Biomass productivity: limited biodiversity, limited nutrient cycling

Vegetation and Soils in Arid Environments

Learning Objective

Explain why biomass productivity is low in arid regions, focusing on (i) limited biodiversity and (ii) restricted nutrient cycling, and evaluate how these constraints interact with climate, landforms, soils, water resources and human activity.

1. Climate, Distribution and Scale

• Aridity definition: Mean annual precipitation (P) < 250 mm and potential evapotranspiration (PET) > P, giving a water‑balance deficit.
• Global distribution (place):
  • Subtropical high‑pressure belts (30° N–30° S) – e.g. Sahara, Arabian, Australian deserts.
  • Rain‑shadow deserts – e.g. Atacama (Leeward of Andes), Great Basin (Leeward of Sierra Nevada).
  • Coastal deserts – e.g. Namib (cold ocean upwelling), Peruvian (cold Humboldt Current).
• Scale of analysis (place):
  • Local – micro‑topography, biological soil crusts, individual plant patches.
  • Regional – desert‑wide climate gradients, land‑use patterns.
  • Global – contribution of arid zones to Earth’s carbon budget (< 5 % of total NPP).
• Inter‑annual climate variability (place): The Sahel experienced a marked rainfall decline of ~30 % between the early 1970s and early 1990s, illustrating how short‑term climate fluctuations can exacerbate water stress, trigger vegetation die‑back and accelerate desert expansion.

2. Physical Processes and Typical Arid Landforms

Arid landforms develop under limited water, high temperature fluctuations and intense but infrequent rainfall.

3. Soil Characteristics, Classification and Pedogenic Processes

• Dominant soil orders (FAO/WRB): Aridisols (desert soils) and Regosols (weakly developed).
• Key pedogenic processes:
  • Calcification – accumulation of calcium carbonate (caliche) in the B‑horizon.
  • Gypsum accumulation – in saline environments.
  • Silcrete formation – cementation of surface grains by silica.
  • Pedogenic carbonate leaching – limited by low moisture, creating hardpans.
• Physical properties:
  • Thin A‑horizon (often < 10 cm) over a hardpan or caliche.
  • Coarse texture: high sand/silt, low clay → low water‑holding capacity.
  • Low organic carbon (< 0.5 % w/w) → low cation‑exchange capacity (CEC).
  • Frequent soluble‑salt accumulations (NaCl, CaSO₄) → salinity/alkalinity problems.
• Soil‑moisture retention (AO2 data interpretation): In arid soils the water‑retention curve is steep; at a matric potential of –10 kPa the volumetric water content often falls below 5 %. A simple graph (not shown) can be used in exam questions to compare sandy Aridisols with more clay‑rich semi‑arid soils.

4. Biomass Productivity in Arid Zones

Net Primary Production (NPP) is the main indicator of biomass productivity.

\( \displaystyle \text{BP} \;=\; \frac{\text{NPP}}{\text{Area}} \)

• Typical arid‑zone NPP: < 100 g C m⁻² yr⁻¹ (vs. 800–1500 g C m⁻² yr⁻¹ in tropical forests).
• Low NPP results from the combined constraints of water scarcity, nutrient limitation and low species richness.

5. Vegetation Adaptations, Functional Groups and Biome‑Scale Types

Biome‑scale vegetation types (place):

• Desert shrubland – scattered shrubs, < 5 % cover (e.g., Sahara, Arabian Desert).
• Desert steppe (semi‑arid) – perennial grasses & dwarf shrubs, 5‑20 % cover (e.g., Mongolian steppe).
• Salt‑marsh / desert salt flats – halophytic herbs, occasional dwarf succulents (e.g., Salar de Uyuni).

Contrasting Case Study (AO3)

• Sahara shrubland (North Africa): Dominated by Artemisia herba‑alba and dwarf acacias; annual NPP ≈ 70 g C m⁻² yr⁻¹; species richness ≈ 15 vascular plant species per 100 km².
• Australian spinifex desert (Central Australia): Dominated by Spinifex spp. (grass‑like hummocks) and scattered Eucalyptus saplings; annual NPP ≈ 90 g C m⁻² yr⁻¹; species richness ≈ 30 vascular plant species per 100 km², but with high functional redundancy.
• Both systems illustrate how low species richness limits canopy development and leaf‑area index (LAI ≈ 0.2‑0.5), yet the Australian example shows slightly higher productivity where occasional summer rains boost pulse growth.

6. Limited Biodiversity and Its Effect on Biomass

• Water scarcity restricts the number of species that can complete a life‑cycle; most plants are long‑lived perennials with slow turnover.
• Specialised adaptations (e.g., CAM, deep roots) reduce interspecific competition but also lower overall species richness.
• Low animal diversity (few large herbivores, many opportunistic nomads) reduces grazing pressure but also limits nutrient redistribution via dung and trampling.
• Resulting sparse canopy gives a low Leaf Area Index (LAI ≈ 0.2‑0.5), directly limiting photosynthetic capacity and NPP.
• Species‑richness–productivity relationship: Meta‑analyses (e.g., Tilman 2001) show a positive correlation between species number and ecosystem productivity; in arid zones the relationship is truncated because the species pool is small, reinforcing low NPP.

7. Limited Nutrient Cycling

Soil nutrient pools are small and turnover is slow because:

1. Low organic‑matter input from sparse vegetation.
2. Intense, infrequent rain events cause rapid leaching of soluble nutrients.
3. Microbial activity is moisture‑limited; respiration and mineralisation are episodic.

Mineralisation rate can be expressed as:

\( \displaystyle \text{Mineralisation Rate} \;\propto\; \text{Soil Moisture} \times \text{Temperature} \)

When soil moisture approaches zero, the proportional term approaches zero, dramatically reducing nutrient availability for plant growth.

8. Systems Thinking: Interactions Between Climate, Soil, Water and Vegetation

• Water–soil feedback: Limited rainfall → thin soil horizons → low water‑holding capacity → further water stress.
• Soil–vegetation feedback: Sparse vegetation → low litter → low organic matter → low CEC → reduced nutrient retention.
• Climate–vegetation feedback: Low NPP → low evapotranspiration → higher surface temperature (albedo effect) → reinforces aridity.
• Groundwater pressure (human‑water link): Over‑extraction for irrigation in the Arabian Peninsula lowers water tables, induces land‑surface subsidence and raises salinity, feeding back to further reduce plant productivity.

Understanding these loops is essential for AO2 (analysis) and AO3 (evaluation) questions.

9. Human–Environment Interactions, Pressures and Management Strategies (AO3)

Main pressures:

• Overgrazing – removal of protective plant cover → wind erosion & desertification.
• Unsustainable irrigation – rising water tables → salinisation of soils.
• Groundwater extraction – depletion of aquifers, reduced baseflow to wadis.
• Mining & infrastructure – habitat fragmentation and loss of limited vegetation.

Case Study – The Sahel (West Africa)

• 1970s–80s: Severe drought + extensive livestock grazing → 30 % decline in shrub cover.
• Consequences: Accelerated soil erosion, increased dust export to the Atlantic, negative feedback on regional rainfall.
• Management response: Farmer‑Managed Natural Regeneration (FMNR) – protection of existing tree stumps, selective pruning, and sowing of nitrogen‑fixing legumes.
• Evaluation:
  • Successes: 30‑50 % increase in tree density, 0.2 % rise in soil organic carbon, higher livestock productivity.
  • Limitations: Requires long‑term community commitment; benefits are site‑specific and may not offset large‑scale climate trends.

Other Management Options

10. Change Over Time and Climate‑Change Implications

• Historical trends: Satellite imagery (1972‑2020) shows a net expansion of desert margins in the Sahel and Central Asia of ~ 5 % per decade, linked to reduced precipitation, increased PET and intensified land‑use pressure.
• Future scenarios (IPCC RCP 8.5):
  • Projected 10‑20 % decline in annual rainfall for many subtropical deserts.
  • Increase in PET by 5‑10 % → larger water‑balance deficit.
  • Potential poleward shift of desert‑steppe boundaries by 200‑400 km.
• Implications for productivity: Further reductions in NPP, more frequent dust storms, heightened risk of salinisation where irrigation expands.
• Adaptation considerations: Promote drought‑resilient crops, expand FMNR, develop water‑harvesting techniques (e.g., zai pits, contour bunds), and protect biological soil crusts.

11. Summary

Arid environments exhibit low biomass productivity because two interlinked constraints dominate:

1. Limited biodiversity: Only highly specialised xerophytic and halophytic species survive, resulting in sparse canopy cover, low leaf‑area index and reduced photosynthetic capacity.
2. Limited nutrient cycling: Minimal organic inputs, low microbial activity and rapid leaching keep soil fertility low, further restricting plant growth.

These biological and pedological constraints are reinforced by climate, landform processes, water‑resource pressures and human activities, creating a fragile system that is highly sensitive to both local management actions and global climate change.

12. Suggested Further Reading

• R. J. N. Miller, Arid Land Ecology, 3rd ed., 2021.
• UNEP, Desertification and Land Degradation, 2020.
• J. H. Brown, “Water‑Use Efficiency in CAM Plants”, Journal of Arid Environments, 2022.
• IPCC, Climate Change 2023: Impacts, Adaptation and Vulnerability, Chapter 5 – Drylands.
• D. Tilman, “Biodiversity and Ecosystem Functioning”, Nature, 2001 – for the species‑richness–productivity relationship.