When is a "forest" a savanna?

I've dealt with this question briefly in earlier posts, but I am prompted to revisit it after coming across a paper with this title from an international group focused primarily in tropical savanna (the authors are from India, South Africa, Australia, North Carolina, England, and Germany). (See reference at end) I was struck that despite the completely different flora and geography, the underlying concepts were almost identical with those of our Midwest savannas.

The discussion in this paper begins with what is called the "savannization" of tropical forests due to logging, often followed by fires. "While such degraded forest areas...may 'look' like savannas due to low tree cover, their functional ecology in terms of which species predominate and how these communities respond to...[disturbance] is entirely different from that of true savannas."

The table here contrasts the characteristics of the savanna and forest.

Table 1 Comparison of physical environments, species composition and traits of dominant tree species in savannas versus forests.

Habitat type	Mesic savanna	Forest
Environmental descriptors	High-light understorey	Low-light understorey
	Frequently burnt	Fires rare, catastrophic
Vegetation composition	Trees	Trees
	Forbs	Forbs
	C4 grasses	C3 grasses
Adult trees
Architecture	Relatively shorter	Relatively taller
	Narrower canopy diameter for a given basal area	Wider canopy diameter for a given basal area
Bark	Thick bark	Thin bark
Canopy	Lower specific leaf area	Higher specific leaf area
	Open crowns and higher light penetration through canopy	Dense crowns and lower light penetration through canopy
	Post-fire recovery of canopy either epicormic, or from protected apical buds	Limited post-fire recovery of canopy
Saplings	Many have vertical pole-like architecture	Varied, branched and unbranched architecture
	High root: shoot ratio	Low root: shoot ratio
	Large underground storage	Low underground storage
	Post-fire resprouting common under frequent, intense fires	Post-fire resprouting rare under frequent, intense fires
Seedlings	Rapid acquisition of resprouting ability through early allocation to root	No obvious acquisition of resprouting ability
	Persist through competition with C4 grasses and repeated fire to sapling stage	Cannot persist through competition with grasses and repeated fires
Reproductive strategy of tree community	No or few species are obligate seeders, reproduction through root-suckering common	Reproduction through root-suckering

Ratnam et al. 2011.

One of the key characteristics is the presence of C4 grasses in the savannas, and C3 grasses in the forests. Given sufficient biomass (brought about by substantial rainfall) C4 grasses are highly flammable so that fire becomes a fundamental feature of a relatively humid (mesic) savanna. As the authors state, there is no need to distinguish between natural and anthropogenic fire, because anthropogenic fire has long replaced natural fire in almost all ecosystems. "What is important is that C4 grasses have high productivity, low decomposition rates,...and a fuel structure that readily carries fire and dries out rapidly in the dry season."

Because of the association with pyrogenic C4 grasses of savanna trees, they are perforce [and have evolved to be ] highly fire-tolerant. On the other hand, under mesic conditions trees associated with C3 grasses are shade tolerant and do not tolerate fire.

There is an exception in the Midwest here, since fire tolerant oaks are able to thrive under mesic conditions, provided fire occurs. As discussed by Abrams and Nowacki, in the absent of fire in eastern North America mesophication occurs and oak forests are replaced by maple, beech, basswood, and other fire sensitive trees.

The underlying concepts in Table 1 are worth detailed analysis.


Ratnam, Bond, Fensham, Hoffmann, Archibald, Lehmann, Anderson, Higgins, and Sankran. 2011. When is a 'forest' a savanna, and why does it matter? Global Ecology and Biogeography 20: 653-660.