Root Zone and the Rhizosphere in the SCP–Scarp Landscapes

Learning goals

Now: Applying session 1a concepts to real WA scenarios

By the end of this block you should be able to:

• Apply effective root zone and PAW concepts to different soils
• Compare Banksia, Jarrah, and pasture root‑zone water use
• Reason about recharge and salinity risk from root‑zone changes on the SCP–Scarp
• Connect root-zone hydrology to your Landscape Management Brief (Assessment 2)”

Scenario overview

You will work in groups with 4 idealised sites:

1. Site 1 – SCP Bassendean dune + Banksia woodland
2. Site 2 – SCP Bassendean dune + pasture (same soil different vegetation)
3. Site 3 – Darling Scarp laterite + Jarrah forest OR pasture (two vegetation scenarios)

For each site we consider:

• Choose the most plausible root distribution (A, B, or C)
• Identify the effective root zone
• Reason about seasonal water use and recharge

Site information (handout)

Task 1 (10 min) in groups: Choose root profiles

For each site and vegetation type:

1. Choose the most plausible root length distribution (A, B, or C) and justify briefly.

   • Site 1: Banksia woodland (Bassendean sand)
   • Site 2: Pasture (on same Bassendean sand)
   • Site 3: Jarrah forest vs pasture (laterite)

2. Mark the effective root zone (depth range supplying most water in late summer).

What to consider?

When selecting root distributions:

Task 2 (10 min in groups): Seasonal water use

Using PAW by depth band and your chosen root distributions:

For each site:

• Identify which depth bands contribute most to transpiration:
  • At the end of winter (soil relatively full)
  • At the end of summer (soil mostly depleted)
  • Decide whether there is unused deep storage (water that remains but is not accessible to roots).

Write answers as short bullet points per site.

Guiding questions

Task 3 (10 mins): Recharge and salinity (Scarp)

Site 3, Scarp laterite and compare: Jarrah forest vs pasture, same soil type. Answer:

1. How does annual ET differ between Jarrah and pasture, given their effective root zones?
2. Which vegetation leads to higher recharge to groundwater? Why?
3. How might this change baseflow and salinity risk in downslope areas?

Write 3–4 bullets, explicitly referring to:

• Effective root-zone depth
• PAW distribution
• Root traits

Task 4 (10 min): Recharge on SCP sands

Sites 1 & 2 Bassendean dunes, compare:

• Site 1: Banksia woodland
• Site 2: Pasture (same soil)

Discuss:

1. Does changing from Banksia to pasture significantly alter root-zone depth?
2. Does it change recharge and leaching? (consider deep watertable vs perched water)
3. How do salinity risks here compare with the Scarp?

Summarise in 3–4 bullets.

Discussion SCP sites (5 mins)

Questions for class discussion:

• What root distribution did you choose for Banksia vs pasture?
• What happens to:
  • Effective root-zone depth?
  • Seasonal water use pattern?
  • Recharge and leaching?

Discussion Scarp site (5 mins)

Questions for class discussion:

• How did Jarrah vs pasture differ in:
  • Root-zone depth
  • Use of deep storage
  • Recharge and salinity risk?

Capture key points:

• Jarrah: deep effective root zone, high ET, lower recharge
• Pasture: shallow roots, lower ET, higher recharge, higher salinity risk

Video relief (4 min) - Life underground: the rhizosphere

Linking to the Landscape Management Brief

These root‑zone insights feed directly into your assessment:

• Diagnosing current state:
  • Root-zone depth and PAW under existing vegetation
  • Where recharge and leaching are likely
• Analysing management options:
  • Clearing vs reforestation vs deep‑rooted perennials
  • Banksia restoration on SCP sands vs Jarrah restoration on the Scarp
• Building evidence-based recommendations:
  • Use root-zone reasoning, not just “more trees = better”

Key takeaways

1. Effective root zone determines which PAW is actually accessible—total storage ≠ usable storage

• Even when soils have similar total PAW (~60 mm), depth distribution + root traits create very different water use patterns

2. Vegetation strategies differ across landscapes:

• SCP Banksia: shallow exploiter (Option B), relies on transient shallow storage, some phreatophytic access
• Scarp Jarrah: deep miner (Option C), dimorphic roots access pallid zone moisture year-round

Key takeaways (continued)

3. Land-use change alters root zone → changes recharge → different outcomes by landscape:

• Scarp: Jarrah → pasture = root zone shallows dramatically → large recharge increase → dryland salinity in valleys
• SCP: Banksia → pasture = smaller root-zone change → moderate recharge increase → nutrient leaching, less salinity risk (deep watertable)

4. Root-zone reasoning is essential for management:

• Predict hydrological impacts of vegetation change (recharge, baseflow, water quality)
• Design revegetation strategies matched to soil + landscape context

Key takeaways (continued)

5. Root-zone insights are important for your Landscape Management Brief:

• WWhat is the effective root zone under existing vegetation?
• How would clearing vs reforestation vs deep-rooted perennials alter root-zone depth, recharge, and salinity risk?
• Build evidence-based recommendations: Use mechanistic reasoning about root zones, not just “more trees = better”

Tomorrow: Rhizosphere biogeochemistry and nutrient cycling (the other half of Week 5)

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