Class 3 - savannas and grasslands Flashcards
Define savannas
Ecosystems that range from mostly grass to a mixture of grass and scattered trees – tropical savanna has a rich scattering of trees
What seasons do savannas usually esperience?
Strong dry seasons and periodic droughts
What is THE factor that maintains savannas and why?
Fire because it protects them from bordering forest
How does dry forest and savanna interact
In a zone of tension
Which other ecosystems do savannas lie between?
Grasslands and dry forests
How much land area is savanna globally?
1/5
What type of grass are in savannas?
C4
How much annual percipitation makes savannas unstable and why?
Above 650 mm, because it could become forest
What makes African savanna unique?
Megafauna, which used to be present in many other areas of teh world
What plants dominate African savannas?
Acasias and baobabs
What plants dominates central american savannas?
Pine, orcid, oaks
What is soil made of in savannas?
Ranges from sandy to claylike - poor soil
What plants dominates Australian savanna?
Eucalyptus, acasia, kangaroos
The three types of savanna and their characteristics
Nonseasonal savanna (poor soils), seasonal savanna (stressful dry season), hyperseasonal savanna (annual water deficiency and saturation)
Two main factors of savanna maintanance
Fire, grazing
How much precipitation does savannas get anually?
Between 50 cm and 250 cm
Are savannas only natural?
No, some savannas have formed where rain forest has been cut and burned
How much annual rainfall does savannas recieve?
Between 500 and 2000 mm - usually in a wet season
How often do savannas get fires?
They experience frequent mild fires and major burns every few years
How do plants adapt to fire in savannas?
Pyrophytes = adapted to withstand occasional burning – grass is a good example
What does fire do to savannas?
Fire burns the above-ground vegetation, fertilises top layer and enhances regrowth
What does less fire lead to?
Less plant species richness
How do trees handle fire?
Rapid growth, thick bark that changes often
What types of soil are in savannas?
Savanna soils are Oxisols and Utisols with low pH and low concentrations of many minerals, where aluminium is high
What characterises the herbivores in Africa?
Over 40 species, either grazers, browsers or mixed
Important African herbivores are:
Thomson’s gazelle, wildebeests
What are grazing lawns?
Grazing lawns appear because of close cropping by herbivores – stimulates plant regrowth (NPP)
What is one of the most significant animal migrations on earth?
The seasonal one on the Serengeti
What happens when wildebeests are born?
Wildebeests are able to walk and run soon after they are born
What are the top-dwn effects on the composition of plants?
Grazing
What are the bottom-up effects on the composition of plants?
Precipitation, water access
What do plants compete for?
Soil N and soil P, water and light
Where do the herbivores prefer to live?
In open areas, rather than with more tree cover, even browsers except for elephants
What makes plants seedlings survive more?
Large animals being present
When did grassy ecosystems spread globally?
11-24 million years ago
What happened 8 million years ago?
C4 grass replaced C3 grass in warm, seasonally arid places
How much land goes grasslands cover and how much does savanna?
25% and savanna (C4) around half of that
Define a savanna
Discontinuos tree layer with a continuous grassy understory AND a system with a continuous C4 grassy understory and and a discontinuous woody overstory
How were the continents when savannas arose and what did it mean?
They were seperated long ago, so savannas difer per continent
What are the woody plants in Africa?
Mimosoid legumes, broad-leafed Detariodeae legumes, Combretacae
What are the woody plants in South America?
Papillionoideae, Detariodeae legumes, Vochysiaceae
What are the woody plants in Australia?
Eucalyptus, Myrtaceae
Are grasslands and savannas the same?
No, they are distinct biomes
What characterises an old-growth savanna?
Ancient and biodiverse
What characterises a derived savanna?
Caused by deforestation
Where are savannas usually found (rainfall + temperature?
Usually found in places with intermediate rainfall, growing season temperatures above 10 degrees
What are the two savanna boundaries?
Mesic, where they transition into forest, and arid, where they transition into arid vegetation
What nutrient limits forest growth in savannas and is less manipulated by vegetation?
Potassium
What is the main requirement of a savanna?
A long dry season
What do long dry seasons do for savannas?
Restrict woody growth and increase fire
At what levels of rainfall does a savanna system lose its continuous grassy understory?
Difers a lot geographically
When do the two types of transitions happen?
Transitions happen either when the system becomes too dry for perennial grass growth or grass can still grow but another vegetation type out-competes it
What is the role of large herbivores in Africa?
They keep the savannas open
What two seasons coincide in tropical savannas?
The growing season and the wet season - because temperatures are not limiting
How is rainfall and soil fertility realated?
Inversely related – wetter regions have more nutrient-poor soils
What are the three biggest threats to savannas?
- The under-appreciation of them and their role in the carbon-cycle, 2. They are targeted for agriculture, 3. Climate change will increase woody cover
What are the fuel loads for savannas and what do they do?
Grazing (reduces fire intensity, favors spread of trees), browsing (surpresses tree growth, aids grass growth, more intense fires, reduces trees)
Why are grassy ecosystems important for food production?
Wheat, rice, corn, barley, oats, sorghum, rye and millet are grasses
How many people are supported by savannas?
Over 1 billion
How much carbon do grasslands store?
15% globally
Define tropical grassy biomes?
C4 grass layer, co-occuring tree layer, grass is shade intolerant, seasonal, tolerant of fire, grazing and browsing
Why are there savannas if the climate is wet and warm enough to support forest?
Deforestation, degraded forest by humans, fire, herbivory
How does grass adapt to garzers and fire?
Inteolerant to shade, tall, high biomass, flammable grass (Tussock grass), or short low aerial cover creeping grass (Stoloniferous grass), high below-ground biomass
How do trees adapt to grazers and fire?
Growing tall quickly, trying again, stay underground, bark growing thick/changing quickly, bud isolation (covered by bark, emerging from bark), spiky plants, bite size control
Types of savanna vegetation?
Woodland (tall trees and grasses), parkland (tall grasses, scattered lower trees), grassland (tall garsses with trees or shhrubs), low tree/shrub (low grasses, low growing trees)
How many stomatas do C4 garsses have?
1
Why do C4 grasses have an advantage?
When water and CO2 levels are low: Delays the need to reopen the stomata, higher water-use efficiency, can tolerate hotter and drier environments
What will higher CO2 levels lead to for C4 grasses?
Less photorespiration, enhanced water use efficiency, higher carbon assimilation rates
Define a forest
More than 10-30% trees with the potential to reach a height of at least 2-5 meters
What is the temp range in savannas?
5-30 degrees
How does grass grow vs trees?
Grass grows from below, trees grow from above
What do trees need to guarantee survival?
Because of fire, it is hard for trees to become adult, as soon as they are adult they generally survive
How do C4 grass photosynthezise?
Plant takes CO2 and via a complicated process creates sugars and oxygen. CO2 and oxygen comes in and leaves through the stomata, but if it is very hot, the plant has to close the stomata. This creates an oxygen buildup. C4 handles that by moving the process away from the oxygen buildup, which delays the need to re-open the stomata and has higher water use efficiency
