Lecture 9- Forests

Overview of Forests

Definition: Forests are large areas dominated by trees and cover approximately 30% of the world's land area.
Importance: They provide essential ecosystem services including biodiversity, carbon storage, soil management, timber production, water conservation, oxygen production, and medicine.

Global Distribution by Climatic Domain:
- Boreal: 1,109,871 ha
- Temperate: 665,803 ha
- Subtropical: 449,122 ha
- Tropical: 1,834,136 ha(Source: FAO Global Forest Resources Assessment 2020)

Layers: Forests typically consist of four distinct layers:
- Emergent Layer: Contains the tallest trees that emerge above the canopy.
- Canopy Layer: The dense layer formed by tree crowns.
- Understory Layer: A darker and more sheltered layer below the canopy.
- Forest Floor: The ground layer containing soil, decomposing matter, and various organisms.

Light Penetration: Different forest types allow varying light levels to reach the forest floor.
- Open Birch Forest: 20-50% light penetration.
- Beech Forest: 2-5% light penetration.
- Tropical Rainforest: 1% light penetration.

Adaptations: Understory plants require less light, needing a minimum light intensity to photosynthesize effectively.
- Compensation Point: This is where the photosynthesis rate equals the respiration rate, crucial for plant survival.
- Sun Plants vs. Shade Plants: Shade plants can begin photosynthesis at much lower light intensities compared to sun plants.

Light Gaps: Tree falls create gaps in the canopy, increasing light availability, thus allowing the growth of understory species.
Shade Evaders: Some species like wild garlic and bluebells have adapted to flower before the tree canopies fully open, exploiting the short period of increased light.
Parasitic Plants: Some plants, such as the Bird’s Nest Orchid, have evolved to rely on host fungi for their nutrition instead of sunlight.
Climbing Plants: Lianas and epiphytes grow on trees to reach more sunlight, enhancing their productivity.

Location: Found in equatorial and sub-equatorial regions.
Characteristics: High biodiversity and productivity; receive consistent rainfall of 2,000-4,000 mm annually and maintain average temperatures of 25-29 °C.
Distribution: Comprise 11% of terrestrial land and contribute to 30% of primary productivity.

Climate: Experience seasonal variations with very cold winters (~-40 °C) and short warm summers (~20 °C).
Vegetation: Dominated by cone-bearing gymnosperms; rainfall averages 300-700 mm yearly.

Location: Predominantly found in mid-latitudes of the Northern Hemisphere.
Characteristics: Seasonal climate with average rainfall of ~700-2,000 mm. The winter temperatures hover around 0 °C, while summers can peak at 35 °C.

Succession: Woodlands in Ireland developed post-glacial retreat (~10,000 years ago) with tree colonization traced back to species like Juniperus, Betula, and Quercus.
Mapping Tree Arrival: Isochrone maps illustrate the timeline of tree presence and species composition.

Native Tree Species: Includes Taxus, Quercus, Fraxinus, and various broadleaved species.
Woodland Types:
- Sessile Oak Woodlands: Acidic soil preferences.
- Ash-Hazel Woodlands: Found on limey soils.
- Wet Woodlands: Transitional ecosystems from peat bogs.

Grazing: Reduces biodiversity and prevents tree regeneration.
Invasive Species: Particularly Rhododendron ponticum, which alters soil chemistry.
Development Pressures: Urban expansion and agricultural practices threaten native forest areas.

Global Importance of Forests: Forests are critical biomes providing significant ecosystem services. Three primary forest ecosystems include tropical, temperate, and boreal forests. Ireland's traditional woodlands have greatly diminished, presenting various conservation challenges.