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Effects of plant cover

Plant cover is effective in preventing erosion to the extent that it absorbs the kinetic energy of raindrops, covers a large proportion of the soil during periods of the year when rainfall is most aggressive, slows down runoff, and keeps the soil surface porous. However, it is difficult to assess the protective action of plant cover without a close look at the farming techniques involved.

Plant cover is certainly the most important factor for erosion, inasmuch as erosion goes from 1 to over 1000 tonnes when, all else being equal, plant cover on a plot falls from 100% to 0% (compare the plots under pineapple and harvest residues left on the surface, with the bare plots in Table 13).

INFLUENCE OF TYPES OF PLANT COVER

Table 13 shows three types of plant cover.

Full cover year-round

This category encompasses closed forest, but also secondary shrub forest, unburnt tree savannah, natural fallow, grasslands with grasses more than one year old, and shrub crops with cover plants or mulching. Erosion is always negligible under such thick cover (E = 0.01 to 1.5 t/ha/yr and runoff very slight (KR % = 0.5-5% on average, 10-25% at most in the case of exceptional rainfall). Erosion and runoff are generally very slight under forest, although there are some exceptions: a forest on a 65% slope on tertiary sand near Abidjan, a plot with a 20% slope on schist-engendered soil at Azaguie, a forest plot in Guyana on Bonidoro schist (Blancaneaux 1979), and a forest plot in a very high-rainfall area in Gabon (Collinet 1971). The maximum runoff observed in these often very moist forests can rise to over 35% for a single rainstorm. On the most usual types of slope, it seems that runoff is considerably stronger on ferralitic schist-engendered soil than on soil resulting from granite or tertiary sediment. With their foliage distributed over several levels, their bushes and their litter of dead leaves, forests provide year-round cover to the soil, protecting it against the energy of falling rain.

Mesofauna (termites and earthworms) keep the soil porous, and infiltration speed remains high throughout the rainy season. The only problem that may arise is saturation of the soil above a relatively impervious horizon with low macroporosity, for example the base of the gravel sheet at Azaguie, and also laterally drained soils in Guyana. Similar results in other words, very little erosion or runoff - have been observed on three plots under closed degraded forest thickets on the Agonkamé station in southern Benin (Verney, Volkoff and Willaime 1967; Roose 1976b). As in forest areas, runoff travels between the soil and the litter and is constantly slowed down by unevenness in the soil and trapped by the holes left by rotted roots and mesofauna. Its flow is broken and its volume reduced in the sequences studied.

Under savannah or old fallows that have been protected for some years, average runoff (Kaar = 0.02-5%) and maximum runoff coefficients are very little higher than under forest. On the other hand, as seen below, if fires occur each year, particularly late in the season, conditions are radically different.

Bare soils, bare fallows or fallows providing little cover during the most aggressive months

Erosion is greater the steeper the slope and the more aggressive the climate. At Adiopodoumé erosion rises from an average of 60 to 138 and 570 t/ha/yr with a slope increase from 4-7 to 23%, and runoff is very heavy (average KR = 25-40% with maximum KR = 70-90%). In principle, farmers never leave their soil uncovered during the rainy season, but grow some crop; otherwise the ground is invaded by weeds. However, if for some reason they sow their crops too late, the soil will be left without cover during the first months of the rainy season and will behave like that on bare plots. Erosion on a late-sown plot is thus roughly 80% that of a bare plot.

TABLE 13
Erosion and runoff at (Adiopodoumé Côte d'Ivoire) as a function of plant cover, cropping practices and slope (1956 to 1972) (cf. Roose 1973)

Plant cover and cropping practices		Annual erosion (t/ha)		Average annual runoff (%)	Maximum runoff* (%)
		Extremes	Average
Secondary forest (23.3% slope)		0.01 to 0.2	0.1	0.7	6 (12)
Bare soil	(S = 4.5)	34 to 74	60	37	71 (98)
	(S = 7)	69 to 150	138	33	66 (87)
	(S = 20-23.3)	266 to 622	570	25	65 (73)
Cover or forage plants (s - 7%)
• 1 st year
- early planting, strong growth starting in 1st year, Pennisetum purpureum, Guatemala grass, Panicum maximum, Cynodon dactylon, Setaria		0.1 to 1.9	0.5	4	25 (29)
- late planting, thin density, poor growth in 1 st year, Crotalaria, Flemingia congesta, Mimosa invisa, Panicum maximum, Digitaria umfolozi, Centrosema, Titonia diversifolia, Stylosanthes		23 to 89	40	20	62 (87)
• 2nd year
- all cover plants - 2nd year		0.05 to 0.7	0.3	1	8 (12)
Natural fallow (S = 4.5%)		-	0.6	8	64
Coffee, oil palm or cacao (S = 7%)
- with a good cover plant		0,01 to 0.5	0.3	2	8 (16)
- with a fairly undeveloped cover plant		5 to 143	-	30	60 (87)
Banana trees with mulch (S = 7%)		0.04 to 0.05	0.04	0.5	4
1st year) flat planted 7%		8 to 20	12	14	51
Pineapple) on mounds 4.5%		-	1.5	9	5
2nd year		0.1 to 0.3	0.2	3	12
Cassava and yam (S = 7%)
• on mounds 1st year		22 to 93	32	22	53 (82)
• on mounds 2nd year		-	2	7	24
Maize 20 × 100 cm (S - 7%) ridged parallel with the slope		(35)** to 131	92	30	75 (86)
Groundnut 20 × 40 cm flat planted (s = 7%)		59 to 120	82	27	73 (87)

* The first column is the probable maximum each year per rainstorm; the second column igure refers to an exceptional rainstorm occurring once every ten years.
** Exceptional rainfall.

Incomplete cover for part of the year

Some food, industrial, cover or forage crops are planted late or simply need time to thrive. Erosion phenomena are clearly intermediate, but depend very much on the planting date, crop spacing, slope and cropping techniques. Table 13 shows that food crops are among those offering the least soil protection. Erosion under cassava or yam amounts to 22 to 93 t/ha/yr on a 7% slope, whereas under maize and groundnut it varies between 35 and 131 t/ha/yr This is with no erosion control measures, late planting, and widely spaced crops on poor soil. In any case, 80% cover only developed after two to five months, in other words after the heaviest rains. This does not happen on traditionally farmed fields, since farmers often plant very soon after the first rains and almost always combine various crops providing complementary cover and succeeding one another in time and space. Under intensive cropping, the risk of having to resow if dry periods follow the first storms is too great, so that planting necessarily follows relatively late after tillage, often two to three weeks after the sowing date in the traditional system. However, tillage does encourage deep rooting, and fertilizer applications make it possible to catch up for the growth lag and to space plants more closely.

Table 13 also shows that erosion - and to a lesser degree runoff - depends largely on the proportion of soil not covered by plants before the heaviest rains. It is not simply a question of the green matter produced on the field, but more specifically of the vertical - or, better, slightly oblique - projection of cover provided. During major rainstorms, the angle of incidence of drops is generally less than 25°, except in the case of certain tornadoes, when it can be as much as 45°. It also depends on the architecture of the plant structure: i.e., the height of foliage above the soil and whether plants are funnel-like and thus concentrate water (e.g. pineapple and maize), or umbrella-like and scatter the drops (e.g. cassava).

There has been little general study of the dynamics of the cover provided by different crops, and there is no reliable technique for measuring every type of plant. Different procedures have therefore been used to assess plant cover on erosion plots (Roose 1973):

• the average diameter of the circle covered by groundnut rosettes, the proportion of surface covered by the circle circumscribed by a tuft of cassava (on a vertical photo);

• the number and surface area of maize leaves;

• the covered or bare geometric areas between rows of Stylosanthes or groundnut or under savannah;

• the quartile points (needles touching the cover or not) for grasses, weeds, stubble and pineapple.

Figure 25 shows that the growth dynamics of plant cover varies considerably depending on the type of plant but also on cropping techniques (spacing, date of planting, fertilizer applications) and climate (rainfall and light). If heavy rainstorms occur one month prior to sowing, erosion will therefore depend as much on the type of plants as on cropping techniques. Hence the concept of "soil-degrading" or "protective" plants, according to the speed at which they cover the soil - a factor that must be tempered by appropriate cropping techniques. Grasses generally protect the soil better than pulses or cassava, although crop protective capacity can be improved a great deal by early planting in relation to the periods of heaviest rain. For example, Stylosanthes reaches the same covering capacity (95%) as Panicum but two months later. Some plants are described as soil-degrading because they cover the soil slowly, for example pineapple and cassava, which gain by only 10 to 20% of cover per month.

FIGURE 25 Development of plant cover of various crops in the course of the year (Adiopodoumé runoff plots, 1966-1975) (cf. Roose 1977c)

TABLE 14
Post-mowing protection by three forage crops (Adiopodoumé 1970-1972)

	Rainfall	Cynodon aethiopicus			Stylosanthes guyanensis		Panicum maximum		Bare soil
	Volume mm	Aggressiveness RUSA	R%	E kg/ha	R%	E kg/ha	R%	E kg/ha	R%	E kg/ha
3.11.1970	41.5	13.8	3.6	47	19.6	10	0	0	39	1843
4.11.1970	mowing
5.11.1970	20.0	4.4	2.3	12	16.6	69	13.3	110	53	1323
7.11.1970	22.0	7.3	2.6	2	14.9	87	25.0	175	74	1111
22.9.1971	mowing
27.9.1971	33.5	18.5	1.9	9	15.2	188	3.3	175	32	1542
15.7.1972	mowing
17.7.1972	65.0	42.3	3.4	10	6.1	16	21.8	335	77	9710
Total after mowing	140.5	72.5	2.8	33	11	360	17	795	62	13686
Plant cover after mowing on 17.7.1972			60 to 80%		42%		8 to 14%		0

Some plants such as groundout, maize and other cereals cover the soil very poorly in the first two months, reaching 80% only at the end of the third month, but since their cycle is fairly short (4 months), for the remainder of the year the bare soil is exposed to rain splash unless there are weeds to cover the soil and absorb raindrop energy. Other plants seen as degrading are simply grown in a way ill-suited to providing ground cover. This applies, for example, to tobacco, which is widely spaced in order to produce more beautiful leaves. This problem can be solved by mulching the spaces between such poor cover crops. It is obviously not possible to use mulch under cotton, another notoriously degrading crop which takes at least two months to cover the soil, and also leaves no later trace of organic matter in the soil as its leaves are grazed and the stalks and roots carefully pulled up and burnt. It is in fact a combination of lack of plant cover and organic imbalance that leads to soil degradation under these different crops. Against this, Panicum maximum and other grasses that grow in large tufts can cover the soil in one month.

TABLE 15
Effect on erosion control of development of a cover plant under bush crops (Adiopoumé, 1961 and 1962, on very desaturated, sandy ferralitic soil with a 7% slope)

Plant cover		Growth of the cover plant	E t/ha/yr	Average runoff %	Peak runoff %
1961	Rainfall: 2289 mm
	Coffee + Flemingia	good	0.4	2.6	8
	Oil palm + Centrosema	almost nil	143.2	2.1	87
	Coffee + Stylosanthes	slow	5.2	1.8	75
1962	Rainfall: 2773 mm
	Coffee + Flemingia 2nd yr	full	0.05	0.7	2
	Oil palm + Centrosema 2nd yr	average	0.08	1.4	4

Table 14 shows the considerable amount of erosion protection provided by the stubble and superficial roots of three forage plants even after mowing:

• runoff is reduced to ½ and erosion to 1/17 and even 1/415 of that observed on the bare plot;

• mown Cynodon is still much more effective than Stylosanthes (sown in rows) and especially Panicum, which grows in tufts; erosion depends on the areas of soil left uncovered after mowing, i.e. 20 to 40% under Cynodon, 60% under Stylosanthes in rows, and almost 90% under Panicum.

In Table 15 it should be noted that results vary widely for incomplete cover. This is fortunate for development experts, for the variability is a result not only of variations in rainfall and imperfections in measurement methods, but primarily of the way in which crops are planted and tended. Experts can therefore manipulate cropping techniques, using biological or mechanical approaches. The first method for soil and water conservation, the biological technique, aims at intensifying production on the best land by increasing plant cover. It entails early, close planting of vigorous species well suited to regional conditions, adequate soil preparation, balanced fertilizer applications, sufficient phytosanitary protection, the use of cover plants or mulching, crop rotation and alternating cover and root and tuber crops.

It is particularly important to ensure soil cover during the heaviest rains, particularly from 15 May to 15 July at Adiopodoumé. On two identical plots with a 7% slope, one month's delay in planting Panicum maximum led to an increase in erosion from 1 to 89 t/ha, and in runoff from 10 to 20% for the rainiest three months of the year.

FIGURE 26 Erosion varies according to the height of plant cover from the ground

FIGURE 27 Combined effect of mulch plus canopy when raindrops fall from an average height of 1 metre or less (cf. Wischmeier and Smith 1978)

The choice of a very vigorous cassava variety and manure supplements reduced erosion from 93 to 30 t/ha/yr on neighbouring plots.

For shrub crops, planting a good cover crop generally solves erosion problems (see coffee, palm oil, cocoa and rubber plantations in Côte d'Ivoire) (Table 15).

INFLUENCE OF THE HEIGHT OF PLANT COVER (Figures 26 and 27)

Figure 26 shows that erosion is dependent not only on plant cover but also on plant height above the ground. For example, when plant cover is 100% but is 4 metres high, erosion will be about 75% of that on a bare plot; if the cover is 2 metres high, erosion will be about 50%; and if it is 50 cm high, erosion will still be about 18%. However, if there is a mulch, erosion will be reduced to 3%. If reduced erosion concerns the percentage of soil covered by mulch, a very fast reduction is seen for a relatively small area. For example, for 10% of soil cover, erosion is no more than 78%, for 20% it is no more than 60%, and for 50% it is only 30% of that found on the bare control plot.

This means that litter has a very quick impact on erosion. Mulching can be of use in erosion control even without covering the whole ground. If even as little as 20% is covered, erosion is reduced by 40%; if the mulch covers about 40%, erosion is reduced by 60%, and if 80% is covered, erosion is reduced by 90% from what would be found on bare soil.

Figure 27 shows the combined effect of mulch and canopy. If there is no canopy, the previous curve is seen for the effect of mulch on erosion, but if 20, 40, 60, 80 or 100% of the ground is also covered by canopy, there is a progressive increase in erosion control. Thus, with mulch covering 20%, erosion is about 60%, but if there is also 100 percent canopy cover, erosion will be no more than 30%. This means that if the mulch is not total, the leaf vault can have a considerable effect in reducing erosion.

Lastly, erosion can be reduced for a catchment if crops that provide poor soil protection (e.g. maize, groundnut, tobacco, cassava, yam) are alternated with grass leys or permanent grassland, or even buffer strips.

PLANT ARCHITECTURE

The architecture of plants can also affect the development of gullying and erosion, for trees with leaves that channel water toward the trunk operate as funnels, and water thus collected at the foot of the trunk can start to shear through the ridges, which will then drain off all the water contained in the furrows, giving rise to a gully. This occurs particularly with pineapple, but also - to a lesser extent - with maize. The other type of architecture is that of umbrella-like plants, which send drops of water outwards and thus scatter their energy; banana and cassava are examples here.

The influence of root formation must also be considered. Fasciculate surface roots hold the surface of the soil. Tap-roots grow in volume to start with, occupying the soil macropores and hence reducing infiltration, but they later rot, leaving tubes stabilized by organic matter, thus encouraging infiltration.

Intensified farming does not necessarily lead to increased soil degradation and erosion. Hudson (1973) indeed demonstrated that the production of one sack of maize caused 50 times more erosion when grown extensively than when thickly planted and combined with fertilizer. This is not only because larger areas had to be cleared in order to produce the same amount of maize, but also because erosion is higher on sparsely planted fields than closely planted ones. Similarly, fertilizers can have a significant protective effect against erosion (Table 16).

FIGURE 28
Effect of various erosion control treatments of Beaujolais grapevine plants (Pommier, France) on runoff, erosion and quality of wine (cf. Gril 1982)

Results obtained under simulated rainfall of 60 mm/in for 1 hour on 1 m²

Treatment		Runoff (%)		Erosion (g/m²) in 1 hour		Weight of harvest (g/vine plant)	Alcoholic strength	pH
		first half hour	second half hour	total dry matter	organic matter
Straw	St	13	26	1.8	0.18	1573	8°33	6.59
Compost (surface)	Cs	24	57	6.9	2.2	1499	8°69	6.71
Compost (dug-in)	Cd	34	66	108	9.7	1514	8 °63	6.76
Tillage	Ti	41	74	142	11	1565	8°38	6.50
Vine-shoots (crushed)	Vc	54	83	190	15	1546	8°33	6.58
Non-tillage	Nt	52	87	137	11	1481	8°38	6.50
Variation coefficient % Repetition = 4		41	13	60	44	9.30	2.80	3.65

Runoff

Soil loss

TABLE 16
Effect of intensification of production on runoff and erosion (cf. Hudson 1973)

	KR %	Erosion in t/ha/yr
Maize without fertilizer	14	18
Maize with fertilizer	8	6.3

INFLUENCE OF CROP RESIDUE MANAGEMENT

Recall the test in Table 11 in which the presence of a cover of pineapple and burnt-off residues reduced erosion on bare soil from 200 t/ha to 25 t/ha and 11 t/ha if the residues are burnt or ploughed in, but 0.4 t/ha if left on the surface. Similarly, runoff fell from an average of 36% on bare, tilled soil to 6.4% under pineapple with burnt-off residues, 2% if the residues are ploughed in, and 0.6% if left on the ground. This clearly indicates that residues left on the soil surface are much more effective in reducing erosion than residues ploughed into the soil to improve its structure.

In France the best vineyards are often located on slopes. Since grapevines provide very little cover during the winter and spring storms (maximum plant cover = 40%), this crop poses serious problems for controlling runoff from steep slopes and preventing it from carrying away too much soil. This led Gril (1982) to set up tests on a Beaujolais grapevine, using an ORSTOM-type rain minisimulator, to assess the influence of six methods of soil preparation and organic matter management on the runoff coefficient (% of the rain), erosion (g/m²/h) and quality of wine (Figure 28). The results clearly showed that:

• tillage reduces the runoff observed on untilled soil (by 15%), but makes little difference to erosion;

• the presence of crushed vine-shoots tends to increase runoff and erosion (+ 37%); curiously, this is the worst possible treatment;

• dug-in compost reduces runoff (by 11%) and erosion (by 24%) as against plain tillage;

• covering the soil with a compost - or, better still, a mulch - is the most effective method, reducing runoff by 65% and erosion by 98% as against the tilled control plot, with no ill-effects on the quantity, pH and alcoholic strength of the wine;

• the variation coefficient is high during the first half hour (13 to 54%), but falls as the rain continues: the condition of the surface is therefore very variable and important during short storms, but differences are really established during the second half hour.

EFFECT OF BUSH FIRES [Plate 4]

Under savannah or old fallow protected for several years, average runoff (Kaar = 0.02 to 5%) and maximum runoff are not much higher than under forest (Saria 1971-1974 and Korhogo 1967-1975: Roose 1979 and 1980a).

TABLE 17
Effect of bush fires on runoff from a plot (Gorse, 1967-1973) (cf. Roose 1979; Roose and Piot 1984)

	Full protection	Early fires	Late fires
Rainfall (mm)	674 and 799	759 and 810	553 to 691
Kaar %	0.2	2.5	15
Max KR %	1	10	50 to 70
Erosion kg/ha/yr	40	140	400
Plant cover %	85 to 95	50 to 85	10 to 55

TABLE 18
Influence on runoff of protection from grazing and fires measured under two fallows* (Saria, Burkina Faso) (cf. Roose, Arrivets and Poulain 1978)

Years	1971	1972	1973	1974
Rainfall mm	602	724	672	714
Runoff:
- on new fallow Kaar %	20	5	6	8
Max KR %	51	29	22	30
- on old fallow Kaar %	10	0,4	0,3	3
Max KR %	41	2	1	8
Erosion kg/ha:
- on new fallow	700	43	19	720*
- on old fallow	17	9	10	35*

* In 1974, before the first storms, removal of the litter and all the stubble.

However, the situation is radically different if fires take place each year. Gonsé is a good example here (Table 17), for there is a sharp difference in soil cover if fire crosses a plot.

If the fire is early (one month after the last useful rain), it passes fast, burning the dried aerial parts, but destroying neither grass clumps nor major tree branches. On the other hand, it does wipe out the young seedlings, the litter of dead leaves, and a good number of insects and pests.

Late fires - as can happen in Sudanian and Sudano-Sahelian savannah in May, just before the rains - are disasters, for the vegetation is so dry that the fire lingers on each clump of grass, destroying every last stalk, the aerial parts of bushes, and sometimes even large trees. The soil is left practically bare, and will have little protection for at least a year. Rainstorms beat freely on the surface, forming a thin, almost totally impervious, slaking surface, giving rise to severe sheet runoff.

On the other hand, if the plot is totally protected from grazing and fires, tall plants and bushes thrive and young saplings take root, covering the land completely in two to four years and producing substantial litter to fully absorb rainfall energy and encourage the activity of the fauna that perforate the surface horizons.

Trials on fallow at Saria at the Mossi Plateau Centre clearly show the effect of stubble left on the soil since the end of 1971 (Table 18). In 1971, runoff was very high, as much as 40 and 50%, for the young fallow had almost no cover as yet, and the older fallow was grazed extensively. During the two years that the plots were protected from grazing and fires, runoff and erosion were kept down to a bare few percent of runoff. In April 1974, before the first storms, all the grass and dry leaves covering the ground on the plots was gathered. The average and especially the peak - runoff immediately rose by several percent, although not returning to its initial level, for the clumps of grass quickly spread again as soon as the rains started, and the mesofauna did not suffer too much.

EFFECT OF FIRE CONTROL SYSTEMS ON THE NATURE OF PLANT COVER

A few kilometres from Bouaké in central Côte d'Ivoire the CTFT set up a very clear demonstration trial in the 1950s on the effect of fire on Guinean savannah (rainfall of 1200 mm spread over four seasons) at the Kokondekro forestry station. On a sloping ferralitic soil, three one-hectare plots were isolated by fire-breaks and subjected each year either to late fire, early, running fire, or total protection from fire. After 30 years, the following observations could be made:

• on the plot subjected to fire (annual, late), the tree vegetation had practically vanished, giving place to a grass savannah;

• on the plot subjected to early, running fire one month after the last useful rainfall, tall plants shared the area with shrubby, fire-tolerant, stunted, misshapen, but fairly abundant shrub vegetation;

• on the plot totally protected from fire (a mere two accidental fires in 30 years), the grass had practically vanished, smothered by a very thick secondary forest rich in creepers and undergrowth, much more vigorous than the surrounding savannah, which was burnt almost every year, and composed of large, dominant trees (10 to 30 trees per hectare) and a mixture of tall plants and numerous shrubs.

Although unfortunately no information is available on changes in soil or on runoff, it is clear that fire has a decisive influence on the development of grasses and trees and on the variety of species present.

EFFECT OF TUFTS OF GRASS

In the absence of fire and grazing, infiltration recovers after a few years on old fallow. While double-ring infiltration tests (Müntz) have shown that infiltration is very slight between tufts of grass on denuded areas (1 to 20 mm/h), it is five to ten times greater under tufts of grass (over 100 mm/h). Such soil provides congenial shelter to termites and other small animals which build very temporary structures and hollow out passages; together with the passages left by rotted roots, these help infiltration (Roose 1979). This means that the better the young plants grow, the more extensively they cover the soil surface and the more they divert raindrops from their trajectory in order to lead them toward the base of the clumps, where they can infiltrate easily. Stalks and subaërial roots, but above all litter, also act as brakes on sheet runoff, slowing it down and thus increasing the time and volume of infiltration although grass stalks are in fact more effective in trapping suspended solids than in reducing the volume of runoff.

CONCLUSION Whatever the slope, cropping technique, extent of soil fragility or climatic aggressiveness, full plant cover (regardless of its architecture and botanical composition, so long as it reaches 80%) ensures a high level of soil and water conservation. The influence of plant cover is greater than that of any other factor. Biological methods that help increase plant cover should therefore have priority in any effort to improve water management, infiltration, biomass production and soil conservation. Elwell (1981) found that even if only 40% of the soil was covered by crops, this reduced erosion by 80% on oxisols in Zimbabwe, which are more resistant than the soils tested in West Africa. This clearly shows the possible interaction between plant cover and soil type with reference to erosion.

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