L2 Flashcards
Biome definition
Areas of vegetation characterised by the same plant life forms
How are Biomes classified?
Classification of terrestrial ecosystems using their structure
Eg what grows there
Differences in vegetation in Biomes?
- Trees vs shrubs vs grasses
- Evergreen vs deciduous
- Broadleaf (angiosperm) vs Needleleaf (coniferous)
4 Tree life forms
Coniferous or angiosperm
Evergreen vs deciduous
Evergreen
Carry a leaf canopy the whole year round
Deciduous
Leafless some parts of the year
2 Ways of Biome differentiation?
Temperature
Water availability (more tropics specific)
Higher northern latitude Biome features…?
More evergreen
More coniferous
Southern latitude Biome features…?
Southern trees often winter deciduous
What are the two responses trees can drop their leaves to?
Temperature
Water availability
Relationships of Biomes with global climate
- Areas of the world with similar climate support vegetation with similar life forms
- At higher latitudes the temperature at a same point at a mountain is at a lower elevation
- Biomes change with altitude
- Results in altitudinal tree bands at lower altitudes
- Same temperature conditions but at different altitudes depending on latitude
Similar climate supports…?
Vegetation with similar life forms
Biomes change with…?
Altitude
Humboldt’s legacy - Eurocentric, climate- centered perspectives on biome distributions
Climate determines vegetation and thus determines biome distributions
Climate can be looked up and the biome can be calculated from known tables etc eg life zones
Two rules for biome distributions (life forms can be predicted off these two rules)
Environmental filters
Competition
Environmental filters (biomes)
Survival of cold or dry environmental extremes requires specialist adaptations
- These can be a trade-off
Competition (Biomes)
Specialists are excluded from warmer and/or wetter environments by stronger competitors
- Not having to tolerate extremities means these plants are less fit compared to competitors
Low temperatures exclude cold- sensitive plants
Eg tropical and sub-tropical plants
- Chilling injuries <10-12C
- Bananas and mangos are chilling sensitive
- Freezing injury and death <0C
- Avocado and palm
The three mechanisms of freezing resistance
Metabolic costs
Structural adaptations
Hypothesised trade-off between climate resistance and growth
Metabolic costs
Accumulate solutes which depress freezing point of tissues
Anti-freeze protein synthesis
Resource allocation competes with growth
Structural adaptations
Eg thick walled cells
Narrow xylem vessels to resist freezing injury
Lower rates of resource capture and growth (photosynthesis)
Trade-off between freezing resistance and growth rate
- In hot climates plants with adaptations to resist freezing grow slower
- Trade-off apparent
What two factors act as filters?
Freezing and drought
Freezing and drought act as filters
-Opposing forces from warm to cold and wet to dry
- Factors exclude species that don’t have adaptation to deal with them
- Competitive exclusion, species that lack adaptations but are competitive outcompete specialists in warmer and wetter environments
- Cold or dry adapted species are excluded from warmer, wetter climates by competition
Competitive exclusion
- Species that lack adaptations but are competitive outcompete specialists
- Cold or dry species are excluded from warmer, wetter climates by competition
Humboldt Review
- Roles of climate and soils in determining biome distributions
- Emphasis on forests as potential and most important vegetation
- View of open landscapes as being degraded by fires and deforestation
- Lead to ideas that open landscapes being degraded landscapes?
The tropical savanna Biome
- Tree-grass coexistence
- Continuous ground layer of grasses and varying tree cover
- Grasses are C4 as in hot environments
Deforestation and fires in tropical forests:
- Deforestation and fires in tropical forests can lead to savanna-like vegetation
- Often referred to as ‘savannisation’
- This looks like savanna according to its definition but is not
Does savannisation result in actual savanna?
No it looks like savanna but is not
Savannas are often portrayed as…?
Degraded ecosystems
Savannas are often portrayed as degraded ecosystems
- Widespread perception that savannas are unnatural, arising from deforestation and fires lit by people
- PROBLEM - large areas of savanna are targeted for restoration programmes for degraded land
Savanna grass communities in Madagascar are adapted to fire or grazing and are rich in endemic species
- Many grass flora are adapted
- Grazing- adapted species evolved in response to now-extinct megafauna
- Grazing-adapted communities are today maintained by domestic cattle
- We can work out fire adapted/ grazing adapted savannas based on characteristics of plants growing
How can we work out fire adapted/ grazing adapted Savannas?
Based on the characteristics of plants growing there
In ancient savanna what decides tree density/
Fire and herbivory
What are the two processes that maintain the ancient savanna Biome
Fire and herbivory
What is the escape height?
The tree must reach a set height to survive fires
What is the escape height determined by?
Frequency of fires and rate of tree growth
What is browse height?
Protection from herbivores
If animals can reach growing tips they can stop tree growth
Diversity of savanna ecosystems?
- Previous ecosystems characterised as forest can be defined as savannas based off the functional definition
- Even savannas with high tree cover have dense ground cover and fire adapted trees
- They all have a deep layer for continuous grasses and fire adapted trees
How are savanna mosaics maintained?
By fire-promoting grasses and fire-tolerant trees
How are forest mosaics maintained?
- Forests maintained by shading which excludes shade intolerant grasses
- Forests have no light for grass to establish so no fuel for fire
Mosaics of grassy and forest vegetation
- Not climatically determined
- Sharp boundaries between savanna and forest
- Savannas are maintained by fire-promoting grasses and fire-tolerant trees
- Forests maintained by shading which excludes shade intolerant grasses
- Forests have no light for grass to establish so no fuel for fire
- Forests and savannas coexist in the same climate, with abrupt transitions
Misclassification of Asian savannas
- Long history of savanna use and management (old vegetation types)
- When colonisers came and saw mosaics they presumed these were degraded ecosystems so planted trees
- In reality this is misclassification and leads to miss management
- Fire and grazing suppression in ‘dry forests’
- Leads to elimination of fire tolerant vegetation and forest establishment
Outcomes of misclassification of Asian savannas?
- Fire and grazing suppression in ‘dry forests’
- Leads to elimination of fire tolerant vegetation and forest establishment
Rising CO2 promotes growth of…?
Savanna trees
Tree growth and root storage depends on atmospheric CO2
Trees escape fire at high CO2, therefore reach escape height quicker, therefore reaching higher tree density
Climate change impacts in savannas
Rising CO2 promotes growth of savanna trees
- Savanna trees are C3
- Grasses in savannas are C4
- Tree growth and root storage depends on atmospheric CO2,
- Trees escape fire at high CO2, therefore reach escape height quicker, therefore reaching higher tree density
- Rising CO2 benefits C3 trees more than C4 savanna grasses
- Promotes growth of trees over grasses
Climate change promotes… in savannas?
Growth of trees over grasses
Consequences of climate change on Savanna:
- Woody plant encroachment into savannas
- Open savanna is covered in greater tree cover compared to less CO2
- Woody plants deplete savanna groundwater and threaten soil carbon stocks
Woody plant densities impact livelihoods
- Less pasture for grazing domestic animals
- Some charismatic animals require open landscapes
- Firewood harvesting and charcoal production
- Trees are often spiky and sharp, this is not good for cattle
- Land cannot therefore be used for grazing
Degradation arises through opposing processes in forests and savannas
- Tropical forests are rich everywhere
- Tree removal doesn’t suddenly result in rich ground flora
- In an open savanna tree there is poor canopy diversity etc however ground flora is diverse
- When savanna is encroached/ planted, diversity of ground layer is reduced and replaced with species poor trees etc
- If tall trees grow or invade, this grassy layer is then competitively excluded to form a species poor canopy
In an open savanna is there more or less canopy diversity than forest?
Less
Conclusions:
- Fires and herbivores have ancient roles in maintaining savannas
- Tree encroachment or planting in savannas degrades ecosystem functions and services
- Colonial misclassification of these ecosystems led to mismanagement
