I CATENA vol. t7, p. 111-126 Cremlingen 1990

EFFECTS OF TREETHROW M I C R O T O P O G R A P H Y ON THE CHARACTERISTICS A N D GENESIS

OF SPODOSOLS, MICHIGAN, USA

R.J. Schaetzl, East Lansing

Summary that impede infiltration of snowmelt wa- ters, whereas pit soils remain unfrozen

This study examined the pedogenic el- or acquire only a porous, granular frost fects of pit and mound microtopography, layer. Thus, saturated flow of snowmelt formed by tree uprooting, in a Spodosol within pits is relatively unrestricted, re- (Haplorthods) landscape. Most soils in sulting in maximal leaching and profile treethrow pits were Entic Haplorthods or differentiation. Spodic Upidsamments whereas mound soils usually classified as Typic Udipsam- ments, suggesting that degree of profile 1 Introduction development is: Undisturbed _> Pit > Mound. Given that pit and mound soils The effect of relief on soil develop- are substantially younger than pedons on ment has received considerable study; "undisturbed" sites, rates of pedogenesis reviews are provided by RU H E (1969),

G E R R A R D (1981), and BIRKELAND are thought to be: Pit > Undisturbed > Mound. Strong pedogenesis in pits was (1984). Many of these studies examine explained by: soil variability along a drainage sequence

(e.g. DALSGAARD et al. 1981). At this 1. greater water contents in the upper scale, however, water table relations, as-

sola, which may facilitate weather- pect, erosion, microclimate, and variabil- ing processes, ity in vegetation and lithology may in-

troduce error into the state-factor equa- 2. thicker O horizons, which may lead tion (JENNY 1941). The present study

to increased production of organic focusses on microrelief as a pedogenic acids, and factor, in an attempt to minimize extra-

neous factors and effects. 3. greater insulation by thick litter and snow cover, which reduces the inci- Relatively few studies have focussed

on the effects of microrelief on soil dence of soil freezing, formation (e.g. LAG 1951, VENE-

In winter, mound soils may develop im- MAN et al. 1984, A LEX A N D ER 1986, permeable layers of concrete soil frost KNUTESON et al. 1989). Microto-

ISSN 0341-8162 pography influences pedogenesis primar- @1990 by CATENA VERLAG, ily through the redistribution of energy D-3302 Cremlingen-Destedt, W. Germany and materials, including water, litter, 0341 8162/90/5011851/US$2.00+0.25 and snow. In this study I examined

CAFENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY GEOMORPHOLOGY

112 Schaetzl

microrelief formed by the uprooting of 2 Study area trees, often called "pit/mound microto- pography". The study of pit/mound mi- The research was conducted in Baraga crorelief allows for comparisons between County, Michigan, USA (48 °, 39' N lat., pit, mound, and "control" or undis- 88 °, 28' W long.). Baraga County bor- turbed pedons (VENEMAN et al. 1984, ders the southern shore of Lake Supe- SCHAETZL et al. 1990). rior. Surficial sediments consist of glacial

Mounds are typically drier and outwash, tills and glaciolacustrine sedi- less variable in temperature than pit ments deposited approximately 11,000~ sites (DWYER & MERRIAM 1981, 10,000 BP (SAARNISTO 1974). BEATTY 1984, BEATTY & STONE The climate is cool, humid continen- 1986), and often have weak profile de- tal. Snow cover is usually observed from velopment (DENNY & GOODLETT late November through mid April; snow- 1956, SCHAETZL 1986, SCHAETZL pack depths in excess of one meter are et al. 1990). Overthickened, tongue-like common. Locations near Lake Supe- E horizons are commonly observed be- rior receive less snow than do sites in- neath pits (LAG 1951, IVES et al. 1972, land, where orographic effects are ev- WANG et al. 1978). Organic (O) hori- ident. Early autumn snowfall coupled zons tend to be overthickened in pits, pri- with thick litter accumulations on the marily because of litter redistribution by soil surface can insulate the soil from wind and water (LUTZ 1940, LYFORD cold temperatures that lead to deep freez- 1973, ARMSON & FESSENDEN 1973, ing; soil frost is thus sporadic and thin. SHUBAYEVA & KARPACHEVSKIY When snow cover is thin, however, soil 1983, BEATTY & STONE t986). These frost is more pronounced. observations suggest that pits are com- Mixed deciduous and coniferous forest monly zones of stronger leaching than typifies this region, with Tsuga canaden- mounded sites (LUTZ 1940, IVES et al. sis, Acer saccharum, Tilia americana, Be- 1972, VENEMAN et al. 1984). tula lutea, and Pinus strobis trees be-

This study examined: ing the most common species. Entic and Typic Haplorthods are the prevalent

1. the morphology of mound, pit, and soil subgroups (SOIL SURVEY STAFF undisturbed pedons, 1975). Udipsamments and Histosols oc-

2. the effect of pit/mound microtopog- cupy xeric and hydric sites, respectively.

raphy on soil water content and soil freezing, 3 F ie ld and l a b o r a t o r y m e t h o d s

3. the spatial and temporal variabil- Six pit/mound pairs in sandy, mixed, ity of pedogenesis as indicated by frigid Typic and Entic Haplorthod soils cations of Fe, A1 and H in soil so- were excavated by digging trenches, ap- lutions, and proximately 0.7 m wide, parallel to the

4. the relationships between microto- pit-mound axes. Three pedons were described and sampled (at pit center, pography, soil frost, and infiltration, mound crest, and in an undisturbed, or as they relate to pedogenesis. "control" pedon) in each trench. Labora- tory analyses were performed on the air-

CATENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLO(IY GEOMORPHOLOGY

Treethrow and Soil Genesis 113

Subgroup classification" Pit Mound Undisturbe, No. pedons

I Typic Udipsamments 1 4 I

Increasing Spodic Udipsamments 3 1 1 Spodic Horizon Entic Haplorthods 2 3

Development I Typic Haplorthods 1 2 V

l " Most pedons are in sandy, mixed, frigid families. H Classification follows the SOIL SURVEY STAFF (1975).

Tab. 1 : Field microtopography and soil genesis.

dried, <2 mm size fraction. Particle size Corp., Santa Barbara, CA) during the fractionation was by pipette. Soil pH was spring snowmelt and summertime peri- measured on 2:1 water:mineral soil mix- ods of 1986 and 1987 from O, A, E, tures (8:1 for O horizons). Organic car- Bsl, Bhs, Bsm, Bs2, and BC horizons bon analyses were performed by acid di- of pit, mound, and undisturbed pedons. gestion (ALLISON 1965), and Fe and AI Field installation is described elsewhere extractions by sodium citrate-dithionite (SCHAETZL & ISARD 1989). Sprin- and sodium pyrophosphate (HOLM- kling of deionized water onto the sur- G R E N 1967, USDA-SCS 1982). Modifi- face of several pedons was necessary on cations of the former method from that approximately 25% of the sites, during of H O L M G R E N (1967)include: the 1986 summer sampling period. In

all, 193 horizons in 84 pedons were sam- 1. use of 1 g Na-dithionite instead of pled, lessening problems of pedon or site

2 g; representativeness. Water samples were analyzed for pH, and for Fe and AI con-

2. use of 10 g Na-citrate instead of tents by atomic absorption spectropho- 2(~25 g; and

tometry (1986 samples) and ICP spec-

3. centrifugation in preference to addi- trophotometry (1987 samples). Finally, tions of Superfloc. general characteristics and spatial conti-

nuity of soil frost in the winter and sping Fe and Al contents of the extracts were of 1985-86 and 1986-87 were noted. determined by atomic absorption spec-

trophotometry. 4 Results Contents of soil water were mea-

sured in the field, on two pit-mound- 4.1 Soil characteristics undisturbed pedon triplets, at 2 12 cm increments, during a dry period (Site #1) Profile development, as indicated by and 24 hours after a moderate rain (Site the field observations, morphologic and #2). Soil water was also collected in situ chemical data (tab.l), and Soil Taxon- (samplers from Soilmoisture Equipment omy (SOIL SURVEY STAFF 1975) is:

C A ' l E N A An Interdisciplinary Journal of S O I L S C I E N C E H Y D R O L O G Y G E O M O R P H O L O G Y

114 Schaetzl

Hori- Depth Color" pH Org. Fe A1 Fe A1 Gravel Sand*" Silt Clay zon Carbon C-D C-D Pyro Pyro

(cm) (H20) % g kg -1 %

U N D I S T U R B E D P E D O N Oe 4-0 - - 5.90 nd . . . . . . . A 0-9 N 2/0 5.67 7.1 3.3 0.5 0.5 0.4 0 69.2 26.5 4.3 E 9-19 5YR 5/2 4.71 0.4 3.3 0.2 0.2 0.2 0.7 86.0 12.8 1.2 Bsl 19-26 5YR 4/4 4.97 0.9 6.2 nd 2.5 3.2 3.7 89.4 9.6 1.0 Bs2 26-33 7.5YR 5/4 5.02 0.4 4.3 1.7 1.2 1.8 0.9 85.5 13.4 1.1 BC 33 54 7.5YR 6/4 4.91 0.4 4.3 1.6 1.4 2.3 0.4 84.7 14.5 0.9 Bsl' 54-74 7.5YR 5/6 5.23 0.4 5.4 2.8 0.8 1.8 0 90.7 9.0 0.3 Bs2' 74-104 7.5YR 4/6 5.49 0.1 1.9 nd 0.4 0.4 0 95.6 3.8 0.6 BC' 104 117 7.5YR 5/4 5.71 0 1.1 0.6 0.3 0.5 0 97.8 1.7 0.5 C 117-186+ 7.5YR 6/2 5.70 0.1 0.7 0.4 0.2 0.5 0 97.8 1.7 0.5

PIT P E D O N Oe 8~) - - 5.79 nd . . . . A/Oa 0-10 N 2/0 4.83 nd - - - - - - E 10-38 5YR 6/2 4.81 0.5 2.7 0.1 0.1 0.1 1.6 92.1 7.2 0.7 Bhs 38-55 2.5YR 3/4 5.16 0.7 4.2 1.7 1.5 1.2 0.1 94.7 5.3 0 Bsl 55-72 5YR 4/6 5.30 0.3 1.9 0.9 0.7 0.7 0 98.1 1.2 0.6 Bs2 72-94 5YR 5/6 5.52 0.2 2.0 0.1 0.3 0.6 0 98.3 1.3 0.4 Bs3 94-122 5YR 5/4 5.85 0.4 0.8 0.5 0.3 0.5 0 98.5 1.0 0.5 BC 122-135+ 7.5YR 5/4 5.92 0 1.2 0.4 0.2 0.8 0 98.7 0.8 0.5

M O U N D P E D O N A/Oe 0-2 7.5YR 3/0 5.23 8.4 5.3 1.9 1.8 1.5 0 - - - - - - Bsl 2-46 5YR 4/6 5.00 0.5 6.3 2.5 1.6 1.9 0.1 90.2 5.6 4.3 Bs2 46-59 5YR 5/3 5.02 0.3 5.2 1.7 1.8 1.9 0.7 89.2 10.0 0.8 Bs3 59-88 5YR 4/6 5.15 0.3 4.0 nd 0.8 1.4 0.1 nd nd nd BC 88-114 7.5YR 5/4 5.78 0.1 1.6 nd 0.4 0.8 0 98.0 1.5 0.6 C 114-151+ 7.5YR 6/4 6.06 0 0.8 0.5 0.2 0.5 0 98.6 1.3 0.1

* Colors are for moist soil "" Typically, 75-90% of the sands are in the medium and fine size classes nd = not determined

Tab. 2: Selected physical and chemical soil properties of a pit, mound, and an undis- turbed soil.

U n d i s t u r b e d > Pi t > M o u n d (see a lso ( 1 5 - 7 0 + c m ) t o n g u e s o f E h o r i z o n b e l o w

S C H A E T Z L 1987). o v e r t h i c k e n e d O a n d A h o r i z o n s (fig.l).

M o s t pi t soils were c lass i f ied e i the r as U n d i s t u r b e d si tes s h o w t o n g u e i n g less

E n t i c H a p l o r t h o d s or S p o d i c U d i p s a m - f r equen t ly , a n d the t o n g u e s are usua l ly

m e n t s ( tab . l ) . T h e s e soils were m o r e m u c h smal ler . Bhs h o r i z o n s , c o m m o n l y

deep ly l e ached t h a n m o u n d soils, as ev- o b s e r v e d b e l o w E h o r i z o n tongues , a re

i d e n c e d by th i cke r E h o r i z o n s t h a t a lso c o m m o n l y a b s e n t in m a n y (ad jacent )

h a d lower m e a n c o n t e n t s o f Fe, A1, u n d i s t u r b e d soils. U n d i s t u r b e d p e d o n s

a n d o r g a n i c c a r b o n , a n d were gene r - were s t r o n g to i n t e r m e d i a t e wi th r e g a r d

ally h ighe r in c h r o m a ( tabs.2 a n d 3). to m o s t p r o p e r t i e s ; m o s t w e r e i n the Spo-

T h e s e p e d o n s c o m m o n l y exh ib i t d e e p d o s o l o rder .

CATENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY GEOMORPHOLOGY

Treethrow and Soil Genesis 115

Parameter Units Pit Mound Undisturbed

O Thickness cm 12.9 3.6 3.4 E Thickness cm 32.3 2.4 15.0 Depth to top of B ° cm 55.8 8.6 18.5 E Pyrophosphate Fe+A1 g kg i 0.2 0.6 0.5 E Organic Carbon % 0.6 0.8 0.5 E Munsell color** 5YR 5/2 5YR 5/3 5YR 5/2 B Pyrophosphate Fe+AI g kg 1 2.8 3.0 5.6

" Depth from the soil surface to the top of the first pedogenic B horizon ** The most commonly observed Munsell color for the E horizon

T a b . 3: Comparative soils data (mean values)from six trench sites.

Distance (cm) 0 40 80 120 160 200 240 280

0 i I I i ~ L I I I I i i , I i

L

" " O a

120 t - ~ B h s m

160 ~ . BC

200 C - - we

C

240

Fig. 1" Cross section through a pit/mound pair, illustrating deep tongueing of" the E horizon in the pit. Soil horizon clasts are mixed in the center of mound, and do not reflect post-uprooting pedogenesis..

( A T E N A An Interdisciplinary Journal o f S O I L S C I E N C E H Y D R O L O G Y G E O M O R P H O L O G Y

116 Schaetzl

Mound soils ordinarily were Entisols, 4.3 Soil water content and unless they have low relief (gener- ally implying greater age), have thin E Pit soils were wetter than mound or and B horizons. Higher amounts of Fe undisturbed soils during the warm sea- and AI in B horizons of mounds than son, near the surface (fig.2). Mound and of pits (tab.2, 3) were inherited from the undisturbed pedons were drier near the

surface and showed little change in wa- pre-uprooting soil and do not reflect con- temporary pedogenesis, ter content below about 25 cm. Within

lower soil, water conditions were Pit = Undisturbed = Mound.

4.2 Snow cover and soil frost

Snow cover, at near record depths for 4.4 Soil solution analyses much of the winter of 1985 86, especially in November-December, was thinnest on Aluminum concentrations in soil water mounds and thickest above pits. Lay- samples were generally 2 6 times greater ers (2 10 cm thickness) of impermeable, than those of iron (figs.3, 4). Maximum concrete frost (POST & DREIBELBIS concentrations of Fe and A1, as sampled 1942) were observed in the mineral soil during summers, occurred in O/A hori- of large mounds, usually where the snow zons (pits), in E horizons (undisturbed), depth was less than 30 cm. Pit and undis- and in upper B horizons (mounds). Max- turbed soils were unfrozen in 1986. imum concentrations observed during

During the snowmelt period, mounds snowmelt were in the upper B horizon became snow-free first, pits last. Con- for undisturbed and pit soils, and in the crete frost in mounds was retained for E horizons of mounds. at least one week after the overlying A nearly steady increase in pH with snow cover had melted. Snowmelt wa- depth was observed for soil solutions of ters thawed only a thin, surficial layer mound and undisturbed soils, collected of soil in these mounds. Soils at pit and during the summer period (fig.5); pit soil undisturbed sites were unfrozen and per- solutions were the most acidic, pH's of meable during snowmelt in 1986. soil solutions collected during snowmelt

The winter of 1986-87 had near record were less predictable, but pit soils were minimum amounts of snowfall. Frozen commonly the most acidic at this time mineral soil horizons were common be- as well. neath thin snowpacks. By March, 1987, concrete frost had penetrated to >20 cm within mounds. O, A and upper E hori- 5 Discussion zons of undisturbed soils had frozen, but

S C HAETZL & F O L L M E R (n.d.) pro- only the E horizons contained concrete

vided evidence that most mounds in frost. Porous granular frost (POST &

this area are less than 2500 radiocarbon DREIBELBIS 1942) had formed in O and A horizons of undisturbed and pit years old. This finding, in conjunction

other data (tabs.2, 3), implies that soils soils; concrete frost was not observed. in pits have attained equal (sometimes greater) profile development than undis- turbed pedons in a geologically short period of time, and that pedogenic pro-

CAIENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY GEOMORPHOLOGY

Treethrow and Soil Genesis I 17

10-i

2 o . ~,

~ 3 0 - = d

~ 40- - - P i t

M o u n d 5 0 . . . . . . . . . . . . . Und is tu rbed

60--~.

7 0 I i I I I

0 50 100 150 200 250 300 Soil Water (percent by weight)

o 0 . . . . - t - - w - - - " ' " ~ B

lO-

20- J

~ 30- e. .-

~" 40- O I I - - , Pit J' . Mound !

5 0 - l . . . . . . . . . . . . Und is tu rbed

60"

7 0 i I I I I 0 50 100 150 200 250 30~

Soil Water (percent by weight)

Fig. 2: Upper: Soil water contents, determined gravimetrically, on a pit, mound, and undisturbed pedon during a dry period. Lower." Similar soil water contents, determined on a nearby mound/pit pair, 12 hours after a 27 mm rainfall event.

CATENA An Interdisciplinar3, Journal of SOIL SCIENCE HYDROLOGY ( ;EOM(/RPHOLOQ' t

118 Schactzl

A: Spring Snowmelt O,A

\ E

C:: BsmlBhs i J i / / / / / / e 0 N .s ~'~S~S~

"E B s l

0 / S/ t S~' "1"

s/S / / / " /, # / - - Pit

. . . . . M o u n d

BC/C If// / I " / . . . . . . . . . U n d i s t u r b e d

i I I I t 0 1 2 3 4 5 6

AI (mg k -1)

B: Summer O~A ¶ .... .

E I S

~ s f

"E B s h / B s l . ~ . . . . ~ O

sJ S ~ _ ss S ~ . . ~s "~ Pit

/s I _,~,~" . . . . . M o u n d

Bs2./BC e ' / e "~ '~ '~ - . . . . . . . . U n d i s t u r b e d

I I I I I 0 1 2 3 4 5 6

AI (mg L -1)

Fig. 3: Aluminum contents of soil solutions (mean values)for pit, mound and undis- turbed pedons. A. Spring snowmelt. B. Summer values.

C A I ' E N A A n Interdisciplinary Journal of SOIl_ S C I E N C E H Y D R O L O G Y - - G E O M O R P H O L O G Y

Treethrow and Soil Genesis 119

A: Spring Snowmelt

e~

BsmlBhs p ¢- o • ~_ Bsl ! t - - ' - / ° / / / -r

; ]/ . . . . . Mound BC/C ~ o"~ .......... Undisturbed

I I I I F I I 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fe (mg L -1)

O/A ¶,!~ / B: Summer

8 .

"r

/ # /

/ / - - Pit ~ / / / . . . . . M o u n d

. . . . . . . . . . U n d i s t u r b e d Bs2/BC

q I I I I I 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fe (rag L -t)

Fig. 4: Iron contents of soil solutions (mean values)for pit, mound and undisturbed pedons. A. Spring snowmelt. B. Summer values.

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120 Schactzl

0/A A: Spn, '9 qr ' \ "~" "~ '~ "~"~ ° Snowmelt

E

I~" Bsm/Bhs Q, 0

.N Bsl O z ~

I I I I I 4.0 4.5 5.0 5.5 6.0 6.5 7.0

pH

01A o, ~ / , B: Summer

E J " k 7" ]" %,~

Bhs,Bst % T \ ~ \ . Pit

\ \ , Mound \ \ "~ . . . . . . . . . . Undisturbed

\ " \ .

Bs2IBC ke " " ~

I I I I I 4.0 4.5 5.0 5.5 6.0 6.5 7.0

pH

Fig. 5: pH's of soil solutions (mean values)Jbr pit, mound and undisturbed pedons. A. Spring snowmelt values. B. Summer values.

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Treethrow and Soil Genesis 121

cesses in pits may be operating at faster lutions thus remain in contact with the rates than on undisturbed sites. Thus, matrix for long periods of time. Long rates of pedogenesis appear to be: Pit > contact times have led, in most instances, Undisturbed > Mound. to great similarity between soil solution

and solid phase pH for the warm sea- son (tab.2, fig.5), as the solid-solvent

5.1 Pedogenesis during the warm system approaches chemical equilibrium season

(TRUDGILL et al. 1983).

Soils in this region have low water Retention of water in the mor-like contents at many periods throughout litter of overthickened O horizons pro- the warm season, primarily because of motes decay of organic materials within, the low water-holding capacities of the thereby releasing abundant acids. High sandy soils, large amounts of water up- concentrations of such acids could e×- take by the forest, and the high ca- plain summer pH values below 5.5 in pacity of the forest canopy and mor- soil solutions from the upper sola of like forest floor to intercept significant pits (MANLEY et al. 1987, Fig.5B). amounts of precipitation (ANDERSON The acids are probably translocated into et al. 1969, DWYER & MERRIAM lower horizons only during major "flush- 1981, SCHAETZL & ISARD 1989). ing events" (snowmelt and large sum- Whereas deep horizons attain low wa- mer storms) (UGOLINI et al. 1982, ter contents during dry climatic periods STONER 1984). Thick O horizons in at all microtopographic sites (fig.2), wa- pits are thus viewed as organic acid ter contents of O and A horizons dur- "repositories". ing the warm season are: Pit > Undis- O horizons in pits have a greater ca- turbed > Mound, due to high organic pacity to produce organic acids than do matter content that aids in water re- mound or undisturbed O horizons, be- tention (see also BEATTY 1984, and cause the former horizons are thicker BEATTY & STONE 1986). This rela- and do not achieve similar levels of dry- tionship, observed by DWYER & MER- ness. Pit and undisturbed pedons are RIAM (1981) as: Pit >> Level > Mound, highly anisotropic with regard to or- is reflected by the density of fine roots ganic carbon, having abundant organic within the upper sola (data not presented compounds in both A and B horizons, here). Mound soils have relatively few coupled with well-defined eluvial zones roots, whereas the root mat within the (tab.2). Organic carbon amounts within upper sola of pits is often very dense, mound soils, however, are typically very Soil water is retained for longer periods low, and B horizons with illuvial OC are of time in O and organic-rich A hori- not observed (tab.2). These findings are zons (DWYER & MERRIAM 1981). suggestive of slower organic acid pro- Because O horizons are thickest in pits duction and transloeation within mound (tab.2) large amounts of water are re- soils than in pits or on undisturbed sites. tained there. The above findings agree with those of

Between periods of deep wetting, sum- several other studies. DWYER & MER- met soil solutions flow slowly due to RIAM (1981) and SHUBAYEVA & low unsaturated hydraulic conductivities KARPACHEVSKIY (1983) tbund that within sands (HILLEL 1982). Soil so- comparative rates of litter decomposi-

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122 Schaetzl

tion and humus formation were: Pit > profile, many small to moderate precip- Undisturbed > Mound, and attributed itation events probably generate wetting these differences to inequalities in soil fronts that only penetrate the forest floor water and temperature, since soil water or upper solum (ANDERSON et al. is a major factor in the decomposition 1969). Successive (back-to-back) pulses process (DE BOOIS 1974). Both VENE- of water through the soil are most ef- MAN et al. (1984) and IVES et al. (1972) fective at leaching organo-metallic corn- concluded that increased pedogenesis in pounds (STONER 1984); these seldom pits is primarily due to increased volume occur during summer because large rain- and acidity of leaches. The abundance of fall events are isolated, both spatially organic acids was ascribed to decompo- and temporally (SCHAETZL & ISARD sition of thick litter layers. VENEMAN 1989). A second explanation for high Fe et al. (1984) thought that the increases and A1 concentrations in soil solutions in volume of leachate in pits were due to of eluvial horizons may involve release lateral flow within O horizons, whereas of cations by weathering, accompanied IVES et al. (1972) ascribed it to overland by a lack of sufficient organic acids to flow. Overland flow was not observed in complex the ions and render them mo- the Baraga County study area, however, bile. even during torrential rainstorms. Lack Fe and A1 contents of mound soil so- of runoff may be due to highly perme- lutions for summer show a peak in the B able sandy soils and porous litter mats. horizon, with low concentrations in elu-

Infiltration results in translocation vialhorizons (figs.3B, 4B),indicating that of organo-metallic complexes and, ulti- eluviation is keeping pace with release of mately, profle differentiation. Ion con- ions into the soil solution via weather- centrations in soil solutions, being a ing. The paucity of litter cover, the thin- function of additions (via weathering) ness ofeluvial horizons, and comparative minus losses (via translocation), is a scarcity of roots within mounds probably means of estimating leaching intensity, allows more wetting fronts to penetrate High Fe and A1 concentrations in the the B horizons than in undisturbed and upper sola of pit and undisturbed soils pit soils. In the latter, the depth to illu- were observed in summer (figs.3B 4B), vial horizons is greater (tab.3); wetting suggesting that cations in solution may fronts must first penetrate a thick litter not be eluviating from these horizons, or layer containing may roots and an elu- that weathering rates and release of ions vial mineral horizon. Thus, more wetting into solution exceed eluviation, fronts in undisturbed and pit soils proba-

A possible explanation for the lack bly terminate above or within eluvial lay- of eluviation is an inadequate number ers, rendering them ineffective at translo- of deep percolation events, as much of cation of organo-metallic complexes into the rainfall incident on the forest is in- the B horizon. Because mound soils are tercepted by the coniferous canopy and often dry and have relatively high pH's, retained in thick litter layers (WALSH comparatively slow weathering rates are & VOIGT 1977, DWYER & MER- also likely. RIAM 1981, SCHAETZL & ISARD 1989). Whereas infrequent, heavy pre- cipitation events may saturate the entire

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Treethrow and Soil Genesis 123

5.2 Pedogenesis during snowmeit 40 cm of snow (BEATTY 1984). Thus, when mound soils become frost-free dur-

Soil frost has a pronounced effect on ing latter phases of snowmelt, infiltra- saturated flow, and hence pedogenesis, tion cannot occur due to lack of a water during spring snowmelt (BENOIT & source. Pit and undisturbed soils there- BORNSTEIN 1970, PRICE & HEN- fore areleached by meltwater throughout DRIE 1983). Observations indicate that the snowmelt period, whereas mounds the probability of a pedon freezing is: may receive little or no water input. Even Mound > Undisturbed > Pit, because if pit soils did freeze, and the soils be- the two insulating agents, litter and came impermeable, this situation would snow cover (KIENHOLZ 1940, AN- perch water above pit, which could then DERSON 1947), are cumulatively thick- infiltrate into the pit soil at the onset of est above pits and thinnest on mounds melting. (BEATTY 1984). Even if frozen, pit and In winters with deep snow, frozen soil undisturbed soils develop porous, gran- is rare and if present is only found as ular frost layers, probably because of thin, discontinuous layers in mounds. In the inherent, low-density, soil structure, such years, infiltration potential into soils TRIMBLE et al. (1958) found that soil during snowmelt would be: Pit > Undis- with granular frost was actually more turbed > Mound, not because inhibition permeable than unfrozen soil. of infiltration by frost, but due to differ-

During most winters, mound soils ential snow depths above the microtopo- commonly develop layers (0-30 cm) of graphic sites. concrete frost. The formation of con- Repeated freezing of upper horizons crete frost in sandy, otherwise porous in mound soils (BEATTY & STONE soils is not uncommon (HELMREICH 1986) may also act to destroy incipient E & CLARK 1962); surficial freezing ini- horizons and thus retard soil formation tiates upward movement of substantial (GOODLETT 1954). These processes amounts of water via vapor flux, thereby would be insignificant in pits, as the min- adding volume to the frozen crust, filling eral soil rarely freezes to the depth of the in pore spaces, and "freeze-drying" the E horizon. soil below (GARSTKA 1944, ANDER- Experimental evidence suggests that SON 1947). Meltwater cannot infiltrate slow percolation (DAVIES 1971) and through the impermeable, concretely- progressive leaching episodes (STONER frozen surface. Snowmelt waters flow 1984) are more effective at translocation across the top of the frozen soil, however, of mobile complexes from eluvial zones and can subsequently thaw the upper- than are similar amounts of infiltration most few cm (HELMREICH & CLARK that occur at widely disparate intervals. 1962). In most instances, concretely- The former condition is attained dur- frozen soil remains as an aquiclude be- ing the snowmelt period (SCHAETZL low the thawed surface layer, and little & ISARD 1989). Low concentrations of or no vertical infiltration into the center Fe and AI in eluvial horizons of pit and of frozen mounds occurs, undisturbed soils, coupled with high con-

In addition, many mounds become centrations in upper B horizons (figs.3A, snow-free shortly after the snowmelt pe- 4A) suggest that these soils are expe- riod begins, while pits still retain 10- rieneing free leaching and translocation

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124 Schactzl

of organo-metallic complexes into the B Ford Forestry Center, L'Anse, MI, and horizon. The eluvial/illuvial relationship the Depts. of Agronomy and Geography, is best expressed in pit soils (figs.3A, 4A). Univ. of Illinois. Cartographic work was Low pH values in the upper sola of pit supplied by the Center for Cartographic soils (fig.5A), suggest that pit sites are Research and Spatial Analysis, Dept. of also regions of strong organic acid pro- Geography, Michigan State. Comments duction during snowmelt, on earlier drafts of this paper from L.R.

The data above indicate that the Barrett, F.D. Hole, D.L. Mokma, and translocation of organo-metallic corn- anonymous reviewers are greatly appre- plexes from upper horizons (i.e., pod- ciated. zolization) may be more pronounced during snowmelt than the warm sea- References son. BRUCKERT (1970) found that

ALEXANDER, M.J. (1986): Micro-scale soil a t 0°C, where biological degradation is variability along a short moraine ridge at Ok- slow, transport of organo-metallic c o m - stindan, Northern Norway. Geoderma 3% 341-

plexes involving small organic acids was 360. more effective than at warmer tempera- ALLISON, L.E. (1965): Organic carbon. In: C.A. tures . Black (Ed.), Methods of Soil Analysis. American

Society of Agronomy, Agronomy Series Publ Back-to-back pulses of meltwater No. 9. Madison, WI, 1367-1378.

could effectively leach organo-metallic ANDERSON, H.W. (1947): Soil freezing and complexes from E horizons into B hori- thawing as related to some vegetation, climatic, zons of pit and undisturbed soils. I sug- and soil variables. Journal of Forestry 45, 94- gest that this temporal and spatial dis- 101. parity is the main difference among the ANDERSON, R.C., LOUCKS, O.L. & SWAIN,

A.M. (1969): Herbaceous response to canopy genesis of pit, mound, and undisturbed cover, light intensity, and throughfall precipita- soils. Except during the very deep wet- tion in coniferous forests. Ecology 50, 255-263. ting episodes, translocation of organo- ARMSON, K.A. & FESSENDEN, R.J. (1973): metallic complexes in summer is inhib- Forest windthrows and their influence on soil ited by low concentrations of organic morphology. Soil Science Society of America

Proceedings 37, 781-783. acids and high pH's. High concentra- tions of Fe and AI in mound E hori- BEATTY, S.W. (1984): Influence of microtopog-

raphy and canopy species on spatial patterns of zons support the contention that these forest understoryplants. Ecology65, 1406-1419. horizons are minimally leached during BEATTY, S.W. & STONE, E.L. (1986): The va- snowmelt (figs.3A 4A). riety of soil mocrosites created by tree falls.

Canadian Journal of Forest Research 16, 539- 548.

Acknowledgements BENOIT, G.R. & BORNSTEIN, J. (1970): Freezing and thawing effects on drainage. Soil

The following persons provided assis- Science Society of America Proceedings 34, 551 tance and encouragement: R.G. Dar- 557. mody, N. Hanson, G.J.D. Hewings, D.L. BIRKELAND, P.W, (1984): Soils and Geomor- Johnson, S.G. Shetron, T.W. Small, W.M. phology. Oxford, New York, NY. Wendland. The research was supported BRUCKERT, S. (1970): Influence des composes by grants from the Univ. of Illinois, organiques soluble sur la pedogenese en milieu

acide. If. Experiences de laboratoire: modali- Sigma Xi, and Michigan State Univer- ties d'action des agents complexants. Annales sity. Equipment was provided by the Agronomiques 21, 725-757.

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