Lecture 1 - Chronosequence

Question 1

why is fraser isand a model ecological system?

Answer 1

> it has a very distinct soil age sequence (chronosequence)

Question 2

where does fraser island’s sand orginate from?

Answer 2

> NSW (Hawkesbury, hunter, clarence river catchments)
it is at least 350 000 years old (mainland Great Sandy National Park, some of the region 730 000 years old)
transport of sand still occurs today

Question 3

how does ‘time travel’ occur on Fraser Island?

Answer 3

> 500 year East Coast young dunes to 700 000 year old dunes form soils
dune chonosequence spans a short distance
ecosystems are shaped by, and shape, the soils
biomass build - up and subsequent decline characterise the ecosystems

Question 4

what are the stages of succession on Fraser?

Answer 4

> earliest succession:
- Pioneer beach vegetation
- low diversity , low biomass
Early succession:
- Scribbly gum (Eucalyptus racemosa) woodland
- greater species diversity and biomass than pioneer community
late succession (‘climax’):
- Eucalyptus pilularis and Syncarpia hillii
- even higher biodiversity and biomass (?)
late succession (maybe):
- rainforest
- often not considered strictly part of the chronosequence due to greater water availability, others argue that it is
retrogression (declining phase):
- Banksia dominated low woodland and heathland
- low biodiversity (?) and biomass

Question 5

what is the concept of ecological (primary) succession?

Answer 5

> in the absence of soil renewal, climax vegetation ‘degrades’ to form retrogression (declining) vegetation. Nutrient stocks decline (leach), from the soil over geological time spans, resulting in nutrient-impoverished soils.

Question 6

Describe the Fraser island chronosequence

Answer 6

> succession (build up stages)
- coastal pioneers, woodland, tall eucalypt forest, (rainforest)
retrogression (declining)
- wallum woodland, heathland
mangroves: marine and terrestrial influences

Question 7

what shapes the vegetation of Fraser Island

Answer 7

> distinct changes in soil and associated nutrient availability.
nitrogen enters soil, availability increases and then declines
initial soil phosphorous stocks become available and then decline
as soils age, nutrients become available (sand to soil) but decline in very old soils due to net loss in the absence of soil renewal (volcano, glaciation)

Question 8

describe the important plant-sol interactions

Answer 8

> plants acquire essential elements from soil (in addition to C, O, H from air and water)
in most ecosystems, N and P most limit plant growth
macro-nutrients are essential for plant survival
- >1mg/g of plant dry weight
- N: proteins
- S: proteins
- P: DNA, energy
- K: water relations
- Ca: cell wall
- Mg: Chlorophyll
- Si: Structure

Question 9

describe plant root adaptations and symbiosis

Answer 9

> allow access to nutrients in the depauperate soils
Early succession: Casuarina, Acacia
- symbiosis with N2 fixing bacteria (“root nodules”)
- biological nitrogen fixation is prevalent in low N soils
climax: Acacia, Eucalyptus
- symbiosis with mycorrhizal fungi
- 80% of plants have mycorrhizas, different types (endo/ecto)
- Ectomycorrhizal fungi shown here occur only in certain woody species
- organic matter breakdown, nutrient acquisition
- higher surface area
REtrogression: Proteaceae ( banksia, hakea, grevillea)
- cluster roots
- proteaceae, sedges and some other species do not use mycorrhizas but cluster roots
- enhance access to P

Question 10

what are characteristic of the youngest dunes?

Answer 10

> youngest dunes are unstable, exposed to wind and salt, no soil has developed, minimal vegetation and yellow sand
herbs and grassers are first colonisers, shrubs and trees follow

Question 11

What are characterisitic species from behind the beach? (pioneer communities)

Answer 11

> Casuarina - nitrogen fixing
acacia sophorae - nitrogen fixing
Banksia integrifolia
Pandanus tectorius

Question 12

which species use nitrogen fixation?

Answer 12

> approx. 10% of extant plant species form symbiosis with N-fixing procaryotes (bacteria, cyanobacteria)
many species in the legume family (Fabaceae) fix N
also species in Casuarinaceae, other flowering plant families but also ancient plants such as cycads

Question 13

how does nitrogen fixation occur?

Answer 13

> casuarina root nodules house bacteria in genus frankia
red colour in nodules is indicative of active nodule (leghaemoglobin), low oxygen levels protect nitrogenase enzyme which fixes N2 into N for plants
Nitrogen enters terrestrial ecosystems via biological N fixation
only certain bacteria and cynobacteria (symbiotic, associated or free living) possess the enzyme nitrogenase and associates biochemical machinery to convert inert N2 to reactive NH3
nitrogen accounts for approx. 80% of nutrients acquired by plants from soil, only N-fixing plants can supplement their N demands with N2 derived from air

Question 14

what are some plants represented in the older dunes?

Answer 14

> woodland communities
> Eucalyptus
> Bloodwoods
> Banksia
 - cluster roots

Question 15

describe cluster roots

Answer 15

> cluster roots explore a small volume of soil intensively by excluding numerous chemicals and enzymes to release nutrients from soil

Question 16

describe the types of Mycorrhizals

Answer 16

> 80% of terrestrial plants have mycorrhizal (root fungus) symbiosis 
> fungus explores soil and exchanges nutrients for photosynthates
>ecto (woody plants)
 - outside root
> endo (woody and herbaceous)
 - inside root cells
> ericoid (ERicaceae)
 - coils inside epidermal cells
 - large
 - enzymatic capabilities

Question 17

describe the sand and nutrients that start off succession

Answer 17

> Aeolian sand deposition formed Fraser island
the deposited sand has coating of mineral nutrients
inside = Quartz grain
outside = sesquioxide (clay) coating contains essential nutrients, p, Mg, Ca, K for plant growth

Question 18

what are the sstages of soil development?

Answer 18

> plants and microbes strip nutrient coatings from sand grains and incorporate nutrients into biomass
colour of sand goes from yellow to white (1-> 2)
accumulation of organic matter results in grey colour
A1 horizon contains nutrients in organic matter
A2 horizon is leached (vertical movement of nutrients with water movement)
B horizon accumulates nutrients as Fe-Al-oxides and other minerals

Question 19

how can the fraser island chronosequence be reset?

Answer 19

> disturbance, such as fire, volcanoes, glaciation etc
if the disturbance it to great, it can cause collapse of the system
otherwise, the system can make a partial recovery