Grasslands & Savannas

1. Fire-Grazing-Drought Triad

Grasses tolerate fire because their growth meristems are at or below the soil surface, protected from thermal pulses. A typical savanna fire consumes herbaceous biomass in minutes, releases nutrient-rich ash, and spares roots and crowns; grass regrowth begins within days. Woody seedlings, by contrast, lack protected meristems and are killed. Fire interval 2–10 years suppresses woody expansion; longer intervals allow shrub/tree encroachment.

Grazing by large herbivores reduces fuel loads in some areas while creating disturbance mosaics in others. Fire and grazing are antagonistic for biomass but synergistic for maintaining open grassland vs. woodland. Drought aligns with fire season and pushes the woody–grassy balance toward grass. The three drivers interlock in what Bond 2005 called the “black world” of fire-maintained open ecosystems.

2. Below-Ground Biomass

In tallgrass prairie, 50–80% of total biomass sits below ground in dense root networks reaching 2–4 m depth. This underwrites the exceptional drought resistance, the long recovery time from overgrazing, and the enormous soil-carbon stocks (Mollisols) that made US Great Plains agriculture possible in the 19th century. Hart 2021 estimates global grassland soils store ~300 Gt C — more than the atmosphere.

3. C3 vs C4 Photosynthesis

Most temperate grasses use the C3 pathway; most tropical and warm-season grasses use C4. The C4 pathway uses PEP carboxylase in mesophyll cells to concentrate CO₂ in bundle-sheath cells, effectively eliminating photorespiration:

\[ \text{PEP} + \text{HCO}_3^- \xrightarrow{\text{PEPcase}} \text{OAA} \xrightarrow{\text{NADP-MDH}} \text{Malate} \xrightarrow{\text{bundle sheath}} \text{CO}_2 + \text{Calvin cycle} \]

C4 grasses achieve higher photosynthesis at high temperature + low CO₂and have ~2× the water-use efficiency of C3. The global expansion of C4 grasslands 8–6 Mya — during the Miocene CO₂ decline — was a biogeographic revolution visible in tooth-wear and stable-isotope records of fossil ungulates (Cerling 1997).

Simulation: C3/C4 Response Curves

4. Grassland Types

5. Serengeti Megafauna

The Serengeti–Mara ecosystem supports the world’s largest remaining terrestrial mammal migration: ~1.5 M wildebeest, ~250 000 zebra, ~400 000 Thomson’s gazelle following rainfall gradients across 30 000 km². The migration itself is a major disturbance force: mowing grass, depositing nutrients, sustaining lion / cheetah / wild dog / hyena predator guilds, and driving successional vegetation patches. North America once had a comparable system (bison, pronghorn, elk); it was extirpated during the 19th century.

Key References