Terrestrial Biomes Student Notes
High-Yield Notes Unit 3.3 Study Sheet
Terrestrial Biomes: Climate, Adaptations & Ecology
AP / General Biology Page 1 of 2
Core Ecological Framework: Primary Climatic Drivers
Whittaker Biome Concept
A biome is a regional biological community defined by characteristic temperature ranges and annual precipitation. Because climate shapes selective pressures, geographically separated regions with analogous climates host ecologically equivalent communities (convergent evolution).
1. Temperature Governs enzymatic rates, frost mortality, and length of thermal growing season. Driven by latitude and insolation.
2. Precipitation Dictates water balance and plant transpiration. Controlled by Hadley cell circulation and mountain rain shadows.
3. Limiting Factors Cold limits polar biomes; aridity limits deserts; light limits forest floors; soil nutrients limit tropical/boreal systems.
1
Tropical Rainforest
0°–10° N/S
Temp: 20°–34°C (stable) Precip: 125–660 cm/yr
• Photoperiod: 11–12 hrs constant daily sunlight; year-round continuous primary productivity (highest terrestrial NPP).
• Vertical Stratification: 4 distinct tiers (Forest Floor <2% light \(\rightarrow\) Understory shrubs \(\rightarrow\) Continuous Upper Canopy 30–40m \(\rightarrow\) Emergent Giants).
• Soil Leaching Paradox: Oxisol soils are severely nutrient-deficient. Torrential rain leaches minerals; nutrients are trapped in living biomass and instantly re-absorbed upon decay.
• Ecological Value & Threats: Known as "Nature's Pharmacy" for medicinal compounds; heavily threatened by slash-and-burn logging and agriculture.
2
Subtropical Deserts
15°–30° N/S
Temp: 0°C (night) to >60°C (day) Precip: <30 cm/yr
• Climatic Origin: Centered on Tropics due to descending high-pressure dry air (Hadley cells) and leeward rain shadow effects.
• Xerophytic Plant Adaptations: Succulent stems store water; modified spine leaves minimize transpiration; ephemeral seeds remain dormant for years until brief rains.
• Faunal Traits: Nocturnal and fossorial (burrowing) behavior avoids daytime heat; highly concentrated excretion conserves water.
• Cold Deserts: Great Basin, Gobi; experience sub-zero winter temperatures with frozen snowfall precipitation.
3
Savannas (Tropical Grasslands)
Equatorial Margins
Temp: 24°–29°C (warm) Precip: 50–127 cm/yr
• Bimodal Seasonality: Brief rainy season followed by severe dry season lasting up to 8–9 months; regular seasonal drought.
• Vegetation Structure: Open grassland expanse dotted with scattered drought-deciduous trees (Acacia, Baobab) with deep root systems.
• Fire & Herbivory: Perennial grasses regenerate rapidly from underground roots and rhizomes after grazing and dry-season wildfires.
• Trophic Webs: Hosts massive migratory herbivore herds (wildebeest, zebras) followed by apex carnivores.
4
Chaparral (Scrub Forest)
Coastal Mid-Latitudes
Temp: Mild winter, hot dry summer Precip: 65–75 cm/yr
• Mediterranean Regime: Almost all precipitation falls during winter; summer is extremely arid, driving summer plant dormancy.
• Sclerophyllous Flora: Tough evergreen shrubs (chamise, scrub oak) with thick, leathery, waxy leaves that minimize transpiration.
• Fire Dependency: Natural fires occur periodically. Many plant seeds require smoke chemicals or heat shock to germinate; nutrient-rich ash fertilizes the topsoil.
• Geography: Coastal California, Mediterranean basin, central Chile, and SW Australia.
Topic 3.3 • Terrestrial Biomes Student Study Notes Page 1 of 2
2
Temperate, Boreal & Tundra Biomes + Master Synthesis
Page 2 of 2
5 Temperate Grasslands (Prairies/Steppes) 25–89 cm rain
• Climate: Extreme annual swings: hot summers (>35°C), cold freezing winters (<-20°C).
• Treeless Filter: Treeless due to low precipitation, periodic fires, and herbivore grazing.
• Fertile Mollisols: Deep underground root networks and rhizomes die back annually, creating a massive layer of nutrient-rich, moisture-holding humus (global breadbaskets).
6 Temperate Deciduous Forest 75–150 cm rain
• Climate: -30°C to 30°C; defined 4 seasons; reliable precipitation year-round.
• Phenology: Broadleaf deciduous trees (oak, maple) drop leaves in autumn to endure winter dormancy and freeze-induced drought; open canopy lets sun reach understory.
• Soil Fertility: Thick deciduous leaf litter breaks down gradually into rich humus; low leaching preserves minerals, sustaining fertile alfisols.
7 Boreal Forest (Taiga) 50°–60° N • 40–100 cm snow
• Climate: Long, frigid subzero winters; brief cool summers; low evaporation.
• Conifer Adaptations: Evergreen needles have thick waxy cuticles and sunken stomata. Needles warm rapidly in spring sunlight, allowing immediate photosynthesis; retaining needles year-round conserves scarce nitrogen.
• Acidic Soils: Needles decompose slowly; produces acidic, nitrogen-poor spodosols.
8 Arctic Tundra >60° N • 15–25 cm rain
• Climate: Winter -34°C; short 50–60 day summer (3°–12°C) with ~24 hrs daylight.
• Prostrate Flora: Low-growing mats (mosses, lichens, dwarf shrubs) escape freezing winds.
• Permafrost Mechanics: Perennially frozen subsoil blocks root penetration and drainage (creating summer bog pools); cold prevents decay, locking massive global carbon stores.
Master Biome Comparison Matrix
| Biome | Temperature | Precipitation | NPP | Biomass | Soil Quality & Limiting Factors |
|---|
| Tropical Rainforest | 20°–34°C (stable) | 125–660 cm | Highest | Very High | Poor (heavy leaching); limited by canopy light/minerals |
| Savanna | 24°–29°C (warm) | 50–127 cm (seasonal) | Moderate | Moderate | Moderate fertility; limited by prolonged dry season |
| Subtropical Desert | 0°C night / >60°C day | <30 cm (variable) | Lowest | Very Low | Poor (low organic humus); limited strictly by water |
| Chaparral | Mild winter / hot sum. | 65–75 cm (winter) | Low–Med | Low–Med | Poor until fire ash release; limited by summer drought |
| Temperate Grassland | -20°C to 35°C | 25.4–88.9 cm | Moderate | Moderate | Highest (rich humus mollisols); winter cold / fire filter |
| Temperate Forest | -30°C to 30°C | 75–150 cm | High | High | Very Rich (leaf litter decomposition); freezing winter pause |
| Boreal Forest (Taiga) | Severe winter / cool sum. | 40–100 cm (snow) | Low–Med | High | Acidic & nitrogen-poor (slow decay); short season / cold |
| Arctic Tundra | -34°C winter / 3°–12°C sum. | 15–25 cm | Very Low | Very Low | Permafrost locked; limited by extreme cold & short season |
Fire Regimes & Suppression Natural fires maintain grasslands and chaparral by suppressing woody succession. Fire suppression allows combustible fuel loads to build up, resulting in catastrophic mega-fires. Controlled burns recycle nutrients into ash and sustain native biodiversity.
NPP vs. Standing Biomass \(NPP = GPP - R\) measures the net carbon fixed annually. Tropical forests maximize NPP with continuous warmth. In contrast, Boreal forests have low annual NPP but massive standing biomass because persistent woody conifers accumulate over centuries.
Global Feedback Loops • Rainforests: Deforestation breaks evapotranspiration cycles, inducing drought.
• Tundra Permafrost Thaw: Melting permafrost permits microbial decay of ancient organic pools, venting \(\text{CH}_4\) and \(\text{CO}_2\) (positive warming feedback).
Topic 3.3 • Terrestrial Biomes Student Study Notes Page 2 of 2
