Savannas (rainfall greater than evaporation) in wet seasons

Savannas cover
around an eighth of the world’s landmass and are one of the world’s largest
biomes 1. These ecosystems mainly occur within
subtropical and tropical zones around the world and are often dictated by a
strong alternation of a positive water regime (rainfall greater than
evaporation) in wet seasons and a negative balance in dry seasons 18. Their defining characteristic is the codominance
that occurs between woody vegetation and grasses. As mentioned prior, the
factors that influence the relative codominance of grasses and trees within
savannas and the dynamic feedbacks that exist between the two are still in the
process of being understood 1-3. The exact mechanisms that allow
coexistence without displacement and the exact factors that determine the
tree/grass ratios in various African savanna ecosystems remain unclear 1, 19. Attempts have been made to
model tree/grass codominance, but researchers differ on the characteristics
that result in stable or unstable systems 1. These different models for savanna tree-grass
dynamics have been grouped into two major categories, competition-based models
and demographic-bottleneck models.

 

The first
category is that of competition-based models, which argue the fundamental role
of competitive interactions in fostering tree-grass coexistence 20. This approach states that trees and grasses
coexist in savannas because they occupy different niches, allowing them to
acquire and partition limiting resources 20. This stems from root niche theory, developed
in the 1970’s, which states that tree cover is determined by the stratification
of root profiles between grasses and trees when competing for water in the soil
21. Trees tend to have deep root structures that
allow them to gain access to deeper soil water while relatively shallow-rooted
grasses are superior in accessing surface soil water 21. Thus, the major limiting factors in the
function and structure of all savannas are believed to be water and nutrients in
the first category 22.

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The second
category is that of demographic-bottleneck models, which states that tree/grass
codominance within savannas persists due to various disturbance factors
(climate, fire, and grazing) limiting tree germination, sapling establishment,
and eventual tree maturity 2, 20, 23-25. The demographic-bottleneck
approach states that the equilibrium state of many savannas should either be
forest or grassland, and that the co-existence of grasses and trees represents
a state of disequilibrium stemming from disturbances that limit the transition
to either state. More realistically, the recruitment of trees in particular is
inhibited due to these various environmental factors, preventing a continuous
closed canopy from ever truly forming. 26. This approach implies that coexistence between
trees and grasses does not result from a long-term and stable equilibrium 18.