Soil conditions in three recent landslides in Southeast Alaska

Forest Ecology and Management, 18 (1987) 93-102

93

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Soil C o n d i t i o n s in T h r e e R e c e n t L a n d s l i d e s in Southeast Alaska P A U L W. ADAMS 1 and ROY C. S I D L E 2

1Department of Forest Engineering, Oregon State University, Corvallis, OR 97331 (U.S.A.) '~USDA Forest Service, Forestry Sciences Laboratory, 860 N. 12th East, Logan, UT 84321 (U.S.A.) (Accepted 24 July 1986)

ABSTRACT Adams, P.W. and Sidle, R.C., 1987. Soil conditions in three recent landslides in southeast Alaska. For. Ecol. Manage., 18: 93-102. Soils in some recent landslides were studied to better understand limitations to revegetation and management. Large areas of the landslides, particularly the scour zones, had limited rooting potential due to exposed rock or large organic debris. Soil bulk density was generally higher in the scour zones than in the deposit areas. Soil chemistry in the landslides was highly variable, probably due to uneven mixing of deposited material and post-landslide sloughing of surface soil and organic debris into scour areas. Soil fertility, however, was generally greater in deposit areas than in scour zones, where fertility was often comparable to mineral horizons of similar undisturbed soils. Although revegetation and plant growth in these and similar landslide units are likely to respond more favorably to soil conditions in the deposit area, they are still expected to be highly variable within a deposit or scour area and among different landslides.

INTRODUCTION

Soil mass movement is the dominant erosion process on the steep forest lands common to southeast Alaska (Swanston, 1974), and logging and road construction can significantly accelerate landslide activity (Bishop and Stevens, 1964; Sidle et al., 1985). These mass movements can seriously retard forest regeneration by first removing the soil mantle down to bedrock or glacial till on upper slopes, and then depositing the debris over productive soils on lower slopes and valley bottoms (Harris, 1967). M a n y of the larger debris avalanche and flow scars in southeast Alaska are still barren more than 30 years after the slides occurred (Swanston, 1974). Although a relatively small percentage of the total land area in the region is in this condition, it is likely to increase as harvesting continues and because, under sub-arctic conditions, natural recovery processes are slow. Furthermore, the common occurrence of

0378-1127/87/$03.50

© 1987 Elsevier Science Publishers B.V.

94

landslide scars in V-notch side drainages (Bishop and Stevens, 1964; Swanston, 1974 ) most likely renders them important and persistent sources of headwall and bank sloughing, surface erosion, and associated stream sedimentation. Despite the frequency and likely impacts of natural and management-related mass movements in southeast Alaska, only limited study has been made (Gregory, 1960; Harris, 1967 ) of the soil conditions and the associated revegetation patterns found in landslide scars in the region. This study was initiated to obtain some basic information about physical and chemical soil characteristics in some representative southeast Alaska landslides and, thus, help improve our understanding of the potential limitations to revegetation, forest development and management. STUDY AREA AND METHODS

Three landslides less than a year old, ranging in slope area from 0.06 to 0.57 ha (Table 1), were studied within 2 km of each other in the Kennel Creek drainage on northeast Chichagof Island (57°52'N, 135°45'W). The mass movements occurred as in-unit debris avalanche-debris flows (shallow, rapid failures) that took place within a year of clearcut logging. Undisturbed soils in this remote area have not been mapped in detail, but are shallow to bedrock ( < 1 m) and are tentatively classified as loamy, skeletal Humic to Lithic Cryorthods (USDA Soil Taxonomy). Each of the three landslides was stratified into scour and deposit areas; five sampling transects were established in each of the two areas perpendicular to the long axis of the landslide. Along each transect, four bulk surface soil (0-10 cm) samples were taken at random for later chemical analysis, producing a total of 40 bulk samples for each landslide. Three random samples for bulk density and particle size analysis were also taken in the scour and deposit areas at each site, using an irregular hole excavation technique suitable for steep, skeletal forest soils (Flint and Childs, 1984). The ten soil sampling transects at each landslide were also used to evaluate general vegetation rooting potential. At 30-cm intervals along each transect, a tile spade was used in a manner similar to a planting bar for reforestation work. Rooting potential was considered positive when the spade could be inserted to about a 10 cm depth with little resistance. Results were tallied and converted to approximate rooting area as a percent of the total area of the landslide stratum being considered. General observations were also made in the field of the various limitations to rooting that dominated each area, including coarse fragments, exposed bedrock, large organic debris, and very dense soil. Laboratory analysis of the collected soil samples followed standard procedures (Day, 1965; Berg and Gardner, 1978). Tests included dry bulk density and particle size analysis (sieving and hydrometer), soil pH (electrodes in 1:2 soil:water suspension ), exchangeable (1 N ammonium acetate) K, Ca and Mg,

95 TABLE 1 General characteristics of landslide study sites in southeast Alaska Landslide Slopearea (ha)

Scour area (%)

Deposit area (%)

Averageslope (%)

A B C

19 49 82

81 51 18

36 35 4O

0.27 0.57 O.O6

available (Bray No. 1) P, total (micro-Kjeldahl) N, and total (Walkley-Black) organic matter. Results of the soil physical and chemical analyses for each site were then stratified by scour and deposit area, the mean values calculated, and paired t-test comparisons ( at P < 0.05 and P < 0.01 ) of soil chemistry between the areas were made. RESULTS AND DISCUSSION

Physical characteristics Total landslide area and the relative proportions of scour and deposit area varied widely among the three landslides (Table 1 ), despite the generally similar soil and site conditions in the Kennel Creek drainage. Although sources of slope failure and overall landslide behavior may be relatively uniform for a given location or soil type, net areas of depletion and deposition appear to be considerably influenced by site-specific factors like micro-topography. Large variation in landslide characteristics, such as area or volume, is not unusual (Fredriksen, 1965; Ketcheson and Froehlich, 1978) and probably represents the rule rather than the exception. Soil physical conditions show both similarities and differences between scour and deposit areas at each site (Table 2). Soil texture was fairly uniform at each site, with the fine ( < 2 m m ) fraction typically characterized by sandy loam. Both the scour and deposit zones had high coarse fragment ( > 2 m m ) contents, all exceeding 50% of total soil weight. There was, therefore, little evidence at these sites of particle sorting during m a s s m o v e m e n t and/or large differences in the initial surface ( now in deposit areas) and subsoil ( remaining in scour areas) textures. Both whole and fine soil bulk densities were typically higher in the scour area than in the deposit zone (Table 2). Fine soil bulk density in the scour zones averaged 0.51 g/cm 3, whereas the deposit zones averaged 0.37 g/cm 3. These differences were consistent with field observations that relatively dense subsoil material was common in the scour zones, whereas fairly loose and organic-rich surface soil was transferred to deposit areas. Overall, the fine soil

0.20_+0.03 82_+10

1.38_+0.40 0.69_+0.25 69_+ 2 1.09 _+0.51 0.50 _+0.38 69 _+12

0.93_+0.30

0.88_+0.26 0.27_+0.12 75_+18

1.16_+0.50 0.58_+0.34 67_+ 5 0.84+0.27 0.42-+0.08 58-+10

20_+ 3 20 _+ 7

12_+ 6

17_+11

19_+ 5 26-+ 4

Sand (0.05-2 mm)

7_+1 7 _+2

4_+2

6_+5

11_+2 11-+5

Silt (0.002-0.05 ram)

Coarse fragments ( > 2 ram)

Whole soil

Fine soil

Particle size distribution ( % of soil weight)

Bulk densitF (g/cm :~)

4_+2 4 _+3

2_+1

2_+1

3_+1 5_+2

Clay ( <0.002 mm)

Gravelly sandy loam Gravelly sandy loam

sandy loam

sandy loam Cobbly/gravelly

Cobbly/gravelly

Gravelly sandy loam Gravellysandyloam

USDA textural class

an=3 for each value listed, except rooting areas which were based upon five transects (see text). bVariability shown is among transects. CWhole soil bulk density includes coarse fragments > 2 mm. Fine soil bulk density values are corrected for coarse fragments, assuming particle density = 2.65 g/cm :~.

47_+ 9 52 _+11

(C) Scour Deposit

34_+19

(B) Scour

53_+ 9

54+13 83+ 6

(A) Scour Deposit

Deposit

Potential rooting area L' (% of total)

Site and sample area

Physical characteristics of soils in landslide study sites in southeast Alaskaa

TABLE 2

97 bulk densities were comparable, while the whole soil values were generally higher than reported elsewhere for undisturbed surface soils in southeast Alaska (Stephens et al., 1969). The latter contrast probably simply reflects our chosen method of sampling large soil volumes containing significant amounts of coarse fragments (Flint and Childs, 1984). The scour zone at each landslide had a consistently lower percentage of potential rooting area than the deposit zone (Table 2). Values for potential rooting area in the scour zone ranged from 34 to 55% and averaged 45%, whereas values in the deposit zones ranged from 52 to 83% and averaged 63%. Potential rooting sites in the scour zones were typically limited by exposures of large rocks or bedrock, whereas large organic debris and rocks typically reduced the potential rooting area in deposit zones. Because relatively small-scale conditions sometimes produced a negative tally during the potential rooting area survey, all of the potential rooting area percentages should be considered as approximate and as totals of numerous non-contiguous areas at each site. When applied to the entire area of each of the three landslides (Table 1), net potential rooting area ranged from 44 to 77% and averaged 56% of the total landslide area. Although not directly comparable, these values are generally consistent with the results of a survey of 25 landslides in western Oregon ( Miles et al., 1984). On those units the existing forest regeneration and substrate conditions in unstocked areas indicated an average potential stocking level of about 67%. Chemical characterization Soil chemical characteristics showed wide variation within and among the three landslide units (Table 3). This follows the typically heterogeneous appearance of the soil and organic debris in the units. Surface soil and organic layers moved to the deposit areas were unevenly mixed during transport. The scour areas, while often exposing relatively undisturbed subsoil material, showed considerable evidence of post-landslide sloughing of adjacent topsoil and organic material. Soil pH values in the landslides ( Table 3 ) were somewhat higher than those reported elsewhere for undisturbed mineral soils in southeast Alaska (Stephens et al., 1969; Heilman and Gass, 1974; Sidle and Shaw, 1983; and Table 4). Scour zone pH values were typically greater than those in deposit areas, which is consistent with observed pH differences between highly organic surface soil and mineral subsoil. Southeast Alaska soils show the usual pattern of increasing pH with depth in the soil profile (Table 4) on a variety of parent materials (Heilman and Gass, 1974). Soil organic matter levels appeared to be inversely related to soil pH (Table 3 ). Relatively high levels of organic matter were found in the landslides, even in the scour areas. Transport and mixing of the litter layers and logging debris

5.2** 5.0

5.4** 5.2

5.0 5.3

(A) Scour Deposit

(B) Scour Deposit

(C) Scour Deposit

18.1 _+1 4 . 0 16.5_+11.0

19.4-+ 6.8 25.0 _+ 7.4*

9.8_+ 4.7 16.0_+ 5.7**

Organic matter (%)

0.32_+0.19 0.37_+0.22

0.32-+0.10 0.40 _+0.16

0.13_+0.09 0.27_+0.11"*

Total nitrogen (%)

32 _+14" 25-+ 6

36_+ 8 38 -+ 10

50___14"* 37_+ 9

C/N ratio

0.1 _+0.1 0.3__0.1"*

0.2_+0.1 0.3 _+0.1"

0.3_+0.1 0.3-+0.1

K

2.6-+2.0 5.3-+2.5**

2.9-+3.4 4.0 _+1.5

1.4_+1.3 1.6-+0.8

Ca

Exchangeable cations (meq per 100 g)

0.6_+0.6 1.2_+0.8"

0.7-+0.4 1.0 -+0.4*

0.3_+0.1 0.6_+0.3**

Mg

4_+4 8_+3**

1_+1 1 _+0

3+2 4-+3

Avalaible P (ppm)

an = 20 for each listed value; t-test of inequality of paired means; pH data were converted to hydrogen concentrations for statistical calculations. *Significantly greater than other sample area at 95% confidence level. **99% confidence level.

pH

Site and sample area

Soil chemical characteristics in landslide study sites in southeast Alaska a

TABLE 3

¢,D OO

99 TABLE 4 Physical and chemicalproperties of Cryorthod soils sampled throughout southeast Alaska~ Soil pHb horizon

O E B21 B22 B3

3.4 3.7 4.1 4.5 4.7

Organic matter (%) 6.6 16.3 11.7 4.5

Total C/N nitrogen ratio (%) 1.29 0.19 0.32 0.20 0.16

21 29 26 29 27

Exchangeablecations (meqper 100 g)

Available P (ppm)

K

Ca

Mg

0.8 0.1 0.1 0.1 0.1

5.7 0.5 0.7 0.4 0.4

6.5 0.5 0.6 0.3 0.2

28 2 2 2 2

aUnpublisheddata, USDA Forest Service,Tongass National Forest, Juneau, AK. Valuesare means of 21-39 samples, except for organic matter which are means of 5-9 samples. Analyticalmethods were comparable to those used in this study. bpH data were convertedto hydrogenconcentrations to calculate mean values. undoubtedly contributed organic m a t t e r to the deposit areas, along with sloughing of topsoil and litter around the margins of the scour zone. Organic matter levels were higher in the deposit areas of two of the three recent landslides, however. Amounts of total nitrogen in the landslide soils (Table 3) were also fairly high compared to expectations from data for representative undisturbed soils (Heilman and Gass, 1974; and Table 4). Soil C / N ratios were higher in the scour zones of two of the three study sites, and overall were somewhat higher t h a n average values for undisturbed Cryorthods in southeast Alaska (Table 4). These observations suggest a relatively low nitrogen availability in landslide soils, particularly for scour zones. Exchangeable K levels in the landslides (Table 3) were similar to those reported for the mineral layers of undisturbed soils (Table 4). Exchangeable Ca was consistently higher t h a n in the undisturbed soils, partly because of mixed and sloughed organic matter in the landslides. Exchangeable Mg and available P showed a wide range of values (Table 3 ), with the higher quantities exceeding the averages reported for undisturbed soils in the region (Table 4 ). This may again indicate contributions from organic matter present in the landslide soils, particularly in the deposit areas. At the other extreme, the lower Mg and P levels observed were comparable to the modest subsoil concentrations in the undisturbed soils. Most of the available P and exchangeable cation levels, however, were higher in the deposit area at each landslide.

Implications for revegetation and management Because soil moisture normally remains high throughout the year in southeast Alaska, revegetation and tree growth in soil mass movement areas should

100

be largely controlled by the availability of favorable rooting sites and their fertility. The study results lead, therefore, to some expected revegetation responses on similar landslide scars. For example, incomplete revegetation and/or uneven forest stocking are likely to be greater problems in scour areas than in deposit areas. Landslide deposits lack the bedrock exposures common in scour areas, and the large organic debris typical of deposit areas will eventually decompose to provide more rooting sites. Although western hemlock and sitka spruce (both are preferred commercial species) germinate on both mineral and organic seed beds, establishment and subsequent growth is much greater on soils with abundant organic matter (Ruth and Harris, 1979). In scour zones, not only is a lower potential rooting area expected, but bank sloughing can also inhibit revegetation (Harris, 1967). A period of active sloughing followed by stabilization could explain differences seen elsewhere between landslide age and the age of natural forest regeneration on the landslides (Miles et al., 1984). Although very high soil densities occurred infrequently at the study sites, dense soil in scour zones in some areas could also impede the establishment and growth of sitka spruce and western hemlock, whose roots are unable to penetrate soil with a bulk density of 1.45 g/cm 3 (Minore et al., 1969). Red alder can better tolerate dense soil conditions (Minore et al., 1969), and this or another Alnus species is likely to be more successful in areas of exposed compact till or other high density soil layers. Soil chemistry differences between scour and deposit areas are also likely to contribute to poorer regeneration and growth in scour areas. Such contrasts in tree growth within landslide scars have been observed for Douglas-fir in western Oregon, although relationships with specific soil properties were not evaluated (Miles et al., 1984). In areas where landsliding exposes calcareous till, high pH and Ca levels could inhibit establishment and growth of sitka spruce and western hemlock (Heilman and Ekuan, 1973; Van den Driessche, 1978). Low nutrient levels in landslide scour areas are expected to be a more widespread problem for revegetation and tree growth; however, the observed variability in nutrients among sites makes it difficult to predict which specific nutrient might become limiting in a given location. Furthermore, critical levels of nutrients for major tree species and other vegetation in southeast Alaska have not been defined. Soil and litter layer total N, C/N ratios, exchangeable cations, and available P, however, have shown some clear relationships with the growth of sitka spruce and western hemlock (Stephens et al., 1969; Heilman, 1978; Heilman and Ekuan, 1980; Blyth and Macleod, 1981). Although not examined in this study, several other soil and site conditions could influence revegetation and growth in southeast Alaska landslides. High light, high transpiration demands, and temperature extremes can inhibit revegetation of landslides (Flaccus, 1959), and similar problems with high temperatures (Gregory, 1956), dry conditions (Harris, 1967) and frost damage and heaving (Patric, 1967; Ruth and Harris, 1979) have been noted in south-

101 east Alaska clearcuts. Landslide areas with exposed subsoil or large logs may also lack suitable populations of mycorrhizal fungi (Shaw and Sidle, 1983). Mycorrhizae have been shown to be i m p o r t a n t in the growth and nutrition of sitka spruce (James et al., 1978), whereas western hemlock can survive at least one growing season without t h e m (Christy et al., 1982 ). All of the evaluations in the study were of surface soil (0-10 cm ), and deeper soil layers or limited soil depth could eventually affect developing vegetation. Hemlock and spruce tend to be shallow-rooted (Farr et al., 1977; Minore, 1979) and require only limited mineral soil ( 25 cm ) for m a x i m u m production ( Heilman and Gass, 1974 ) ; however, the lack of what are normally deep and continuous organic layers may place greater importance on mineral soil depth in landslide areas. Recent landslides were the focus of this study because resource managers are often concerned about prompt revegetation and stabilization of disturbed areas. Questions remain, however, about the expected changes in soil properties over time and relationships with revegetation. Studies of a 125-year-old landslide (Gregory, 1960) and recently deglaciated areas ( Crocker and Dickson, 1957) suggest t h a t some fairly rapid and significant changes in both physical and chemical soil characteristics will occur in landslide areas. Finally, as suggested by the previous discussions and supporting data, perhaps the most striking characteristic of the soils in the southeast Alaska landslides was the variability both within and among the mass movements. Therefore, despite the expected general trends, revegetation and growth are likely to be highly variable among landslides and even within the scour or deposit area of a given landslide. If land managers want to better understand and improve revegetation success in mass movement areas, they will need to seek expert soil and ecological opinion on a site-by-site basis. REFERENCES Berg, M.G. and Gardner, E.H., 1978. Methods of soil analysisused in the Soil Testing Laboratory at Oregon State University. Spec. Rep. 321 (revised), AgriculturalExperiment Station, Oregon State University, Corvallis, OR, 44 pp. Bishop, D.M. and Stevens, M.E., 1964. Landslides on logged areas in southeast Alaska. USDA For. Serv. Res. Pap. NOR-l, 18 pp. Blyth, J.F. and Macleod, D.A., 1981. Sitka spruce (Picea sitchensis) in northeast Scotland. I. Relationships between site factors and growth. Forestry, 54: 41-62. Christy,E.J., Sollins,P. and Trappe, J.M., 1982. First-year survivalof Tsuga heterophyUa without mycorrhiza and subsequent ectomycorrhizaldevelopmenton decayinglogs and mineral soil. Can. J. Bot., 60: 1601-1605. Crocker, R.L. and Dickson, B.A., 1957. Soil developmentof the recessional moraines of the Herbert and MendenhallGlaciers, southeastern Alaska. Ecology,45: 169-185. Day, P.R., 1965. Particle fractionation and particle-size analysis. In: C.A. Black (Editor) Methods of SoilAnalysis,Part 1. Agron.9, AmericanSocietyof Agronomy,Madison,WI, pp. 545-567. Farr, W.A., Smith, H.A. and Benzian, B., 1977. Nutrient concentrations in naturally regenerated seedlings of Picea sitchensis in southeast Alaska. Forestry, 50:103-112.

102 Flaccus, E., 1959. Revegetation of landslides in the White Mountains of New Hampshire. Ecology, 40: 692-703. Flint, A.L. and Childs, S., 1984. Development and calibration of an irregular hole bulk density sampler. Soil Sci. Soc. Am. J., 48: 374-378. Frederiksen, R.L., 1965. Christmas 1964 storm damage on the H.J. Andrews Experimental Forest. USDA For. Serv. Res. Pap. PNW-29, 11 pp. Gregory, R.A., 1956. The effect of clearcutting and soil disturbance on temperatures near the soil surface in southeast Alaska. USDA For. Serv. Stn. Pap. 7, 22 pp. Gregory, R.A., 1960. The development of forest soil organic layers in relation to time in southeast Alaska. USDA For. Serv. Tech. Note 47, 3 pp. Harris, A.S., 1967. Natural reforestation on a mile-square clearcut in southeast Alaska. USDA For. Serv. Res. Pap. PNW-52, 11 pp. Heilman, P., 1978. Soil and site index in coastal hemlock forests of Washington and Alaska. In: W.A. Atkinson and R.J. Zasoski (Editors), Proc. Western Hemlock Management Conf., College of Forest Resources, University of Washington, Seattle, WA, pp. 39-48. Heilman, P.E. and Ekuan, G.L., 1973. Response of Douglas-fir and western hemlock seedlings to lime. For. Sci., 19: 220-224. Heilman, P.E. and Ekvan, G.L., 1980. Phosphorus response of western hemlock seedlings in Pacific coastal soils from Washington. Soil Sci. Soc. Am. J., 44: 392-395. Heilman, P.E. and Gass, C.R., 1974. Parent materials and chemical properties of mineral soils in southeast Alaska. Soil Sci., 117: 21-27. James, H., Court, M.N. and Macleod, D.A., 1978. Relationships between growth of Sitka spruce (Picea sitchensis), soil factors and mycorrhizal activity on basaltic soils in western Scotland. Forestry, 54: 105-119. Ketcheson, G. and Froehlich, H.A., 1978. Hydrologic factors and environmental impacts of mass soil movements in the Oregon Coast Range. WRRI-56, Water Resources Research Institute, Oregon State University, Corvallis, OR, 94 pp. Miles, D.W.R., Swanson, F.J. and Youngberg, C.T., 1984. Effects of landslide erosion on subsequent Douglas-fir growth and stocking levels in the Western Cascades, Oregon. Soil Sci. Am. J., 48: 667-671. Minore, D., 1979. Comparative autoecological characteristics of northwestern tree species - a literature review. USDA For. Serv. Gen. Tech. Rep. PNW-87, 21 pp. Minore, D., Smith, C.E. and Woollard, R.F., 1969. Effects of high soil density on seedling root growth of seven northwestern tree species. USDA For. Serv. Res. Note PNW-112, 6 pp. Patric, J.H., 1967. Frost depth in forest soils near Juneau, Alaska. USDA For. Serv. Res. Note PNW-60, 7 pp. Ruth, R.H. and Harris, A.S., 1979. Management of Western hemlock-Sitka spruce forests for timber production. USDA For. Serv. Gen. Tech. Rep. PNW-88, 197 pp. Shaw, C.G., III and Sidle, R.C., 1983. Evaluation of planting sites common to a southeast Alaska clearcut II. Available inoculum of the ectomycorrhizal fungus Cannococcum geophilum. Can. J. For. Res., 13: 9-11. Sidle, R.C., Pearce, A.J. and O'Loughlin, C.L., 1985. Hillslope stability and land use. Water Resour. Monogr. 11, American Geophysical Union, Washington, DC, 138 pp. Sidle, R.C. and Shaw, C.G., III, 1983. Evaluation of planting sites common to a southeast Alaska clear-cut I. Nutrient status. Can. J. For. Res., 13: 1-8. Stephens, F.R., Gass, C.R., Billings, R.F. and Paulson, D.E., 1969. Soils and associated ecosystems of the Tongass. USDA Forest Service, Alaska Region, Juneau, AK, 67 pp. Swanston, D.N., 1974. The forest ecosystem of southeast Alaska 5. Soil mass movement. USDA For. Serv. Res. Pap. PNW-17, 22 pp. Van den Driessche, R., 1978. Mineral nutrition of western hemlock. In: W.A. Atkinson and R.J. Zasoski (Editors), Proc. Western Hemlock Management Conf., 1978. College of Forest Resources, University of Washington, Seattle, WA, pp. 56-70.