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DIVISION S-7—FOREST & RANGE SOILS

Soil and Weathered Bedrock

Components of a Jeffrey Pine Plantation Substrate

Soil and Water Sciences Program, Dep. Of Environmental Sciences, Univ. of California, Riverside, CA 92521–0424

* Corresponding author (khubbert{at}fs.fed.us)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Virtually all of the commercial forests in the southern Sierra Nevada are on granitic terrain, where bedrock may be weathered to depths >15 m while soils are <1 m thick. Because plant-available water is depleted in these thin soils by midsummer, study objectives were to characterize the edaphic role of the weathered bedrock relative to the soil. The site was a 30-yr-old Jeffrey pine (Pinus jeffreyi Grev. & Balf.) plantation growing on relatively thin soils (75 cm in depth) overlying weathered granitic bedrock. The average depth to hard bedrock was 350 cm. A trench was excavated and physical and chemical properties of the soil and bedrock were evaluated. Cation-exchange capacities (CEC) were lower in the weathered bedrock (Cr1 horizon = 4.6 cmol kg^-1) than in the soil (A horizon = 13.4 cmol kg^-1), but pH values were similar (4.6–5.5). Organic C content was negligible in the weathered bedrock matrix (<0.1%), but was higher within joint fractures (3.7%), where roots were concentrated, than within the soil A horizon (2.7%). Carbon/N ratios were much lower in the soil A horizon (19.6) than in the bedrock fractures (62.0). Saturated hydraulic conductivities (K_sat) of the soil and the weathered bedrock were similar and high (8–11 cm h^-1). Mean root length density (RLD) was greater within the joint fractures than within the soil, but on a whole rock basis bedrock RLD was much lower (<0.08 cm cm^-3). Total plant-available water storage capacity of 48.8 cm was calculated for the 350 cm thickness of regolith, with 14.7 cm (30%) contributed by soil and 34.1 cm (70%) by weathered bedrock. Weathered bedrock underlying soils is critical to the survival of forest ecosystems, particularly with regard to water supply, and should not be neglected in ecosystem site evaluations and models.

Abbreviations: AWC, available water capacity • CEC, cation-exchange capacity • ECEC, effective cation-exchange capacity • FC, Field capacity • Ksat, saturated hydraulic conductivity • PWP, permanent wilting point • RLD, root length density

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
GRANITIC ROCK CONSTITUTES 20% of California's land area, and underlies >65% of the Sierra Nevada range (Donley et al., 1979; Norris and Webb, 1976). Most of California's forests grow on upland sites where soils are generally thin and are underlain by thick zones of weathered bedrock. The prevalent granitic bedrock is commonly weathered to depths of several to many meters (Wahrhaftig, 1965), whereas overlying soils are often <1 m thick (Fig. 1a). In early to midsummer, the water status of these thin soils indicates that little or no plant-available moisture remains (Anderson et al., 1995; Sternberg et al., 1996).

View larger version (150K): [in this window] [in a new window]   Fig. 1. Photographs of (a) the roadcut near study area showing thin soil and thick weathered rock zone, with roots extending out of fractures (person is 1.7 m tall); and (b) the sampled trench face showing joint factures with roots and the soil/bedrock boundary. Tile spade is 1.2 m long. Camera angle makes soil appear thicker than its actual 80 cm thickness.

Weathered bedrock can act as a rooting medium for shrubs and trees (Hellmers et al., 1955), providing plant-available water during the summer dry season (Arkley, 1981; Anderson et al., 1995; Sternberg et al., 1996). Pine species have root systems that are deep and widespread and can exploit water held in bedrock by growing into and following joint fractures. Conifer roots have been observed growing in joint fractures of weathered bedrock to depths >25 m (Stone and Kalisz, 1990). Roots form thick mats and develop severely flattened cortexes within the joint fractures, allowing for greater surface area contact between the roots and fracture wall (Zwieniecki and Newton, 1995).

It is clear from the literature, as well as observations of roadcuts, that both soil and weathered bedrock are components of the substrate supporting forests in the mountains of California. The relative contributions of these two components with regard to supporting plant growth have not been specifically investigated. In this study, we evaluated the physical and chemical properties of the weathered bedrock and the overlying soil in order to assess their respective potential contributions to forest vegetation. Greater understanding of weathered bedrock will help maintain the health of the soil, water, and vegetation in the Sierran mixed conifer forests. The conifer forests on the west flank of the southern Sierra Nevada comprise the largest area of commercial timberland on granitic terrain in California (Forest and Rangeland Resources Assessment Program, 1988).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description
The study site is on the Sequoia National Forest in eastern Tulare County, California. It is at an elevation of about 1950 m in the Greenhorn Mountains in the southern Sierra Nevada, just south of County Road M50 at Parker Pass (35.957° N, 118.630° W). The site is on the northeast-facing footslope (gradient <10%) of a knoll. The bedrock is quartz monzonite of the late Mesozoic Kaweah Peaks pluton of the Sierra Nevada batholith (Jennings, 1977). Soils mapped in the general area of the study site are the Chaix series (coarse-loamy, mixed, superactive, mesic Typic Dystroxerepts) and the Chawanakee series (loamy, mixed, active, mesic, shallow Typic Dystroxerepts) (Hanes et al., 1981). These soils have Cr horizons of weathered granitic bedrock (paralithic material). Most precipitation falls between November and April, primarily as snow, averaging 760 mm annually (Rantz, 1972). Summers are warm and dry. Vegetation at the site consisted of 30-yr-old Jeffrey pine planted after a fire, some younger ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) from natural regeneration, and greenleaf manzanita (Arctostaphylos patula Greene) as the principal understory shrub. Trees were mostly 8 to 10 m high.

Field Methods
Ten auger holes bored within the 0.15 ha encompassing the study area revealed that the mean soil depth was 75 cm (to the Cr horizon) and total depth to hard rock (soil plus Cr horizon) was 350 cm. In addition, ground penetrating radar was used to help describe the subsurface spatial variability within the site. Based on these findings, a representative site was chosen for intensive sampling and analysis. A trench measuring 2.4 m deep and 5 m long was excavated by backhoe in late October 1996 (Fig. 1b). Additional trenching would have provided more information, but it is also expensive and destructive to the site. The trench face was near three Jeffrey pine trees ranging in height from 8 to 10 m. To determine RLD in bedrock fractures, three samples from each root-containing fracture (seven total) were taken. Sample volumes were 10 cm wide and 15 cm long, with thickness determined by the largest root diameter in each fracture. To determine soil RLD, triplicate core samples (7.1 cm diam. and 10 cm long) were taken at 15-cm depth increments to the 60-cm depth immediately upslope from the trench. Rooting characteristics of Jeffrey pine were visually observed in the trench face and in nearby roadcuts. Roots in fractures were exposed by digging into the wall face. Root matting, root flattening, and root suberization were described.

A morphological description of the soil and weathered bedrock was made near the center of the trench face (Soil Survey Staff, 1993). Intact core samples (5.3 cm diam. and 5.9 cm long) for measurement of K_sat were taken in sets of five from the A, Bw1, and Bw2 horizons, and in sets of seven from the Cr1 and Cr2 horizons across the trench face. A combination of hand-carving and gentle pressure applied to the core sampler was needed to obtain close-fitting, intact cores from the trench face. For bulk density measurements, intact clods 3 to 5 cm in diameter were removed from the soil horizons, and similar sized chunks of bedrock were cut from the Cr horizons using a soil knife.

Laboratory Methods
Root length densities were determined using the modified line intersect method (Marsh, 1971) after washing the sample over a set of progressively smaller sieves to minimize loss of fine roots. Root diameter and degree of flattening were measured using digital calipers. Roots were examined for evidence of dimorphic branching of laterals that would indicate ectomycorrhizal infection (Brundett et al., 1989). Bulk density was determined using the paraffin-coated clod method (Blake and Hartge, 1986). Porosity was calculated from the bulk density values using an assumed particle density of 2.65 g cm^-3 (Danielson and Sutherland, 1986). Particle-size distribution was measured by the pipette method using samples that were air-dried and sieved to remove rock fragments >2 mm in diameter (Gee and Bauder, 1986). Saturated hydraulic conductivities were calculated from measured water flow and cross-sectional area under constant head using the intact cores (Klute and Dirksen, 1986).

Moisture retention curves were determined for the soil (Bw horizon) and the weathered bedrock (Cr1 horizon). Soil and weathered bedrock water potentials >-0.01 MPa were determined using a hanging water balance, and a mid-range pressure plate system was used for water potentials of -0.01 to -0.1 MPa (Klute, 1986). Soil and bedrock water potentials less than -0.1 MPa were determined using the filter paper method (Campbell and Gee, 1986). Water content measurements were made gravimetrically after oven drying (Gardner, 1986). Gravimetric water contents were converted to volumetric using measured bulk densities. We used limits of -0.01 MPa (field capacity, FC) and -2.2 MPa (permanent wilting point, PWP) to calculate available water capacity (AWC) as suggested for coarse-textured soils in a natural system (Cassel and Nielsen, 1986; Savage et al., 1996).

The equivalent diameter of pores was determined using water characteristic curves and the capillary rise equation:

(1)

where h is height (m), is surface tension (N m^-1), _w is density of water (kg m^-3), g is acceleration of gravity (m s^-2), and R is radius (m).

Total C and N were measured by dry combustion using a Carlo Erba NA1500 C/N/S analyzer (CE Instruments, Austin, TX)¹. Total C was assumed equivalent to organic C since carbonates were not present. Plant-available P was approximated using a dilute acid-fluoride extraction (Bray and Kurtz, 1945; Olsen and Sommers, 1982), with P in solution measured photometrically. Soil and bedrock pH was measured using a 1:1 soil/water ratio. Exchangeable base cations were extracted with 1 M NH₄OAc and measured by atomic absorption spectrometry (Thomas, 1986). Exchangeable acidity was determined by the KCl method (Thomas, 1986). Cation-exchange capacity was measured by NaOAc saturation followed by NH₄Oac extraction at pH 7 (Bower and Hatcher, 1966). Effective cation-exchange capacity (ECEC) was calculated as the sum of the exchangeable cations and exchangeable acidity (Bohn et al., 1985).

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Soil and Bedrock Properties
The soil at the trench site is a coarse-loamy, mixed, superactive, mesic Typic Dystroxerept. Soil textures were sandy loam to gravelly sandy loam, with sand content increasing from 71% in the A horizon to 84% in the Cr2 (Table 1). The A and AB horizons had weak, coarse platy structure, extending downward to Bw horizons with weak to moderate subangular blocky structure. The C horizon was massive, lacking both soil and rock structure. It is separated from the Cr1 horizon by an irregular and broken boundary. The Cr horizons retained rock fabric but could be easily crumbled to individual grains using bare hands, meeting the criteria for Weathering Class 6 of Clayton and Arnold (1972).

Soil moisture characteristic curves for the Bw and Cr1 materials are shown in Fig. 2. At -0.01 MPa, volumetric water content of the soil was 0.246 m³ m^-3, compared with 0.174 m³ m^-3 for the weathered bedrock. The water content at -2.2 MPa was 0.05 m³ m^-3 for both soil and weathered bedrock. Using the limits of -0.01 MPa for FC and -2.2 MPa for PWP, mean AWC values of 0.196 m³ m^-3 for the Bw1 horizon and 0.124 m³ m^-3 for the Cr1 were calculated.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Moisture retention curves for soil (Bw1 horizon) and weathered bedrock (Cr1 horizon). Vertical lines represent limits for field capacity of soil (FCsoil) and weathered bedrock (FCwr), and permanent wilting point (PWP) of both soil and weathered bedrock.

Jeffrey pine roots (distinguished from shrub roots by the strong pine scent of the sap) were present in joint fractures to the maximum depth of the exposed trench face (2.4 m). Actual depth of rooting at the site is much greater, as evidenced by roots recovered in auger borings below 3 m and observed in joint fractures as deep as 6 m in nearby roadcuts. Lateral roots of Jeffrey pine exposed in the trench were >10 m from the bole of the nearest tree. The main laterals grow along the interface between the soil and the weathered bedrock. Sinker roots with diameters as large as 16 mm were confined to the joint fractures and their paths were determined by the orientation of the fracture. Exposed roadcuts show that Jeffrey pine sinker roots, branching off the lower surface of primary laterals, grow along the soil–bedrock interface, and opportunistically enter any joint fracture encountered. Some root remnants protruded from the trench face with bark intact, but with interiors decayed and partially hollowed out. Pine seedlings < 3 yr old that were uprooted during excavation had well developed tap roots >1 m in length.

Sampled RLD in joint fractures ranged from 0.9 to 7.7 cm cm^-3 and averaged 4.3 cm cm^-3, compared with 2.8 cm cm^-3 in the A horizon (Fig. 3). Root mats were observed in joint fractures where RLD was the highest. Within the root mats, many roots were flattened and pressed against the joint fracture walls. For example, a root 0.25 mm thick was 0.89 mm across. In most cases, the thickness of the largest observed root was the same as the fracture width. Root length density varied little in the soil within 0 to 45 cm depth, ranging from 2.4 to 3.0 cm cm^-3 (Fig. 3). Although roots did not grow in the matrix of the weathered bedrock, fracture RLD expressed on a whole rock basis was <0.08 cm cm^-3 for both the Cr1 and Cr2 horizons (Fig. 3). Evidence of ectomycorrhizae associations was observed in joint fractures where some roots exhibited distinct heterorhizy. Close inspection of roots in the joint fractures indicated that most were suberized (roots that have undergone secondary growth and are covered with layers of suberized tissue).

View larger version (22K): [in this window] [in a new window]   Fig. 3. Root length density (RLD) as a function of depth in the soil and weathered bedrock. The RLD was greater within joint fractures than in the soil, but RLD based on bedrock volume as a whole was extremely low. Error bars indicate one standard deviation from the mean.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

The increase in CEC when measured at higher pH reflects the presence of pH-dependent charged materials, such as kaolin, gibbsite, and, in upper horizons, organic matter. These materials were also found in similar Sierra Nevada soils approximately 300 km to the north (Dahlgren et al., 1997). Nutrient base cations are less abundant in the Cr horizons and fractures than in the soil horizons, in part because the CEC is less in the weathered bedrock. The extractable P values, an index of P availability to plants (Olsen and Sommers, 1982), are high in the A horizon and mostly medium throughout the rest of the soil and bedrock profile.

Bulk density increased with depth in the soil–bedrock profile (Table 3), as observed by Graham et al. (1997), due to decreasing weathering intensity. Pore space capable of transmitting gravitational water (>30 mm effective pore diameter) was identical for the Bw1 (0.22 m³ m^-3) and Cr1 (0.22 m³ m^-3) horizons (Table 3). The sources of this porosity differ, however. Gravitational water moves through the weathered bedrock matrix in a pore network consisting of fractures between and through mineral grains, whereas in the soil this water is conducted through root channels and voids between structural units (Frazier, 1997; Graham et al., 1997).

Saturated hydraulic conductivities in the soil and Cr horizons are considered high by USDA standards (Soil Survey Staff, 1993). Nevertheless, it is likely that the K_sat values are underestimates because the larger macropores, consisting of old root channels and faunal burrows in the solum and joint fractures in the bedrock, are not representatively sampled in the relatively small cores used to make the measurements. In situ measurements of K_sat using a constant head permeameter (e.g., Amoozegar, 1989) may give results more representative of field conditions although Graham et al. (1997) found them to be similar to values measured in the lab. In any case, water can move rapidly through the weathered bedrock (K_sat = 10 – 11 cm h^-1) during snowmelt recharge when water content nears saturation (Table 3). High K_sat values for weathered granitic rock were also found by Johnson-Maynard et al. (1994) and Graham et al. (1997).

Previous research has shown that weathered bedrock wets both by matrix-flow from the overlying soil and, under appropriate conditions, by preferential flow down joint fractures (Frazier, 1997). Although K_sat was not measured for the BC and C horizons, we suspect that hydrologic continuity through the regolith is provided by root channel macropores that link the solum to the bedrock fractures, which, in turn, lead to the continuous network of unplugged microfractures of the weathered bedrock matrix, as noted by Frazier and Graham (2000) and Graham et al. (1997).

We calculated an AWC of 0.124 m³ m^-3 for the bedrock, which is greater than the 0.108 m³ m^-3 reported by Jones and Graham (1993) for Weathering Class 6 rock in the San Jacinto Mountains, and less than the 0.15 m³ m^-3 measured by Anderson et al. (1995) for Weathering Class 6 rock in the southern Sierra Nevada. Soil (Bw1 horizon) AWC was determined to be 0.196 m³ m^-3, which is similar to that reported by Anderson et al. (1995). Water retention curves show that the difference in AWC between the soil and weathered bedrock is caused by divergent water contents at FC (-0.01 MPa), not at PWP (–2.2 MPa) (Fig. 2). Based on an average soil thickness of 75 cm and an average weathered bedrock thickness of 275 cm to hard bedrock, a total plant-available water storage capacity of 48.8 cm was estimated for the 350 cm thickness of regolith. About 30% (14.7 cm) of this plant-available water capacity is provided by the soil, while the remaining 70% (34.1 cm) is contributed by weathered bedrock. Although the AWC of weathered bedrock is somewhat lower than that of the soil, the greater thickness of the weathered bedrock gives it a much greater storage capacity for plant-available water relative to the soil.

Joint Fractures and Root Distributions
The joint fractures are generally not empty voids, but are filled with sandy loam material that has sloughed off fracture walls or, near the upper boundary of the Cr1 horizon, has moved down from the overlying soil. Similar joint fractures have been shown to act as conduits for water into and through the weathered bedrock (Frazier, 1997). Dense mats of live and decomposing roots may restrict the flow of water by filling joints, but live roots can shrink on a diurnal or seasonal basis (Huck et al., 1970), creating macropores that may channel water flow. In soils, both decaying roots and old, abandoned root channels act as macropore networks, conveying up to 40% of water input (Mosley, 1981). Partially decayed roots, as observed in this study, may provide for macropore flow within their hollowed interiors (Beven and Germann, 1982).

Sinker roots, such as those extending down from laterals along the C–Cr interface, are important adaptations for deep-water utilization by pine (Strong and La Roi, 1983; Sands and Nambiar, 1984). The deep penetration of Jeffrey pine roots within joint fractures is an adaptation to the seasonal moisture distribution characteristic of Mediterranean climates, in which deep infiltration of water during winter and spring is followed by a drought period during the growing season (Richards, 1986). The weathered bedrock stores plant-available water (Table 3), so deep-rooted species can access this water by penetrating the bedrock joint fractures as paths of least mechanical resistance (Arkley, 1981; Sternberg et al., 1996; Zwieniecki and Newton, 1994). Root distribution observed in the trench face was closely defined by the pattern of joint fractures inherent in the rock, in a manner similar to that observed by Sternberg et al. (1996) for chaparral roots. Roots cannot decrease in diameter to enter pores smaller in cross-sectional area than their root caps, thus the consolidated Cr material remains impenetrable to roots.

While mean RLD was greater within joint fractures than within the soil, on a whole rock basis bedrock RLD was much lower (<0.08 cm cm^-3) (Fig. 3). Similarly, RLDs in an Oregon ponderosa pine stand were >2.0 cm cm^-3 at the 0 to 50 cm soil depth, and <0.05 cm cm^-3 at the 3 m depth in metasedimentary bedrock on a whole rock basis (Zwieniecki and Newton, 1994). In our study, root concentration in the joint fractures varied considerably. It is unclear why not all fractures contained roots, nor to what extent RLD was constant within any one fracture. Joint fractures occupied by one or two dominant large roots appeared to have lower RLDs. High RLD in fractures was indicative of dense root matting and may reflect a more moist environment compared to a fracture without roots. A more moist fracture environment could result from greater AWC of the adjacent Cr material or from increased preferential flow of water down the fracture. Prominent root matting by one species in joint fractures may increase intraspecific competition for the limited water, especially as water deficits develop over the season. Even though RLD of the fractures was very small when expressed on a whole rock basis, these roots obtained >60% of the water taken up during the dry season (Hubbert, 1999).

The degree of root flattening and suberization varied between and within joint fractures. The flattened root faces were typically pressed against the fracture wall, thereby increasing surface contact with the fracture face and forming a bridge for enhanced water transport and uptake (Zwieniecki and Newton, 1995). The preponderance of suberized roots in the joint fractures is significant because suberized roots are active in water uptake. In a study of a 34-yr-old pine stand, suberized roots were shown to furnish about 93% of the total absorbing area, and absorb about three-fourths of the total water taken up from the soil (Kramer and Bullock, 1966).

Because roots in the bedrock are confined to fractures and the fractures are on the order of 50 cm apart, the bulk of the Cr matrix is isolated from the roots. Nevertheless, during the course of the dry season, the entire bedrock matrix was depleted of plant-available water within its upper 0.75 m (Hubbert, 1999), in a manner similar to that described by Sternberg et al. (1996) for bedrock in a chaparral ecosystem. This raises the question of how the plants obtain the water held within the Cr matrix between the joint fractures. While not measured in this study, unsaturated flow in similarly coarse-textured soils at water potentials less than -0.1 MPa is typically on the order of <10^-3 cm h^-1 (Jury et al., 1991). At this rate it would take more than 1042 d for water to move from the center of a matrix block to the roots in the fractures that border it. Clearly, such unsaturated flow is not the sole mechanism by which water reaches roots in the fractures. Root hairs are too short to enter far into the matrix, being 1 mm in length (one elongated epidermal cell) (Cailloux, 1972). Very fine roots (1 mm in diameter) may enter the Cr material, but the distance of penetration may be limited due to lack of contiguous pores large enough for root extension. Ectomycorrhizae fungi, whose hyphae are smaller in diameter (<20 µm) than root hairs and can aggregate to form coarse hyphal strands >2 m long (Fogel, 1983), may penetrate the Cr matrix through a network of microfractures and absorb the available stored water. Although this hypothesis has not been tested directly, the pronounced heterorhizy of roots within joint fractures is interpreted as evidence of ectomycorrhizal associations (Brundett et al., 1989).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Jeffrey pine in a 30-yr-old plantation exploits both soil and underlying weathered granitic bedrock as a rooting medium. These two components of the forest substrate each serve distinct and critical functions in supporting tree growth. Despite having a lower AWC per unit volume, the weathered bedrock stores more than twice as much plant-available water by virtue of its greater thickness compared with the soil. This additional stored water is of utmost importance to forest ecosystems in the summer-drought climatic regime of California. The soil, on the other hand, is the repository of biocycled nutrients, particularly concentrated in the upper horizons. Rooting patterns in the two substrate components are also different. Within the friable soil, roots grow without physical restrictions and are able to directly explore the entire volume. In contrast, roots are limited to joint fractures within the weathered bedrock so that planar concentrations of roots are spaced about 50 cm apart along the roughly vertical and horizontal orientations of the joint sets. Ectomycorrhizae fungi are of sufficiently small diameter to penetrate microfractures in the weathered rock matrix and may enhance water uptake from between fractures. Few studies have addressed plant utilization of weathered bedrock, and bedrock properties are usually not well characterized in soil survey reports. Yet our research shows that weathered bedrock can play a critical role in forested ecosystems. Realistic assessments of rooting depth and water storage capacity, as needed for ecosystem models and site evaluation, must include the weathered bedrock zone.

	ACKNOWLEDGMENTS

 
This work was funded by USDA–NRI Competitive Grant No. 94-37101-0340. We thank April Ulery, Jonathan Wald, Paul Sternberg, and Ralph Strohman for field and laboratory assistance, and Jim Doolittle and Sam Indorante (USDA–NRCS) for GPR analysis. Sequoia National Forest personnel were exceptionally helpful. In particular, we thank Dan Martynn and Nolan Fritz.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
K.R. Hubbert presently at USDA Forest Service, Pacific Southwest Research Station, Forest Fire Laboratory, 4955 Canyon Crest Drive, Riverside, CA 92507.

¹ Mention of a product is for information purposes only and does not imply endorsement by the USDA Forest Service or the University of California.

Received for publication August 21, 2000.

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TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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