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The transport of leaves and sediment over a forest floor
CATENA
Vol. 7, 97-110
THE TRANSPDRITIi EEAVES AN A FORESTFISO'OR
Braunschweig 1980
ENT
A CASE STUDY II~TII~ ~;RAND DUCHYcO.F Kt,,'qXEMBOURG
H.J.M. van Zon, Amsterdam SUMMARY It was found that the litter cover in the forest studied is in transport. Movement is initiated mainly by the wind, rainsplash and animal activity and results in a net downslope transport of organic material (leaves, branches, seedbuds, etc.), provided that slopes are sufficiently steep. Downslope movement of litter was still noticeable on slopes ofabout 6B. Mineral soil material is deposited on the surface of the organic debris mainly by splash and is transported along with it. In the experimental catchment the movement of leaves was studied for a period of one year. The methods used to evaluate the effects of this profess are described, as well as the significance of the leaf transport process in relation to total erosion in the catchment. It appeared that in the present case, this process makes a significant contribution to the sediment redistribution within, and to the total sediment production of the catchment. It is responsible for a denudation rate of 1.2 ram/1000 years, out of a total deriudation rate of about 5.0 mm/1000 years. Keywords: leaf transport, Luxembourg, forest ecology 1. INTRODUCTION Fallen leaves or other loose organic debris lying on a forest floor, may be set in motion, particularly by the wind, but also by raindrop impact, trampling etc. (VAN ZON 1978). If the surface is inclined at a certain angle, this organic material is also subject to gravitational forces, which impart a downslope component to the movement. This results in a selective redistribution of organic material over the forest floor and its evacuation from a catchment if it is transported by flowing water after reaching a river channel at the foot of a slope. The transport of organic material has important implications for the geomorphology. As usually some mineral material is attached to the organic matter (leaves, branches, seeds, etc.) it is conceivable that appreciable quantities of material are transported in a downslope direction, to either accumulate at the foot of the slope, or to be transported by the stream. The processes outlined above are the subject of this paper, insofar as they concern the transport of leaves and material attached to them. A search of the literature failed to reveal any published work on the transport of organic material on slopes. Biologists and ecologists have paid considerable attention to the forest floor with its cover of decaying leaves, twigs and branches, to the population of micro- and macro-organisms which enhance the decomposition process, and to the yearly pattern and amount of organic material production (see e.g. OVINGTON 1962, BRAY & GORHAM 1964, DICKINSON & PUGH 1974). The leaf cover does not seem to have been considered, however, as being in transport.
98
VAN ZON
This leaf movement has various aspects: it is extensive in space, relatively continuous in time and obviously directed downslope especially if slopes are steep. The amount of inorganic material thus transported with the leaves may be small compared to the processes of accelerated erosion, but in wooded areas, where erosional processes like splash and occasional overland flow are curbed by the protective litter cover and the usually permeable forest soils, it is relatively important. The movement of leaves can be initiated by several mechanisms, but only the frequently occurring small scale ones will be considered here. Incidental, major events, such as a severe hurricane or trampling by animals occur unevenly distributed in time and space and are therefore difficult to study. Included under the minor and frequently occurring events are those initiated by foces exerted by regular winds, "normal" animal activity and raindrop impact, which are of a more continuous nature. Not every leaf can be set in motion by the mechanisms under consideration: it has to lie loose on the surface, and should thus not be incorporated in the 0 or AI horizon nor buried under for example splashed soil material of animal mounds. Such a transport susceptible leaf will henceforth be referred to as "transportable".
b
X
Jocationof
$
10
15
2C)
25km
the field area
Fig. I: The Grand-Duchy of Luxembourg and the location of the field area
LEAVES AND SEDIMENT TRANSPORT
99
2. APPROACH 2.1. THE EXPERIMENTAL AREA A catchment was chosen as study area, so that sediment transport by leaves could be related to the sediment yield of the area. It is realised than wind can blow leaves across a watershed boundary, but it is assumed that equal amounts of leaves are blown into and out of the catchment in this way. The catchment chosen (fig. 1), located in the Grand Duchy of Luxembourg, includes a wooded valley approx. 8.5 ha in size, and has steep north and south facing slopes cut into the escarpment slope of the Lias cuesta, one ofthe most prominent features of the geomorphology of Luxembourg. The valley has a well-delined stream channel, which is dry most of the year. These dry reentrant valleys are numerous along the escarpment slope. The rest of the catchment consists of relatively Ilat upper areas which are cultivated. The investigations were concentrated on the wooded parts, but the possible transfer ol'eroded material from the cultiwlted land into the Iorest was also studied. Of the hardwood species present in the forest, especially the beech (Fagus sylvatica) is important (fig. 2). The beech is also found in mixed stands with pine (Hnus sylvestris). Some single species stands of spruce (Hcea abies) are present as well. The geology is characterized by the outcrop of two rock formations, the Luxembourg Sandstone and the marls of the Arieten Formation. From fig. 2 it can be seen that the forest is found mainly on the weathering products of the sandstone.
~ ~
---]
youngspruceplantation
~
rnixedforest (beechand pine)
~
beech
recentlyclearedarea
Iml
.....
river
pasture
contour
leaf trap
. . . . . . surfaceboundary Lux.Sar
Fig. 2: Vegetationmap ofthe field area, location ofthe measurement sites and the surface boundary between Luxembourg Sandstone and Arieten lbrmation
1O0 2.2.
-
-
VAN ZON FACTORS INFLUENCING LEAF TRANSPORT
The leaf transport process can be regarded to be the result of two more or less independently operating subprocesses or components: the transport of the leaves themselves and the supply of inorganic material to the leaves. The leaf transport process might be expected to vary with concomittant differentiation in these two components. Therelbre, the variation in the supply of sediment to the leaves, and the wLriation in the movement of the leaves were studied separately. Supply of sediment to leaves. It has been found that inorganic material attached to the leaf surface is mainly transported and deposited onto it as soil particles by splash (VAN ZON 1978). This superficial transport of inorganic compounds may also be effected by zo6genic activity. Especially the excrements of earthworms are often tbund on leaves, but this transport was not very important in the study area, where dry sandy soils prevailed, which were poor in earthworms. The subsequent attachment of soil material either as single grains or as aggregates to the leaves, is greatly enhanced by the presence ofsome clay and organic matter, atthough also pure adhesive forces may cause even sand grains to stick to a leaf. Material detaches rather easily upon wetting. Therefore, the transport of soil material by leaves will take place largely during dry windy periods. Moreover, leaves are usually most transportable when they are in a dry state. Deposition of the transported material on the soil surface occurs when precipitation washes or splashes the material offthe leaves or when the leaves decompose. In the study area, supply is thus generally controlled by the same t~Lctors as those influencing splash erosion, i.e. erodibility, slope angle, erosivity of the precipitation and the presence or absence of a protecting litter cover, thus the bareness of the surface. A closer examination in the field revealed however, thatit was not necessary to consider all of these factors in detail: some of them could be substituted by surrogate variables with which they are interrelated. In the study area these were: vegetation characteristics, aspect, parent material and slope angle (see for a full review of the interrelations: VAN ZON 1978). Movement of leaves. The pattern of movement of each particular leaf may be very complex and difficult to study. For geomorphological purposes it is sufficient to study bulk movement. Considering the bulk, it will be clear that the type of leaf and especially its form is important: a pine needle for example is generally less transportable than a leaf from a deciduous tree; the importance of the supply of leaves is obvious; microclimatological factors will also be related to the transportability through spatial variations in for example moisture conditions, drying speed, wind effects, etc. It appeared not necessary in the study area to consider theeffect ofach of these variables separately because they could be substituted by such terrain factors as vegetation characteristics and aspect. On a steeper slope the transport rate is expected to be higher so that the angle of slope is also thought to be of significance. Not considered are the degree of decay and the Pact that leaves lying at the top of the litter layer probably move faster than those at the bottom.
2.3.
SELECTION OF THE.OBSERVATION SITES The output from, as well as the redistribution and transport of leaves and attached inor-
LEAVES AND SEDIMENT TRANSPORT
101
ganic material within the catchment need to be considered if the significance of the leaf transport process is to be appreciated. The output was registered at the catchment outlet, by measuring the transport through the river and over the valley floor. Whenever the intermittently flowing river was dry, the river bed was sampled to estimate the storage of leaves and their sediment load in the channel. The transport rates within the catchment were more difficult to study. The procedure which was considered to be most simple and efficient, was to divide the wooded parts of the catchment into units which were thought to be unilorm with respect to the leaftransport process, and to measure the transport in each of these units. This subdivision of the area into sampling units can best be based on terrain qualities which relate to differences in the leaf transport. From the above reasoning there were considered to be the lbllowing: vegetation characteristics, aspect, slope angle and parent material. However, a subdivision of the forest area with regard to slope angle would be unmanageable, because this value fluctuates strongly. For this reason the sampling units are defined on the basis of differences in vegetation characteristics (type and development), parent material and aspect, while variations in the leaf transport process caused by dit~ ferences in slope angle within those sampling units is allowed for by recording the process on different slopes within one unit, thus establishing the relation between slope angle and the process.
The sampling units chosen were: beech vegetation and young spruce plantation, both on the north exposed slopes, beech vegetation, mixed forest, spruce vegetation on the south exposed lopes, all on the sandstone, and beech vegetation on the marls. As the latter area is rath6r fiat, it was not subdivided according to aspect. After this subdivision some areas remain, which need to be considered separately. These are: three small gullies in the marl (.5-1.5 m deep and wide, all approximately 30 m long) and the steep (200-40 °) valley incision, locally up to 5 m deep. Moreover, in the incision and the gullies the soil surface is also rather bare, increasing the effectiveness of the sediment transporting processes.
2.4.
METHODS
in the present case the leaf"discharge" was measured with two, one meter wide folded perspex plates, as illustrated in fig. 3. The edge AB of plate I was placed in the litter cover on the forest floor, perpendicular to the steepest slope. The sides of the leaf trap formed in this way were closed with I cm gauge wire mesh, which also served as a support, being partly dug into the ground. On plate 1 line CD was drawn and the leaves which accumulated in the trap behind this line were collected. Plate 11 was placed over plate I (GH on EF) to protect the leaves in the trap from rain which might wash offthe sediment, and to prohibit the collection of falling leaves. In all the sampling units two leaf traps were erected, of which one on a site having an approximately average slope angle value for that unit. In the spruce vegetation on the south exposed slopes on the sandstone and in the beech vegetation on the marls only one trap was placed since these units show little variation in slope angle. Slope angle values for the different measurement sites are given in table 1. In fig. 2 the locations of the measurement sites are indicated. The measurements were performed at sufficient distance from the sampling unit boundaries to prevent marginal influences, The leaf traps in the valley floor, gully and river bed were different from the others, the
102
VAN ZON 14
i=
iiiiiii!ili!iiiiiii!iiiiiiiiiiiii .... ' ' iiiiiliiiiiiiiiiili~::::E
I-----.3 rn-----4
Tab. l:
.................. ............. y
Fig. 3:
A leaf trap (see text Ior explanation)
SLOPE ANGLE VALUES OF THE MEASUREMENT SITES
sampling unit average slope angle of unit slope angle of measurement sites 18"10' 24*30' beech vegetation - north exp. slope 16"30' 21"10' 30"10' beech vegetation - south exp. slope 20"10' 16*50' 27*20' mixed forest - south exp. slope 15"40' 24*50' 28*00' young plantation - north exp. slope 22*50' 11"40' spruce vegetation - south exp. slope 11"20' beech vegetation - plateau Arieten Fm. 8*30'
one on the valley floor extending over its whole width (2.80 m), and the ones spanning the river bed and gully being constructed ot" wire mesh, allowing water to flow through freely while floating leaves were filtered out. To prevent the collection of bedload, the river channel and gully were covered with a thin sheet of artificial resin over a length of 2 metres upstream from the trap, and this area was kept clean of coarse debris. Besides this, the channel and gully were deepened somewhat immediately behind and underneath the trap, so that all small sized bed load material could move freely through it. Larger stones, if present, were carefully removed from the samples. One major drawback of this method of measurement was that during flow some sediment could wash offthe leaves held in these traps, while on the other hand suspended sediment could be trapped between the leaves. There was no way to overcome this problem with the available apparatus. Field observations seemed to indicate that somewhat more sediment was deposited.between the laves in the traps, than was loosened from them by the flowing water, so that the method tends to overestimate the transported sediment quantity. Leaves were collected at four-weekly intervals from April 1976 to May 1977. The treatment of the collected leaves consisted of first freeze-drying the samples, weighing and then oxidizing with H202 (30°/0) on a steambath, until all the organic material became a white pulp. Due to regular stirring practically all mineral material accumulated at the bottom of the beakers containing the samples, except for the fines. After cooling the mineral material was washed out, dried and weighed (for a detailed description of the procedures reference is made to VAN ZON 1978).
LEAVES AND SEDIMENT TRANSPORT
103
3. RESULTS 3.1. TRANSPORT OF LEAVES WITHIN THE CATCHMENT The results of the leaf discharge measurements are listed in table 2. In table 3, the values for the various sampling units are compared. For the whole period, none of the leaf traps caught leaves in amounts significantly different from those collected at the other traps. However, in the mixed forest somewhat lower amounts were collected. It also appears that leaf discharge is poorly corelated with slope angle (rs= .38, p = . 12, n = 10, computed for annual totals at the different sites), and that leaf discharge can even be larger on sites having lower slope angle values. When monthly fluctuations are considered it appears that the pattern of transport is very similar tbr the traps in any sampling unit, though differences between units are in general also small. Under deciduous woodland, the annual transport cycle shows pronounced seasonality, with a maximum between October and December, during and shortly after leaf t~all. Field observations revealed that in winter the litter layer usually was wet, frozen and/or covered with snow, conditions which tend to prevent movement. A second maximum occurs in late spring and early summer, when the litter layer dries out. In late summer fewer leaves are available tbr transport: they have been either already transported or are incorporated in the non-transportable layer. Under coniferous trees the same pattern is generally tbund, although the peaks are not so pronounced. Only in the young spruce plantation very high values are recorded in the spring because ofthe effects of freezing. In April 1977 a process occurred which could be compared with the "Auffrieren", described by SCHMID ( 1955, 11, 1"2 and 38). The needle cover was moistened by some rain during the day with air temperatures of approximately 4°C; freezing (-9°C) during the night caused the tbrmation of cracks due to differential contraction. Slabs of the frozen material were detached from the underlying surface and slid down the steep (approx. 30 °) slope. The slabs consisted mainly of spruce needles and were .5-1.0 cm thick, about as thick as the litter cover at that particular site. The high values of leaf transport in the young plantation being accounted for, leaf transport could not be considered to be significantly different throughout the catchment during the sampling period; it is theretbre assumed to be unilorm.
Tab. 2: LEAF DISCI lARGE IN GRAMS PER MEASUREMENT PERIOD FORTHE DIFFERENT LEAF TRAPS IN TI IE CATCI IMENT leaf l~ap ~.: period I8.05.76-25.06.76 25.06.76-24.07.76 24,07.76-20.08.76 20.08.76-19.09.76 19.09.76-15.10.76 15.10.76-12.11.76 12.11.76-I0.12.76 I0.12.76-07.01.77 07.01.77-03.02.77 03.02.77-04.03.77 04.03.77-31.03.77 31.03.77-24.04.77
2
3
4
5
6
7
8
9
I0
4.5 110.2 3.9 5.1 2148.7 457.8 -
I
66.8 18.9 14.2 16.0 257.0 655.8
73.9
43.3 169.3 53.5 442.7
23.8 26.3 2.3 13.2 49.0 38.7 68.5 4. I II.2 29.5 20.7 82.1
31.I 28.8 8.1 15.0 59.2 44.6 65.7 I I.I ll.l 23.9 33.1 92.3
48.5 25.7 7.6 22.4 74.5 63.6 57.9 I0.9 18.9 31.3 57.7 43.9
19.1 9.7 5.9 6.6 41.9 59,2 90.7 4.6 15.2 15.1 17.4 8.5
6.2 18.9 2.1 ILl 9.0 21.0 15.2 2.3 2.1 4.4 8.3 7.3
19.3 35.2 6.1 19.2 64.6 80.3 55.5 3.0 3.4 9.8 8.1 19.8
20.7 57.9 28.2 34.4 26.6 25.4 82.7 6.4 9.2 13.7 139.5 152.5
13.9 55.7 7.9 26.2 40.3 22.7 49.6 3.0 6.2 7.5 53,6 123.9
I124.2 171.2
II
12
13.0 20.7 4.4 8.9 42.4 28.0 26.6 2.7 3.7 9.4 34.7 9.0
30.0 13.9 II.0 14.9 65,9 39.6 31.3 5.0 12.6 22.2 19.9 37.5
13
14
56.0 66.2 54.9 42.7 92.1 94.9 198.3 24.0 -
113.1 61.5 41.3 55.5 148.6 107.0 94.7 22.6 34.0 54.4 135.0 187.5
107.0 452.1
sitedescription:
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104
VAN Z O N
Tab. 3: RESULTS OF THE KOLMOGOROV-SMIRNOV TWO-SAMPLE TEST FOR COMPARING THE LEAF DISCHARGE WITHIN AND BETWEEN THE SAMPLING UNITS
comparison within the group 1 group 2 leaf trap nr. 3 4 5 6 7 8 9 10
sampling units p nl n2 12 12 12 12
12 12 12 12
.99 .10 •10 .85
comparison between the sampling units* p group 1 group 2 nl n2 sampling unit hr. .89 1 2 24 24 .01 1 3 24 24 1 4 24 24 .99 .21 1 5 24 12 1 6 24 12 .06 2 3 24 24 .14 .17 2 4 24 24 2 5 24 12 .34 2 6 24 12 .00 .01 3 4 24 24 3 5 24 12 .88 3 6 24 12 .07 .34 4 5 24 12 4 6 24 12 .50 5 6 12 12 .25
sampling unit beech vegetation - north exp. slope beech vegetation - south exp. slope mixed forest - south exp. slope young plantation - north exp. slope sampling unit 1 = beech vegetation - north exp. slope (sandstone) 2 = beech vegetation - south exp. slope (sandstone) 3 = mixed forest - south exp. slope (sandstone) 4 = young plantation - north exp. slope (sandstone) 5 = spruce vegetation - south exp. slope (sandstone)
6 = beech vegetation - plateau (marl) * For comparison between the units the values for the traps in one unit have been combined
3.2.
T H E OUTPUT O F LEAVES FROM T H E C A T C H M E N T
The results of the measurements with leaf traps number 1 and 2 (respectively in the river bed and on the valley floor, see table 2) give the output of leaves from the catchment• Unfortunately, high flood runoff seriously damaged those traps in December 1976 and February 1977. The output of the catchment for these periods was estimated as the sum o f the weight of leaves stored in the river bed (since this was thoroughly cleared o f leaves by the flood water) and supply of leaves to the river during these periods. As the leaves in the river bed were sampled every 25 m each month, the storage could be calculated by multiplying the interpolated weight per unit area of the leaves on the river bed with the surface of the river bed. The results are indicated in table 4. The supply o f leaves was calculated separately for the steep river incision and the small fiat valley bottom. In the 207 m long incision in the supply of leaves per meter river length is assumed to equal the value for the leaf discharge obtained for leaf trap number 14 in this incision, increased with the calculated discharge from the gullies. It appeared that in general, leaves practically stop moving towards the channel when they reach the fiat floodplain developed along the downstream reaches o f the river. However, the river course lies directly adjacent to the south exposed slopes for 56 m and to the north exposed slopes for 18 m, so that along these stretches leaves are supplied directly to the river in
LEAVES AND SEDIMENT TRANSPORT
105
notable quantities. The average value for the leaf discharge obtained for the north and south slopes of the catchment are used as a measure ofsupply for those stretches of the river directly along the slopes, but if the river flows in the centre of the valley floor or.if it has otherwise no appreciable slope segment supplying leaves, this leaf supply is considered to be negligible. Total amouns of estimated leaf supply are indicated in table 4. By summation of the estimated and measured values the total "leaf output" from the catchment is estimated as 408 kg for the year of observation. STORAGE OF LEAVES IN THE CATCHMENT RIVER CHANNEL AND THE ESTIMATED OUTPUT OF THE CATCHMENT
Tab. 4:
estimated stored weight of leaves in the river channel date 18.05.76 12.11.76 03.02.77 04.03.77 25.04.77 stored weight (kg) 330.1 303.0 2.6 7.1 49.6 estimated supply of leaves to the river channel (from the sides and gullies) period 12.11.76-10.12.76 10.12.76-07.01.77 03.02.76-04.03.77 supplied weight (kg) 45.2 9.9 24.3 computed output ( = change in storage of leaves + supply) period 12.11.76-07.01.77 03.02.77-04.03.77 computed weight (kg) (303.6-2.6) + (45.2 + 9.9) = 355.5 (2.6-7.1) + 24.3 = 19.8 total output of leaves from the catchment = coml~uted output via river channel + change in storage over the whole period + measured output via river channel + measured output via valley bottom = 375.3 + 27.1 + 4.1 + 1.7 = 408.2 kg 3.3.
TRANSPORT OF SEDIMENT BY LEAVES WITHIN THE SAMPLING UNITS
The amount of sediment collected in the leaf traps is shown in table 5, as well as the amount transported per 10 grams of leaves collected, this latter value facilitating comparison. The leaf traps in any single sampling unit do not show significant differences in the amounts of material collected, but between the units many differences exist (see table 6). Comparison of the seasonal variation in the weight of transported sediment for each period of measurement showed that within the sampling units similar monthly patterns are found, while between the units again more dissimilarities are present. As the movement of leaves itself is rather uniform throughout the area, the differences between the sampling units in the transport of material by leaves will largely be due to differences in the splash erosion process. Sediment transport by leaves is poorly correlated with slope angle ( r s = . 12, p = .45, n = 10), so differences will largely be governed by differences in erodibility, erosivity and bare ground exposure. Indirectly, the subdivision into sampling units is based upon these differences, and from the results it can be concluded that it is effective with regard to the transport of mineral material by leaves. For the individual units the denudation values are listed in table 7. The difference in erosion rates between the young plantation of spruce and the beech vegetation on the noah exposed slopes in the sandstone implies that material originating from the young plantation, which erodes more quickly, must be deposited in the beech woods which are situated lower on the same slopes. The same applies to the beech vegetation on the plateau in the marls, where erosion is more active than on the lower slopes, and where also material is deposited on the units underlain by the sandstone formation below.
106
VAN ZON
Tab. 5: WEIGHT IN GRAMS OF MINERAL SOIL MATERIAL TRANSPORTED BY LEAVES (ROW A) AND THE WEIGHT OF SOIL MATERIAL TRANSPORTED PER 10 GRAMS OF LEAVES (ROW B) leaf'trap nr.:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
18.05.76-25.06.76
.3 .6
18.0 2.7
1.1 .5
.7 .2
.4 .1
.2 .I
.1 .2
.3 .2
4.1 2.0
2.5 1.8
.3 .2
1.6 .5
32.7 5.8
25.0 2.2
A B
period
25.06.76-24.07.76
51.8 4.7
63.7 32.0
8.2 3.1
2.9 1.0
.6 .2
.3 .3
.3 .2
t.0 .3
21.3 3.7
10.8 2.0
.5 .3
13.9 10.0
102.0 15.4
41.8 6.8
A B
24.07.76-20.08.76
.4 1.0
4.5 3.2
.2 .8
.1 .I
.0 .0
.0 .0
.0 .0
.I .I
7.9 2.8
.6 .7
.I .3
1.8 1.6
45.0 8.2
14.2 3.4
A B
20.08.76-19.09.76
.4 .7
18.5 11.6
2.4 1.8
.2 .2
.2 .1
.1 .1
.0 .0
.2 .1
6.4 1.9
3.4 1.3
.0 .0
.8 .5
14.1 3.3
26.3 4.7
A B
19.09.76-15.10.76
18.6 1.4
53.2 2.1
2.7 .6
.3 ,1
.3 .0
.1 .0
.0 .0
.8 .1
2.8 1.1
.6 .1
.1 .0
4.7 .7
23.1 2.5
31.7 2.1
A R
1.5 .1
15.10.76-12.11.76
14.7 .5
.8 .2
.I .0
.2 .0
.1 .0
.0 .0
.1 .0
.9 .4
.5 .2
.0 .0
.8 .2
18.6 2.0
9.9 .9
A B
12.11.76-10.12.76
-
.1 .0
.1 .0
.0 .0
.2 .0
.0 .0
.2 .0
2.1 .3
.6 .1
.2 .I
.2 .1
56.9 2.9
2.5 .3
A B
10.12.76-07.01.77
-
.1 .3
.0 .0
.1 .1
.0 .I
.0 .1
.0 .0
.5 .8
.2 .8
.0 .0
.3 .5
17.6 7.3
2.0 .9
A B
106.9 14.5
.2 .2
.0 .0
.1 .0
.1 .1
.0 .0
.0 .0
.5 .6
.1 .2
.1 .I
.2 .1
-
4.5 1.3
A B
07.01.77-03.02.77
37.3 8.6
03.02.77-04.03.77
224.4 13.3
1.0 .3
.2 .1
.4 .I
.2 .I
.1 .2
.1 .1
1.8 1.3
.9 1.2
.I .2
1.3 .6
7.9 1.5
A B
04.03.77-31.03.77
855.6 7.6
58.1 10.9
.1 .1
.2 .1
.3 .1
,I .1
.1 .1
.1 .1
20.5 1.5
13.2 2.5
.1 .0
.7 .4
61.1 5.7
5.9 .4
A B
31.03.77-24.04.77
431.3 9.7
78.2 4.6
.8 .1
.3 .0
.2 .1
.I .I
.I .1
.1 .1
17,2 1.1
30.6 2.5
.0 .0
4.1 1.1
487.8 10.8
35.2 1.9
A R
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Tab. 6: RESULTS OF TIlE KOLMOROGOV-SMIRNOV T W O - S A M P L E TEST ON THE WEIGI IT OF MINERAL MATERIAL COLLECTED IN THE LEAF TRAPS
group
nl
n2
P
leaf trap nr. 3 4 5 6 7 8 9 10
I
group
12 12 12 12
12 12 12 12
.10 .25 .25 .25
leaf trap: 3 & 4 = beech vegetation - north exposed slope (sandstone)
3+ 4 3+4 3+4 3+4 3+4 5+6 5+6 5+6 5+6 7+ 8 7+ 8 7+ 8 9+10 9 + l0 I1
24 24 24 24 24 24 24 24 24 24 24 24 24 24 12
24 24 24 12 12 24 24 12 12 24 12 12 12 12 12
.07 .01 .00 .04 .12 .14 .00 .21 .00 .00 .98 .00 .00 .34 .00
5&6 =
5+ 6 7+ 8 9 + 10 11 12 7+8 9 + 10 II 12 9 + 10 11 12 I1 12 12
2
beech vegetation - south exposed slope (sandstone) mixed lbrest - south exposed slope (sandstone)
7&8 =
9 +
10
=
young plantation (sandstone)
-
north exposed slope
II =
spruce vegetation - south exposed slope (sandstone)
12 =
beech vegetation - plateau Arielen Fm. (marl)
LEAVES A N D S E D I M E N T TRANSPORT Tab. 7:
107
DEGRADATION RATES RESULTING FROM THE LEAF TRANSPORT PROCESS
sampling unit beech vegetation - north exp. slope beech vegetation - south exp. slope mixed forest - south exp. slope young plantation - north exp. slope spruce vegetation - south exp. slope beech vegetation - plateau Arieten Fro.
3.4. T H E O U T P U T TRANSPORT
OF
SEDIMENT
degratation rate in mm/1000 year .08 .03 .01 .28 .02 .51
FROM
THE
CATCHMENT
BY L E A F
The output of the catchment will consist of material which has reached the river bed. This material will generally come from areas in close proximity to the river, especially from the steep river incision which has very bare sides and is subject to relatively intensive splash erosion: the leaf trap in the river incision collected significantly more mineral material than all other leaf traps (VAN ZON 1978). Other sources of sediment are the gullies, and, in the downstream reaches, the slopes which directly supply leaves to the river. Figures for the sediment output due to the leaf transport process are listed in table 2"(leaf traps no. 1, river bed, and 2, vally floor). For the periods with missing data, the sediment output is estimated in a similar way as the leaf output (see section 3.2.), and the results are shown in table 8. The denudation of the wooded parts of the catchment as computed from the output of sediment via the leaf transport process amounts to 1.2 m m per 1000 years (specific gravity of the soil assumed to be 2.0).
Tab. 8: STORAGE IN THE RIVER CHANNEL OF SEDIMENT ATI'ACHED TO LEAVES, AND THE ESTIMATED SEDIMENT OUTPUT OF THE CATCHMENT DUE TO LEAF TRANSPORT estimated stored weight of sediment in the channel date 18.05.76 12.11.76 03.02.77 stored weight (kg) 211.5 112.2 2.0
04.03.77 3.3
25.04.77 31.2
estimated supply of sediment to the river channel (from the sides and gullies) period 12.11.76-10.12.76 10.12.76-07.01.77 03.02.77-04.03.77 supplied weight (kg) 1.3 .8 3.3 computed output ( = change in storage of sediment + supply) period 12.11.76-07.01.77 computed weight (kg) (112.2-2.0) + (1.3 + .8) = 112.3
03.02.77 -04.03.77 (2.0-3.3) + 3.3 = 2.0
total output of sediment from the catchment = computed output via river channel + change in storage over the whole period + measured output via river channel + measured output via valley bottom = 114.3 + 99.3 + 1.4 +.6 = 215.6 kg
4.
DISCUSSION
M easu rem ent Several unusual problems were encountered in measuring leaf transport. The measurement apparatus used must have a solid structure from which sediment cannot be
108
VAN ZON
splashed or washed in or out and which does not allow the rainwater to pond (amongst other this influences the decay of the leaves). Furthermore the apparatus should not alter the air movements but must be high enough to catch leaves moving at some height above the ground. Unfortunately, the leaf trap inevitably modifies wind influencs, affecting the measurements of leaf transport. To minimize turbulence wire mesh was used for the sides of the trap and its height was kept to a minimum. However, the techniques applied here were not totally satisfactory, and improvements are still needed.
Leaf transport on the catchment
slopes
It appeared that leaf transport is very constant throughout the forest, though it was expected to differ in an areal sense due to varying environmental factors. The reason for this lack of variation must be sought in the influence of the weather, which overrules the influence of terrain factors (slope, aspect, vegetation and parent material) on leaf transport. During and after frost or rain, the leaf layer is non-transportable. Upon drying, first the superficial layer becomes transportable, and as drying proceeds slowly downward more leaves become available for transport. The layer with fragmented and partly decomposed leaves is always non-transpotable, and forms the lower limit of the layer ofpotentially transportable leaves. It must be concluded that weather conditions are much more important than spatial variations in terrain factors. Only aspect will modify weather conditions on a micro-scale, but this is not important enough to cause significant differences in leaf transport, also because the overall influence of wind under dry conditions is strong enough to counteract the effects of microclimatological differences between north and south exposed slopes: the strongest wind comes from the west and the most frequent winds blow from northeasterly and westerly directions in dry periods (see LAHR 1964, FABER 1971) and these do not seem to have a greatly different effect on north and south exposed slopes. The only place where leaf transport deviates from the general uniformity in the valley floor. Because this is fiat and moist and because wind influences are reduced due to its protected position in the deepest part of the valley, leaf transport is low. In this small area, the supply ofleaves (from the valley sides) is consequently higher than the rate of removal. This is reflected by the presence of a black and humic A horizon in the valley-bottom soils, indicating accumulation of organic material. Comparing the denudation values of the sampling units with the figure for the output, it appears that the latter is higher. This has several reasons. One is that material from the slopes is partly redeposited before it reaches the river. A second reason for this is that the potentially transportable leaf layer is buried every year (or continuously in the case of the coniferous vegetation) by freshly fallen leaves so that it becomes largely non-transportable. The sediment which is stored in this leaf layer will be fixed and redeposited. Deposition may also occur after a leaf has come to rest for other reasons, for example because of a sharp decline in slope, gradual reduction of wind influences, burial under a splashed load in the vicinity of bare spots, or because of zo6genic activity (burial of leaves under mole-hills or earthworm excrements, though this was hardly of any importance in the research area where dry sandy soils with a poor soil fauna prevailed). Furthermore, the higher denudation values, calculated from the output, are for a large part the result of higher leaf transport efficiency in the bare and steep valley incision, the gullies and the otherwise bare river banks. In general, the comparatively high output value implies that the valley should become gradually deeper due to the single influence of this transport.
LEAVES AND SEDIMENT TRANSPORT Leaf transport
109
in t h e r i v e r c h a n n e l
The sediment content of the leaf samples from the river channel is sometimes very high. The distinction between on the one hand leaves with sediment attached'to them and on the other hand sediment with incorporated leaves was not as difficult to make as might be suspected. This can be appreciated from the l~actthat 10 grams of fresh beech foliage has a surface area ofsome. 12 to .22 m 2 (BURGER 1925), so that ifa sample of 10 grams o.fleaves contains 20 grams of sediment (total 30 grams), this means that on the average only .009 to .015 grams of sediment is present per square centimter. Another interesting point is the large decrease in the amount of sediment attached to leaves stored in the bed of the intermittently flowing river (for xample between 18/5/1976 and 12/11/1976). This change in storage is more than 1000 times greater than the sediment output by leaves over the same timespan. From field observations it appeared that a lot of sediment is washed offthe leaves as they are lying in the river channel either by flowing water or by rain. This implies that measuring the output of sediment attached to leaves underestimates the amount of material originally transported into the river by them.
Comparison
with other processes
of erosion
Other erosion processes are or have been active in the catchment, and comparing the effect of leaf transport with some contemporary processes reveals the relative importance of the former. Only processes delivering sediment directly to the river and thus also responsible for the output of sediment are considered here. These are gully erosion, scour and transport by the river, zotigenic erosion (mainly acting in the steep valley incision through the action of worms) and splash erosion. It should be kept in mind that the latter process intimately interacts with the leaf transport process. The forementioned processes give rise to a denudation value of.4 ram/1000 yrs for gully erosion and 5.0 mm/1000 yrs for the combined effects of scour, transport by the river, splash and zofgenic erosion (VAN ZON 1978). From the measurements in the catchment it appears that approximately one quarter of the total output of sediment was generated by the laf transport process during the year of observation. Leaf transport can therefore be considered to make an essential contribution to the output of the catchment. As is the case with other processes, a relatively small part ofth catchment, i.e. the area close to the river, produces the bulk of the output. However, this area adjacent to the river does not become exceptionally bare as the year proceeds, which indicates that leaves from more remote places arc also transported towards the river, effectuating a continuous, sheet-like transport in a downslope direction. It can be concluded that in steep forested areas, a considerable part of the eroded material is transported downslope by leaves, and that this process is worth considering if st:aliment production and transport are to be evaluated. It must be stressed that also the output ofoganic material in the form of leaves, an often neglected factor in research concerning sediment production, is of vital importance. Further research on the leaf transport process should at first be directed towards improving the methods of measuring leaf transport, and to evaluating the importance of (micro-)climatological factors.
110
VAN ZON
ACKNOWLEDGEMENTS I would like to thank Dr. A.C. Imeson and Prof. Dr. P.D. Jungerius ofthe Laboratory of Physical Geography and Soil Science, University of Amsterdam, for their comments and constructive criticism on this paper, and Mrs. M.C.G. Keijzer-v.d.Lubbe for typing the manuscript.
REFERENCES BRAY, J.R., GORHAM, E. (1964): Litter production in forests of the world. Adv. in Ecological Res. 2: 101-157. BURGER, H. (1925): Holz-, Laub- und Nadeluntersuchungen. Schweizer Zeitschr. ftir Forstwesen, 266-310. DICKINSON, C.H., PUGH, G.J.F. (eds.)(1974): Biology of plant litter decomposition. Academic Press, two volumes. FABER, IL (1971): Climatologie du Grand-Duch6 de Luxembourg. Publ. du Mus6e d'Histoire naturelle et de la Soci6t6 des Naturalistes Luxembourgeois, Luxembourg, 48 pp. LAHR, E. (1950/1964): Temps et climat au Grand-Duch6 de Luxembourg. Publ. Min. de l'Agriculture, Luxembourg, 575 pp. OVINGTON, J.D. (1962): Quantitative ecology and the woodland ecosystem concept. Adv. in Ecol. Res. I, 103-192. SCHMID, J. (1955): Der Bodenfrost als morphologischer Faktor. HiJthig Verlag Heidelberg, 144 pp. ZON, H.J.M. van (1978): Litter transport as a geomorphic process. Publ. Fysisch-Geografisch en Bodemkundig Lab., Univ. Amsterdam, nr. 24, 136 pp.
Anschrift des Autors: Dr. H.J.M. van Zon, Laboratory for Physical Geography and Soil Science, University of Amsterdam Dapperstraat 115, 1093 BS Amsterdam
Vol. 7, 97-110
THE TRANSPDRITIi EEAVES AN A FORESTFISO'OR
Braunschweig 1980
ENT
A CASE STUDY II~TII~ ~;RAND DUCHYcO.F Kt,,'qXEMBOURG
H.J.M. van Zon, Amsterdam SUMMARY It was found that the litter cover in the forest studied is in transport. Movement is initiated mainly by the wind, rainsplash and animal activity and results in a net downslope transport of organic material (leaves, branches, seedbuds, etc.), provided that slopes are sufficiently steep. Downslope movement of litter was still noticeable on slopes ofabout 6B. Mineral soil material is deposited on the surface of the organic debris mainly by splash and is transported along with it. In the experimental catchment the movement of leaves was studied for a period of one year. The methods used to evaluate the effects of this profess are described, as well as the significance of the leaf transport process in relation to total erosion in the catchment. It appeared that in the present case, this process makes a significant contribution to the sediment redistribution within, and to the total sediment production of the catchment. It is responsible for a denudation rate of 1.2 ram/1000 years, out of a total deriudation rate of about 5.0 mm/1000 years. Keywords: leaf transport, Luxembourg, forest ecology 1. INTRODUCTION Fallen leaves or other loose organic debris lying on a forest floor, may be set in motion, particularly by the wind, but also by raindrop impact, trampling etc. (VAN ZON 1978). If the surface is inclined at a certain angle, this organic material is also subject to gravitational forces, which impart a downslope component to the movement. This results in a selective redistribution of organic material over the forest floor and its evacuation from a catchment if it is transported by flowing water after reaching a river channel at the foot of a slope. The transport of organic material has important implications for the geomorphology. As usually some mineral material is attached to the organic matter (leaves, branches, seeds, etc.) it is conceivable that appreciable quantities of material are transported in a downslope direction, to either accumulate at the foot of the slope, or to be transported by the stream. The processes outlined above are the subject of this paper, insofar as they concern the transport of leaves and material attached to them. A search of the literature failed to reveal any published work on the transport of organic material on slopes. Biologists and ecologists have paid considerable attention to the forest floor with its cover of decaying leaves, twigs and branches, to the population of micro- and macro-organisms which enhance the decomposition process, and to the yearly pattern and amount of organic material production (see e.g. OVINGTON 1962, BRAY & GORHAM 1964, DICKINSON & PUGH 1974). The leaf cover does not seem to have been considered, however, as being in transport.
98
VAN ZON
This leaf movement has various aspects: it is extensive in space, relatively continuous in time and obviously directed downslope especially if slopes are steep. The amount of inorganic material thus transported with the leaves may be small compared to the processes of accelerated erosion, but in wooded areas, where erosional processes like splash and occasional overland flow are curbed by the protective litter cover and the usually permeable forest soils, it is relatively important. The movement of leaves can be initiated by several mechanisms, but only the frequently occurring small scale ones will be considered here. Incidental, major events, such as a severe hurricane or trampling by animals occur unevenly distributed in time and space and are therefore difficult to study. Included under the minor and frequently occurring events are those initiated by foces exerted by regular winds, "normal" animal activity and raindrop impact, which are of a more continuous nature. Not every leaf can be set in motion by the mechanisms under consideration: it has to lie loose on the surface, and should thus not be incorporated in the 0 or AI horizon nor buried under for example splashed soil material of animal mounds. Such a transport susceptible leaf will henceforth be referred to as "transportable".
b
X
Jocationof
$
10
15
2C)
25km
the field area
Fig. I: The Grand-Duchy of Luxembourg and the location of the field area
LEAVES AND SEDIMENT TRANSPORT
99
2. APPROACH 2.1. THE EXPERIMENTAL AREA A catchment was chosen as study area, so that sediment transport by leaves could be related to the sediment yield of the area. It is realised than wind can blow leaves across a watershed boundary, but it is assumed that equal amounts of leaves are blown into and out of the catchment in this way. The catchment chosen (fig. 1), located in the Grand Duchy of Luxembourg, includes a wooded valley approx. 8.5 ha in size, and has steep north and south facing slopes cut into the escarpment slope of the Lias cuesta, one ofthe most prominent features of the geomorphology of Luxembourg. The valley has a well-delined stream channel, which is dry most of the year. These dry reentrant valleys are numerous along the escarpment slope. The rest of the catchment consists of relatively Ilat upper areas which are cultivated. The investigations were concentrated on the wooded parts, but the possible transfer ol'eroded material from the cultiwlted land into the Iorest was also studied. Of the hardwood species present in the forest, especially the beech (Fagus sylvatica) is important (fig. 2). The beech is also found in mixed stands with pine (Hnus sylvestris). Some single species stands of spruce (Hcea abies) are present as well. The geology is characterized by the outcrop of two rock formations, the Luxembourg Sandstone and the marls of the Arieten Formation. From fig. 2 it can be seen that the forest is found mainly on the weathering products of the sandstone.
~ ~
---]
youngspruceplantation
~
rnixedforest (beechand pine)
~
beech
recentlyclearedarea
Iml
.....
river
pasture
contour
leaf trap
. . . . . . surfaceboundary Lux.Sar
Fig. 2: Vegetationmap ofthe field area, location ofthe measurement sites and the surface boundary between Luxembourg Sandstone and Arieten lbrmation
1O0 2.2.
-
-
VAN ZON FACTORS INFLUENCING LEAF TRANSPORT
The leaf transport process can be regarded to be the result of two more or less independently operating subprocesses or components: the transport of the leaves themselves and the supply of inorganic material to the leaves. The leaf transport process might be expected to vary with concomittant differentiation in these two components. Therelbre, the variation in the supply of sediment to the leaves, and the wLriation in the movement of the leaves were studied separately. Supply of sediment to leaves. It has been found that inorganic material attached to the leaf surface is mainly transported and deposited onto it as soil particles by splash (VAN ZON 1978). This superficial transport of inorganic compounds may also be effected by zo6genic activity. Especially the excrements of earthworms are often tbund on leaves, but this transport was not very important in the study area, where dry sandy soils prevailed, which were poor in earthworms. The subsequent attachment of soil material either as single grains or as aggregates to the leaves, is greatly enhanced by the presence ofsome clay and organic matter, atthough also pure adhesive forces may cause even sand grains to stick to a leaf. Material detaches rather easily upon wetting. Therefore, the transport of soil material by leaves will take place largely during dry windy periods. Moreover, leaves are usually most transportable when they are in a dry state. Deposition of the transported material on the soil surface occurs when precipitation washes or splashes the material offthe leaves or when the leaves decompose. In the study area, supply is thus generally controlled by the same t~Lctors as those influencing splash erosion, i.e. erodibility, slope angle, erosivity of the precipitation and the presence or absence of a protecting litter cover, thus the bareness of the surface. A closer examination in the field revealed however, thatit was not necessary to consider all of these factors in detail: some of them could be substituted by surrogate variables with which they are interrelated. In the study area these were: vegetation characteristics, aspect, parent material and slope angle (see for a full review of the interrelations: VAN ZON 1978). Movement of leaves. The pattern of movement of each particular leaf may be very complex and difficult to study. For geomorphological purposes it is sufficient to study bulk movement. Considering the bulk, it will be clear that the type of leaf and especially its form is important: a pine needle for example is generally less transportable than a leaf from a deciduous tree; the importance of the supply of leaves is obvious; microclimatological factors will also be related to the transportability through spatial variations in for example moisture conditions, drying speed, wind effects, etc. It appeared not necessary in the study area to consider theeffect ofach of these variables separately because they could be substituted by such terrain factors as vegetation characteristics and aspect. On a steeper slope the transport rate is expected to be higher so that the angle of slope is also thought to be of significance. Not considered are the degree of decay and the Pact that leaves lying at the top of the litter layer probably move faster than those at the bottom.
2.3.
SELECTION OF THE.OBSERVATION SITES The output from, as well as the redistribution and transport of leaves and attached inor-
LEAVES AND SEDIMENT TRANSPORT
101
ganic material within the catchment need to be considered if the significance of the leaf transport process is to be appreciated. The output was registered at the catchment outlet, by measuring the transport through the river and over the valley floor. Whenever the intermittently flowing river was dry, the river bed was sampled to estimate the storage of leaves and their sediment load in the channel. The transport rates within the catchment were more difficult to study. The procedure which was considered to be most simple and efficient, was to divide the wooded parts of the catchment into units which were thought to be unilorm with respect to the leaftransport process, and to measure the transport in each of these units. This subdivision of the area into sampling units can best be based on terrain qualities which relate to differences in the leaf transport. From the above reasoning there were considered to be the lbllowing: vegetation characteristics, aspect, slope angle and parent material. However, a subdivision of the forest area with regard to slope angle would be unmanageable, because this value fluctuates strongly. For this reason the sampling units are defined on the basis of differences in vegetation characteristics (type and development), parent material and aspect, while variations in the leaf transport process caused by dit~ ferences in slope angle within those sampling units is allowed for by recording the process on different slopes within one unit, thus establishing the relation between slope angle and the process.
The sampling units chosen were: beech vegetation and young spruce plantation, both on the north exposed slopes, beech vegetation, mixed forest, spruce vegetation on the south exposed lopes, all on the sandstone, and beech vegetation on the marls. As the latter area is rath6r fiat, it was not subdivided according to aspect. After this subdivision some areas remain, which need to be considered separately. These are: three small gullies in the marl (.5-1.5 m deep and wide, all approximately 30 m long) and the steep (200-40 °) valley incision, locally up to 5 m deep. Moreover, in the incision and the gullies the soil surface is also rather bare, increasing the effectiveness of the sediment transporting processes.
2.4.
METHODS
in the present case the leaf"discharge" was measured with two, one meter wide folded perspex plates, as illustrated in fig. 3. The edge AB of plate I was placed in the litter cover on the forest floor, perpendicular to the steepest slope. The sides of the leaf trap formed in this way were closed with I cm gauge wire mesh, which also served as a support, being partly dug into the ground. On plate 1 line CD was drawn and the leaves which accumulated in the trap behind this line were collected. Plate 11 was placed over plate I (GH on EF) to protect the leaves in the trap from rain which might wash offthe sediment, and to prohibit the collection of falling leaves. In all the sampling units two leaf traps were erected, of which one on a site having an approximately average slope angle value for that unit. In the spruce vegetation on the south exposed slopes on the sandstone and in the beech vegetation on the marls only one trap was placed since these units show little variation in slope angle. Slope angle values for the different measurement sites are given in table 1. In fig. 2 the locations of the measurement sites are indicated. The measurements were performed at sufficient distance from the sampling unit boundaries to prevent marginal influences, The leaf traps in the valley floor, gully and river bed were different from the others, the
102
VAN ZON 14
i=
iiiiiii!ili!iiiiiii!iiiiiiiiiiiii .... ' ' iiiiiliiiiiiiiiiili~::::E
I-----.3 rn-----4
Tab. l:
.................. ............. y
Fig. 3:
A leaf trap (see text Ior explanation)
SLOPE ANGLE VALUES OF THE MEASUREMENT SITES
sampling unit average slope angle of unit slope angle of measurement sites 18"10' 24*30' beech vegetation - north exp. slope 16"30' 21"10' 30"10' beech vegetation - south exp. slope 20"10' 16*50' 27*20' mixed forest - south exp. slope 15"40' 24*50' 28*00' young plantation - north exp. slope 22*50' 11"40' spruce vegetation - south exp. slope 11"20' beech vegetation - plateau Arieten Fm. 8*30'
one on the valley floor extending over its whole width (2.80 m), and the ones spanning the river bed and gully being constructed ot" wire mesh, allowing water to flow through freely while floating leaves were filtered out. To prevent the collection of bedload, the river channel and gully were covered with a thin sheet of artificial resin over a length of 2 metres upstream from the trap, and this area was kept clean of coarse debris. Besides this, the channel and gully were deepened somewhat immediately behind and underneath the trap, so that all small sized bed load material could move freely through it. Larger stones, if present, were carefully removed from the samples. One major drawback of this method of measurement was that during flow some sediment could wash offthe leaves held in these traps, while on the other hand suspended sediment could be trapped between the leaves. There was no way to overcome this problem with the available apparatus. Field observations seemed to indicate that somewhat more sediment was deposited.between the laves in the traps, than was loosened from them by the flowing water, so that the method tends to overestimate the transported sediment quantity. Leaves were collected at four-weekly intervals from April 1976 to May 1977. The treatment of the collected leaves consisted of first freeze-drying the samples, weighing and then oxidizing with H202 (30°/0) on a steambath, until all the organic material became a white pulp. Due to regular stirring practically all mineral material accumulated at the bottom of the beakers containing the samples, except for the fines. After cooling the mineral material was washed out, dried and weighed (for a detailed description of the procedures reference is made to VAN ZON 1978).
LEAVES AND SEDIMENT TRANSPORT
103
3. RESULTS 3.1. TRANSPORT OF LEAVES WITHIN THE CATCHMENT The results of the leaf discharge measurements are listed in table 2. In table 3, the values for the various sampling units are compared. For the whole period, none of the leaf traps caught leaves in amounts significantly different from those collected at the other traps. However, in the mixed forest somewhat lower amounts were collected. It also appears that leaf discharge is poorly corelated with slope angle (rs= .38, p = . 12, n = 10, computed for annual totals at the different sites), and that leaf discharge can even be larger on sites having lower slope angle values. When monthly fluctuations are considered it appears that the pattern of transport is very similar tbr the traps in any sampling unit, though differences between units are in general also small. Under deciduous woodland, the annual transport cycle shows pronounced seasonality, with a maximum between October and December, during and shortly after leaf t~all. Field observations revealed that in winter the litter layer usually was wet, frozen and/or covered with snow, conditions which tend to prevent movement. A second maximum occurs in late spring and early summer, when the litter layer dries out. In late summer fewer leaves are available tbr transport: they have been either already transported or are incorporated in the non-transportable layer. Under coniferous trees the same pattern is generally tbund, although the peaks are not so pronounced. Only in the young spruce plantation very high values are recorded in the spring because ofthe effects of freezing. In April 1977 a process occurred which could be compared with the "Auffrieren", described by SCHMID ( 1955, 11, 1"2 and 38). The needle cover was moistened by some rain during the day with air temperatures of approximately 4°C; freezing (-9°C) during the night caused the tbrmation of cracks due to differential contraction. Slabs of the frozen material were detached from the underlying surface and slid down the steep (approx. 30 °) slope. The slabs consisted mainly of spruce needles and were .5-1.0 cm thick, about as thick as the litter cover at that particular site. The high values of leaf transport in the young plantation being accounted for, leaf transport could not be considered to be significantly different throughout the catchment during the sampling period; it is theretbre assumed to be unilorm.
Tab. 2: LEAF DISCI lARGE IN GRAMS PER MEASUREMENT PERIOD FORTHE DIFFERENT LEAF TRAPS IN TI IE CATCI IMENT leaf l~ap ~.: period I8.05.76-25.06.76 25.06.76-24.07.76 24,07.76-20.08.76 20.08.76-19.09.76 19.09.76-15.10.76 15.10.76-12.11.76 12.11.76-I0.12.76 I0.12.76-07.01.77 07.01.77-03.02.77 03.02.77-04.03.77 04.03.77-31.03.77 31.03.77-24.04.77
2
3
4
5
6
7
8
9
I0
4.5 110.2 3.9 5.1 2148.7 457.8 -
I
66.8 18.9 14.2 16.0 257.0 655.8
73.9
43.3 169.3 53.5 442.7
23.8 26.3 2.3 13.2 49.0 38.7 68.5 4. I II.2 29.5 20.7 82.1
31.I 28.8 8.1 15.0 59.2 44.6 65.7 I I.I ll.l 23.9 33.1 92.3
48.5 25.7 7.6 22.4 74.5 63.6 57.9 I0.9 18.9 31.3 57.7 43.9
19.1 9.7 5.9 6.6 41.9 59,2 90.7 4.6 15.2 15.1 17.4 8.5
6.2 18.9 2.1 ILl 9.0 21.0 15.2 2.3 2.1 4.4 8.3 7.3
19.3 35.2 6.1 19.2 64.6 80.3 55.5 3.0 3.4 9.8 8.1 19.8
20.7 57.9 28.2 34.4 26.6 25.4 82.7 6.4 9.2 13.7 139.5 152.5
13.9 55.7 7.9 26.2 40.3 22.7 49.6 3.0 6.2 7.5 53,6 123.9
I124.2 171.2
II
12
13.0 20.7 4.4 8.9 42.4 28.0 26.6 2.7 3.7 9.4 34.7 9.0
30.0 13.9 II.0 14.9 65,9 39.6 31.3 5.0 12.6 22.2 19.9 37.5
13
14
56.0 66.2 54.9 42.7 92.1 94.9 198.3 24.0 -
113.1 61.5 41.3 55.5 148.6 107.0 94.7 22.6 34.0 54.4 135.0 187.5
107.0 452.1
sitedescription:
-
~
. . . . . . ~ . . E. -~
~
~
~-
~-
"g
~
~
~
~
~
~
E~
~'~
~
~
~ .
104
VAN Z O N
Tab. 3: RESULTS OF THE KOLMOGOROV-SMIRNOV TWO-SAMPLE TEST FOR COMPARING THE LEAF DISCHARGE WITHIN AND BETWEEN THE SAMPLING UNITS
comparison within the group 1 group 2 leaf trap nr. 3 4 5 6 7 8 9 10
sampling units p nl n2 12 12 12 12
12 12 12 12
.99 .10 •10 .85
comparison between the sampling units* p group 1 group 2 nl n2 sampling unit hr. .89 1 2 24 24 .01 1 3 24 24 1 4 24 24 .99 .21 1 5 24 12 1 6 24 12 .06 2 3 24 24 .14 .17 2 4 24 24 2 5 24 12 .34 2 6 24 12 .00 .01 3 4 24 24 3 5 24 12 .88 3 6 24 12 .07 .34 4 5 24 12 4 6 24 12 .50 5 6 12 12 .25
sampling unit beech vegetation - north exp. slope beech vegetation - south exp. slope mixed forest - south exp. slope young plantation - north exp. slope sampling unit 1 = beech vegetation - north exp. slope (sandstone) 2 = beech vegetation - south exp. slope (sandstone) 3 = mixed forest - south exp. slope (sandstone) 4 = young plantation - north exp. slope (sandstone) 5 = spruce vegetation - south exp. slope (sandstone)
6 = beech vegetation - plateau (marl) * For comparison between the units the values for the traps in one unit have been combined
3.2.
T H E OUTPUT O F LEAVES FROM T H E C A T C H M E N T
The results of the measurements with leaf traps number 1 and 2 (respectively in the river bed and on the valley floor, see table 2) give the output of leaves from the catchment• Unfortunately, high flood runoff seriously damaged those traps in December 1976 and February 1977. The output of the catchment for these periods was estimated as the sum o f the weight of leaves stored in the river bed (since this was thoroughly cleared o f leaves by the flood water) and supply of leaves to the river during these periods. As the leaves in the river bed were sampled every 25 m each month, the storage could be calculated by multiplying the interpolated weight per unit area of the leaves on the river bed with the surface of the river bed. The results are indicated in table 4. The supply o f leaves was calculated separately for the steep river incision and the small fiat valley bottom. In the 207 m long incision in the supply of leaves per meter river length is assumed to equal the value for the leaf discharge obtained for leaf trap number 14 in this incision, increased with the calculated discharge from the gullies. It appeared that in general, leaves practically stop moving towards the channel when they reach the fiat floodplain developed along the downstream reaches o f the river. However, the river course lies directly adjacent to the south exposed slopes for 56 m and to the north exposed slopes for 18 m, so that along these stretches leaves are supplied directly to the river in
LEAVES AND SEDIMENT TRANSPORT
105
notable quantities. The average value for the leaf discharge obtained for the north and south slopes of the catchment are used as a measure ofsupply for those stretches of the river directly along the slopes, but if the river flows in the centre of the valley floor or.if it has otherwise no appreciable slope segment supplying leaves, this leaf supply is considered to be negligible. Total amouns of estimated leaf supply are indicated in table 4. By summation of the estimated and measured values the total "leaf output" from the catchment is estimated as 408 kg for the year of observation. STORAGE OF LEAVES IN THE CATCHMENT RIVER CHANNEL AND THE ESTIMATED OUTPUT OF THE CATCHMENT
Tab. 4:
estimated stored weight of leaves in the river channel date 18.05.76 12.11.76 03.02.77 04.03.77 25.04.77 stored weight (kg) 330.1 303.0 2.6 7.1 49.6 estimated supply of leaves to the river channel (from the sides and gullies) period 12.11.76-10.12.76 10.12.76-07.01.77 03.02.76-04.03.77 supplied weight (kg) 45.2 9.9 24.3 computed output ( = change in storage of leaves + supply) period 12.11.76-07.01.77 03.02.77-04.03.77 computed weight (kg) (303.6-2.6) + (45.2 + 9.9) = 355.5 (2.6-7.1) + 24.3 = 19.8 total output of leaves from the catchment = coml~uted output via river channel + change in storage over the whole period + measured output via river channel + measured output via valley bottom = 375.3 + 27.1 + 4.1 + 1.7 = 408.2 kg 3.3.
TRANSPORT OF SEDIMENT BY LEAVES WITHIN THE SAMPLING UNITS
The amount of sediment collected in the leaf traps is shown in table 5, as well as the amount transported per 10 grams of leaves collected, this latter value facilitating comparison. The leaf traps in any single sampling unit do not show significant differences in the amounts of material collected, but between the units many differences exist (see table 6). Comparison of the seasonal variation in the weight of transported sediment for each period of measurement showed that within the sampling units similar monthly patterns are found, while between the units again more dissimilarities are present. As the movement of leaves itself is rather uniform throughout the area, the differences between the sampling units in the transport of material by leaves will largely be due to differences in the splash erosion process. Sediment transport by leaves is poorly correlated with slope angle ( r s = . 12, p = .45, n = 10), so differences will largely be governed by differences in erodibility, erosivity and bare ground exposure. Indirectly, the subdivision into sampling units is based upon these differences, and from the results it can be concluded that it is effective with regard to the transport of mineral material by leaves. For the individual units the denudation values are listed in table 7. The difference in erosion rates between the young plantation of spruce and the beech vegetation on the noah exposed slopes in the sandstone implies that material originating from the young plantation, which erodes more quickly, must be deposited in the beech woods which are situated lower on the same slopes. The same applies to the beech vegetation on the plateau in the marls, where erosion is more active than on the lower slopes, and where also material is deposited on the units underlain by the sandstone formation below.
106
VAN ZON
Tab. 5: WEIGHT IN GRAMS OF MINERAL SOIL MATERIAL TRANSPORTED BY LEAVES (ROW A) AND THE WEIGHT OF SOIL MATERIAL TRANSPORTED PER 10 GRAMS OF LEAVES (ROW B) leaf'trap nr.:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
18.05.76-25.06.76
.3 .6
18.0 2.7
1.1 .5
.7 .2
.4 .1
.2 .I
.1 .2
.3 .2
4.1 2.0
2.5 1.8
.3 .2
1.6 .5
32.7 5.8
25.0 2.2
A B
period
25.06.76-24.07.76
51.8 4.7
63.7 32.0
8.2 3.1
2.9 1.0
.6 .2
.3 .3
.3 .2
t.0 .3
21.3 3.7
10.8 2.0
.5 .3
13.9 10.0
102.0 15.4
41.8 6.8
A B
24.07.76-20.08.76
.4 1.0
4.5 3.2
.2 .8
.1 .I
.0 .0
.0 .0
.0 .0
.I .I
7.9 2.8
.6 .7
.I .3
1.8 1.6
45.0 8.2
14.2 3.4
A B
20.08.76-19.09.76
.4 .7
18.5 11.6
2.4 1.8
.2 .2
.2 .1
.1 .1
.0 .0
.2 .1
6.4 1.9
3.4 1.3
.0 .0
.8 .5
14.1 3.3
26.3 4.7
A B
19.09.76-15.10.76
18.6 1.4
53.2 2.1
2.7 .6
.3 ,1
.3 .0
.1 .0
.0 .0
.8 .1
2.8 1.1
.6 .1
.1 .0
4.7 .7
23.1 2.5
31.7 2.1
A R
1.5 .1
15.10.76-12.11.76
14.7 .5
.8 .2
.I .0
.2 .0
.1 .0
.0 .0
.1 .0
.9 .4
.5 .2
.0 .0
.8 .2
18.6 2.0
9.9 .9
A B
12.11.76-10.12.76
-
.1 .0
.1 .0
.0 .0
.2 .0
.0 .0
.2 .0
2.1 .3
.6 .1
.2 .I
.2 .1
56.9 2.9
2.5 .3
A B
10.12.76-07.01.77
-
.1 .3
.0 .0
.1 .1
.0 .I
.0 .1
.0 .0
.5 .8
.2 .8
.0 .0
.3 .5
17.6 7.3
2.0 .9
A B
106.9 14.5
.2 .2
.0 .0
.1 .0
.1 .1
.0 .0
.0 .0
.5 .6
.1 .2
.1 .I
.2 .1
-
4.5 1.3
A B
07.01.77-03.02.77
37.3 8.6
03.02.77-04.03.77
224.4 13.3
1.0 .3
.2 .1
.4 .I
.2 .I
.1 .2
.1 .1
1.8 1.3
.9 1.2
.I .2
1.3 .6
7.9 1.5
A B
04.03.77-31.03.77
855.6 7.6
58.1 10.9
.1 .1
.2 .1
.3 .1
,I .1
.1 .1
.1 .1
20.5 1.5
13.2 2.5
.1 .0
.7 .4
61.1 5.7
5.9 .4
A B
31.03.77-24.04.77
431.3 9.7
78.2 4.6
.8 .1
.3 .0
.2 .1
.I .I
.I .1
.1 .1
17,2 1.1
30.6 2.5
.0 .0
4.1 1.1
487.8 10.8
35.2 1.9
A R
=-=.
=-=.
=-=.
=-g
site
-
description:
-
=,. ~-
=-~
_.=-
¢D.<
xr0
==
-
......
,_-_=.
=,
~.g.
~g.
=
"-~
=o~=
Tab. 6: RESULTS OF TIlE KOLMOROGOV-SMIRNOV T W O - S A M P L E TEST ON THE WEIGI IT OF MINERAL MATERIAL COLLECTED IN THE LEAF TRAPS
group
nl
n2
P
leaf trap nr. 3 4 5 6 7 8 9 10
I
group
12 12 12 12
12 12 12 12
.10 .25 .25 .25
leaf trap: 3 & 4 = beech vegetation - north exposed slope (sandstone)
3+ 4 3+4 3+4 3+4 3+4 5+6 5+6 5+6 5+6 7+ 8 7+ 8 7+ 8 9+10 9 + l0 I1
24 24 24 24 24 24 24 24 24 24 24 24 24 24 12
24 24 24 12 12 24 24 12 12 24 12 12 12 12 12
.07 .01 .00 .04 .12 .14 .00 .21 .00 .00 .98 .00 .00 .34 .00
5&6 =
5+ 6 7+ 8 9 + 10 11 12 7+8 9 + 10 II 12 9 + 10 11 12 I1 12 12
2
beech vegetation - south exposed slope (sandstone) mixed lbrest - south exposed slope (sandstone)
7&8 =
9 +
10
=
young plantation (sandstone)
-
north exposed slope
II =
spruce vegetation - south exposed slope (sandstone)
12 =
beech vegetation - plateau Arielen Fm. (marl)
LEAVES A N D S E D I M E N T TRANSPORT Tab. 7:
107
DEGRADATION RATES RESULTING FROM THE LEAF TRANSPORT PROCESS
sampling unit beech vegetation - north exp. slope beech vegetation - south exp. slope mixed forest - south exp. slope young plantation - north exp. slope spruce vegetation - south exp. slope beech vegetation - plateau Arieten Fro.
3.4. T H E O U T P U T TRANSPORT
OF
SEDIMENT
degratation rate in mm/1000 year .08 .03 .01 .28 .02 .51
FROM
THE
CATCHMENT
BY L E A F
The output of the catchment will consist of material which has reached the river bed. This material will generally come from areas in close proximity to the river, especially from the steep river incision which has very bare sides and is subject to relatively intensive splash erosion: the leaf trap in the river incision collected significantly more mineral material than all other leaf traps (VAN ZON 1978). Other sources of sediment are the gullies, and, in the downstream reaches, the slopes which directly supply leaves to the river. Figures for the sediment output due to the leaf transport process are listed in table 2"(leaf traps no. 1, river bed, and 2, vally floor). For the periods with missing data, the sediment output is estimated in a similar way as the leaf output (see section 3.2.), and the results are shown in table 8. The denudation of the wooded parts of the catchment as computed from the output of sediment via the leaf transport process amounts to 1.2 m m per 1000 years (specific gravity of the soil assumed to be 2.0).
Tab. 8: STORAGE IN THE RIVER CHANNEL OF SEDIMENT ATI'ACHED TO LEAVES, AND THE ESTIMATED SEDIMENT OUTPUT OF THE CATCHMENT DUE TO LEAF TRANSPORT estimated stored weight of sediment in the channel date 18.05.76 12.11.76 03.02.77 stored weight (kg) 211.5 112.2 2.0
04.03.77 3.3
25.04.77 31.2
estimated supply of sediment to the river channel (from the sides and gullies) period 12.11.76-10.12.76 10.12.76-07.01.77 03.02.77-04.03.77 supplied weight (kg) 1.3 .8 3.3 computed output ( = change in storage of sediment + supply) period 12.11.76-07.01.77 computed weight (kg) (112.2-2.0) + (1.3 + .8) = 112.3
03.02.77 -04.03.77 (2.0-3.3) + 3.3 = 2.0
total output of sediment from the catchment = computed output via river channel + change in storage over the whole period + measured output via river channel + measured output via valley bottom = 114.3 + 99.3 + 1.4 +.6 = 215.6 kg
4.
DISCUSSION
M easu rem ent Several unusual problems were encountered in measuring leaf transport. The measurement apparatus used must have a solid structure from which sediment cannot be
108
VAN ZON
splashed or washed in or out and which does not allow the rainwater to pond (amongst other this influences the decay of the leaves). Furthermore the apparatus should not alter the air movements but must be high enough to catch leaves moving at some height above the ground. Unfortunately, the leaf trap inevitably modifies wind influencs, affecting the measurements of leaf transport. To minimize turbulence wire mesh was used for the sides of the trap and its height was kept to a minimum. However, the techniques applied here were not totally satisfactory, and improvements are still needed.
Leaf transport on the catchment
slopes
It appeared that leaf transport is very constant throughout the forest, though it was expected to differ in an areal sense due to varying environmental factors. The reason for this lack of variation must be sought in the influence of the weather, which overrules the influence of terrain factors (slope, aspect, vegetation and parent material) on leaf transport. During and after frost or rain, the leaf layer is non-transportable. Upon drying, first the superficial layer becomes transportable, and as drying proceeds slowly downward more leaves become available for transport. The layer with fragmented and partly decomposed leaves is always non-transpotable, and forms the lower limit of the layer ofpotentially transportable leaves. It must be concluded that weather conditions are much more important than spatial variations in terrain factors. Only aspect will modify weather conditions on a micro-scale, but this is not important enough to cause significant differences in leaf transport, also because the overall influence of wind under dry conditions is strong enough to counteract the effects of microclimatological differences between north and south exposed slopes: the strongest wind comes from the west and the most frequent winds blow from northeasterly and westerly directions in dry periods (see LAHR 1964, FABER 1971) and these do not seem to have a greatly different effect on north and south exposed slopes. The only place where leaf transport deviates from the general uniformity in the valley floor. Because this is fiat and moist and because wind influences are reduced due to its protected position in the deepest part of the valley, leaf transport is low. In this small area, the supply ofleaves (from the valley sides) is consequently higher than the rate of removal. This is reflected by the presence of a black and humic A horizon in the valley-bottom soils, indicating accumulation of organic material. Comparing the denudation values of the sampling units with the figure for the output, it appears that the latter is higher. This has several reasons. One is that material from the slopes is partly redeposited before it reaches the river. A second reason for this is that the potentially transportable leaf layer is buried every year (or continuously in the case of the coniferous vegetation) by freshly fallen leaves so that it becomes largely non-transportable. The sediment which is stored in this leaf layer will be fixed and redeposited. Deposition may also occur after a leaf has come to rest for other reasons, for example because of a sharp decline in slope, gradual reduction of wind influences, burial under a splashed load in the vicinity of bare spots, or because of zo6genic activity (burial of leaves under mole-hills or earthworm excrements, though this was hardly of any importance in the research area where dry sandy soils with a poor soil fauna prevailed). Furthermore, the higher denudation values, calculated from the output, are for a large part the result of higher leaf transport efficiency in the bare and steep valley incision, the gullies and the otherwise bare river banks. In general, the comparatively high output value implies that the valley should become gradually deeper due to the single influence of this transport.
LEAVES AND SEDIMENT TRANSPORT Leaf transport
109
in t h e r i v e r c h a n n e l
The sediment content of the leaf samples from the river channel is sometimes very high. The distinction between on the one hand leaves with sediment attached'to them and on the other hand sediment with incorporated leaves was not as difficult to make as might be suspected. This can be appreciated from the l~actthat 10 grams of fresh beech foliage has a surface area ofsome. 12 to .22 m 2 (BURGER 1925), so that ifa sample of 10 grams o.fleaves contains 20 grams of sediment (total 30 grams), this means that on the average only .009 to .015 grams of sediment is present per square centimter. Another interesting point is the large decrease in the amount of sediment attached to leaves stored in the bed of the intermittently flowing river (for xample between 18/5/1976 and 12/11/1976). This change in storage is more than 1000 times greater than the sediment output by leaves over the same timespan. From field observations it appeared that a lot of sediment is washed offthe leaves as they are lying in the river channel either by flowing water or by rain. This implies that measuring the output of sediment attached to leaves underestimates the amount of material originally transported into the river by them.
Comparison
with other processes
of erosion
Other erosion processes are or have been active in the catchment, and comparing the effect of leaf transport with some contemporary processes reveals the relative importance of the former. Only processes delivering sediment directly to the river and thus also responsible for the output of sediment are considered here. These are gully erosion, scour and transport by the river, zotigenic erosion (mainly acting in the steep valley incision through the action of worms) and splash erosion. It should be kept in mind that the latter process intimately interacts with the leaf transport process. The forementioned processes give rise to a denudation value of.4 ram/1000 yrs for gully erosion and 5.0 mm/1000 yrs for the combined effects of scour, transport by the river, splash and zofgenic erosion (VAN ZON 1978). From the measurements in the catchment it appears that approximately one quarter of the total output of sediment was generated by the laf transport process during the year of observation. Leaf transport can therefore be considered to make an essential contribution to the output of the catchment. As is the case with other processes, a relatively small part ofth catchment, i.e. the area close to the river, produces the bulk of the output. However, this area adjacent to the river does not become exceptionally bare as the year proceeds, which indicates that leaves from more remote places arc also transported towards the river, effectuating a continuous, sheet-like transport in a downslope direction. It can be concluded that in steep forested areas, a considerable part of the eroded material is transported downslope by leaves, and that this process is worth considering if st:aliment production and transport are to be evaluated. It must be stressed that also the output ofoganic material in the form of leaves, an often neglected factor in research concerning sediment production, is of vital importance. Further research on the leaf transport process should at first be directed towards improving the methods of measuring leaf transport, and to evaluating the importance of (micro-)climatological factors.
110
VAN ZON
ACKNOWLEDGEMENTS I would like to thank Dr. A.C. Imeson and Prof. Dr. P.D. Jungerius ofthe Laboratory of Physical Geography and Soil Science, University of Amsterdam, for their comments and constructive criticism on this paper, and Mrs. M.C.G. Keijzer-v.d.Lubbe for typing the manuscript.
REFERENCES BRAY, J.R., GORHAM, E. (1964): Litter production in forests of the world. Adv. in Ecological Res. 2: 101-157. BURGER, H. (1925): Holz-, Laub- und Nadeluntersuchungen. Schweizer Zeitschr. ftir Forstwesen, 266-310. DICKINSON, C.H., PUGH, G.J.F. (eds.)(1974): Biology of plant litter decomposition. Academic Press, two volumes. FABER, IL (1971): Climatologie du Grand-Duch6 de Luxembourg. Publ. du Mus6e d'Histoire naturelle et de la Soci6t6 des Naturalistes Luxembourgeois, Luxembourg, 48 pp. LAHR, E. (1950/1964): Temps et climat au Grand-Duch6 de Luxembourg. Publ. Min. de l'Agriculture, Luxembourg, 575 pp. OVINGTON, J.D. (1962): Quantitative ecology and the woodland ecosystem concept. Adv. in Ecol. Res. I, 103-192. SCHMID, J. (1955): Der Bodenfrost als morphologischer Faktor. HiJthig Verlag Heidelberg, 144 pp. ZON, H.J.M. van (1978): Litter transport as a geomorphic process. Publ. Fysisch-Geografisch en Bodemkundig Lab., Univ. Amsterdam, nr. 24, 136 pp.
Anschrift des Autors: Dr. H.J.M. van Zon, Laboratory for Physical Geography and Soil Science, University of Amsterdam Dapperstraat 115, 1093 BS Amsterdam