How Do Forests Change Over Time?

Forests are dynamic systems that evolve through interacting natural and human-driven processes:

1. Ecological Succession

• Primary: On bare rock/substrate (lava, glacial till) → lichens/mosses → grasses → shrubs → pioneer trees (aspen, pine) → climax community.
• Secondary: After disturbance (fire, windthrow, logging) where soil remains → faster progression through similar stages.

2. Growth, Competition & Stand Dynamics

• Trees compete for light, water, nutrients. Shade-intolerant pioneers give way to shade-tolerant climax species.
• Self-thinning: density decreases as individual trees grow larger (–3/2 power law).
• Canopy stratification: emergent, canopy, understory, shrub, herb layers develop.

3. Natural Disturbance Regimes

• Fire, wind (hurricanes, derechos), ice storms, insect outbreaks (bark beetles, gypsy moths), disease, floods.
• Disturbance frequency/severity shapes species composition, age structure, and landscape pattern.

4. Climate & Environmental Change

• Gradual shifts (warming, precipitation changes) alter species ranges, phenology, productivity.
• Extreme events (droughts, heatwaves) cause dieback, mortality, regime shifts.
• CO₂ fertilization may increase growth where water/nutrients not limiting.

5. Human Influence

• Deforestation/Fragmentation: Agriculture, urbanization, roads → habitat loss, edge effects.
• Management: Plantation forestry, selective logging, thinning, prescribed fire, restoration.
• Invasive Species: Plants, pests, pathogens (e.g., emerald ash borer, chestnut blight).
• Pollution: Acid rain, ozone, nitrogen deposition.

Forests are never static; they are perpetually becoming.