The influence of surface volcaniclastic layers from Haleakala (Maui, Hawaii) on soil water conservation

Catena 38 Ž2000. 301–332 www.elsevier.comrlocatercatena

The influence of surface volcaniclastic layers from Haleakala žMaui, Hawaii / on soil water conservation Francisco L. Perez ´

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Department of Geography, UniÕersity of Texas, Austin, TX 78712-1098, USA Received 13 January 1999; received in revised form 23 May 1999; accepted 6 July 1999

Abstract This study focuses on the soils and surficial volcaniclastic layers of Haleakala’s crater ŽMaui, Hawaii.. The main objective is to assess the effects of covers with fragments of various sizes Žash, cinder, lapilli. on soil water conservation. Soil and gravel samples were collected in Haleakala National Park from a site at 2505 m where the Hawaiian silversword, a giant rosette-plant, grows densely on pyroclastic materials. An evaporation experiment lasting 22 days showed a gradual drop in seven pairs of soil samples initially at field capacity. One set of samples was left bare, the other was covered by gravel mulches ŽGM.; these resulted in lengthening of T100% Žtotal desiccation time. by a factor of 2.3–5.2. Bare soils dried after 70–140 h, but drying time under gravel was 246–509 h. Mean grain size Ž Mz . and sorting Ž fs . had the greatest influence on evaporation rates. Coarse lapilli Ž Mz : 13.8 mm. were less effective than fine ash Ž Mz : 2.9–3.8 mm. in preventing water losses, while medium-grained cinder Ž Mz : 4–5.2 mm. produced the greatest water savings. Lapilli Žmean fs : 0.48. and ash Ž fs : 0.6. were moderately- to well-sorted; cinders, with a broader grain-size range Ž fs : 1.13., were poorly sorted. This allowed infilling of large interstices between coarse fragments by smaller grains, effectively reducing pore size and therefore evaporation rates. A second experiment determined water storage and rates of water loss by mulches. Wetting occurred swiftly, within 3–4 min. All gravel types were dry within 26 h. This drying process is important for water conservation, as it effectively prevents further water loss from the mulch surface. Larger fragments stored less water. This is related to their ‘surface arearweight’ ratio, which increases for smaller particles. Thus, fine ash or cinder grains, with a high total area, intercept more water than larger lapilli. A 5-cm thick layer of ash intercepted an average of ; 6.8 mm of rain, slightly more than the same depth of cinder Ž6.3 mm., but lapilli

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retained only 4.7 mm. Therefore, light rainfall events are more likely to contribute water to soil under lapilli than below finer pyroclastic material. Volcaniclastic covers serve an important ecological role in Haleakala by prolonging periods of water availability to plants, thus allowing Hawaiian silverswords to grow for longer time spans. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Haleakala; Lapilli; Soil desiccation; Soil moisture; Vegetation ecology; Volcaniclastics

1. Introduction In recent years, several effects of rock fragments on soils have been the focus of numerous studies. Different processes that rock fragments on or within soils may influence include runoff generation and sediment yield, infiltration and percolation rates, soil erosion and the gradual physical degradation of top soils, aeolian deposition, soil temperatures and heat flow, subsurface water condensation, plant growth and biomass production, and evaporation of soil water Žsee detailed reviews in Poesen and Lavee, 1994a,b; Poesen and Bunte, 1996.. I recently discussed in this journal ŽPerez, 1998. ´ different effects of stone covers on soil–water relations; in this report, I will specifically focus on the influence that surface stone fragments Ž) 2 mm. have on water losses by eÕaporation from the underlying soil. 1.1. Influence of graÕel mulch characteristics on soil desiccation Since ancient times, farmers in arid zones have known that a cover of stone fragments can greatly reduce water losses from the soil below. Use of such ‘gravel mulches’ ŽGM. ŽWeaver, 1919. made possible agriculture in marginal, dry areas of Argentina, the Canary Islands, China, Israel, Italy, Peru, New Zealand, and the USA, where ‘lithic-mulch agriculture’ has been-so far-documented ŽGale et al., 1993; Lightfoot, 1994; Poesen and Lavee, 1997; Doolittle, 1998.. Many authors have demonstrated the ability of a GM to reduce evaporative losses from the soil, but the efficiency of this effect varies widely, depending on several mulch characteristics. Some researchers ŽAdams, 1965; Ingelmo-Sanchez et al., 1980. report ´ only a modest reduction Ž; 20–30%. in desiccation losses below a GM. Most studies indicate more substantial water savings. Corey and Kemper Ž1968. found that, after a whole summer, surface soils beneath a GM in Colorado still had about 80% more water than control soils. Fairbourn Ž1973. measured in the same area, over a 17-day period, a water depletion twice as high in bare soils than below a GM. Gee et al. Ž1994. determined that groundwater recharge in western USA deserts was F 50% of the annual precipitation under soils with abundant rock fragments, but recharge was absent under fine loamy soils. In a laboratory experiment, Keller Ž1954. found that, after 4 months, uncovered soils had lost three times more water than those under a GM. In the dry season in Kenya, Othieno Ž1980. measured a water content 3.3 times greater below a layer of volcanic rock chippings than in adjacent bare soil. Modaihsh et al. Ž1985. noted that, after 35 days of desiccation, bare soil had lost 4.5 times more water than soil under

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a 6-cm thick sandy mulch. Weaver Ž1919. experimentally determined that, over a 30-h period, bare talus soils lost 8.4 times more water than those covered by a GM. Lightfoot and Eddy Ž1994. studied 500-year old GM ‘gardens’ in New Mexico, where soils had an average water content 3.5 times greater than contiguous unmulched soils; contemporary agricultural soils also showed water contents 3–4 times greater beneath a lithic mulch. This last study and others ŽCorey and Kemper, 1968; Benoit and Kirkham, 1963; Othieno, 1980; Perez, 1991a, 1998. also indicate that water differences due to a GM are ´ most pronounced in the upper 5 cm of the soil, and that variation in water content swiftly disappears with depth, usually within 10 to 20 cm. Different aspects of a mulch cover have been investigated; thickness of the GM has a direct effect on soil water. A layer of just 6–15 mm may suffice to reduce water losses ŽHanks and Woodruff, 1958; Bleak and Keller, 1974.. Thus, Corey and Kemper Ž1968. Žp. 14. felt that ‘‘ . . . thickness wof a gravel mulchx should exceed 1r2 in. but . . . need not exceed about 1 in.’’, as evaporation may decrease no further with a thicker mulch. In fact, many reports indicate that thicker GM layers are more efficient in arresting desiccation. Hanks and Woodruff Ž1958. recorded evaporation rates 3.6 times greater with a 6.3 mm-thick GM than under 38.1 mm of gravel. Modaihsh et al. Ž1985. and Kemper et al. Ž1994. noted that GM layers 5–6 cm thick effected greater reduction in water losses than thinner Ž1–2 cm. ones. Lamb and Chapman Ž1943. and Perez ´ Ž1991a. also showed that stone layers 20–25 cm thick suppressed evaporative losses better than shallower Ž10–15 cm. stone covers. Size of gravel fragments can greatly affect desiccation rates. Lemon Ž1956. determined that a surface layer coarser than the underlying soil is needed to produce a diffusional barrier to both the liquid and vapor flow of water; thus, grain-size stratification in ‘layered soils’ significantly affects evaporation ŽWillis, 1960.. In fact, it is not the diameter of gravel fragments that matters, but the size and continuity of intergranular pores ŽPerez, 1998., as capillary flow will be interrupted only if the majority of the soil ´ pores are smaller than those within the overlying mulch. Considering this, it is surprising that few controlled studies have considered the role of GM grain size on evaporation. Corey and Kemper Ž1968. compared mulches of ‘fine gravel’ and ‘sawdust’; 17 days of desiccation resulted in water losses 60–115% greater under the latter, but no data on grain sizes were given. Benoit and Kirkham Ž1963. also contrasted ‘dust’ and ‘gravel’ mulches; their dust provided the least effective evaporation barrier because water was able to flow through it, while transfer within gravel occurred exclusively in the vapor phase. Modaihsh et al. Ž1985. tested two sand mulches Ž0.12 vs. 0.25 mm. and noted the coarser one was more effective in decreasing losses because water flowed more readily through finer sand. Studies comparing coarser particles, which should prevent liquid flow, appear to be scarce. Keller Ž1954. thought that ‘gravel chips’ Ž- 12.7 mm. were better than larger ‘pea gravel’ in preventing evaporation from soil, but as the chip mulch was ; 67% thicker, his conclusions are invalid. It is not always possible to separate the effects of gravel size from those due to thickness of the mulch layer. I sampled high-elevation soils in California ŽPerez, 1998. where water content in summer was ´ Ž directly correlated with the median size D50 . of their stone cover Žpebbles, cobbles, blocks.. However, as these stone layers also differed in thickness, the influence of size and thickness was confounded.

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1.2. Geoecological significance of surface Õolcaniclastic layers in Haleakala One of the most striking features of Haleakala is the extensive volcaniclastic layers that litter the crater floor. The first recorded ascent by haoles Žwhite foreigners. of Haleakala was in 1828, but this missionary party did not venture into the summit crater ŽMissionary Herald, 1829.. In 1841, three scientists of the Wilkes Expedition made the first descent into the crater ŽWilkes, 1845, p. 254.. These explorers already noted that ‘‘The gravel that occurred on the top was composed of small angular pieces of cellular lava, resembling comminuted mineral coal . . . containing irregular cavities rather than vesicles’’. Other early visitors commented on the widespread occurrence of volcaniclastic covers and their association with the Hawaiian silversword. Alexander Ž1870. noticed that these plants were most abundant over the ‘ volcanic sand’ Ži.e., cinder. in the central part of the crater. The silversword Ž Argyroxiphium sandwicense DC.. is the most spectacular plant in Haleakala’s crater. This giant rosette-plant has a single, short, 5–8 cm thick woody stem that ends in a dense crown of shiny, long, narrow, leaves; it produces attractive, tall stalks that contain up to 500 purple flower heads ŽFig. 1.. Human impact and browsing by feral goats had nearly brought this species to extinction by the 1920s, but protection within Haleakala National Park has resulted in a resurgence of crater populations. Still, little is known about the geoecological requirements of this endangered plant. Kobayashi Ž1973. Žpp. 48–49. noted silverswords were found on ‘pyroclastic ejecta’ and that high rosette regeneration was restricted to areas with ‘‘ . . . a complete cover of . . . fragments 2.5 to 5.0 cm thick’’, but after a few exploratory samples, he simply reported ‘‘ . . . a several-fold increase in moisture . . . a few cm below the . . . rock fragments’’ when rock-covered soils were compared to bare adjacent ones. The literature often mentions this silversword association with ‘cinder’ or ‘ volcanic gravel’ ŽRuhle, 1959; Siegel et al., 1970; Bruegmann, 1995. and several obvious plant adaptations to survive drought ŽCarlquist, 1970., but further data on Haleakala’s volcaniclastic layers are lacking, as far as I know. I surveyed 500 silverswords in five populations ŽPerez, unpubl. data.; most ´ plants had germinated on cobble or pebble areas ŽFig. 2., only ; 12% on fine gravel, and F 5% on bare sand devoid of stone fragments. A similar substrate association is found in related giant rosette-plants Ž Espeletia spp.. in the high equatorial Andes, as soil water below stone fragments is much greater than in bare sandy soils, where rosettes may not grow at all ŽPerez, 1991a, 1994.. ´ I have been unable to find comparable research on the ecological influence of volcaniclastic layers on soil water. Yet, in Lanzarote ŽCanary Islands., in a subtropical hyperarid climate Ž- 100 mmryear., farmers are able to grow grapes, maize, and other crops by using volcanic-ash mulches ŽLightfoot, 1994.. Arana ˜ and Lopez ´ Ž1974. Žp. 205. explain how the enarenados Žsand covers. are built as ‘‘ . . . very porous and refractory . . . pyroclastic fragments Žpumice and lapilli. are spread in layers over cultivated land to maintain its humidity and protect it from rapid evaporation’’. Eruptions in the 1730s blanketed much of Lanzarote with volcaniclastic ejecta, which farmers carry to their fields. A similar technique was used by the Sinagua Žliterally, ‘without water’. people near Sunset Crater ŽArizona. after eruptions around 1065 AD deposited vast amounts of ash Ž‘black sand’. and cinder, which they used to protect their crops from

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Fig. 1. Flowering Hawaiian silversword Ž Argyroxiphium sandwicense. along the Sliding Sands trail, at 2505 m altitude, Haleakala National Park. This plant is part of population 6 in Kobayashi, 1973; the leaf rosette is about 55 cm wide, height to the tip of the inflorescence is ;138 cm. Note the coarse lapilli and block cover on the right side of the photo; the fine-soil area devoid of fragments on the lower left is the Žeroded. trail. August 1998 ŽHK-98.0.3..

rapid desiccation ŽLightfoot, 1994; Doolittle, 1998.. Laboratory experiments using volcanic fragments are rare, but Groenevelt et al. Ž1989. found that a black volcanic

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Fig. 2. A silversword rosette-about 45–50 cm diameter-growing on an area covered by reddish scoriaceous lapilli; median fragment size Ž D50 . here is about 14–15 mm. Haleakala’s crater, 2505 m elevation Žpopulation 6 site; Kobayashi, 1973.. July 1996 ŽHK-96.27..

scoria Ž2–5-mm diameter. layer was nearly as efficient in reducing evaporation from the soil below as a sand mulch. I am now investigating the spatial distribution of soil water in Haleakala during the dry season, in bare ground and adjacent areas covered by pyroclastic fragments ŽPerez, ´ unpubl. data.; this laboratory study will complement the field data. Although evaporation experiments may be carried out in the field ŽUnger, 1971a; Fairbourn, 1973; Perez, ´ 1998. it was decided to take measurements in the laboratory because: Ža. lack of sufficient field time precluded following observations for an extended period, Žb. a range of soils and stone fragments of predetermined characteristics could be used, and Žc. a controlled environment could allow manipulation of some variables to avoid confounding their effects. The goals of this study were to Ž1. analyze some relevant soil properties and the sedimentological characteristics of pyroclastic covers from a Haleakala site; Ž2. experimentally determine and compare evaporation losses among soils with different water contents at field capacity, while they remain bare; Ž3. study the effects of surficial layers of volcaniclastic fragments of various sizes Žash, cinder, lapilli. on the rates of water depletion from these soils; Ž4. assess both the water-storage capacity and rates of desiccation exhibited by fragment layers themselves; Ž5. investigate water retention by individual volcaniclastic fragments in relation to their mass Žsize..

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2. Characteristics of the study area Maui, the second largest island of the Hawaiian chain, is centered at 20850X N and 156820X W ŽFig. 3a.. Maui reaches an elevation of 3055 m at Haleakala ŽFig. 3b.; its summit is occupied by the ‘crater’, a huge depression ; 12 by 4 km, and ) 800 m deep. This crater is not volcanic, but owes its origin to stream erosion, which caused the upslope recession of two valleys ŽKaupo and Koolau Gaps. until they coalesced at the summit ŽStearns, 1942.. The crater floor, at 2000–2600 m, contains several cinder cones and lava flows, erupted during the most recent period ŽHana. of volcanism ŽStearns, 1946.. Upland climate in Maui is semi-arid due to isolation from oceanic moisture sources by a persistent subsidence inversion that forms at 1200 to 2400 m ŽBlumenstock and Price, 1967.. Tall mountains that pierce this inversion layer force the trade-wind circulation to split and flow around the high peaks ŽLeopold, 1949.. Skies remain clear, allowing high diurnal insolation and rapid night longwave losses into the thin atmo-

Fig. 3. Location of the study area. ŽA. Hawaiian Islands; the island of Maui is shown in black. Latitude ŽN. and longitude ŽW. are indicated. ŽB. Maui. Altitude is in meters; contour interval is 305 m Ž s1000 ft.. Scale is in km. Stippled area in east Maui shows the location of Haleakala National Park; the island summit is indicated by a dark triangle. Source map: State of Hawaii, Principal Islands, 1972, scale 1:500,000, United States Geological Survey.

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sphere ŽGiambelluca and Nullet, 1991.. Highland air is very dry; relative humidity Ž R H . is usually - 40%, and may commonly drop to just 5–10% ŽWhiteaker, 1983.. Precipitation in Haleakala is highly variable in space and time. Park Headquarters Ž2143 m. show a mean of ; 1300 mmryear ŽYocom, 1967; Kobayashi, 1973. and a strong seasonality. Nearly 75% of the rain falls in October–March, while the dry period ŽApril–September. averages - 75 mmrmonth. Interannual variability is high: rain in 1980 reached 3081 mm, but only 420 mm in 1983 ŽLeuschner and Schulte, 1991.. Records for the crater are deficient. The summit could get ; 500 mmryear, but some areas on the crater floor may receive - 130 mmryear ŽYocom, 1967.. Interpolating from various sources ŽKobayashi, 1973; Noguchi et al., 1987; Giambelluca and Nullet, 1991., the study site could get 700–900 mmryear. Yearly temperature range in Haleakala is narrow, only 3.88C ŽWhiteaker, 1983., but-as in other tropical mountains-diurnal amplitude may exceed 208C ŽRundel and Witter, 1994.. Dry-season highs reach 28–298C at the study site, and R H drops to 28–32%. Freezing can occur any month in the crater, where 121–187 freeze–thaw eventsryear occur at ground surface ŽNoguchi et al., 1987.. Less is known about soil temperatures, but the dark ash soils in the high-elevation alpine desert reach temperatures of 31.08C to 47.28C Žat 5 cm depth. during the dry season ŽKobayashi, 1973; Perez, unpubl. data.. ´ Soils in Haleakala are poor, coarse, porous, lacking in organic matter, with high infiltration rates, and low water-holding capacity; these result in edaphic aridity and frequent soil–water stress to plants ŽRuhle, 1959; Leuschner and Schulte, 1991.. The thin, sandy, weakly-developed, profiles over loose cinder or lava are classified as Inceptisols or Andisols ŽCline, 1955.. Soils from the crater have scarcely been studied. Noguchi et al. Ž1987. found just 6–8% fines in soils from the Sliding Sands trail, often covered by a 1–3 cm layer of vesicular gravel. Red and black cinder are slightly weathered and differ little from each other or from the rock; thus Kobayashi Ž1973. Žp. 75. thought that ‘‘ . . . physical, rather than chemical differences of the substrate . . . are important . . . in determining the distribution of plants’’. Vegetation changes sharply with altitude. Rising aridity above the inversion level causes a drought-induced timberline at ; 2200 m, and the dense subalpine scrub changes into a sparsely covered alpine desert ŽWhiteaker, 1983; Leuschner and Schulte, 1991.. The summit and most of the crater lie within this desert, which contains about 35 vascular species, including several native grasses and shrubs ŽRuhle, 1959; Yocom, 1967., but the most conspicuous plant in the crater is the Hawaiian silversword. The study site is at 2505 m next to the Sliding Sands trail, 4.2 km from the rim trailhead and ; 1 km after the trail fork to Kalua o ka Oo, the westernmost cinder cone ŽSinton, 1979.. The site is on the gently sloping Ž5–118. crater floor and has a N198NE aspect. The substrate is ‘‘Ash, largely blown but some water washed . . . primarily fresh and gray to black on the surface’’. ŽMacDonald, 1978.. The coarse sandy soils are covered by discontinuous pyroclastic-fragment layers that range from ash Ž- 4 mm diameter. and cinder Ž4–6.4 mm. to lapilli Ž6.4–38 mm., but also contain many small volcanic blocks Ž) 38 mm. ŽAmerican Geological Institute, 1976; MacDonald and Abbott, 1977.. Fragment layer thickness was 1 to 3 q cm in the finer ash and cinder deposits and 4 to 9 cm Žmean of 5–6 cm. in lapilli. Rosette density was very high at the

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Fig. 4. Site sampled in Haleakala’s crater Žpopulation 7; Kobayashi, 1973. at 2510 m elevation. This plot is covered by a 4- to 7.5-cm thick layer of reddish scoriaceous lapilli; median fragment size Ž D50 . here is 20–21 mm. This site has very high silversword regeneration; the area shown in the photo, ; 2.5 m wide, contains about 45 seedlings Ž ; 3 to 9 cm tall., clearly distinguishable by their high albedo. August 1996 ŽHK-96.35..

site, with several hundred adult plants and seedlings growing on gravel-insulated ground ŽFig. 4.; plants correspond to populations 6 and 7 in the map of Kobayashi Ž1973..

3. Field and laboratory methods 3.1. Field sampling As part of a broader geoecological study ŽPerez, in preparation., three sets of 12 soil ´ samples were taken during Aug. 1996 from the upper 10 cm in three positions. One set ŽR: ‘rosette’. was collected under live rosette-plants. A second group ŽC: ‘control’. was obtained ; 100 cm away from silverswords, on bare ground devoid of litter or plant cover. A third set ŽD: ‘dead rosette’. was gathered from below the remains of adjacent dead, but still standing, plants. Individual soil samples are identified with numbers Ži.e., C 1 , C 2 . . . . within each set. Presence of coarse fragments, and the tightly appressed rosette foliage against the ground, precluded sampling by the core method, which would have allowed determination of field bulk density ŽBlake and Hartge, 1986.. Thus, soils were gathered with a small trowel. Five samples of volcaniclastic layers covering a

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broad range of grain sizes were collected in areas of dense rosette growth; all fragments within a 15 = 15 cm area were gathered ŽPerez, 1990.. Gravel samples are labeled ´ according to type ŽA: ash, Ci: cinder, L: lapilli., and numbered. Any gravel subsamples are further identified with lower-case letters Ži.e., L a , L b .. Depth of fragment layers was measured with a ruler. Air and soil temperatures Žat a depth of 5 cm., and air R H were taken at noon with thermometer, digital thermocouple probe, and sling psychrometer. 3.2. Laboratory analysis All samples were air-dried for several weeks, oven-dried for 48 h at 1058C, and weighed. Samples were triturated by hand and sifted through a 2-mm sieve to separate the soil Ž- 2 mm. and gravel fractions. Soil grain size was assessed by sieving through a 10-mesh series Ž1.4 to 0.063 mm.; finer grain content was analyzed with ASTM-152H hydrometers and a Calgone solution as dispersing agent. Sand and fines Ž- 0.063 mm. content is given as a percentage by weight. Gravel samples were sieved through a 15-mesh sequence Ž45.0–2.0 mm. to determine particle-size distribution. Soil carbon percentage was assessed with a CHN analyzer; this is an indicator of organic-matter content. Dry soil and gravel colors were determined with Munsell Soil Color Charts Ž1992.. Bulk density was estimated for the soil fraction by filling a 35-cm3 cylinder with oven-dry soil, tapping it on the side ; 20 times, and adding soil until level with the container top; samples were then weighed. This procedure was repeated thrice per sample; results were replicable within narrow limits. Soil porosity was calculated with a formula ŽSegalen, 1984. after estimating true density of 10 gravel samples by displacement of water ŽBlake and Hartge, 1986.. Water content at field capacity Ž FC . was measured in 36 soil samples ŽPerez, in preparation. by saturating 30 g of soil with ´ distilled water in covered, ribbed-glass funnels lined with filter paper. Samples were left to equilibrate in a stable room Ž22 " 18C, 65 " 4% R H . until ‘excess’ pore water was removed by gravity ŽSmith and Atkinson, 1975.. This point was empirically attained 15–16 h after saturation ŽPerez, 1992a., when samples had reached field capacity ŽPitty, ´ 1979.. Soil water content was gravimetrically determined by oven-drying samples G 24 h at 1058C. Water content was calculated as a percentage of the dry soil weight with the formula ŽBrady, 1974.:

Ž wet soil weighty dry soil weight. r Ž dry soil weight. . 3.3. Experimental techniques Two desiccation experiments were conducted with the substrate samples from Haleakala. In the first experiment, seven soil samples were selected from among the 36 examined to cover a broad range Ž15.8% to 48.3%. of water-storage capacity. Three specimens came from bare Žcontrol: C. areas; three samples were collected below live rosette-plants ŽR., and the last soil from under a dead silversword ŽD.. The seven soils were split into equal portions, thus creating two sets Ž7 = 2. of samples.

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Transparent cylinders Ž‘cores’. of sturdy, lightweight Ž; 15 g. plastic, sealed at the bottom, were used as containers ŽBenoit and Kirkham, 1963; Corey and Kemper, 1968.. The 10-cm tall cores had an inner diameter of 73 mm, providing an exposed area of 41.8 cm2 . The maximum allowable weight for all components Žsoil, gravel, distilled water, core. was carefully determined, as their combined weight could not exceed 405 g, the capacity of the electronic analytical balance utilized. The same amount of dry soil Ž180 g. was used in all 14 samples, but as their bulk density differed Ž1.13–1.56 mg my3 . ŽTable 1., soil depth within cores varied from 37 to 49 mm Žmean: 42.0 " 3.9 mm.. All soils were brought to field capacity following lab procedures above and gently introduced into the cores. Two of the five pyroclastic samples Žhenceforth called ‘gravel mulches’. were split, and the resultant seven subsamples placed over one set of soils; the other seven soil samples were left uncovered. Mulch types were randomly assigned to soils, with the following provisions: Ž1. not all soil samples under a given mulch type could belong to the same category; Ž2. pairs of mulch subsamples ŽA 2a , A 2b ; L a , L b . could not be coupled with the same soil category. Ash samples were paired with C, C, R soils; cinder with R, C; and lapilli with R, D soils. Dry gravel weight varied between 120 and 150 g, and was slightly adjusted to produce uniform layers, ; 46 mm thick, in all samples. This thickness was selected because silversword germination in Haleakala is hindered by layers thicker than 5 cm ŽKobayashi, 1973.. Gravel D50 varied from 2.7 to 14.4 mm; core diameter limited the maximum size of fragments tested Ž; 37.5 mm., although some lapilli covers of Haleakala have larger median sizes Ž D50 F 25 mm; Perez, unpubl. ´ data.. Fragments were loosely placed Žnot embedded. on top of soils, creating an openwork layer 43 to 47 mm thick Žmean: 45.7 " 1.5 mm.. The 14 cores plus one mechanical thermograph Žsee below. were placed on reflective aluminum trays and arranged in a 3 = 5 Latin square design ŽSokal and Rohlf, 1969.. All sample positions were randomized to minimize any possible ‘edge effects’; cores were randomly changed a few times during the experiment to ensure homogeneity of results. A heat source was provided by a battery of six infrared lamps Ž100–150 W each. placed ; 60 cm above samples. Floodlight position could be regulated, and lamps were periodically adjusted, or briefly turned off, to ensure uniform temperature conditions during the test. An oscillating fan constantly recirculated air in the room to ensure steady evaporation, but the fan was not directly aimed at the drying samples. Lights and fan were kept on continuously during the desiccation period. Air temperature was recorded on 8C paper for 22 days by a clockwork thermograph ŽWeathertronics 4120. with an empirically-determined measurement error of "0.58C. The sensor was at q10 cm, the approximate height of the gravel surface in the mulch-covered soils. A digital thermometer-hygrometer also monitored R H next to the desiccating samples. Gradual water depletion from samples was assessed by directly weighing cores ŽWillis, 1962; van Wesemael et al., 1996. on a digital balance Ž0.01 g precision.; this allows for accurate determination of water evaporated from each soil between readings. Knowing the water lost made possible calculation of evaporation rates Žmmrh. for each desiccation stage. Moist soils were weighed at the beginning of the test and then every hour for the first 12 h. Samples were reweighed every 2 to 4 h afterwards, until all had reached their initial Ž‘constant’. dry weight ŽPerez, 1991b., when the test was termi´

312 Table 1 General characteristics of the soils used in the desiccation experiments. Key: C s control soils; R ssoils from beneath live silversword rosettes; Dssoil collected beneath a dead rosette-plant. Colors indicated are for dry soil samples ŽMunsell Soil Color Charts, 1992.. D50 is the grain diameter of the 50th percentile Soil properties

C2

C3

R1

R2

R3

D1

94.1

92.8

91.8

94.4

97.2

95.6

92.0

5.9 0.55

7.2 0.53

8.2 0.38

5.6 0.43

2.8 0.51

4.4 0.62

8.0 0.40

0.06 15.8

0.17 24.4

0.14 28.8

0.28 26.2

0.58 31.5

0.39 35.4

2.57 48.3

1.56

1.53

1.52

1.40

1.31

1.35

1.13

35.5 10YR 4r2 Ždark grayish brown.

37.3 10YR 4r4 Ždark yellowish brown.

37.2 10YR 3r3 Ždark brown.

41.9 10YR 3r1 Žvery dark gray.

46.1 10YR 2r1.5 Žblack to very dark brown.

44.6 10YR 3r1 Žvery dark gray.

54.1 10YR 2.5r1 Žblack to very dark gray.

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Particle-size distribution: sand Ž%. fines Ž%. Ž - 0.063 mm. Median soil grain size Ž D50 . Žmm. Carbon content Ž%. Water content Ž%. at field capacity Bulk density Žmg my3 . Porosity Ž%. Soil color

Soil samples C1

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nated. One mulched soil needed ; 509 h to fully desiccate; thus the experiment lasted 22 days, during which ; 1500 soil–water measurements were obtained. To facilitate comparison among soils and between bare and mulch-covered sets, two values ŽT50 % and T100% . were calculated; these indicate the time required to evaporate 50 and 100% of the soil water at field capacity, respectively. T50 % , a concept comparable to the ‘half-life of stored moisture’ ŽHillel, 1998, p. 468., was chosen because it may have greater physiological meaning when contrasting soil–water use by plants than T100% . As the test above focused on water decline in the soils, a second experiment following similar procedures was carried out to assess both the water-storage capacity and rates of evaporative loss in the gravels. Two 50-g specimens were obtained from each of the five samples collected in Haleakala. The 10 dry subsamples were placed in glass funnels double-lined with a40 filter paper Žslow filtration rate.; this allowed ponding and saturation of samples by full immersion. The saturation process was repeated three times to ensure thorough water absorption by all fragments. Excess water was allowed to drip, and after 80–90 min, all samples had equilibrated at field capacity. Wet gravels were weighed, and their water content calculated. Fragments were then introduced into 81-mm tall cylinders, with a 55-mm diameter and an exposed surface of 23.8 cm2 . Thickness of mulch layers ranged from 28 to 42 mm Žmean: 34.0 " 5.2 mm.. Gravels were placed under floodlights and their decline in water content determined by weighing them at hourly intervals during the first 16 h, then every 2 or 3 h until all samples had reached constant Ždry. weight ; 26 h after the test started. Water absorption was investigated in 35 gravel particles in relation to their mass. Fragments were selected from sample L, a reddish scoriaceous lapilli with a rough surface and marked vesicularity; dry particles spanned a wide weight range Ž0.36–39.02 g.. The lapilli were individually placed on a Petri dish with 3–5 mm standing water and allowed to saturate by gradual absorption. Fragments were later placed on deep ceramic plates and wetted from above until immersed. Particles were turned until air bubbles ceased emerging from vesicles, and were left submerged for 30 min. They were then picked up with tweezers, allowed to drip any excess water for 15 s, and weighed Žcf. Perez, 1991b.. ´ 3.4. Analytic sedimentology techniques Most previous reports provide scanty sedimentological detail about GM, and only rarely Že.g., Corey and Kemper, 1968. have thorough particle analyses been carried out. Special attention was paid in this study to granulometric parameters of Haleakala’s volcaniclastic layers, as these might help elucidate the role of grain size in water conservation, and make it easier to compare results with later studies elsewhere. Sieved gravel fractions were combined in three size intervals, following USA ASTM standards ŽDackombe and Gardiner, 1983; Poesen and Lavee, 1994b.: fine graÕel Ž- 4.75 mm., medium graÕel Ž4.75–19 mm. and coarse graÕel Ž) 19 mm.. Cumulative particle-size distributions were plotted on log paper and different numerical indices obtained from these in the phi Ž f . scale ŽFolk, 1980.; f units were converted to mm, and are reported here. Several measures of central tendency were used to analyze gravels. The first was the median particle size Ž D50 or f 50 ., the 50th percentile diameter. This measure is

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useful insofar as it denotes the most abundant grain size in a sample ŽInman, 1952.; in addition, it has been commonly used in other studies ŽCorey and Kemper, 1968; Perez, ´ 1998. and can be used for comparisons. Inman Ž1952. devised the mean particle size Ž M . as a better measure of central tendency; this is calculated as M s Ž f 16 q f 84 .r2, where f 16 is the f size at percentile 16 Žthe point in the curve where 16% of the grains are smaller., and f 84 is the point at which 84% of the particles are smaller. Folk Ž1966. combined indices in a graphic mean particle size Ž Mz ., computed as Mz s Ž f 16 q f 50 q f 84 .r3; this index is more appropriate for bimodal andror strongly skewed curves, and provides a better overall granulometric picture. Two statistics of grain-size dispersion Žsorting. were also calculated: a f deÕiation measure Ž fs . ŽInman, 1952. given in phi Ž f . units and calculated as: fs s Ž f 84 – f 16 .r2, and Hazen’s uniformity coefficient Ž m ., the ratio being m s Ž D60 rD10 . ŽCorey and Kemper, 1968.. Additional details are given in the excellent reviews by Inman Ž1952. and Folk Ž1966; 1980.. As the average particle size of a GM is believed to be significant primarily in relation to the texture of the underlying soil ŽPerez, 1998., median grain size Ž D50 . was also calculated for the ´ soils, and used to determine the median-size ratio Ž Mr . of the seven mulchrsoil combinations; this is simply computed as Mr s Ž D50 gravelrD50 soil. ŽCorey and Kemper, 1968..

4. Results 4.1. Characteristics of the soils and Õolcaniclastic layers from Haleakala Due to their high average sand Ž; 94%. and gravel Ž) 15%. fractions, Haleakala’s soils were classified as graÕelly sands ŽSoil Survey Staff, 1975.. Content of clay particles was extremely low ŽF 1%. and barely detectable by a hydrometer. All samples had a low and relatively uniform content of fines Žsilt q clay: 2.8%–8.2%. and their overall particle-size distributions, regardless of sampling position, varied little ŽFig. 5.. Soils showed a much broader variation of carbon content Ž0.06%–2.57%. associated with sampling position: bare soils had an average of only ; 0.12% C, while soils below live rosettes contained 3.5 times more C Ž0.42%., and the soil gathered beneath a dead plant had 2.57% C ŽTable 1.. All the other soil properties examined were closely dependent on carbon content. Soil color under plants was 1 value unit, and 2 chroma units, darker than in bare soils ŽPerez, 1996.. Bulk density of control soils was ; 19% ´ higher than below rosettes, where porosity was 27% greater than in controls. As a result of the structural effects of organic matter, average water content Žfield capacity. of soils under silverswords was 35.4%, about 54% higher than in control areas Žwith 23%. ŽTable 1.. A logarithmic regression showed that 85.4% Žr 2 . Ž p - 0.005. of the variation in water content at FC was explained by C content alone, but a multiple regression adding texture as a second X variable raised this value to just 86.3%. It is obvious that organic matter controlled water storage in Haleakala’s soils, while the role of textural variation was strictly secondary. These sharp contrasts underline the extreme pedological significance of tropical-alpine rosettes as sources of organic matter ŽPerez, 1992b.. ´

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Fig. 5. Graphic envelopes of particle-size distributions Žmm. for the Haleakala soils used in the desiccation experiments. ŽA. Control ŽC. soils; ns 3. ŽB.: Soils under Hawaiian silverswords; ns 4 Ž3 under live rosette-plants wRx, and one below a dead plant wDx..

Based on Mz size values, pyroclastic layers were classified as ash Žsamples A 1 , A 2a , A 2b ., cinder ŽCi 1 , Ci 2 ., and lapilli ŽL a , L b . ŽTable 2.. Ash and cinder fragments had a brown Ž7.5YR 4r3. to black Ž10YR 2r1. color, while the lapilli were a vivid reddish brown Ž5YR 4r4.. These color variations in Hawaiian pyroclasts are caused by oxidation during and immediately after the eruptions depositing cinders ŽMacDonald, 1967, p. 50. and cause little, if any, significant chemical differences ŽKobayashi, 1973, p. 58.. Fragments of all volcaniclastic types contained numerous vesicles; their size was related to that of particles. Ash and cinder vesicles ranged up to ; 1 mm, but most were tiny, 0.1–0.25 mm. Those in lapilli were larger, normally 0.4–2 mm, but some reached 9-mm diameter. Textural comparisons show wide mulch variation and a gradation of particle content within the different fractions ŽTable 2.. All ashes and the finest cinder lacked coarse gravel and had the bulk of their mass in the fine-gravel fraction, while the lapilli had almost no fine gravel and most particles in the medium-gravel size-class. Graphic particle-size distributions indicate that pyroclastic samples had a ‘bimodal’ distribution and fell into two broad categories: a group of ‘fine’ ash and cinder samples-albeit with considerable variation- and the ‘coarse’ lapilli ŽFig. 6..

316 Table 2 Textural parameters and colors of pyroclastic layers used in the desiccation experiments. Pyroclastic samples are arranged by increasing graphic mean size Ž Mz . and textural coarseness. Key: A sash, Ci s cinder, L s lapilli. See text for terminology details and calculation of sedimentological parameters. Colors, shown for identification purposes, are for dry samples ŽMunsell Soil Color Charts, 1992.. Paired soils correspond to those in Table 1, and refer to samples under pyroclastic mulches during the first desiccation experiment Mulch characteristics

Particle-size distribution Ž%. fine gravel Ž - 4.75 mm. medium gravel Ž4.75–19 mm. Coarse gravel Ž )19 mm. Graphic mean Ž Mz . Žmm. Median particle size Ž D50 . Žmm. Mean size Ž M . Žmm. Deviation Ž s ., f units and Žmm. Uniformity ratio Žm . Paired soil samples Median size ratio Ž Mr .

A1

A 2a

A 2b

Ci 1

Ci 2

La

Lb

10YR 3r2 Žvery dark grayish brown.

10YR 2r1 Žblack.

10YR 2r1 Žblack.

7.5YR 4r3 Žbrown.

7.5YR 3r1 Žvery dark gray.

5YR 4r4 Žreddish brown.

5YR 4r4 Žreddish brown.

92.2

71.3

69.3

63.7

52.2

0.3

0.6

7.8

28.7

30.7

36.3

42.7

92.2

78.3

0

0

0

0

5.1

7.5

21.1

2.9 2.7

3.6 3.7

3.8 3.8

4.0 3.6

5.2 4.5

13.7 14.4

13.8 13.8

2.9 0.48 Ž1.4.

3.6 0.66 Ž1.6.

3.8 0.65 Ž1.6.

4.3 0.96 Ž1.9.

5.6 1.30 Ž2.5.

13.4 0.44 Ž1.4.

13.8 0.52 Ž1.4.

1.5 C1 5.0

1.9 C3 9.8

1.9 R2 7.5

2.1 R1 8.4

3.0 C2 8.5

1.8 R3 23.1

1.8 D1 34.1

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Color

Mulch samples

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Fig. 6. Particle size distributions Žmm. for the five layers of pyroclastic fragments collected from Haleakala for the desiccation experiments. Dry fragment colors determined with Munsell Soil Color Charts Ž1992.. Key: A 1 : very dark grayish brown ash, Ci 1: brown cinder, A 2 : black ash Žsubsample A 2a ., Ci 2 : very dark gray cinder, L: reddish brown scoriaceous lapilli Žsubsample L a .. Specimens A 2b and L b , respectively similar to A 2a and L a , are not included in the diagram.

Mz grain size varied from 2.9 to 13.8 mm; other size measures Ž D50 , M . gave similar values, with only minor differences. However, all graphic-mean values lay between those of M and D50 . This suggests that Mz indices provide a more balanced assessment of grain size in the Haleakala samples. Particle-size dispersion indices showed comparable trends: both the fs and m ratios gradually increase from the finest ash ŽA 1 . to the coarsest cinder ŽCi 2 ., then drop abruptly in the lapilli samples ŽTable 2.. Using the f sorting scale of Folk Ž1980. Žp. 42., all ash and lapilli samples were ‘well to moderately well’ sorted, while cinder specimens were ‘moderately’ ŽCi 1 . to ‘poorly’ sorted ŽCi 2 .. These trends reflect the grain-size ranges of the mulches discussed above. Cinder contains a broad diversity of particle sizes, both coarse and fine. Ash contains mostly small grains, while lapilli is almost exclusively composed of coarse particles. However, fragments in lapilli L b , with a greater fs and diversity of grain sizes than L a ŽTable 2. naturally settled into a well-packed layer, as smaller pebbles neatly fitted into the openings between larger fragments; this effectively reduced the average intergranular pore size. Due to the narrow textural range of Haleakala’s soils ŽFig. 5., their D50 varied little, just from 0.38 to 0.62 mm Žmean: 0.49 mm.. However, as the D50 of mulches fluctuated widely, median-size ratios Ž Mr . of gravels and soils showed a high dispersion, from 5.0

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to 34.1 ŽTable 2.. Mr increased along with the particle size of the mulch, but the difference between the average Mr for ash samples Ž7.4. and for cinder Ž8.5. was small, while the lapilli displayed a much greater Ž28.6. average Mr . 4.2. EÕaporation from bare soils Ambient conditions during the desiccation experiment were conducive to rapid soil water loss. Air R H fluctuated between 20 and 39%, with a mode of 28 " 2% prevalent during 52% of the time. Temperatures remained high and varied little, from 30 to 368C Žmode of 33 " 28C for 95.3% of the time.. These climatic conditions resembled closely those found at the study site Žsee above.. Rates of water decline in bare soils varied sharply. Plotting the drop in water content with time ŽFig. 7. indicated three distinct stages of evaporation ŽLemon, 1956., a characteristic response of desiccating soils reported by several studies ŽBenoit and Kirkham, 1963; Modaihsh et al., 1985; Perez, 1991b, 1997a.. The mechanisms involved ´ in these stages are discussed below. The first phase showed a linear and steady decline of soil water that lasted 15 to 18 h. During this stage, absolute rates of evaporation were higher in finer soils, with greater initial water reserves ŽAlizai and Hulbert, 1970; Ingelmo-Sanchez et al., 1980., but relatiÕe water losses were higher in soils with less ´ water at FC . A comparison between D1 , with the highest FC water, and C 1 , with the lowest, illustrates these trends. D 1’s water fell from 48.3 to 19.7% in ; 17.8 h, a drop

Fig. 7. Temporal decline in the gravimetric water content of seven soils from Haleakala; all samples were initially at field capacity and remained bare Ži.e., without a mulch cover. during the experiment. Key: C: Control soils Ž n:3.; R: Soils under live silversword rosettes Ž n:3.; D: Soil sample from beneath a dead rosette Ž n:1.. Consult text for further details.

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equal to 2.08 mm. In contrast, C 1 had 15.8% water at the start, but after 15 h, this had diminished to barely 2.7%, which equals a loss of just 0.68 mm. Water in the other five samples had dropped by now to an intermediate level, of ; 6%–14%. These calculations show that, although evaporation during phase one occurred 2.6 times faster in D 1 than in C 1 , soil D 1 still retained at the end ; 41% of its original water, while only 17% remained in C 1. The second desiccation stage was characterized by a swift deceleration of water loss, and curves for all soils became noticeably less steep ŽFig. 7.. The second phase was over some 32–44 h after the experiment started. This stage merged gradually into the next and longest one, when all curves became flattened; subsequent evaporation rates remained low for a long time, until soils were dry 70 to 140 h after time zero ŽTable 3.. Desiccation times were closely dependent on FC water; linear regressions indicate that soils with more water took significantly longer to reach both T50 % Ž r 2 s 0.84, p - 0.025. and T100% Ž r 2 s 0.928, p - 0.001. Žcf. Ingelmo-Sanchez et al., 1980.. As initial water ´ retention was controlled by carbon content, it follows that high organic matter under silverswords should result in extended periods of water supply; Table 3 indicates that control soils did reach both their T50 % and T100% much faster than soils under plants. In contrast, a regression using soil D50 Žgrain size. showed that texture had only a marginal, nonsignificant influence on times of desiccation. 4.3. EÕaporation from mulch-coÕered soils Presence of GM resulted in all cases in substantially lower rates of evaporation than in bare soils, but the efficiency of this effect varied with the granulometric characteristics of mulches ŽTable 4.. Linear regressions between evaporation rates and different gravel size-classes indicated that water losses were inversely correlated with fine gravel, but directly associated with medium and coarse gravel fractions Ž r 2 s 0.78, p - 0.05.; thus, as mean gravel size increased, so did evaporation. Similar correlations with the three measures of central tendency also suggest that coarser mulches were less efficient in controlling evaporation. Surprisingly, D50 was the best evaporation predictor Ž r 2 s 0.67, p - 0.05., with Mz closely behind Ž r 2 s 0.62., and M in third place Ž r 2 s 0.59. Žboth at p - 0.05.. An evaporation time factor Ž Ef . was devised to compare soil runs with and without mulch; this indicates how many times longer the period needed for desiccation was in

Table 3 Results of the first desiccation experiment, for bare soils. Soil labels correspond to those in Table 1 and Table 2. T50 % , T100% s times required to evaporate 50% and 100% of the soil water at field capacity, respectively. All desiccation times are rounded to the closest 5 min. Samples are arranged by increasing time for T100% . Thickness of the soil layers used in the experiment is given in millimetres Soil samples

C1

C2

C3

R1

R2

R3

D1

Soil thickness Žmm. T50 % Žh:min. T100% Žh: min.

37 8:45 70:20

40 11:20 72:30

38 11:10 83:35

42 12:30 94:00

44 11:40 101:30

44 13:45 107:35

49 13:40 139:40

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Table 4 Results of the first desiccation experiment, for mulch-covered soils. Pyroclastic samples are arranged by increasing graphic mean size Ž Mz .. Key: A sash, Ci s cinder, L s lapilli; see text for terminology details. Thickness of the gravel–mulch layers is given in millimetres. T50 % , T100% s times required to evaporate 50% and 100% of the soil water at field capacity, respectively. All desiccation times are rounded to the closest 5 min. Ef compares soil runs with and without mulch, and indicates how many times longer the periods needed for T50 % or T100% were in samples covered with gravel than in bare ones. Compare with evaporation data for bare soils ŽTable 3. Mulch samples

Paired soil samples Graphic mean Ž Mz . Žmm. Gravel mulch thickness Žmm. T50 % Žh:min. T100% Žh:min. Ef 50% Ef 100%

A1

A 2a

A 2b

Ci 1

Ci 2

La

Lb

C1 2.9 47 111:55 260:40 12.8 3.7

C3 3.6 47 111:35 306:05 10.0 3.7

R2 3.8 45 190:50 480:50 16.4 4.75

R1 4.0 43 174:45 436:35 14.0 4.65

C2 5.2 46 138:45 379:20 12.25 5.2

R3 13.7 47 56:35 246:10 4.1 2.3

D1 13.8 45 156:30 508:25 11.4 3.65

samples covered with gravel than in bare ones. Ef was computed Žboth for T50% and T100% . for all soilrgravel combinations as: Ef s Ž Evaporation time of a soil with GM . r Ž Evaporation time of same soil alone . . Cinders were most efficient in extending desiccation times, as soils beneath them took, on the average, 13.1 and 5.0 times longer to reach T50 % and T100% , respectively, than bare ones ŽTable 4.. Ash mulches prolonged evaporation periods nearly as much, as soils under ash took 13.1 and 4.0 times longer to attain T50 % and T100% than bare samples. In contrast, a lapilli cover only extended T50 % and T100% periods by an average factor of 7.7 and 3.0, respectively. These differences among mulches are primarily ascribed to grain-size dispersion, as average fs sorting values for gravel types showed a similar ranking: cinder Ž1.13 fs . ) ash Ž0.60 fs . ) lapilli Ž0.48 fs .. Thus, poorly sorted cinders, with a broad particle-size range, postponed soil desiccation for a longer period than the better sorted, uniformly-sized ash and lapilli. A linear regression using fs and Ef 100% values gave an r 2 s 0.641 Ž p - 0.05.. A multiple regression with D50 as second independent variable accounted for 77% Ž p - 0.05. of the variability in Ef 100% . In contrast to fs , uniformity Ž m . ratios were not useful predictors of water losses ŽCorey and Kemper, 1968., even though average values were also higher for cinder Ž2.6. than for ash or lapilli Žboth 1.8.; further use of this ratio is not recommended. Mr ratios were closely related to overall evaporation rates; a polynomial regression for the 7 soilrgravel pairs Ž r s 0.922, p - 0.025. accounted for 85.1% of the variation in water loss from mulched soils. Presence of GM not only extended the time of soil drying, but drastically affected the shape of the evaporation curves; the three desiccation stages discussed above for bare soils were reduced to just two ŽFig. 8.. In all cases, initial rates of water loss were considerably less steep than in control specimens. Most mulched soil samples ŽC 1 , C 2 , C 3 , R 1 , R 2 . displayed perfectly linear rates of water loss from the start, and maintained

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Fig. 8. Temporal decline in the gravimetric water content of seven soils from Haleakala; all the samples were initially at field capacity and remained covered by a gravel mulch layer during the experiment. Key as in Fig. 7. Consult text for additional details on pyroclastic fragments.

them for 208–356 h. A point was reached when soils had only 2–5% water. Curves flattened afterwards and soils continued losing the remaining meager water at very slow rates; this corresponds to phase three, as discussed above. One sample ŽD 1 . went through a short Ž32 h. phase of steep water decline but quickly reached stage two. Afterwards, it followed the pattern of other samples, outlasting them all and retaining water for ) 21 days. Sample R3 departed sharply from the rest ŽFig. 8.; it showed an initially fast rate of drying, which gradually declined after ; 45 h, but the soil dried completely in only 10 days ŽTable 4.. Some general trends are evident: Ž1. the main effect of GM mulches was to eliminate— or radically shorten— the first, rapid-desiccation, phase seen in control samples; Ž2. as a result, mulched soils remained in ‘stage two’ for a greater fraction of the experiment Ž70%–80% of the time. than bare soils Ž20%–25%.; Ž3. soil R3, covered by unpacked lapilli ŽL a ., showed an anomalous desiccation pattern Žsee discussion below.. 4.4. Water retention and rates of water loss by Õolcaniclastic layers The experiment above involved dry fragments placed over saturated soils. Adding water directly to mulched samples was avoided because a cover of porous gravel would interfere with infiltration by absorbing an unknown amount of water ŽUnger, 1971b; Lightfoot and Eddy, 1994.; this would have prevented precise determination of soil water content during the first test.

322

Mulch samples

Mulch layer thickness Žmm.

Water content at field capacity Ž%.

Water intercepted by mulch Žmm.

Water intercepted by a 50-mm mulch Žmm.

T50 % Žh:min.

T100% Žh:min.

A 1a A 1b A 2a A 2b Ci 1a Ci 1b Ci 2a Ci 2b La Lb Averages Ž"S.D..

32 29 35 36 28 28 34 33 42 42 34.0"5.2

23.6 26.3 16.6 16.4 21.8 16.2 18.8 16.0 21.2 16.5 19.35"3.65

5.0 5.5 3.5 3.5 4.6 3.4 4.0 3.4 4.5 3.5 4.07"0.77

7.8 9.6 5.0 4.8 8.2 6.1 5.7 5.1 5.3 4.1 6.17"1.75

6:55 6:15 6:05 6:15 5:25 4:40 6:45 5:15 5:25 4:50 5:45"46 min

25:10 24:50 22:35 22:35 22:35 22:20 25:15 24:50 19:05 19:10 22:50"137 min

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Table 5 Water-retention capacity and evaporation losses of pyroclastic mulch layers. Pyroclastic samples are arranged by textural type. Key: A sash, Ci s cinder, L s lapilli; see text for additional terminology details. T50 % , T100% s times required to evaporate 50% and 100% of the soil water at field capacity, respectively

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323

The second experiment showed that water absorption by mulches did vary between 16.0 and 26.3% Žpercentage by dry weight.. The finest ash ŽA 1 . retained the most water, while one lapilli subsample ŽL b . intercepted the least amount of water ŽTable 5.. However, regressions with all measures of central tendency and dispersion indicated that neither water absorption nor desiccation time were significantly associated with grain size or sorting. Water storage by gravels was converted to millimeters Ždepth.; this approximates the amount of precipitation that would be intercepted by each volcaniclastic material in the field. Because depth of samples used in the experiment varied, values were adjusted to show the amount of water Žmm. that would have been intercepted by ‘uniform’, 5-cm thick gravel layers. These calibrated values indicate that 5 cm of ash would store, on the average, only slightly more rain Ž6.8 mm. than cinder Ž6.3 mm., but about 45% more than a 5-cm layer of lapilli Ž4.7 mm.. Rates of desiccation were dependent on initial water storage. A logarithmic regression Ž r 2 s 0.71, p - 0.0025. established that samples with greater FC values lost water faster. Water decline occurred swiftly in all samples, but due to contrasts in water content, mean desiccation time ŽT100% . was shorter for lapilli Ž19:05 h. than for ash or cinder Ž; 23:45 h for both.. The shape of the desiccation curves for most mulches ŽFig. 9. was comparable to that seen for bare soils, although initial drop in water content was not as steep as in these Žcf. Fig. 7.. A period of steady water loss ensued at the start; this continued for 6 to 9 h, when water had decreased to ; 5–8%. Rates of water loss slowed then gradually, until gravel water had dropped below ; 2%; evaporation rates

Fig. 9. Temporal decline in gravimetric water content of five pyroclastic samples from Haleakala; two subsamples were analyzed for each mulch type. All samples were initially at their saturation point. Key as in Fig. 6.

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afterwards remained low for several hours, until mulches were dry. Cinder and lapilli subsamples exhibited a sharp variance, while ash subsamples did not ŽFig. 9.. Evaporation patterns in the finest ash ŽA 1 . differed slightly from the others: they were lower at the start, then accelerated, but gradually dropped again during the last stage. 4.5. Water storage by lapilli fragments Water absorption by lapilli fragments was a swift process. When placed on standing water, the wetting front migrated to the top of some pyroclasts in as little as 15 s, although observations under a magnifying glass indicated that water was absorbed by most fragments during a 3–4 min period. Water storage at saturation fluctuated from 9% to 71%; average water content was 31.7 " 14.7%. Lapilli fragments showed a pronounced pattern of diminishing water retention with increasing size. A polynomial regression exhibited high scattering about the regression line, but given the ample number of data points used Ž35., indicated a highly significant probability Ž r 2 s 0.531, p - 0.0001. for the relationship between water content and particle weight ŽFig. 10.. Loss of water from individual lapilli fragments was not analyzed in detail, but periodic weighing indicated that — predictably — the largest particles remained moist for longer time periods. However, even these fragments were nearly dry 6 to 7 h after saturation, when their water content had already dropped to between - 1% and 3%.

Fig. 10. Gravimetric water storage by 35 fragments of reddish brown scoriaceous lapilli ŽL in Figs. 6 and 9.. All particles were initially at their saturation point. Dotted line indicates the slope of the Žpolynomial. regression: Y s 42.944y35.594 X q14.136 X 2 ; ns 35, r 2 s 0.531, p- 0.0001.

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5. Discussion and integration of results Each segment of the evaporation curves in Fig. 7 corresponds to three well-known phases of water loss from soils ŽGhildyal and Tripathi, 1987; Marshall et al., 1996; Hillel, 1998.. In the initial constant-rate stage, water migrates upward by capillary flow. Gradual loss of pore water leads to a sharp break in the curves; this indicates the start of the falling-rate stage, characterized by a progressive shift from liquid to vapor flow, although capillary condensation within partially-filled soil pores may also occur ŽLemon, 1956.. The slow-rate stage is associated with vapor diffusion and residual water loss. Presence of a mulch is known to primarily affect the constant-rate stage, when much of the total water loss occurs ŽBond and Willis, 1969; Hillel, 1998., thus Hanks and Woodruff Ž1958. defined a mulch as ‘a medium that transports water only in the vapor phase.’ Indeed, the desiccation curves for gravel-covered soils ŽFig. 8. indicate a total absence of the initial capillary-flow phase that reduced water so quickly in bare samples. Steady vapor diffusion prevailed during desiccation of mulched specimens, but as these became very dry, minor resistance to further evaporation also occurred. Soil core examination showed that vapor condensation occurred within the pore spaces and vesicles of all pyroclastic gravels; this appeared as 8–22 mm wide bands of tiny water droplets immediately above the soilrgravel interface. Condensation bands were twice as thick Ž17–22 mm. in lapilli than in the finest ash Ž8–12 mm.; this was a function of mulch pore size, which was estimated through the transparent cores. Most intergranular pores in ash and cinder were - 1–1.5 mm in diameter, but were normally 4–9 mm Žmode: ; 6 mm. in lapilli. It seems the smaller openwork spaces in ash and cinder slowed down vapor diffusion and caused efficient condensation near the soil surface, while the more porous lapilli permitted unchecked vapor flow through a thicker layer. Although more commonly reported for soils ŽLemon, 1956; Russell, 1973., condensation within a gravel layer appears important in retarding water movement, and shows that simple vapor diffusion is not the only process of water transport through a rock fragment layer ŽPoesen and Lavee, 1994b.. As expected, average grain size was a crucial variable controlling evaporation, but its influence was combined with that of sorting, discussed below. Together, these granulometric factors can adequately explain the ranking of mulch types Žlapilli ) ash ) cinder. observed with respect to evaporation losses. The median-size ratio Ž Mr . was the only parameter that involved the evaluation of grain size in both the mulch and soil; thus, it seemed useful in predicting water loss. However, the Mr is an imperfect index because the pore-size ratios of the gravel and the underlying soil, the true controls of water transport, are not assessed. Comparing T50 % and T100% between the two lapilli ŽTable 4. shows that packing and the inherently smaller pore size in poorly-sorted L b resulted in greater water conservation than in L a , even though this last had a lower Mr Ž23.1. than L b Ž34.1. ŽTable 2.. It would be desirable to develop an index that compares actual intergranular-pore sizes; this entails complex lab procedures involving impregnation with fluorescent resins and microscopic analysis ŽFitzPatrick, 1985.. The GM literature is surprisingly devoid of discussion on the effects of grain sorting or packing, yet one of the earliest researchers ŽWeaver, 1919, p. 89. clearly stated after an experiment that ‘‘In nature, finer particles occupy the interstices between larger ones,

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and hence the GM must be much more efficient . . . in protecting the underlying soil from high evaporation’’. As noted, it is precisely this infilling of large pores by smaller grains that might explain why the two lapilli, with nearly the same mean grain size, displayed strikingly different desiccation curves ŽFig. 8.. Grain size and sorting, through their effect on pore dimensions, control evaporation losses from mulched soils, as Weaver Ž1919. correctly surmised. If a very fine-grain mulch Ž‘dust’. with a modal pore size close to that of the underlying soil is used, the mulch may still allow some capillary flow, and water loss is not greatly curtailed ŽBenoit and Kirkham, 1963, p. 497.. As discussed above, the evaporation curves indicate that this did not occur even with the finest Haleakala ash. A poorly sorted, medium-grain mulch with an effective pore size larger than the soil beneath interrupts capillary rise at the soil surface, but this remains largely covered by pebbles, which retard evaporation. In this case, water loss occurs mainly by slow vapor diffusion through the small open pores. The cinders used here should fit this category perfectly. Lastly, if a coarse, well-sorted gravel with pore spaces much greater than those of the soil below is applied, some openings may be too large, and rock fragments will now insulate a smaller fraction of the soil surface. This leads to rapid water loss through tiny but numerous exposed soil patches, where much water is lost due to wind turbulence and convection, rather than by the slower vapor diffusion that is characteristic of smaller pores ŽCorey and Kemper, 1968, p. 12.. This last description seems to apply well to the lapilli, particularly to the better sorted one ŽL a .. Why did smaller gravel particles in both absorption experiments ŽTable 5, Fig. 10. store more water than coarser ones? Smaller fragments usually store more water than larger rocks because the former are normally more weathered and therefore more porous ŽChilds and Flint, 1990; Poesen and Bunte, 1996., but this only refers to absorbed water. Water can be held by gravels in two compartments: as ‘internal’ water, retained in vesicles and pores within particles, and ‘external’ water, which clings to the surface of stone fragments or fills surface irregularities and depressions. In the case of whole layers, external water is also stored in the pore spaces between grains. External water retention by small ectohydric cryptogams ŽLarson, 1981; Kershaw, 1985; Perez, 1997a,b. ´ is inversely correlated with size. This is due to their ‘surface area to weight ratio’ Ž ArW . ŽLarson, 1981., which increases as objects — lichens or pebbles — decrease in size. Thus, small ash and cinder particles, with a large total surface area, hold more external water than larger lapilli. In contrast, lapilli fragments might store a greater amount of internal water due to their pronounced vesicularity, but given the fact that they dried faster than ash or cinder ŽTable 5., their internal water must also have been low. Discussion of GM in the literature invariably focuses on the beneficial aspects to the soil below, but seldom includes the fact that presence of gravel may also preÕent the underlying soil from becoming wetted. A mulch layer will interfere with infiltration because, as shown, it absorbs water ŽUnger, 1971b; Lightfoot and Eddy, 1994.. This also applies to the soil profile; low water-storage capacity in the topsoil results in deeper penetration of the wetting front, thus greater water conservation ŽKosmas et al., 1993; van Wesemael et al., 1996.. The absorption tests suggest that any rain falling on a dry pyroclastic layer would be readily imbibed in a matter of seconds. Water storage by

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graÕel may offer an explanation to the oft-cited drop in mulch efficiency or plant growth with deeper volcanic gravels ŽMaule, 1963; Colton, 1965; Doolittle, 1998.. Similarly, the finding of Kobayashi Ž1973. Žp. 69. that silversword seedlings in Haleakala do not germinate or survive with a fragment cover ) 75 mm thick may be related to excessive water interception by deeper volcaniclastic layers. During a light rainfall, it is unlikely that any water would reach the soil under a continuous gravel cover before this becomes nearly saturated. A precipitation threshold Žminimum or ‘effective rainfall’. must be exceeded in order for soil under gravel to be wetted ŽGardner et al., 1970; Hillel, 1998.. With an average mulch thickness of 5 cm, the effective rain needed for soil moistening is F 8–10 mm for ash or cinder, but only 4–5 mm for lapilli ŽTable 5.. Deeper volcaniclastic layers would, of course, intercept even more rain. These absorptive patterns may help explain why Hawaiian silversword regeneration is noticeably better in coarse red lapilli ŽKobayashi, 1973, p. 16; Perez, ´ pers. observ.., even though the best evaporation protection is offered by finer cinder. Water absorption by soil under mulch would depend on rainfall received during each precipitation event and on rain frequency; unfortunately, precipitation-intensity data for Haleakala’s crater are not available. The benefits of a gravel cover should increase with rain intensity and with more closely spaced events ŽCorey and Kemper, 1968; Alizai and Hulbert, 1970; Lightfoot and Eddy, 1994., although as Haleakala gravels become totally dry within a day, rain frequency would be less important than intensity in this locality. In fact, this rapid drying, or ‘self-mulching’ ŽLemon, 1956; Willis, 1962. may be an effective way of discouraging evaporation from the gravel surface. Ultimately, it would be desirable to understand the actual ecological significance of water conservation by pyroclastic covers for the Hawaiian silversword and related Haleakala vegetation; regrettably, ecophysiological data for these plants are lacking ŽSmith and Young, 1987; Meinzer et al., 1994.. I have shown elsewhere ŽPerez, 1991a. ´ that giant Espeletia rosettes of the desert paramo — a comparable dry high-altitude ´ zone in the equatorial Andes — are restricted to stone-covered areas, where soil water remains above the permanent wilting point ŽPWP.. During the dry season, Andean – desert rosettes reach their lowest leaf water-potential at about y15 bars ŽBaruch, 1976.; the coarsest paramo soils have at this point just ; 2% to 5% water ŽPerez, 1987.. Fig. 8 ´ ´ shows that Haleakala’s soils had a similar water content at the start of the third evaporation phase, when they probably also approached their PWP. The ranges of leaf water-potential for different African and South American giant-rosette species are very similar ŽSchulze et al., 1985; Meinzer et al., 1994.; thus, it seems reasonable to assume that the Hawaiian silversword should have comparable physiological limits. If this is the case, the observed desiccation curves can be interpreted as follows. Bare soils quickly lose water, attaining T50 % in - 12 h, on the average ŽTable 3.; any silverswords growing there would reach their PWP only 1–3 days after saturation. In contrast, rosettes on pyroclastic materials would benefit from a much longer period of readilyavailable water, as median T50 % occurs after ; 6 days ŽTable 4.; plants should still be able to obtain some soil water for an additional 10 days, when the PWP is finally approached! As net photosynthetic rates decline with increasing water stress and drop to zero near the PWP ŽSmith and Young, 1987; Barbour et al., 1987., silverswords on gravel areas should grow faster and have a greater chance of survival during the dry

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season. In essence, the most significant ecological role of volcaniclastic covers is that they substantially extend the periods over which Hawaiian silverswords can obtain sufficient water from their substrate to sustain growth.

6. Conclusions The main findings of this study can be summarized by the following list of desirable characteristics of an efficient volcaniclastic cover in Haleakala Žcf. Corey and Kemper, 1968.. Ž1. Mulch thickness should probably not exceed 5 cm, to avoid excessive water interception by the gravel during low-intensity precipitation events. As Maule Ž1963. Žp. 31. put it: ‘‘The optimum depth wof a cinder coverx is the smallest significant accumulation which successfully stops . . . evaporation.’’ Ž2. Percentage of ground surface coÕered by gravel should be near 100% ŽLamb and Chapman, 1943; Poesen et al., 1997., to effectively prevent water loss through any exposed soil patches. Ž3. Type of pyroclastic material should be cinder or fine lapilli with a small average particle size, ) 5 mm but probably smaller than ; 10 mm diameter. Such gravel would have pores large enough to disrupt all capillary flow, but small enough to prevent rapid vapor diffusion. Cinder should provide a more efficient evaporation control, but the reduced surface area of larger lapilli fragments might actually prove more significant by minimizing water retention within the volcaniclastic layer ŽFig. 4.. Ž4. Gravel particles should be poorly sorted, with a high dispersion value of fs G 1.3. This would allow smaller grains to close off large pore spaces between coarse fragments, thus effectively reducing vapor flow through the largest openings. However, fs should not be too high, less the abundant fine material allows capillary rise through gravel ŽCorey and Kemper, 1968; van Wesemael et al., 1996.. Ž5. The median-size ratio Ž Mr . of gravel and soil particles should be kept low, preferably no larger than 12 to 18. Given the uniform particle-size distribution of Haleakala’s soils Žmean D50 : 0.49 mm., cinder would provide an average Mr of 10 to 13, while fine lapilli should result in an Mr of 13 to 18. Ž6. Gravel color. Although not explicitly investigated here, field temperature data ŽPerez, unpubl. data. indicate that color may also be important Žcf. Othieno and Ahn, ´ 1980; Kemper et al., 1994.. Red and light-brown particles should effect a greater water conservation than black or dark-brown grains, as the lower reflection values of the latter allow soils in Haleakala to attain temperatures ; 48C higher.

Acknowledgements I thank Ronald J. Nagata ŽChief, Resources Management. and Lloyd L. Loope ŽResearch Scientist. for kindly providing the necessary permits to collect soils and plant data at Haleakala National Park. I am grateful to my wife, Ines, ´ and our two sons,

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Andres ´ and Alejandro, for trekking with me down Haleakala’s crater, and for their vital help with fieldwork. Funds to analyze soils were provided through a grant ŽSRG-242. from the Research Institute ŽUniversity of Texas, Austin.. M.J. Schabel ŽSoils and Physical Geography Laboratory, University of Wisconsin, Milwaukee. analyzed several chemical soil properties. Critical comments and suggestions by Drs. I.L. Bergquist, P.W. Unger, and B. van Wesemael were very helpful in improving the original manuscript.

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