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EEFECT OF RELIEF /TOPOGRAPHY ON SOIL

FORMATION

Relief or topography is one of the five main factors that influence the way soilsdevelop. The influence of topography on soil formation operates through threemain elements: slope angle, slope position, and altitude.

Slope angle influences processes such as the movement of the soil and water.Slope position governs whether soils are net receivers or net losers of soil andwater. Altitude affects climate, which is also an important soil-forming factor.

Areas of great relative relief, such as mountain areas, often experience differentclimate characteristics from lower to upper slopes, which are usually expressed interms of different rainfall and temperature regimes. This generally results in avertical zonation of soil types, which can correspond to a zonation of vegetationtypes. Thus, a typical sequence on tropical mountains is from lowland tropical

forest, through submontane, montane, and subalpine vegetation zones to thealpine zones of grasses and shrubs.

It has been known for some time that soils vary systematically along a transectfrom the top to the bottom of a hill slope. The effect of topography is seen best onslopes developed on a single rock-type; it may otherwise be difficult todifferentiate the effect of topography from that of rock-type. Soil differences areusually related to changes in soil moisture and availability of water. In anyexplanation of the way in which soils vary with topography, emphasis is placed onthe difference between freely drained upper parts of hill slopes and imperfectly topoorly drained lower portions.

There is a continuum between those parts of a slope where the influence of soilmoisture is at a minimum and those parts where maximum influence of soilmoisture is felt. Slope steepness is one of the most important factors affecting soilmoisture, because it influences the balance between the amount of water thatinfiltrates into the soil and the amount of water that runs off as surface flow. Onsteep slopes relatively less water infiltrates, thus reducing percolation and theintensity of leaching processes within the soil. On steep slopes more water withinthe soil will also flow downslope as through flow. If less water infiltrates, morewill flow across the surface, perhaps resulting in soil erosion. Soluble minerals

are leached from soils on upper slopes, move down the slope, and are oftendeposited at the foot of the slope (Fig. 1). This leads to different trends in soilproperties on upper and lower slopes. Upper slopes are associated with processesof removal of soil and water, whereas lower slopes are associated essentially withdeposition and accumulation. Some workers have made the distinction betweennon-cumulative soils on upper slopes and cumulative soils on lower slopes. Theoperation of such effects across the entire hill slope produces a series ofsystematic changes in soil properties and soil profiles. The greatest

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concentrations of certain soil properties, such as the amount of organic matterpresent, would be expected on the gentler areas at the top and bottom of hillslopes.

Fig. 1.Processes that lead to the differentiation of soil profiles on hill slopes.

Variation in soil colour often provides a clue to the processes that are operating.Colour changes are especially prominent on many tropical hill slopes.

Upland, well-drained soils are reddish-brown, the colour indicating the presenceof non-hydrated iron oxide. Drainage is slower on middle and lower parts of hillslopes, partly because of moisture seeping downslope from upper soils. Middle-to lower-slope soils remain moist longer and dry out less frequently and lesscompletely. This causes the iron to become increasingly hydrated, and the redcolour changes to brown or yellow. Drainage is poor on the lowest slopes andpart, or all, of the soil profile may be waterlogged, leading to chemical reductionof iron. Under waterlogged conditions bacteria obtain their oxygen from oxygen-containing compounds, which are then reduced to other compounds (see soildevelopment). Such waterlogged soils are usually bluish-grey, greenish-grey, orneutral grey in colour, although if the water-table fluctuates, alternate oxidizingand reducing conditions will lead to the formation of red and grey mottles.

The recognition that there are clear patterns of soils on slopes and that these patterns

repeat themselves on similar slopes in similar environments led to the formulation of the

catena concept. A catena is a grouping of soils which are linked in their occurrence byconditions of topography and are found in the same relationships to each other wherever

the same conditions are met. The concept was first developed in the 1930s by G. Milne,working in East Africa, and was used as a way of mapping soils over wide areas. The

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assumption made was that where slope patterns were similar, soil patterns would also be

similar. Since then catenas have been recognized in a variety of areas and under a variety

of climatic conditions, an indication of the strength of the relationship between soil andtopography.

SOILS and NUTRIENT CYCLING IN THE RAINFOREST

Understanding the basic composition of forest soils helps explain the concept of nutrient cyclingin the rainforest; why there are problems with clearing rainforest lands for agriculture; and howsoils are an important factor influencing forest complexity.

SOIL COMPOSITION

Over two-thirds of the world's rainforests, and three-fourths of the Amazonianrainforest can be considered "wet-deserts" in that they grow on red and yellow clay-likelaterite

soils which are acidic and low in nutrients. Many tropical forest soils are very old andimpoverished, especially in regionslike the Amazon basinwhere there has been no recentvolcanic activity to bring up new nutrients. Amazonian soils are so weathered that they are largelydevoid of minerals like phosphorus, potassium, calcium, and magnesium, which come from "rock"sources, but are rich with aluminum oxide and iron oxide, which give tropical soils their distinctivereddish or yellowish coloration and are toxic in high amounts. Under such conditions, onewonders how these poor soils can appear to support such vigorous growth.

Where are the Rocks in the Lower Amazon?

Rainforests are tremendously vegetated. Early European settlers in the tropics were convinced(and even assured by scientists at the time) that the lushness of the "jungle" was due to the richsoils, so they cut down large patches of forest to create croplands. The cleared land supported

vigorous agricultural growth, but only for one to four years, when mysteriously, plant growthdeclined to a point where copious amounts of fertilizer were required for any growth. Settlerswondered why their crops perished and how such poor soil could support the luxuriant growth oftropical rainforest. The answer lies in the rapid nutrient cycling in the rainforest.

NUTRIENT CYCLING

The colonial settlers did not realize that they were dealing with an entirely different ecosystemfrom their temperate forests where most of the nutrients exist in the soil. In the rainforest, most ofthe carbon and essential nutrients are locked up in the living vegetation, dead wood, anddecaying leaves. As organic material decays, it is recycled so quickly that few nutrients everreach the soil, leaving it nearly sterile.

Decaying matter (dead wood and leaf litter) is processed soefficiently because of the abundance of decomposersincluding bacteria, fungi, and termites. These organismstake up nutrients, which are released as wastes whenorganisms die. Virtually all organic matter is rapidlyprocessed, even fecal matter and perspiration. It is only amatter of minutes, in many rainforests, before dung isdiscovered and utilized by various insects. Excrement canbe covered with brightly colored butterflies, beetles, andflies, while dung beetles feverishly roll portions of the waste

Dung-mimics

The attractiveness of dung to smallrainforest insects has led to thedevelopment of dung-mimics bothamoung predators and prey. Theseanimals, generally insects andspiders, sit motionless for hours ata time trying to look as dung-like aspossible to avoid detection.

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into balls for use later as larval food. Insects are not only attracted to dung for the energy value,but often for the presence of nutrients like calcium salts. Human sweat is a treasure for severalspecies of butterflies, which gather on the necks and hats of tourists, and for annoying sweatbees, which can cover seemingly every inch of exposed skin in some forests.

As vegetation dies, the nutrients are rapidly broken down and almost immediately returned to thesystem as they are taken up by living plants. Uptake of nutrients by plant roots is facilitated by aunique relationship between the roots and a fungi, mycorrhizae. The mycorrhizae attach to plantroots and are specialized to increase the efficiency of nutrient uptake nutrient from the soil. Inreturn, plants provide the fungi with sugars and shelter among their roots. Studies have alsoshown that mycorrhizae can help a tree resist drought and disease.

TREE ROOT SYSTEMS

Tropical rainforest trees are well-adapted to their environment and have mastered the problem ofpoor soils. Since the first six to eight inches (15-20 cm) of soil is a compost of decaying leaves,wood, and other organic matter, it is the richest source of nutrients on the ground. To tap thisresource, canopy trees are shallow rooted, whereas most temperate tree roots extend more than5 feet (1.5 m) deep. Many tropical specieshave roots that actually grow out of the groundto form a mat on the forest floor in order tomore efficiently collect nutrients. These tinyroots form a network that, along with themycorrhizae fungi, rapidly absorb nutrients.

The configuration of shallow roots and greatheight causes a great deal of instability forrainforest trees, especially with wet soils andstrong winds of the upper canopy that canaccompany tropical storms. To counter this,many tree species have extensive root

systems that in some cases may run for over325 feet (100 m). Other trees, especially tallemergent species, have evolved buttress rootslarge, thin extensions of the trunk that beginsome 20 feet from the ground. Thesestructures are thought to also aid in wateruptake and storage, increase surface area forgas exchange, and collect leaf litter foradditional nutrition. Some trees, expeciallypalms, have stilt roots for support.

Thus when colonists cut the forest and burnedthe vegetation, they were destroying the

delicate rainforest system which allowsvigorous growth on such poor soil. Burning thedead wood and vegetation released enoughnutrients into the soil to allow crops to grow forseveral years, but without the mycorrhizae, and other soil organisms to fix nutrients, soils wererapidly leached by the harsh tropical sun or washed away by heavy rains. Essential mineralswere not replaced by new decaying matter since there was no longer forest above to drop leavesand wood. Within a few years, the soil becomes nutrient deficient and can no longer supportproductive yields of conventional crops. Essentially, the colonists destroyed several links in thesemi-closed nutrient system of the rainforest and had to abandon the site for another forested

Tree roots, Brazil 1999

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patch. While this seems similar to the "slash-and-burn" technique of native indigenous peoples,the difference is in the scale and form of the cleared sites. By clearing large areas, the colonistscreated a major break in the rainforest nutrient cycling system; something which takesgenerations to recover. In the smaller patches cleared by traditional forest dwellers, forest canquickly recolonize after agriculture is abandoned, especially if the patch is left surrounded byforest. Within 20 years, relatively well-developed secondary forest can reclaim such anagricultural plot.

Not all rainforest soils are so poor; some rainforests grow on nutrient-rich floodplain and volcanicsoils. Some of the best soils are found on steep slopes because minerals are released when theexhausted topsoils erode. Such rich soils are found in the Amazonian floodplains, Andeanfoothills, and volcanic areas of Southeast Asia (Java), Africa, Central America, and theCaribbean. However, without proper management, these soils as well can be rapidly leached ofnutrients by heavy rains and the sun. For example, a hectare of tropical rainforest rarely losesmore than 1 ton of soil annually. However, when the forest is leveled and planted with variouscrops, the erosion increases drastically. If the forest is replaced with dense vegetation like acoffee plantation, the hectare loses between 20 and 160 tons, whereas if it is replaced with fieldcrops, the patch can lose more than 1,000 tons annually.

REGROWTH AFTER CLEARING

When Europeans cleared the rainforest for agriculture, they probably assumed that the forestswould regrow relatively quickly like the temperate forests of Europe and North America. Butrainforest will not readily return on lands with agricultural monocultures that have been devoid offorest for several years and have highly degraded soils. Tropical soils rapidly becomeinhospitable to growth due to swift leaching of nutrients caused by heavy rains and intensesunlight. In addition, many tropical hardwood trees are dependent on certain animal species forpollination, seed dispersal,and seed processing. Theseeds of many tropicalrainforest species are large

(since they germinate in theshade of the canopy andmust have enough foodreserves to grow in the lowlight conditions of the forestfloor) and require animaldispersers (wind or othermechanical means oftenare not sufficient fordispersing seeds of thissize). The loss of thesedispersal species whenforest is leveled, means tree

seeds are unlikely to bedispersed into clearedareas. Therefore these important forest tree species will not return.

The seeds and seedlings of those tree species not limited by animal dispersal and pollinationagents are often specifically adapted to the light and climate conditions of the shaded rainforestfloor. These seeds usually will not germinate in the hot, arid conditions of clearings, and thosethat do sprout rapidly succumb to the sun and poor soils. Tropical rainforest plants areaccustomed to the stability of the rainforest, where they are robust. When they and their seedsare confronted with the entirely different set of conditions presented by forest clearing, they do not

Carpet of moss in Perinet, Madagascar 1997

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fare well.Their seeds have little or no capacity to remain dormant since they have no need undernormal forest conditions.

The dry air of the forest clearing also dessicates the leaf litter causing the mycorrhizae to die. Theelimination of the symbiotic mycorrhizae reduces the capacity of trees to take up nutrients fromthe soil. This fungi is especially difficult to replace since each species of tree may have its ownsymbiotic species of mycorrhizae. Regeneration is further stunted by the rapid encroachment oftough grasses and shrubs after the clearing of forest.

The situation is different when a cleared area is left surrounded by forest and the soils have notbe decimated. Under these conditions, forest may rapidly reclaim the barren patch as fast-growing, weedy pioneer plants like forest grasses, bananas (Heliconia), gingers, and vines moveinto the clearing. Pioneer tree specieswhich require such conditions of bright sunlight and lowerhumidity for growthlike cecropia (neotropics) and macaranga (Asia) quickly colonize forestgaps. After several years, the small number of pioneer plants and trees like kapok and cecropiaare gradually replaced with diverse species more characteristic of older-growth primary forest.The necessary mycorrhizae fungi can recolonize from the surrounding forest and facilitatenutrient uptake. After 20 years the formerly cleared land may again support vigorous growth,although this forest is only a shadow of the original primary forest. The new secondary forest, isfar less diverse, has a less developed canopy, fewer animals, and thicker ground vegetation. It is

unknown how long it takes for secondary forest to return to the complexity of primary forest, butthe estimates range from hundreds to thousands of years.

Review questions:

Why does traditional agriculture often fail in rainforests?

How are nutrients recycled in the rainforest environment?

How are tropical forest soils different from soils in temperate forests?