward water flow only if most of its pores are larger than those of the subsurface soil. Modaihsh et al. Ž1985., comparing the effects of ‘coarse’ Ž0.25 mm. and ‘fine’ Ž0.1–0.15 mm. sand mulches on suppression of evaporation, noted that texture variation was less important than thickness of the sand cover. Ingelmo-Sanchez et al. Ž1980. compared evaporation in a sandy soil with and ´ without 23% gravel Ž2–3 mm diameter., and found that sand alone lost, over a 27-day period, about 28% more water than sand with gravel. Water losses were lower even when the gravel was mixed with the soil and not added as a cover ŽIngelmo-Sanchez, ´ 1997, pers. comm... Perez ´ Ž1991. sampled water content in sandy talus under large single boulders, where moisture was higher than in bare contiguous soil; the difference in water content between the sand and the soil under blocks increased nearly 8 times with block size Žrange: 46 to 88 cm.; this may have resulted from a more effective

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insulation and evaporation reduction by the larger boulders. Valentin and Casenave Ž1992. compared infiltration rates in soils with a broad range of stone-cover sizes Ž2–20, 21–75, and 76–150 mm. in the Sahelian zone. Infiltration rates increased along with the size of free Žnon-embedded. gravels up to a median of 29 mm, then dropped with larger fragment size; unfortunately, no clear reasons for this were evident. Several authors ŽAbrahams and Parsons, 1991; Poesen and Lavee, 1994; Brakensiek and Rawls, 1994; Valentin, 1994. have also commented on the ambivalent effects of clast layers on infiltration and runoff generation, and have concluded that this complex relationship depends on a number of variables in addition to fragment size. Most studies agree that greater thickness of the fragment layer results in higher soil moisture. Corey and Kemper Ž1968. noted in Colorado that a 12-mm gravel mulch effected greater water savings, by preventing evaporation, than a 6-mm layer, but water conservation increased no further with a 25-mm layer. Adams Ž1967. compared thin gravel layers Ž2.5 and 5 cm. in Texas, and found that plots with gravel had more available water than bare areas, but moisture under the thinner mulch was slightly greater than below the thicker one, although this may have been caused by differences in plant growth. Kemper et al. Ž1994. observed greater water conservation over a 388-day period in Colorado soils under 5 cm-thick gravel than beneath thinner layers Ž1 and 2 cm.. Modaihsh et al. Ž1985. also reported greater water conservation in Iowa soils with a 6 cm-thick Žvs. 2 cm. layer of sand. Perez ´ Ž1991. measured a water content in Andean talus which was 75% higher below block layers 15–25 cm-thick than under 8–15 cm-thick clastic epipedons. Lamb and Chapman Ž1943., over a 20-day period in New York state, also recorded lower evaporation losses with a 20-cm stone layer than with a 10-cm one. Percentage of ground area covered by stone fragments is, predictably, positively correlated with soil moisture. This may be due to greater infiltration rates in stone-covered plots, at least on bare areas, since plants can affect and greatly complicate this relationship ŽAbrahams and Parsons, 1991.. Hillel and Tadmor Ž1962. and Kadmon et al. Ž1989. found that water availability in the Negev depends on the exposed Žbed.rock to soil cover ratio, as wetting depth near rocks is higher ŽG 50 cm. than in bare soils, where the infiltration front seldom exceeds 30 cm. Vogel Ž1955. found similar contrasts next to rocks in South African deserts. This is caused by the aforementioned concentration of rockflow into adjoining soil patches; with greater percentage of fragment cover, all inputs to the soil—including water—become increasingly concentrated in a smaller mass of soil ŽPerez, 1991; Poesen and Lavee, 1994., thus the deeper infiltration. In ´ addition, a greater fragment-cover percentage will decrease evaporative losses, as Lamb and Chapman Ž1943. determined when comparing cover percentages Ž18, 65, 100%.. The color of surface rocks produces additional effects on soil moisture, as lighter clasts induce lower evaporation rates than darker ones ŽFairbourn, 1973; Kemper et al., 1994.. This is due to the greater albedo of light-colored stones, which causes them to heat up less quickly than dark ones and results in diurnal soil temperatures 5–68C lower beneath them ŽLarmuth, 1978; Kemper et al., 1994.. Many researchers have used field-irrigation or rainfall-simulation experiments to clarify the effects of stone covers ŽPoesen, 1986; Abrahams and Parsons, 1991; Poesen and Ingelmo-Sanchez, 1992; Valentin and Casenave, 1992; Van Wesemael et al., 1996.. ´

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Numerous water-application systems have been devised for such purpose, but most have practical constraints that restrict their usefulness, the most important being limited mobility. ‘Mobile’ infiltrometers have been developed for use in deserts or rangelands ŽBlackburn et al., 1974; Yair and Lavee, 1976; Rostagno, 1989. but trucks are needed to transport the water supply, motors, pumps, etc. utilized by these heavy systems, which can only be used on flat areas or near the base of slopes. ‘Hand-portable’ rainfall simulators are still problematic. Many are bulky and cumbersome ŽMalekuti and Gifford, 1978.; they may still need a truck to carry a watertank trailer ŽWilcox et al., 1986., their only truly hand-portable part being the nozzle, connected to a water source by a lengthy hose. Munn and Huntington Ž1976. used a hand-portable simulator purportedly for rugged terrain, but the apparatus was still quite bulky, and was only ‘‘ . . . successfully used to measure the infiltration rates . . . on slopes up to 30% w; 16.58x’’. Truly transportable infiltrometers exist, but they deliver water to a very small area, usually a circular plot - 15 cm diameter ŽAdams et al., 1957; McQueen, 1963.. Presumably at least in part due to these limitations, no rainfall-simulation experiments have ever been carried out—to my knowledge—in alpine talus slopes. As an alternative to the use of complex equipment, several field studies concerned with evaporation rates from soils under rock covers have simply relied on methodical irrigation or application of specific amounts of water to test plots of known size ŽUnger, 1971; Fairbourn, 1973; Jury and Bellantuoni, 1976a; Modaihsh et al., 1985; see also Koon et al., 1970.. This approach does not provide data on variables such as median drop size, kinetic energy of raindrops, or rainfall intensity rates, which may be critical for rainsplash and runoff-erosion research Že.g., Abrahams and Parsons, 1991; Valentin, 1991; Van Wesemael et al., 1996., but are largely irrelevant in evaporation studies. The specific objectives of this study are to: Ž1. contrast the dry-season Žsummer. field moisture of bare and stone-covered soils in talus slopes of Lassen ŽCalifornia.; Ž2. evaluate the role of stone size in water conservation by comparing soil moisture under layers of stones with different sizes; Ž3. replicate with an irrigation field experiment the moisture conditions found in bare talus and under stone covers; and Ž4. monitor evaporation losses from these experimental plots for an extended time period.

3. The study area Lassen Volcanic National Park ŽLVNP. is in northern California ŽFig. 1.. Its highest point is Lassen Peak Ž3193 m., the southernmost volcano of the Cascade Range, at 408 29X N and 121830X W ŽFig. 2.. Lassen is a dacite dome extruded ; 11,000 years BP. The Chaos Crags Ž2592 m., a cluster of domes 5 km north of Lassen, also formed by lava extrusion about 1200 years ago. Peaks in LVNP have enormous talus banks. Much debris originated during dome extrusion, as spines rose through the rubble generated by expansion and fracturing of the outer layers. Glaciers covered Lassen Peak during the Late Tioga advance Ž11,000–9000 BP., removing most of the original clast cover; thus, taluses on Lassen have been produced by rockwall weathering during the last nine millennia ŽCrandell, 1972; Perez, 1989a.. A catastrophic rockfall ca. 1675 AD left a ´ huge scar on the NW flank of the Chaos Crags, now occupied by talus deposits which

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Fig. 1. Location of the study area ŽLassen Volcanic National Park. in northern California. Dark circles identify main cities: SF: San Francisco, LA: Los Angeles, SD: San Diego.

have accumulated during the past 325 years ŽCrandell et al., 1974; Perez, 1998.. ´ Bedrock at both sites is a porphyritic hornblende–mica dacite, which breaks easily owing to its high content of interstitial, fractured glass ŽWilliams, 1931; Perez, 1988.. ´ This study focuses on the alpine talus slopes—above timberline—on both Lassen Peak Žfrom 2720 to 3055 m. and the Chaos Crags Žfrom 2030 to 2270 m.. The sedimentology and general geomorphic characteristics of these talus deposits have been described in detail elsewhere ŽPerez, 1989a, 1998.. ´ LVNP has a typical Mediterranean climate characterized by warm, dry summers, and cool winters with heavy snowfall. During summer, Lassen is under the influence of the North Pacific High, which blocks precipitation for several months. As this high migrates south in winter, storms move south-easterly into the region from the Aleutian low. Accordingly, most precipitation falls between October and April as snow, which is redistributed by the prevailing westerlies to produce sizable snowfields on talus slopes. The nearest station ŽManzanita Lake, 1750 m, ; 3 km from the Chaos Crags. has an annual 30-yr mean of 1049 mm, with 91% of this received in the October–May period. Average monthly temperatures at Manzanita Lake vary from y1.18C in January to 16.98C in July ŽParker, 1992.. During summer, broad daily fluctuations occur on Lassen and other high peaks, where maxima can reach 258C and minima may dip below freezing at night ŽPerez, 1988.. ´ Alpine soils have scarcely been studied at LVNP. Talus soils on Lassen Peak are sandy–skeletal Lithic and Typic Cryorthents with a high gravel content—up to 60%— and much sand—77 to 93%—in the soil fraction, and thus are catalogued as graÕelly

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Fig. 2. View of the southern flank of Lassen Peak, showing the upper edge of the subalpine forest and the extensive talus slopes which fringe the mountain above timberline. Note the sizable dome spines which protrude through several areas of the volcano; the prominent cylindrical mass left of the center is called Vulcan’s Eye. Talus moisture was sampled on the taluses below the dacite spines on the upper left side. Photograph taken Sept. 1983.

sand ŽSoil Survey Staff, 1994.. Silt content is only 6 to 21%, and clay content is very low, just 0.3 to 2.0% ŽPerez, 1989a.. This texture suggests that fines are produced ´ mainly by recurrent frost weathering of the easily fragmented dacite. The regional forest line—upper limit of continuous forest—in LVNP is at G 2440 m, but small ‘tree islands’ of whitebark pine Ž Pinus albicaulis. or mountain hemlock ŽTsuga mertensiana. are found up to ; 3060 m ŽPerez, 1990.. All the talus slopes ´ investigated on Lassen Peak were well above the forest line. Due to the northerly aspect of the talus on the Chaos Crags, the undisturbed local forest line is found there at ; 2130 m altitude; but because of its recent origin, the talus investigated is being slowly colonized by western white Ž Pinus monticola. and ponderosa Ž P. Ponderosa. pine trees ŽPerez, 1998. and remains still largely bare; thus, it lies right within the present ´ timberline zone.

4. Methods 4.1. Field sampling Soil samples were collected from gravelly-sand areas on the talus slopes of Lassen Peak Ž n: 30. and the Chaos Crags Ž n: 33. by pressing a 125-cc iron cylinder into the upper 10 cm of the soil profile. All samples were gathered in bare areas away from any plants, as presence of vegetation can strongly influence several pedological variables ŽLee and Hewitt, 1982; Perez, 1990, 1991; Abrahams and Parsons, 1991.. Altitude was ´

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measured at each location with an altimeter—estimated error: "10 m—and crosschecked on USGS topographic maps. Slope aspect was determined with a compass; several measurements of slope angle were obtained with a clinometer. Field moisture content was sampled at three depths Ž0–5, 5–10, 10–15 cm. Žcf. Jury and Bellantuoni, 1976a; Perez, 1995. on the southwestern flank Ž2408SW. of Lassen ´ Peak on July 24 and 25, 1990, in a zone between 2740 and 2775 m where the talus surface showed a broad size range of stone covers. Slope angle varied from 24 to 36.58 Žmean: 30.88.. Gravimetric water content Žpercent by dry soil weight. was determined in four types of talus covers: Ž1. graÕelly sand ŽG.: clasts absent to nearly absent. The other talus types had a stone cover: Ž2. pebbles ŽP.: with a mean size Žlongest w ax axis. of - 50 mm; Ž3. cobbles ŽC.: size: 50–100 mm; Ž4. blocks ŽB.: size: ) 100 mm ŽFig. 3. Žcf. Iwata, 1983; Valentin and Casenave, 1992.. Sampling sites were selected using a 50 = 50 cm portable wooden frame with a stringed 10 = 10 cm grid, which was superimposed on the talus to estimate average stone size ŽPerez, 1986.. Chosen sites had ´ a uniform cover type, and—in P, C, B areas—clasts covered plots completely; 15 sets

Fig. 3. Appearance of the four ground cover types on the talus surface of Lassen Peak: A. Gravelly sand. B. Pebbles. C. Cobbles. D. Blocks. The Žstring. grid scale is 10=10 cm.

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each with four samples ŽG, P, C, B. were sequentially obtained every 5 to 7 m from bare talus, starting at the slope base and proceeding upward. Samples within each set were gathered in the same order ŽG–P–C–B., at a similar elevation, and as close to each other as possible. Distance between set ‘end-points’ ŽG–B. ranged from 2.4 to 7.0 m; average within-set sample spread was 4.22 m. In most plots, particularly those with small stones ŽP, C., clasts formed a discrete openwork layer Žwith empty voids. over the underlying finer material, while the talus in B plots usually contained clasts at depth also. The thickness of this surficial horizon was measured several times with ruler or vernier gage at each ŽP, C, B. plot; the size of an average or ‘representative’ block was also determined along three Ž a, b, c . axes at each B plot. The soil samples for the 60 Ž15 = 4. replicate profiles Žand for the field experiment; see below. were kept in hermetically sealed and taped plastic containers Ž35 cc. until processed in the laboratory about 8 days after collection. Soil temperatures were continuously recorded on 8C paper with sealed mechanical thermographs Ža 4120 Weathertronics. in two contiguous, non-vegetated plots ŽG, B. on the SW Lassen talus Žaspect: 2318, angle: 298.. Thermographs were cross-calibrated prior to fieldwork and checked again in the field; the estimated measurement error after empirical tests is "0.58C ŽPerez, 1989b.. Thermograph sensors were at y15 cm ŽG. ´ and y29 cm ŽB: 14 cm openworkq 15 cm fine matrix; mean a axis: ; 17 cm.. Air ground temperatures Žq10 cm. were taken with a digital maximum–minimum thermometer placed on sand under a white shelter, around which air could circulate freely. Stations were installed on July 19, 1990, but when the site was revisited on July 29, widespread mass-wasting activity had clearly affected the talus, and all equipment was lost. A similar set of new instruments was installed a year later on a nearby site; these functioned successfully during an 11-day period ŽAug. 7–17, 1991.. 4.2. Experimental procedures A field irrigation experiment was carried out at 2110 m on the flank Ž3298NW. talus of the Crags from July 18 to Aug. 2, 1993, in a bare homogeneous area of gravelly sand Žmean slope angle: 34.38.. Four 100 = 75 cm plots were chosen about 6 m apart from each other, with their long side parallel to contours. Plots were demarcated with a sturdy, flexible 22 cm-wide plastic edge inserted 11–14 cm vertically into the ground. Several iron bars Ž60 cm long, 10 mm diameter. were staked around each plot outside its edge, and a few sizable dacite blocks were also placed there to help stabilize the experimental parcels during the test, as this steep talus is prone to dry avalanching ŽPerez, 1998.. Extreme caution was exercised every time plots were approached to avoid ´ further talus disturbance. Before experiment initiation, antecedent gravimetric soil moisture was determined at three depths Ž0–5, 5–10, 10–15 cm. with four replicate sets of samples per plot. Altitude, aspect, and slope angle were measured as above. Clasts were gathered and transported from nearby talus areas to the experimental plots, where a uniform openwork layer of pebbles, cobbles, or blocks ŽP and C: 7–8 cm thick; B: 10–12 cm s about two stones’ thickness. was placed resting Žnot embedded. on the slope surface. The fourth plot was left as originally found, thus serving as the G quadrat.

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Particle-size distribution was assessed for the G and P plots with 125 and 250 cc cores of surface material, respectively. Stone-size distribution on the C and B plots was determined by measuring Ž a axis. 120 clasts for each plot. A rainfall event was simulated on the morning of July 18 by manual irrigation with a hand-portable plastic container equipped with a single 7 cm-diameter, downward-oriented nozzle placed 50 cm above the plot surface, to minimize splash. The 0.5 mm-wide nozzle openings delivered a fine steady shower; each plot continuously received 22.5 mm of water during a ; 30-min period; thus, water-supply rate was about 45 mm hy1 . Prior empirical tests had suggested that this rate would not produce runoff on the coarse talus Žcf. Kemper et al., 1994., therefore no overland flow outside of plots occurred during the experiment. Water was obtained from Crags Lake, a large seasonal pond at the talus base. This, coupled with good site accessibility, was the main reason for choosing this talus for the experiment. The progression of soil moisture was thereafter determined in all four plots by direct soil sampling every few days, for a period of 15 days. Three sets of samples were taken per plot at three depths on five dates ŽJuly 20, 23, 26, 29; Aug. 2. around noontime, following procedures as above. Sampling took place each time on a different sector of the quadrats, in order not to disturb the rest of the plot. Stones were removed from a small area, and carefully replaced after gathering soil replicates from the underlying fine material Žcf. Jury and Bellantuoni, 1976a.. Samples were collected at least 15 cm away from any plot edges, thus creating a buffer strip to avoid any possible unwanted ‘edge effect’. Thermographs recorded air and soil temperatures in sand at the experiment site from July 17 to 29, 1993. One instrument was below a white shelter Žq10 cm., the other had its sensor buried at y10 cm. 4.3. Laboratory and analytical techniques Soils were air-dried for several days, then oven-dried overnight ŽG 16 h at 1058C.. Samples were rubbed by hand and sieved to remove the graÕel fraction Ž) 2 mm., given as a percentage by weight. Gravel of the samples from the G and P experimental plots was sieved through a 12-mesh series Ž45, 37.5, 31.5, 25, 22.4, 19, 16, 12.5, 8, 6.3, 4.75, 3.35 mm.. Organic matter content was assessed with the loss of weight on ignition at 3758C. The mineral soil fraction Ž- 2 mm. of all samples was then sieved through a 9-mesh series Ž1.4, 1.0, 0.7, 0.5, 0.355, 0.25, 0.18, 0.125, 0.09 mm.. The content of finer grains was determined with a Bouyoucos ŽASTM 152H. hydrometer using a Calgone solution as dispersing agent. Soil moisture in Lassen and the Chaos Crags was measured gravimetrically by oven-drying samples as above; water content was calculated with the equation:

Ž wet soil weighty dry soil weight. r Ž dry soil weight. and is expressed as a percentage of the dry soil weight. Statistical within-horizon comparisons of soil water content between different talus covers were made with the non-parametric Mann–Whitney ŽU . test, because the small data sets produced mostly non-normal distributions.

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5. Results and interpretation 5.1. Characteristics of talus stone coÕers and soils Mean particle size on the talus of both Lassen Peak and the Chaos Crags varies widely, from fine gravelly-sand to large blocks ŽFig. 3., which may reach an average length of ; 65 cm ŽPerez, 1989a, 1998.. Representative particle-size distributions for ´ the four talus types examined are shown in Fig. 4, which corresponds to the experimental plots on the Chaos Crags, where median particle size ŽD50 : Corey and Kemper, 1968. for the quadrats was 0.64 mm ŽG., 16.8 mm ŽP., 73.5 mm ŽC., and 149.6 mm ŽB.. Sandy areas, which become more common toward the steep talus apex, are very unstable and slide easily. Mean clast size rises toward the talus base in both areas due to gravitational sorting, since larger blocks bounce and roll to the footslope. Superimposed on this stratification, there is a complex lateral zonation, caused by constant downstreaming of debris on the mobile talus surface ŽPerez, 1986.; thus, all talus-cover types ´ can be often found next to each other, as in the area sampled for moisture on Lassen Peak. Excavation of stone-covered areas shows they only contain empty voids near the surface, and sand and smaller stones occupy them at depth. This packing is promoted by frequent talus sliding, as moving debris gradually fall into and in-fill voids between large clasts—the ‘sieving effect’ of Brunner and Scheidegger Ž1974.. Mean depth of the openwork layer on Lassen Peak was 4.1 cm Žrange: 2 to 9.5 cm. in P areas, 8.9 cm Ž5 to

Fig. 4. Particle size distributions Žmm. of surface material from the different talus covers, as measured at the experimental plots on the Chaos Crags talus ŽNW flank, at 2110 m.. Key, from left to right: A. Gravelly sand. B. Pebbles. C. Cobbles. D. Blocks.

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Table 1 Selected physical properties of soils in ‘gravel’ areas of talus slopes on Lassen Peak Žaltitude range: 2720 to 3055 m. and the Chaos Crags Žaltitude range: 2030 to 2270 m. Property

Lassen Peak

Chaos Crags

Gravel, percent by weight

24.3"8.43

13.7"6.9

Particle size (soil fraction: - 2 mm) Percent Sand Percent Silt Percent Clay

86.4"4.73 11.3"5.21 2.3"1.32

95.8"1.14 3.3"0.89 0.9"0.27

Percent Organic matter

1.73"0.81

0.42"0.06

Values are averages Ž"S.D.. of 30 ŽLassen. or 33 ŽChaos Crags. samples.

14.5 cm. in C zones, and 16 cm Ž12.5 to 22 cm. in B sites; mean block size was 48.3 = 35.6 = 22.5 cm Ž a axis range: 28 to 63 cm.. Talus soils were sampled on Lassen only on the southern peak side Ž1398SE–2458W. because the northern side was disturbed by recent Ž1914–1921. pyroclastic explosions and lahars ŽWilliams, 1932.. Sites sampled ranged in elevation from 2720 to 3055 m, and had a slope between 22 and 398 Žmean: 29.68.. Soil samples from the Crags were obtained from the collapsed NW flank Ž311–3208. between 2030 and 2270 m, on talus with a slope gradient of 24 to 35.58 Žmean: 30.68.. Sampling sites had an uncrusted soil surface, and clastic fragments above were free Žnon-embedded.. Soil samples from both Lassen and the Crags were quite coarse, but showed a pronounced inter-site textural variation ŽTable 1.. Gravel content was ; 75% higher in Lassen, but the fines fraction was three times greater Ž13.6%. there than in the Crags Ž4.2%.. This might be partially due to petrographic variance of the dacite bedrock but is more likely that such sharp contrast in fines results from the considerable age difference between study sites. Graphic envelopes of particle-size distribution indicate that the two populations of samples displayed characteristic texture curves with only partial overlap, but also that intra-site textural variation was significant ŽFig. 5.. Lassen soils contained four times as much organic matter as those on the Chaos ŽTable 1.. This could also be attributed to site differences in age and plant density, although the finer soils in Lassen would, in any case, be able to adsorb more organic matter than the coarser soils on the Crags talus ŽRostagno, 1989.. A relevant point stemming from these data is that talus soils on both peaks, due to their coarse texture and low organic matter have a restricted capacity to store water or to provide even a modest moisture supply during the dry summer ŽPerez, ´ 1987.. 5.2. Field moisture content on Lassen Peak Water content was sampled on the SW flank of Lassen because it is the warmest and driest on the peak; this is even reflected in the timberline, which reaches its greatest elevation Ž3060 m. here ŽPerez, 1990.. When sampled in late July 1990, soils in ´ gravelly-sand talus were considerably drier than under stone covers. Surface Ž0 to 5 cm.

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Fig. 5. Particle size distributions Žmm. for the soil fraction of surface samples collected on sandy talus areas of Lassen Peak Žaltitude range: 2720 to 3055 m. and the Chaos Crags Žaltitude range: 2030 to 2270 m.. Graphic envelopes show 30 samples from Lassen Peak Žlight shading. and 33 samples from the Chaos Crags Ždash lines..

soils in sandy areas were totally dry and dusty, while those below rocks were visibly moist and contained, on average, 6 to 14 times more water ŽTable 2.; statistical differences between sand and all stone covers were highly significant Ž p - 0.001.. Water content increased with depth. This happened gradually under stone layers but more abruptly in sandy talus ŽFig. 6.; however, subsoil samples in the latter were still

Table 2 Average moisture content Žgiven as a percentage of dry soil weight"S.D.. of the soil fraction Ž - 2 mm. in different types of talus surfaces ŽSW flank, between 2740 and 2775 m altitude. on Lassen Peak Soil depth Žcm.

Talus covers Gravelly sand

0–5 5–10 10–15

0.8"0.34 6.3"2.35 10.8"2.37

Pebbles

Cobbles a

4.9"2.62 9.0"2.90 c 11.5"2.23

Blocks a,d

8.7"2.52 11.1"3.12 a 12.3"2.94

11.0"2.70 a,d 12.0"2.27 a,e 13.3"3.52 b

Numbers indicate averages for data sets of 15 samples collected on July 24 and 25, 1990. Significance levels correspond to within-horizon comparisons ŽMann–Whitney wU x test. between talus covers. Comparison with gravelly sand: a : p- 0.001; b : p- 0.05; c : p- 0.01. Comparison with pebbles: d : p- 0.001; e : p- 0.01.

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Fig. 6. Variation of gravimetric field moisture content with depth in the talus soils of Lassen Peak. Soil moisture is expressed as a percentage of dry soil weight. Samples taken July 24 and 25, 1990 between 2740 and 2775 m altitude on the SW flank of the volcano. Each graphic envelope shows the spread of moisture values, at three depths Ž0–5 cm, 5–10 cm, 10–15 cm., of 15 sampled soil profiles. The dark dash lines within the envelopes indicate the median water content for the cover type; thinner solid lines separate statistical quartiles. Key: A. Gravelly sand. B. Pebbles. C. Cobbles. D. Blocks.

drier than those below all stone covers at the same depth. The substantial moisture rise in sand resulted in smaller differences among mean values at depth, so that at 5–10 cm depth, soils under blocks had just 90% more water than those in sandy areas Ž p - 0.001., and by 10–15 cm this difference had been reduced to only 23% and was, accordingly, less significant Ž p - 0.05.. Nevertheless, it is noteworthy that soils in sand plots still contained a little less water at 10–15 cm depth than did surface soils in block areas. Average values on Table 2 mask several important aspects of moisture variation within talus types. Fig. 6, which shows data envelopes as well as the median, clearly illustrates the following points: 1. Moisture variance at the surface was similarly high with all stone covers, but much narrower in sandy plots, which were all quite dry Ž- 2% water. at the time of sampling. 2. Moisture variance generally increased with depth in all talus types, including gravelly sand. 3. Overall variation of subsoil water content in sand plots resembled that observed in pebbles. The rate of moisture change down the soil profile was equally pronounced in both areas, where the median values attained at 10–15 cm were comparable ŽFig. 6A and B.. 4. In contrast, water content rose less dramatically with depth in cobble- and blockcovered plots. These types also displayed a similarly broad range of moisture variance, especially at the surface and at 10–15 cm depth ŽFig. 6C and D., where several samples reached water contents of nearly 20%.

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In summary, field data from Lassen Peak show two well-defined general trends: moisture increased steadily down the soil profiles, as well as with greater particle size on the talus surface. 5.3. Daily temperature fluctuations on Lassen Peak Data from the 1991 summer provide useful information on the likely conditions during sampling in 1990. It is pertinent to note that summer temperatures on Lassen Peak in other years ŽPerez, 1988., and measurements obtained during extensive annual ´ field work since 1982, indicate little inter-annual weather variation should be expected in LVNP: summer here is simply an extremely hot, dry period.

Fig. 7. Daily temperature fluctuations Ž8C. at the study sites. A. SW flank of Lassen Peak, 2720 m; records from Aug. 7 to 17, 1991. Solid line: soil temperature Žy15 cm. in sandy talus; dashed line: soil temperature Žy15 cmq14 cm ‘openwork’ layer. in an adjacent area beneath blocks Ž14 cm-thick layer.. B. Temperature cycles from July 17 to 29, 1993 in a sandy talus plot, NW flank of the Chaos Crags, 2110 m. Solid line: air temperature Žq10 cm.; dashed line: soil temperature Žy10 cm..

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Soil temperature was consistently higher in bare sandy talus than in the block plot ŽFig. 7A.. Sandy soil reached maxima between 17.9 and 22.28C Žmedian: 20.78., while soil below blocks only attained highs of 13 to 16.18C Žmedian: 15.18.; thus average daily difference between plots was 5.68C. In contrast, nightly minima hovered about the same level in both plots; temperatures in sand dropped to 10.0–12.18C Žmedian: 118C. but reached 9.3–12.48C Žmedian: 10.88C. beneath blocks. During the same period, air temperatures climbed to a maximum of 26.68C, and dipped to 7.08C. Diurnal amplitude in blocks was only ; 4.38C, vs. 9.78C in sand; clearly, this contrast resulted primarily from the efficient reduction of maxima by stones ŽMehuys et al., 1975.. Thermal fluctuations were dependent in both plots on cloud cover: extreme amplitudes occur under clear skies Že.g., Aug. 8–12., but overcast days ŽAug. 13, 14. substantially reduced temperature ranges ŽPerez, 1995.. The ability of the rocks to insulate the ground ´ underneath is also evident in the lag times between talus types, as maxima and minima were reached in sandy talus 3 to 3 1r2 h before than under blocks ŽFig. 7A.. 5.4. Irrigation experiment at the Chaos Crags Sample examination revealed no significant textural soil differences among experimental quadrats. Antecedent moisture prior to placing the stone covers was similar in all four plots ŽTable 3.. Soils were uniformly dry at the surface Ž0.4–0.9%., and water percentages increased sharply downward, but moisture at depth Ž2.7 to 6.7%, mean: 4.2%. was somewhat lower than that found 3 years earlier in Lassen ŽTable 1.. This may

Table 3 Average gravimetric moisture content Žgiven as a percentage of dry soil weight"S.D.. of the soil fraction Ž - 2 mm. in four experimental plots on the talus slope ŽNW flank, 2110 m altitude. of the Chaos Crags, between July 18 and August 2, 1993 Talus cover

Depth Žcm.

day 0 ŽJuly 18.

day 2 ŽJuly 20.

day 5 ŽJuly 23.

day 8 ŽJuly 26.

day 11 ŽJuly 29.

day 15 ŽAug. 2.

Gravelly sand Ž0.64.

0–5 5–10 10–15 0–5 5–10 10–15 0–5 5–10 10–15 0–5 5–10 10–15

0.4"0.17 3.2"1.7 3.2"0.6 0.6"0.48 3.9"0.97 4.1"2.0 0.8"0.28 1.8"0.24 2.7"1.5 0.9"0.87 3.1"1.2 6.7"0.41a

0.5"0.32 5.0"0.4 6.5"0.59 7.3"0.88 a 7.3"0.25a 7.5"0.4 a 5.1"0.15a 6.5"0.86 a 7.5"0.42 a 5.7"0.2 a 7.4"0.36 a 8.6"1.8 a

0.3"0.06 4.4"0.28 4.8"0.44 4.9"0.89 a 5.7"0.63 a 6.5"0.51a 3.7"0.37 a 4.1"0.44y 4.1"0.67y 4.5"0.28 a 5.6"0.68 a 7.9"0.91a

0.3"0.03 2.7"1.6 4.2"0.33 4.6"0.96 a 5.5"0.57 a 5.9"0.98 a 2.8"0.68 a 5.7"1.4 a 5.1"0.46 a 3.9"0.22 a 5.8"0.44 a 6.1"0.95a

0.3"0.11 2.9"0.65 3.4"1.4 4.2"0.47 a 4.9"0.64 a 4.8"0.3y 2.2"0.43 a 4.7"0.4 a 4.9"1.1y 3.8"1.3 a 4.9"0.78 a 6.4"0.18 a

0.3"0.04 0.7"0.34 3.5"1.0 1.6"0.9 a 5.0"0.74 a 6.3"0.42 a 0.2"0.04y 3.5"0.89 a 4.0"0.36y 4.4"1.8 a 6.7"0.26 a 8.3"0.77 a

Pebbles Ž16.8.

Cobbles Ž73.5.

Blocks Ž149.6.

Water content values for day 0 Ž‘antecedent moisture’. are averages of four samples; other data sets include three samples. Significance levels correspond to within-horizon comparisons ŽMann–Whitney wU x test. between gravelly sand and the other talus covers. Median particle size in mm ŽD50 . for all plots is indicated in parenthesis. a : p- 0.05; y: no difference.

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be ascribed to the lack of nearby water sources for the Crags talus at the time Žsee discussion below. and to the coarser texture of the soils there. Soils were sampled 48 h after irrigation in order to allow enough time for water to percolate through the upper soil layers ŽBirkeland, 1974.. At this time, the overall distribution of water was similar to that found on Lassen Peak ŽTable 2.: moisture increased with depth in all four plots, and also under larger stones. However, all soil depthrcover combinations—with the sole exception of the surfacerpebble soil—contained less water than comparable samples on Lassen. The surfacerpebble soil also had a greater moisture content at the Crags than any other surface soils; at depth, it held nearly the same, or even more water, than soil under cobbles or blocks ŽTable 3.. After July 20, moisture content dropped gradually in all plots and soil depths, although the rates at which this took place differed among test treatments. Fig. 8 provides a useful visual comparison of progressive water losses; the main trends can be summarized as follows. Ž1. Fastest evaporation occurred, as expected, in exposed sand. Remarkably, the surface of this plot was already dry Ž0.5% water. only two days after irrigation. By day 5, moisture had dropped to 0.3%, a meager water content maintained until the end of the observation period. Deeper soils Ž10–15 cm. became desiccated more gradually, but had nearly returned to pre-test conditions after 15 days. Ž2. As anticipated, water was conserved most efficiently by blocks. Water losses were modest, and by the end of the experiment, soil in this quadrat retained 77%, 91%, and

Fig. 8. Gravimetric soil moisture changes, at three different depths Ž0–5 cm, 5–10 cm, 10–15 cm., in four experimental talus plots during 15 days Žfrom July 18 to August 2nd, 1993. at the Chaos Crags, 2110 m. A. Gravelly sand. B. Pebbles. C. Cobbles. D. Blocks. For each talus cover, the frontal polygon indicates moisture changes at 0–5 cm depth, the middle one at 5–10 cm, the polygon on the back at 10–15 cm. Soil moisture is expressed as a percentage of dry soil weight. The first measurement Žon day 0, July 18. indicates antecedent soil moisture before watering. Day 2 ŽJuly 20. shows the first moisture measurement after initiation of the experiment; soil water is then gradually depleted throughout the remainder of the observation period.

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97% Žat 0–5, 5–10, and 10–15 cm, respectively. of the maximum moisture content on day 2. Ž3. Surprisingly, the pebble-covered soil lost water only slightly faster than that under blocks, and after 15 days still contained 22%, 68%, and 84% Žat 0–5, 5–10, and 10–15 cm. of the water on day 2. Ž4. Cobbles were much less effective than the other stone covers, and only marginally better than bare sand, in preventing soil evaporation. Upon test termination, the soil beneath cobbles conserved only 4%, 54%, and 53% Žat 0–5, 5–10, and 10–15 cm. of the water it contained on July 20 ŽFig. 8.. Ž5. A few measurements were higher than those in a previous sampling period. This, of course, does not mean that soil moisture increased, but simply shows the inherent variability of moisture data. Ž6. By the end of the field experiment, all soils at the Chaos Crags were much drier than their counterparts on Lassen Peak. 5.5. Diurnal temperature cycles on the Chaos Crags The climate during the evaporation experiment was very hot and dry. Air temperatures on the talus climbed during the day to 18.3–24.18C Žmedian: 20.98C.; peak temperatures were reached around 1430 h. At night, minima stayed rather low, between 2.6 and 7.98C Žmedian: 6.08C.; the lowest temperatures occurred right before sunrise Ž0625 h.. As a result, the average daily thermal amplitude was broad Ž14.98C. ŽFig. 7B.. Despite the northern aspect of the site, and due to lack of any nearby obstructions, the experimental plots were directly illuminated for at least 9 1r2 h every day Žfrom ; 0730 to after 1700 h.; this allowed the talus substrate to reach high temperatures. The exposed gravel attained a maximum between 18.9 and 23.08C Žmedian: 20.18C. around 1645 h, thus exhibiting a time lag of about 2 1r4 h with the air temperatures. The minima dropped to only 10.5–13.98C Žmedian: 12.18C., and were attained about 2 1r2 h after those in the air Žaround 0850 h.. Because the sandy talus experienced slightly lower maxima but substantially higher minima than the air, its median diurnal amplitude was only 8.08C. The weather remained exceptionally hot, with clear skies throughout most of the test period, but two overcast days ŽJuly 21 and 22. strongly influenced temperatures, by depressing maxima 2 or 38C and raising minima by several degrees ŽFig. 7B.. 6. Discussion and conclusions It is relevant to emphasize that talus slopes at LVNP are highly unstable due to frequent activity of numerous geomorphic processes, including rockfalls, dry debris slides Žgrain flows., debris flows, snow avalanches, needle-ice heave and creep, and block gliding over the snow ŽPerez, 1989a, 1998.. All these processes are crucial in ´ determining the characteristics both of stone covers and of the underlying soils, as slope mobility appears to prevent the development of a compacted layer under and between the clasts, which would limit infiltration depth. This is in contrast with gravelly surfaces in arid areas, which often show a very limited infiltration capacity due to the occurrence

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of a strongly compacted layer immediately below the clasts ŽYair and Lavee, 1974, 1976; Yair, 1997, personal comm... Stone covers must have influenced soil moisture in LVNP in different ways. First, infiltration in the Lassen Peak talus—at least in block areas, where clasts were more intermingled with the matrix than in the other talus covers—was probably affected by rockflow, which would have concentrated water in the interstitial fine matrix and presumably increased the depth of the percolation front. However, as pebbles and cobbles had a tendency to simply form a discrete layer over sand, infiltration in these areas was probably more uniform. For similar reasons, infiltration variance should have been minimal in the Crags talus, as rocks on all experimental plots were just laid down resting on the ground surface. Some authors have emphasized the role of ‘subterranean dew’ condensation ŽBoyko, 1967; Evenari et al., 1982. in arid areas, caused by temperature differences between soil layers. This ‘distillation’ process ŽStark, 1970. is an important source of moisture for desert plants ŽEvenari et al., 1975, 1982.. Rapidly falling temperatures at sunset in high-equatorial areas such as Mt. Kenya ŽCoe, 1969. or the Andes ŽPerez, 1991. may ´ also cause condensation in the spaces sheltered by talus blocks. Considering the pronounced lag times between air and soil temperatures observed at LVNP, both processes could have added some moisture to upper soil horizons in both study sites. Jury and Bellantuoni Ž1976b. found that rock covers induce lateral heat flow and water-vapor movement from the surrounding soil to the areas under stones, resulting in condensation in the rock-sheltered soil. Other studies ŽStark and Love, 1969; Mehuys et al., 1975. have also detected condensation on the urdersides of surface stones, but this process is restricted to dry soils, and wet ground undergoing evaporative cooling should not be affected by it ŽJury and Bellantuoni, 1976b.. It is therefore improbable that the moist rock-covered soils in the study sites allowed much lateral heat flow and condensation. In all likelihood, the moisture differences observed in both taluses resulted mainly from reduced evaporation rates under stones. The rocks and the intervening air spaces had a dual role in restricting water losses: they prevented capillary movement to the soil surface, and effectively insulated the ground from high diurnal temperatures. In contrast, unrestricted evaporation produced a swift water loss in the exposed sandy soils, which were dry 2 days after watering. This is in agreement with many similar studies ŽCorey and Kemper, 1968; Unger, 1971; Fairbourn, 1973; Modaihsh et al., 1985. which indicate that most of the evaporative loss from bare soils occurs shortly Ž1 to 4 days. after moistening. At LVNP, it was possible to hold constant some of the features of a stone cover Žsee Section 1. that may have an impact on water conservation, and therefore they could not have caused any moisture variation. Thus, all plots at Lassen or the Crags had a 100% cover of non-embedded, uniformly-colored particles, as care was taken to exclude any areas with glistening black lava blocks Žerupted in 1915 wWilliams, 1932x. on Lassen Peak, or any deep red oxidized stones, often found at the Chaos Crags ŽWilliams, 1929.. In retrospect, however, it appears that the sharp contrasts in water content among talus types on Lassen Peak must have been caused by variation of both particle size and openwork-layer thickness. Both parameters ranged widely Žsize: - 1 mm in G to 63 cm

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in B; thickness: G 2 cm in P to F 22 cm in B.; however, since they exhibited a strong covariance—larger rock sizes were associated with a thicker cover—whatever their individual effects may have been, these could not be isolated. This problem was minimized in the experimental plots, where only particle size differed markedly among parcels Žfrom 0.64 mm to 14.96 cm.. As thickness of clast cover was nearly uniform Ž; 8 to 12 cm. in the Crags, this factor must have introduced little variance in the irrigation test. It is not clear why soils at the Crags remained more moist under pebbles than below cobbles, but the lack of strict correspondence between clast size and moisture content there suggests that the sharp variance of field moisture on Lassen Peak may have been caused more by the different thicknesses of clast layers than by stone size. As noted above Žin 5.1., the average depth of pebbles on Lassen Peak was substantially lower Ž4.1 cm. than that of either cobbles Ž8.9 cm. or blocks Ž16 cm.. In summary, mean clast size may have been less important that clast-layer thickness in preventing water losses. When the two sites are compared, the overall greater water content of rock-protected soils at Lassen ŽTables 2 and 3. might be attributed to the generally larger rock sizes and thicker clast covers found there, were it not for the fact that bare sand was also more moist at Lassen than in the Crags. This may have resulted from insufficient watering of the Chaos Crags soils at the outset of the experiment, but this is unlikely. Jenny Ž1980. Žp. 40. notes that—in absence of runoff—infiltration depth is controlled by field capacity, and gives a ‘rule of thumb’ which states that 1 cm of water would suffice to wet a sand to a depth of 6–9 cm or more. Based on this, the 22.5 mm applied to the Crags plots were probably enough to moisten the soil down to the maximum sampled depth Ž15 cm.; the fact that no ‘dry’ spots were found when digging supports Jenny’s data, and suggests that insufficient watering was not a factor. What was the source of water on Lassen Peak, where moisture was sampled two, or even three months, into the dry season? Snowfields, common at higher elevations, help produce many small debris flows which, along with snowmelt, contribute water to high-mountain slopes in early summer. Although snowfields and debris flows also affect the Crags slopes ŽPerez, 1998., snow disappears earlier from this lower elevation site, ´ whereas on Lassen much snow can linger on talus slopes, sometimes until late August ŽPerez, 1988, 1989a.. The most likely explanation for the lower moisture of the Crags ´ soils is that their coarse texture and lesser organic content simply do not allow them to store more water than they actually did. Extensive sampling on both mountains indicates that the soils at the two research sites are fairly representative of general pedological conditions on these volcanic peaks, and helps underline the importance of stone covers for water conservation in this Mediterranean-climate area, where the coarse sandy soils on alpine taluses become quickly desiccated unless protected by rocks. Acknowledgements Financial assistance was contributed by the Univ. of Texas Research Institute in 1990, and by the Committee for Research and Exploration of the National Geographic Society Žgrant 4592-91., during the 1991 and 1993 seasons. I thank G.E. Blinn ŽPark

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Superintendent. and E. Knight ŽChief Park Naturalist. for the research permits to work in Lassen Volcanic National Park. I am especially indebted to my sons, Andres ´ and Alejandro, for their vital help with field work. Dr. K.W. Butzer generously allowed the use of the SoilsrGeoarcheology Laboratory ŽUniv. of Texas.. Dr. J. Poesen ŽCatholic University of Leuven. kindly provided several relevant publications. The original manuscript was critically reviewed and improved by Drs. I.L. Bergquist ŽUniv. of Texas. and A.J. Parsons ŽUniv. of Leicester..

References Abrahams, A.D., Parsons, A.J., 1991. Relation between infiltration and stone cover on a semiarid hillslope, southern Arizona. Journal of Hydrology 122, 49–59. Adams, J.E., 1967. Effect of mulches on soil temperature and grain sorghum development. Agronomy Journal 57, 471–474. Adams, J.E., Kirkham, D., Nielsen, D.R., 1957. A portable rainfall-simulator infiltrometer and physical measurements of soil in place. Soil Science Society of America Proceedings 21, 473–477. Arana, J., 1974. Volcanismo. Dinamica y Petrologıa ˜ V., Lopez, ´ ´ ´ de sus Productos. Ed. Itsmo, Madrid, 481 pp. Birkeland, P.W., 1974. Pedology, Weathering and Geomorphological Research. Oxford Univ. Press, New York, 285 pp. Blackburn, W.H., Meeuwig, R.O., Skau, C.M., 1974. A mobile infiltrometer for use on rangeland. Journal of Range Management 27, 322–323. Boyko, H., 1967. Salt-water agriculture. Scientific American 216 Ž3., 89–96. Brakensiek, D.L., Rawls, W.J., 1994. Soil containing rock fragments: effects on infiltration. Catena 23, 99–110. Brunner, F.K., Scheidegger, A.E., 1974. Kinematics of a scree slope. Rivista Italiana di Geofisica 23, 89–94. Coe, M.J., 1969. Microclimate and animal life in the Equatorial mountains. Zoologica Africana 4, 101–128. Cooper, W.S., 1916. Plant successions in the Mount Robson region, British Columbia. Plant World 19, 211–238. Corey, A.T., Kemper, W.D., 1968. Conservation of Soil Water by Gravel Mulches. Colorado State University Hydrology Papers, 30, 23 pp. Cox, C.F., 1933. Alpine plant succession on James Peak, Colorado. Ecological Monographs 3, 300–371. Crandell, D.R., 1972. Glaciation near Lassen Peak, northern California. United States Geological Survey Professional Paper 800 C, 179–188. Crandell, D.R., Mullineaux, D.R., Sigafoos, R.S., Rubin, M., 1974. Chaos Crags eruptions and rockfallavalanches, Lassen Volcanic National Park, California. United States Geological Survey Journal of Research 2, 49–59. Evenari, M., Schulze, E.D., Kappen, L., Buschbom, U., Lange, O.L., 1975. Adaptive mechanisms in desert plants. In: Vernberg, F.J. ŽEd.., Physiological Adaptation to the Environment. Intext, New York, pp. 111–129. Evenari, M., Shanan, L., Tadmor, N., 1982. The Negev. The Challenge of a Desert, 2nd edn. Harvard Univ. Press, Cambridge, 345 pp. Fairbourn, M.L., 1973. Effect of gravel mulch on crop yields. Agronomy Journal 65, 925–928. Fisher, F.J.F., 1952. Observations on the vegetation of screes in Canterbury, New Zealand. Journal of Ecology 40, 156–167. Harshberger, J.W., 1929. The vegetation of the screes, or talus slopes, of western North America. Proceedings, American Philosophical Society 68, 13–26. Hillel, D., Tadmor, N., 1962. Water regime and vegetation in the central Negev highlands of Israel. Ecology 43, 33–41. Ingelmo-Sanchez, F., 1997. Pers. comm. ´ Ingelmo-Sanchez, F., Cuadrado-Sanchez, S., Blanco de Pablos, A., 1980. Evaporacion ´ ´ ´ de agua en suelos de distinta textura. Anuario, Centro Edafologıa ´ y Biologıa ´ Aplicada, Salamanca 6, 255–280.

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