Land Ecosystems: Adaptations, Soil Formation, and Light Dynamics in Terrestrial Biology

Life on Land Imposes Unique Constraints

Terrestrial organisms must overcome desiccation, gravity, and variable temperature and moisture.

Desiccation

The process by which organisms lose water to the air; a major constraint on land.

Water balance

Maintaining hydration by replacing water lost to the air; crucial for terrestrial life.

Adaptations for water conservation

Waxy cuticles and stomata in plants, behavioral and physiological strategies in animals.

Structural support in terrestrial organisms

Investment in materials like cellulose (plants) or skeletons (animals) to resist gravity.

Variability in terrestrial environments

Greater fluctuations in temperature and moisture compared to aquatic environments.

Gravitational force

The need for land organisms to stay upright due to low buoyancy in air.

Geographic variation and climate

Large-scale plant and ecosystem distribution is governed by climate gradients.

Plant Cover and Light Distribution

Leaves absorb and reflect light, creating vertical gradients in light availability.

Leaf Area Index (LAI)

The total leaf surface area per unit ground area; determines canopy light penetration.

Leaf orientation

Angled leaves intercept less midday sun, affecting photosynthesis and adaptation to environment.

Spectral quality changes in canopy

Ratio of red to far-red light decreases with depth, impacting plant growth responses.

Sunflecks

Brief, intense bursts of sunlight reaching the forest floor through canopy gaps.

Seasonal changes in canopy light

In deciduous forests, more light reaches the ground in spring than in summer.

Beer's Law

Describes vertical attenuation of light in a plant canopy as a function of LAI.

Soil definition

A natural product of mineral and organic matter, serving as medium for plant growth and habitat.

Soil as an ecosystem

Contains billions of organisms; the interaction between biotic and abiotic elements makes it living.

Soil formation

Begins with weathering, the breakdown of rocks and minerals into smaller particles.

Mechanical weathering

Physical breakdown of rocks by water, wind, temperature, and plants.

Chemical weathering

Alteration of rocks by water, acids, and biological activity.

Parent material

The mineral substrate from which soil develops; influences soil properties.

Five factors in soil formation

Parent material, climate, biotic factors, topography, and time.

Role of climate

Affects rates of weathering, plant growth, and leaching of minerals.

Biotic factors in soils

Plants, animals, bacteria, and fungi mix and add organic matter to soil.

Topography and soil

Landscape shape impacts water flow, erosion rates, and soil depth.

Time in soil development

Full soil profiles can require 2,000 to 20,000 years to form.

Soil color

Indicates properties like organic matter (black), mineral content (red/yellow for iron, purple/black for manganese).

Soil texture

The proportion of sand, silt, and clay particles; affects water movement and compaction.

Soil textural classes

Loam, clay, sand, silt, clay loam, sandy loam—defined by percentages of particle types.

Soil depth variations

Influenced by vegetation, slope, parent material, and erosion.

Soil horizons

Layers in soil profile: O (organic), A (topsoil), sometimes E (leaching zone), B (subsoil), C (unconsolidated).

O Horizon

Organic layer of partially decomposed plant material.

A Horizon (Topsoil)

Dark mineral layer with humus and leaching of minerals.

E Horizon

Zone of maximum leaching, common under forests.

B Horizon (Subsoil)

Accumulation of minerals, clay, and salts.

C Horizon

Unconsolidated material close to parent material, below zones of biological activity.

Bedrock

The solid rock beneath the soil profile.

Moisture-holding capacity

Determined by soil texture; sand drains quickly, clay retains more water.

Field capacity

Maximum water soil can hold after drainage; measured at 0.33 bar suction.

Wilting point

The moisture level at which plants cannot extract water; measured at 15 bar suction.

Available water capacity (AWC)

The difference between field capacity and wilting point.

Capillary water

Water held between soil particles by capillary forces.

Effect of soil texture on water

Fine-textured soils (clay) have higher field capacity and wilting point; loams have highest AWC.

Cation Exchange Capacity (CEC)

The total number of negatively charged sites that attract nutrient cations.

Soil pH

Influences nutrient availability; acidic soils can result in aluminum toxicity.

Regional soil formation processes

Include laterization, calcification, salinization, podzolization, and gleization.

Laterization

Rapid weathering and leaching in humid tropics, forming acidic, iron-rich soils.

Calcification

Upward accumulation of calcium salts in arid/semi-arid regions.

Salinization

Accumulation of soluble salts near the surface, common in deserts or irrigated land.

Podzolization

Acidic leaching and removal of minerals in cool, wet climates (coniferous forests).

Gleization

Water-logged conditions leading to bluish-black soils high in organic matter.

Soil erosion

Loss of topsoil due to wind and water, exacerbated by removal of vegetation and tillage.

Conservation practices

Crop rotation, contour farming, no-till farming, and grass strips help prevent erosion.

Ecological consequences of erosion

30% of arable land lost, threatens food production and sustainability.