Tropical Rainforests and Mangroves Study Notes

Natural Vegetation and Tropical Climate

• Natural Vegetation: Plant life covering land areas that develops without human interference.

• Climate: Average weather conditions over a long period (typically more than $30$ years).

• Tropical Climate Characteristics:

  • Found between the Tropic of Cancer ($23.5^\circ \text{ N}$) and the Tropic of Capricorn ($23.5^\circ \text{ S}$).

  • High temperatures throughout the year ($27\,^\circ \text{C}$ and above).

  • High annual rainfall (average $2000\,\text{mm}$).

Characteristics and Distribution of Tropical Rainforests (TRFs)

• Global Distribution: Generally found along the Equator in South America (Amazon), Southeast Asia (Borneo), and Africa.

• Evergreen Nature: Trees do not shed all leaves simultaneously; new leaves replace old ones year-round due to high temperature and rainfall.

• High Biodiversity: Large variety of species (e.g., the Amazon has over $300$ plant species within $0.01\,\text{km}^2$).

• Three-Layered Vertical Structure:

  • Emergent Layer: Uppermost layer ($30\,\text{m}$ and above); crowns of tallest trees like the Kapok Tree.

  • Canopy Layer: $20-30\,\text{m}$ tall; interlocking crowns block sunlight; includes epiphytes, lianas, and Rubber trees.

  • Undergrowth Layer: $20\,\text{m}$ and below; sparse vegetation due to low sunlight; contains seedlings and fungi.

Characteristics and Distribution of Mangrove Forests

• Distribution: Found along sheltered coasts, shallow river mouths, or behind islands where water is calm.

• Salinity Tolerance: Adapted to higher salinity environments compared to TRFs.

• Uniform Height: Lower density of trees results in less competition for sunlight, creating a relatively uniform height.

• Horizontal Zonation: Structure determined by high and low tide levels:

  • Coastal Zone: Closer to low tide; flooded longer; contains species more tolerant of high salinity.

  • Inland Zone: Closer to high tide; flooded for shorter periods; contains species less tolerant of salinity.

Plant Adaptations

• Tropical Rainforest Adaptations:

  • Broad Leaves: Large surface area to maximize sunlight absorption for photosynthesis.

  • Waxy Surfaces: Coating to repel water and minimize transpiration loss.

  • Drip Tips: Narrow tips allow water to flow off easily to prevent fungi growth.

  • Buttress Roots: Roots growing up to $5\,\text{m}$ above ground to provide structural support.

  • Shallow Roots: Quickly absorb nutrients from decomposing matter in the topsoil.

• Mangrove Adaptations:

  • Salt-secreting Leaves: Avicennia species remove salt through leaves; salt crystals are then washed away.

  • Salt-excluding Roots: Bruguiera species prevent salt from entering the plant.

  • Aerial Roots: Roots (e.g., Avicennia and Sonneratia) grow above soil to take in oxygen directly from the air in waterlogged, oxygen-poor soil.

Environmental Functions and Human Uses

• Oxygen Generation: Plants absorb $\text{CO}_2$ and release oxygen through photosynthesis year-round.

• Carbon Storage: $\text{CO}_2$ is stored in leaves, stems, and roots. Mangrove soils are particularly effective as waterlogged conditions slow down bacteria decomposition.

• Habitat Provision: Supports biodiversity like the Raffles’ banded langur, Malayan colugo, and mudskippers.

• Erosion Protection:

  • Soil Erosion: Leaves intercept raindrops to reduce impact force; roots bind soil.

  • Coastal Erosion: Mangrove roots trap sediment and create friction to reduce wave energy.

• Human Uses: Recreation (trekking), habitation (forest shelters), food (fruits), and raw materials (timber).

Resource Extraction and Sustainability

• Deforestation: Permanent removal of forests leads to species extinction (e.g., up to $57\%$ of tree species in the Amazon face threat).

• Enhanced Greenhouse Effect: Burning/cutting trees releases stored carbon as $\text{CO}_2$. Fewer trees remain to absorb greenhouse gases ($\text{CO}_2$, methane, nitrous oxide), leading to global warming.

  • Impact: Antarctica loses ice mass at an average rate of $136 \text{ billion tons}$ per year.

• Management Strategies:

  • Protected Areas: Establishing reserves like Bukit Timah Nature Reserve in Singapore.

  • Regulated Forestry: Selective logging (practiced in Malaysia) or patch logging (practiced in Columbia).

  • Rehabilitation: Reforestation efforts like Singapore’s OneMillionTrees Movement.

  • Public Education: Guided walks and workshops organized by NParks or Nature Society Singapore.

Questions & Discussion

• Q: Describe the distribution of tropical rainforests shown in Fig. 1.

• A: Generally found along the Equator between $23.5^\circ \text{ N}$ and $23.5^\circ \text{ S}$. Largest forests are in South America (Amazon), with smaller patches in South and Southeast Asia.

• Q: Where can we find mangroves in Singapore?

• A: (Open for discussion/exploration of Singaporean coastal areas).

• Q: What did you see and hear in the Borneo Rainforest video?

• A: (Student-led observations reflecting biodiversity and forest density).

• Q: How do the polar bear and cacti adapt to their respective environments?

• A: (Comparison activity for understanding general biological adaptation).