Vegetation, soils and ecosystems in tropical environments

The diagram shows three nutrient stores drawn as circles whose size represents the quantity of nutrients held in each.

The Biomass store is by far the largest circle, showing that most of the ecosystem's nutrients are held in the living vegetation and animals.

The Litter store and the Soil store are both much smaller, with the Soil store the smallest of the three. So the order of store size shown is Biomass (largest) > Litter > Soil (smallest).

The profile is very deep, showing that intense chemical weathering has broken down rock to a great depth in the hot, wet conditions.

The subsoil is a strong red colour. This is because leaching and laterisation have removed soluble silica and left behind insoluble iron and aluminium oxides, which give the soil its red colour.

The surface organic (litter/humus) layer is very thin, because litter decomposes extremely quickly on the hot, humid forest floor rather than building up. Together these features — great depth, red colour and a thin organic layer — are typical of a heavily leached, infertile latosol.

Savanna vegetation must survive a long dry season and the fires that often sweep through it, and its adaptations are responses to these two stresses.

To cope with drought, acacia trees develop deep tap roots that reach groundwater far below the surface, and small, waxy leaves with an umbrella-shaped crown that reduce water loss by transpiration and shade the ground. The baobab stores water in its swollen trunk and is deciduous, dropping its leaves in the dry season to cut water loss. The grasses grow quickly in the wet season, then die back and survive the drought as seeds and underground roots, shooting up again when the rains return.

To survive fire, many savanna trees have thick, corky bark that insulates the living tissue from the heat of grass fires, and they are able to re-sprout from protected buds after burning. The baobab's thick trunk is also fire-resistant.

Together these adaptations allow the vegetation to endure both the seasonal water shortage and the recurrent fires that characterise the savanna climate.

Nutrients cycle rapidly in the rainforest because the hot, humid climate speeds up the processes that move them between stores.

Litter falling onto the warm, moist forest floor is broken down very quickly by decomposers, so the litter store stays small and nutrients are released into the soil almost continuously. These released nutrients are then taken up almost immediately by the dense, shallow root systems of the vegetation, so they do not remain in the soil for long. Because uptake is so fast, the soil store also stays small, and most of the nutrients end up locked in the living biomass.

The poor soil reinforces this. Heavy, year-round rainfall leaches soluble nutrients downwards out of the soil, so any nutrients not quickly taken up by roots are washed away. The plants therefore capture nutrients rapidly before they can be lost, which is why the biomass store is the largest and the soil store the smallest.

On a Gersmehl diagram this appears as a large Biomass circle with small Litter and Soil circles, and large, fast flows of decomposition and uptake between them.

Tropical soils and the nutrient cycles they support are shaped by both natural climatic processes and by human activity. The climate of the tropics drives the soil-forming processes of leaching and laterisation and the rapid nutrient cycle of the rainforest, while human activities such as deforestation, burning and grazing modify these soils and cycles, sometimes dramatically. This essay assesses the relative importance of the two and argues that, while climate sets the underlying character of tropical soils and nutrient cycling, human activity becomes the dominant influence where clearance is large-scale and permanent — so the view is correct only in part and depends on scale.

Climate is the fundamental control on tropical soils. In the rainforest, high, year-round rainfall drives intense leaching, washing soluble nutrients down out of the topsoil, while laterisation removes silica and leaves the red iron and aluminium oxides of the deep, infertile oxisol or latosol. Constant heat and humidity also drive very rapid decomposition and root uptake, so that nutrients are held in the biomass and the soil store remains small — the cause of the famous paradox of lush forest on poor soil. In the savanna, the marked dry season reduces net leaching and, with thinner litter, produces the less-leached but organic-poor red and brown earths. None of these soils or cycles could exist without their particular climate, so climate sets the baseline that any human activity then acts upon.

Human activity, however, can transform these soils and cycles rapidly and is clearly the more important influence in many places. Because rainforest fertility is held in the biomass, deforestation removes the main nutrient store and breaks the cycle that sustained it. Slash-and-burn clearance for shifting cultivation, as practised across the Amazon and the Congo basin, releases a pulse of nutrients to the soil as ash, but much is lost in smoke, and once the protective canopy is gone heavy rain leaches and erodes the bare soil so that fertility collapses within a few years. In the savanna, a large proportion of the grassland is a fire and grazing plagioclimax: regular burning, much of it started by people, together with grazing by domestic herds, kills tree seedlings and keeps the land as grassland that would otherwise tend towards woodland. Here human activity does not merely modify the soil and cycle but actively determines the vegetation and, through repeated burning and overgrazing, can degrade the soils over time.

The most accurate assessment is that climate and human activity operate at different scales and interact, so neither can be ranked the 'main' influence in isolation. Climate creates the soils and the nutrient cycle over long periods; human activity changes them locally and quickly. The balance between them depends above all on the scale and permanence of human interference. Traditional small-scale shifting cultivation, where a plot is cleared, cropped briefly and then left to recover for many years, works largely within the natural cycle and is broadly sustainable, so climate remains dominant. Large-scale, permanent clearance for ranching or plantations, with no fallow period, overwhelms the natural cycle, exposes and degrades the soil irreversibly (including the hardening of laterite into a brick-like crust), and makes human activity the controlling influence.

In conclusion, human activity is not, as a blanket statement, the main influence on tropical soils and nutrient cycling. Climate sets the underlying character of both, and where human use is small-scale and temporary it works within natural limits. But where clearance and burning are large-scale and permanent, human activity becomes the dominant control, breaking the nutrient cycle and degrading soils faster than climate alone ever would. The view is therefore justified only at large scales of human interference; the most accurate judgement is that climate sets the potential and human activity, at scale, determines the outcome.

Tropical rainforest grows in extraordinary abundance, yet the soil beneath it is notoriously poor. Explaining this apparent contradiction requires looking both at the soil itself — its leaching and laterisation — and at the nutrient cycle, in which most nutrients are held in the biomass rather than the ground. This essay assesses whether the nutrient cycle or the soil better explains the infertility, and argues that the two explanations are really one: the nutrient cycle and the soil properties are products of the same climate and reinforce each other, so the infertility is best understood as the outcome of their interaction.

The case for the nutrient cycle is strong. On a Gersmehl diagram the rainforest shows a very large Biomass store and small Litter and Soil stores. Because the hot, humid climate makes litter decompose extremely rapidly and the dense shallow roots take up the released nutrients almost at once, nutrients spend most of their time in the living vegetation and very little in the soil. The soil therefore appears infertile simply because the cycle keeps nutrients moving through the biomass rather than allowing them to accumulate in the ground. By this argument the 'infertility' is not really a property of the soil at all but a feature of where the nutrients are stored — overwhelmingly in the biomass. This is powerfully supported by what happens after clearance: remove the biomass store and the fertility disappears with it, showing that the fertility was never in the soil.

The case for the soil itself is also strong, however. The rainforest's soils are genuinely impoverished by soil-forming processes. The high, year-round rainfall drives intense leaching, washing soluble nutrients such as calcium, potassium and nitrates down out of reach of plant roots, while laterisation strips out silica and leaves behind the chemically inert iron and aluminium oxides of the deep red latosol. These oxisols have a low capacity to hold nutrients in the first place. So even setting the cycle aside, the soil is intrinsically poor and prone to losing what little it has, which suggests the soil itself is a real and independent part of the explanation.

The most accurate assessment is that the cycle and the soil cannot be separated, because both are produced by the same climate and each reinforces the other. The heavy rainfall that leaches the soil is the same rainfall whose warmth and moisture drive the rapid decomposition and uptake that concentrate nutrients in the biomass. The poor, leaching-prone soil is precisely why the plants have evolved to capture nutrients so quickly through shallow roots and rapid recycling — the cycle is an adaptation to the soil, and the soil's emptiness is partly a result of the cycle removing nutrients into the biomass. To ask which is 'more important' is therefore slightly artificial; they are two expressions of the same climatic system.

In conclusion, the nutrient cycle gives the more complete explanation of why a lush forest can grow on poor ground, because it shows the fertility is held in the biomass and explains why clearance is so damaging. But it does not replace the soil explanation: leaching and laterisation make the soil genuinely poor and incapable of storing nutrients, and this is what forces the cycle to depend on the biomass in the first place. The infertility is best explained as the interaction of the two — a climate that both empties the soil and concentrates nutrients in the biomass — and any answer that credits only one oversimplifies the system.