NATF320 - L10 Grassland Savannas

what is a grassy ecosystem

ecosytem with grass

• grasslands, steppes, savannas, open woodlands

can have trees

open ecosystems with sunlight

what is a savanna

grassy ecosystem

• heterogenous

continuous grass layer

• c4 grasses (often)

• c3 grasses (sometimes)

discontinous tree cover

• varianle tree density

disturbance

• herbivores, fire

dry and wet season

• infleunces growing season, influences fire and herbivore

subtropics

savannas are home to species that love

• sunlight, drought, disturbance (fire/herbivory)

• c4 grasses need light

what is a savanna

there is a debate about whether the minimum and maximum tree cover can be used to define savannas

mimum tree cover: savanna vs grasslands

maximum tree cover: savanna vs forest

history of savannas

76 Ma BP grasses apear (late Cretaceous

55-33 Ma BP C3 grassy ecosystems start to appear

8-10 Ma BP savannas become common in tropical Africa and Asia

grassy ecosystems evolutionary younger than forests

arose after continets separated (200 Ma BP)

functionally similar but different species

dry tropical forest does not equal savanna

dry tropical forest lack grasses

dry tropical forest contain different species

wet tropical forest does not equal savanna

wet tropical forest lack grasses

wet tropical forest contain different species

savannas are diverse

savannas are variable

can have dense tree and shrub cover

look at:

• structure

• species composition

  • function

what determines savanna distribution (global)

intermediate rainfall

growing season temperature >10C

whatever determines growth rate of trees and grass

seasonality

long dry season limits tree growth

limits growth days

increases fire frequency and intensity

not useful on small scales

soil nutrients

low concentrations of N and P limit forest growth

• savanna lower nutrient content

• forests higher nutrient content

forest can grow on savanna soil

fire depletes and N and P

forest cover increases N and P

causality is unknown

nutrient differences could be symptom rather than cause

disturbance: alternative biome states )ANS)

when disturbance disappears

when herbivores disappear trees appear

fire kills young trees

when fire disappears, trees appear

what determines savanna distribution

canopy closure vs disturbance

abiotic controls determine growth rates of woody vegetation

growth rate determines time till canopy closure

• high resource environments → high growth rate

• low resource environments → low growth rates

disturbance decreases → canopy closes → shade increases → grasses decreases → forest → disturbance decreases

photosynthesis in plants

calvin cycle

CO2 → sugars + O2

Sugars used for growth

different methods

• C3, C4, CAM

photosynthesis in plants; C3

original pathway, occurs in most plants

creates 3-carbon compound

photorespiration: when hot and dry

1. stomata close ot preserve water

2. inside leaf: concentration O2 increases, concentration CO2 decreases

3. rubico (the compound capture CO2) captures O2 instead

   1. photosynthesis decreases + energy loss

photosynthesis in plants: C4

solotuion to photorespiration

3% of plants

concentrates CO2 in bundle sheath cell

CO2 delivered directly to Rubisco, no contact with O2

Creates 4-carbon compound during photosynthesis

photosynthesis in plants: CAM

collecting sunlight during the day and fixes CO2 at night

adaptation to arid environments (avoids water loss)

example: cacti, pineapple

C3 vs C4 grasses

C3

• evolutionarly older

• photosynthesis ineffienct

• less productive than C4

• shade-tolerant

C4

• evolutionary younger

• photosynthesis efficient

• C4 is better adapted to

  • drought and high temperatures

  • low CO2

• more productive than C3

• shade-intolerant

C4 grasses create ideal savanna conditions

adapted to sunny and dry conditoins

highly productivity → increases fuel load

form continuous layer → helps fire spread

dry easily + high C:N ratio → highly flammable

shade-intolerance → do not survive in forestsq

different types of fire

rarity, rare vs common

intensity, cool vs intense

extent, small vs large

mesic savanna fire

grass burns

low intensity

regular (2-3 year interval)

species adaptedfi

fire in grassy ecosystems

process of renewal

• removes (dead) biomass

• removes shade

burned grass regrows each year

• carbon neutral (over yearly basis)

  • cooling due to smoke

fire and biodiversity

savanna species are adapted to fire, some need it

savannas with diverse fire regime have higher diversity of mammals and birds

burned grasslands attract grazing herbivores

some species need fire

temminck’s courser - only nests in recently burned grasslands

fire in grass ecosystems

oldest form of land management, as early as 40-60 ka BP

savanna fires are declining (even though forests fires increases)

savannas and large herbivores

most diverse large hebirvore assemblage of any biome

highest densities of wild herbivorescon

consequences of herbivore extinctions

herbivores disappeared from many savannas

when herbivory disappeared → fire increased

herbivore body size and herbivory

small herbivores

• can only digest-high quality forage

• strongly influenced by predation

large herbivores

• can digest low quality forage

  • less influenced by predationM

Megaherbivores

herbivores > 1000 kg

unique and large impacts

large body size, can physically modify vegetation

high (absolute) energy requirements , need a lot of food

can digest low-quality vegetatoin, majority of vegetation accesible

mostly immune to (non-human) predation

• can attain high population sizes

growth rate decoupled from seasonality due to long gestation

• can attain high population sizes

two types of forage

grass, browse

grass

structure is homogenous

close ot the ground

high in fiber

abrasive

few chemical defences

browse

strucutrally heterogenous, found at variable height

non abrisive

high in protein

high in pectin

often woody

chemically and physically defeded

herbivores adapted to different forage

browser, mixed feeder or grazer

how do herbivores affect vegetation

browsers and mixed feeders remove woody vegetation

mixed feeders attain highest densities

• they switch food sources based on availability

• key to keeping the savannas open

grazers create grassland heterogeneity

• remove tall bunch grasses and create grazing lawns

migratory grazer attain high densities

• may have been important in the past

diversity ofs peceis is key to keeping savannas open

grazing lawns

short grass pathces with continuous grazing

feature of functional savannas

nutrient hotspots for grazers

grazing lawns does not equal overgrazed

meters to hectares

often iniated by largest herbivores such as megaherbivores )herbivores > 1000 kg(

grazing lawns: good grazing

vegetetaion quality and productivity increases

• grasses have high leaf to stem ration (Measure of palatability)

• fresh regrowth

• ferilisation by herbivore excretement and urine

  • light availability increases → photosynthesis increasesg

grazing lawns: effects

creates grassland heterogeneity

influences fire spread

attracts and facilitates (smaller) grazers

several species depend on lawns

lower tree regeneration

mixed feeders eat seedlings

trampling

low water availability

migration

many savannas likely used to have large migrations

allows herbivores to maintain large population sizes

migrations are disappearing

threats and challenges

climate cahnge and shrub encroachement

conversion into agriculture

displacement of indigenous people

misidentification of savannas

climate change and shrub encroachement

savannas are becoming more shrubby, unclear why

high grazing may faciitate tree growth

high grazing pressure reduces fuel load and can increase tree growth

more common with livestock

• high densities, confied to area, in unfenced areas, wild herbivores migrate

climate change: CO2 increases, and temperate increases and water?

• savannas evolved in a low CO2 world, C3 grass and trees perform better when concentration of CO2 are high

changes in ecological processes/land management

• loss of megaherbivores and fire

  • forest increases and savanna decreases

conversion into agriculture

growing population in Africa, South America and Asia

oppourtinities for agricultural expension

easy to convert

• no tress, perfect for livestock grazing

  • erroneoulsy believed to have low biodiveristy costs )conservation fouccses on forests)

displacement of indigenous peoples

savannas are not wilderness

humans evolved in savannas, they have lived in savannas for millions of years

national parks were not empty spaces

people were evicted to create national parks → colonial invention

today, displacement continuous in the name of conservatoin

maasai fight to remain on their lands

herders living in kenya and tanzania

cultural heritage is celetebrated in europe conservation despite a much shorter shared history

misidentified savannas

savannas are seen as degraded forests

perceived as in need of restoration

• remove disturbances (herbivores and fire)

• plant trees (not savanna trees)

legacy of european colonialism

many europeans believed the untouched, pristine nature was forests

in europe, ceasing management led to forest growth

early modern europe had very little forest left due to poor land management

degraded forests look like savannas

european colonist viewed open ecosystems as degraded forests, due to poor land management by natives

differentiating savannas and degraded forest

there are old-growth savannas (like old growth forest)

• old growth savannas have unique species

species ocmposition is key

• species found degraded forests poorly adapted to savanna dynamics

• leftover forest species

degraded forest can develop into old-growth savannas