13. Tropical grasslands

Question 1

What is special about plant cells? 1

Answer 1

1. They are totipotent and can therefore grow a new one

Question 2

What is the meristem? 1

Answer 2

1. The area of the plant that is actively dividing to produce new, healthy tissue

Question 3

What climates are grasslands found in?

Answer 3

1. dry regions - annual rainfall of about 100cm 2. tropical grasses found in (sub)tropical zones 3. Often in areas with a wet and dry season

Question 4

Where are grasslands found globally?

Answer 4

1. temperate grasslands are found in north and south america, europe, and far east asia 2. tropical grasslands are found in south america, africa, india and australia 3. dry weather favours grass ecosysems

Question 5

What are the threats to grasses and how do they survive? 2

Answer 5

1. To grow and complete their lifecycles, plants must gain biomass faster than they lose it. 2. Major selective pressures acting on plants in tropical grasslands: Grazers and Browsers Fire Drought and Heat

Question 6

What is the difference between grazers and browsers?

Answer 6

1, Grazers eat grass eg. antelope 2. browsers eat trees eg. giraffe 3. some animals eg. elephants fall in the middle of this spectrum

Question 7

What are the advantages and disadvantages of being a tree on a savanna? 5

Answer 7

1. trees outgrow grazers, creating a browse line at the maximum height a browser can reach 2. tree germinates at ground - must grow beyond browse line to survive 3. once it has, tree is stable 4. too many grazers can mean all seedlings eaten before trees established 5. some eg. elephants may just knock tree down

Question 8

What are the advantages of being grass on savanna? 6

Answer 8

1. herbivore is selective pressure, rather than compeition 2. grass have basal meristem, rather than at end of branch as in trees 3. In trees, the dividing cells are in bud-producing nodes at the end of the branch, so if it gets eaten growth stops 4. in grass, cells divide behind other matter in the meristem and push everything up 5. this means they can continue to grow if the top is eaten with low/no recovery time needed 6. slight slowing of growth due to reduced photosynth as plant matter gone

Question 9

What are some defences against herbivory? 5

Answer 9

1. Woody plants eg. acacia – thorns, secondary metabolites 2. some spp aren’t bothered by this 3. Grasses - phytoliths 4. These are like small shards of glass in plant 5. some species have very long teeth eg. giraffes so can keep gnawing at cellulose without difficulty

Question 10

How did rinderpest affect the savanna composition in Africa? 4

Answer 10

1. 19th Century Rinderpest (cattle disease) introduced from british colonial cows to no resistance local wild cattle 2. Tree population rose as cattle wiped out 3. 1960s - Rinderpest eradicated but Tree population rose 4. Changes in population, changes in climate, decreased disturbance… e.g. Human induced fire impact also

Question 11

What is the importance of fire in grasslands? 8

Answer 11

Fire – natural or human triggered 1. Grasses tend to be fire tolerant (basal meristem) 2. Hottest part is 1m above ground so meristem is safe 3. Growth resumes immediately if water available 4. Trees tend to be fire sensitive 5. Increased grazing > reduced fuel >less/cooler fire (good for trees) 6. fires started to manage lanscapes and help farmed grazers - growth of new produce 7. tall trees are ok but intermediate sized trees are wiped out 8. some trees have evolved to tolerate fire but it still favours grass

Question 12

What interactions determine the tree-grass balance? 5

Answer 12

Question 13

What are the effects of water and co2 availability on plant growth? 6

Answer 13

1. Weter is limiting in the savanna
2. Decrease water availability lowers Leaf Water Potential
3. Reduces cell expansion > reduced growth
4. Plant closes stomata to retain water
5. Reduced water loss but reduced CO2 entry
6. convergent evolution of C4 photosynthesis to reduce this imapact
7. in places where temp is high therefore not limiting, co2 can become a stronger limiting factor

Question 14

What is C4 photosynthesis? 4

Answer 14

1. co2 sucked in
2. can keep stomata closed by pulling in through cells
3. lose less water
4. bundle sheath around xylem that fix carbon from co2 taken from the cells surrounding them

Question 15

When is C3 or C4 photosynthesis favoured most? 5

Answer 15

1. when grasses evolved, the atmos had a lower co2 conc
2. created a selection pressure particlarly prevalent in high temp
3. C4 favoured with increasing temp
4. C3 favoured with increasing co2
5. Graph in two triangles with x - temp and y - co2
6. c4 is uncommon in non-grasses
7. C3 favoured at lower light
8. acacia trees can create own ecosystem with c3 under shade of tree (cooler and more humid) and c4 outer