Case Study of a Tropical Rainforest

Tropical rainforests: core characteristics and system context

Global distribution & climate

Tropical rainforests occupy a broad equatorial belt across the Amazon Basin, Congo Basin, Indonesia–New Guinea region, and parts of northern Australia. Year-round high insolation, high humidity and mean temperatures of around 27 °C create ideal conditions for plant growth and intense biological activity. Most areas receive over 2,000 mm of rainfall a year, often delivered in short, heavy convective storms.

Why they matter for the water cycle

Dense multi-layered canopies intercept a large proportion of rainfall. A significant percentage of this water is returned to the atmosphere by evapotranspiration (ET), helping to “recycle” moisture and generate further rainfall downwind. In many rainforest regions, this recycling process can supply a large share of total rainfall. The remaining rainfall infiltrates the soil, supports root uptake, and feeds river systems with relatively stable baseflow.

Why they matter for the carbon cycle

Continuous warmth and moisture drive very high primary productivity. Carbon stored in living biomass is passed through food webs and eventually into the soil. In an undisturbed system, most tropical forests function as net carbon sinks. However, the strength of this sink can decline sharply when forests are fragmented, stressed by drought, or damaged by fire. Long-term studies in several regions show that some tropical forests are now close to carbon neutrality, and in parts of the Amazon, deforested and fire-affected areas have become net carbon sources.

Human pressure

Logging, road building, mining and conversion to agriculture disrupt interception, reduce ET and increase overland flow. Carbon stored in trees and soils is rapidly transferred to the atmosphere through burning and oxidation. Drier, fragmented forests become more vulnerable to fire, which can lead to further carbon release and vegetation loss. As more forest is cleared, recycled rainfall decreases, leading to reduced precipitation, longer dry seasons, and higher risk of drought.

CASE STUDY: Indonesia

A. Location, scale and significance

• Indonesia once held the third-largest area of tropical rainforest on Earth after the Amazon and Congo.
• It also contains over 13 million hectares of tropical peatland, especially in Sumatra, Borneo (Kalimantan) and Papua. Peatlands store up to 10 times more carbon per hectare than mineral soils.
• In some regions of Kalimantan, peat deposits exceed 10 metres in depth, storing thousands of years of accumulated carbon.

B. Water cycle: natural baseline

• In intact peat swamp forest:
  • Up to 70% of annual rainfall is returned to the atmosphere through evapotranspiration.
  • Water tables are naturally within 20–30 cm of the surface.
  • Slow drainage from peat maintains stable dry-season river flows.
• Peat acts as a natural reservoir — a long-term water store preventing both floods and drought.

C. Drivers of change

• Rapid land-use change for palm oil, timber plantations, mining and settlements.
• Between 1990 and 2020, Indonesia lost over 28 million hectares of primary forest, an area roughly the size of Italy.
• Drainage canals are cut through dry peat for agricultural purposes; water tables can fall by more than 1 metre in the first dry season.

D. Impacts on the water cycle

• When peat dries, soil compaction reduces infiltration, and overland flow increases dramatically.
• During the 2015 El Niño event, prolonged drought and drained peat caused:
  • Over 2.6 million hectares of land to burn
  • Severe river decline in Central Kalimantan
  • Thousands of wells and village water sources to dry up
• Flooding risk increases during the wet season: in South Sumatra, peak discharge in rivers receiving runoff from plantations has been recorded at 2–3 times the pre-deforestation levels.

E. Impacts on the carbon cycle

• Drained peat becomes exposed to oxygen and releases 50–100 tonnes of carbon per hectare, per year, even without fire.
• Fire dramatically increases emissions:
  • 2015 Indonesian peat fires released an estimated 1.6 billion tonnes of CO₂, temporarily exceeding the daily CO₂ emissions of the USA and China combined.
  • In severe fire years, 85% of Indonesia’s greenhouse emissions come from peatland degradation.
• Burnt peat can smoulder underground for weeks to months, making emissions long-lasting.

F. Climate change signals and feedbacks

• As peat dries, decomposition and fire increase → more CO₂ is released → climate warms → peat becomes drier and more flammable.
• Smoke haze travels across Singapore, Malaysia and Thailand, creating regional atmospheric impacts and suppressing incoming sunlight.
• Some peat domes have subsided more than 2 metres in 20–25 years, permanently lowering storage capacity.

G. Soil and vegetation changes

• Natural peat swamps, dominated by ramin, meranti, and sago palm, are replaced by single-crop plantations.
• Organic carbon in soils declines significantly: studies show that 50–60% of peat carbon is lost within the first decade of drainage and burning.
• Biodiversity impact:
  • Borneo has lost significant portions of its orangutan habitat
  • Species requiring a swamp forest cannot survive in plantation landscapes

H. Mitigation and management

• Indonesia has introduced a moratorium on new peatland development and a Peatland Restoration Agency, aiming to restore more than 2 million hectares.
• Canal-blocking projects re-wet peat, raising water tables back towards the surface and slowing oxidation.
• Community-based fire patrols, early-warning systems, and restrictions on slash-and-burn have reduced fire in some regions since 2017.
• Long-term success is mixed: where water tables stay above 40 cm, peat stabilises; where drainage continues, emissions remain high.

CASE STUDY: Amazon Basin

A. Location and scale

• Covers over 6 million km² across nine countries, with Brazil containing about 60% of the forest.
• The Amazon stores around 90–120 billion tonnes of carbon in vegetation and soils.
• Roughly 15–20% of global river freshwater entering the oceans flows from the Amazon into the Atlantic.

B. Water cycle: baseline

• High evaporation and transpiration maintain a humid atmosphere; 30–50% of rainfall in the basin is generated by recycled water.
• Average rainfall: 2,300 mm a year, but exceeds 6,000 mm in the northwest.
• Interception loss under dense forest can be as high as 25% of total rainfall.

C. Drivers of change

• Between 2000 and 2020, the Amazon lost around 50,000 km² of forest each year at peak periods, mostly for cattle ranching and soy.
• Fire is often used to clear land; during drought years (2005, 2010, 2015), satellite data recorded tens of thousands of hotspots.
• Road building opens up remote regions to mining, logging and settlement.

D. Impacts on the water cycle

• Removal of trees reduces evapotranspiration → less rainfall is returned to the atmosphere.
• Air arriving at the eastern Amazon now contains less moisture, contributing to longer dry seasons in southern and eastern areas.
• Runoff increases: In some converted pastures, overland flow can be four times higher than in adjacent forests.
• Larger sediment loads enter rivers, reducing water quality and altering aquatic habitats.

E. Impacts on the carbon cycle

• Each hectare of forest cleared releases 100–200 tonnes of carbon through burning and decomposition.
• Severe drought years have pushed parts of the eastern Amazon from a carbon sink to a carbon source.
• In 2019, Amazon fires released enough CO₂ to cancel out the annual carbon absorption of many remaining intact forest regions.

F. Climate change signals

• Average temperatures across the basin have risen.
• Studies show more frequent megadrought years since the mid-2000s.
• Some climate models suggest that if 20–25% of the forest is lost, rainfall recycling could weaken enough to trigger a shift towards a drier savanna-like system in parts of the basin.

G. Vegetation changes

• Large emergent trees, which store the most carbon, are the most vulnerable to drought and wind damage near cleared land.
• Forest edges dry faster and burn more easily.
• In repeatedly burned regions of Mato Grosso and Pará, forest has been replaced with scrub, grass and fire-adapted species with much lower biomass.

H. Soil changes

• Forest soils can contain 4–9 kg of carbon per m² in the upper 50 cm.
• Under pasture, this can fall to 1 kg per m².
• After clearance, 30–60% of soil carbon can be lost within 10 years.
• Nutrient-rich topsoil is washed into rivers, reducing fertility and increasing sediment loading.

I. Mitigation and governance

• Satellite monitoring and rapid-response enforcement have significantly reduced deforestation in some periods.
• Indigenous reserves and national parks protect large continuous areas; they show some of the lowest deforestation rates.
• Reforestation projects in the Xingu, Acre and Rondônia regions aim to reconnect fragmented landscapes.
• Enforcement fluctuates by government policy — meaning long-term protection remains uncertain.