Tom's Blog

Sunday, March 27, 2016

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
Environmental descriptors
High-light understorey
Low-light understorey

Frequently burnt
Fires rare, catastrophic
Vegetation composition


C4 grasses
C3 grasses
Adult trees

Relatively shorter
Relatively taller

Narrower canopy diameter for a given basal area
Wider canopy diameter for a given basal area
Thick bark
Thin bark
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
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
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.


Post a Comment

Subscribe to Post Comments [Atom]

<< Home