Exploring the tug of war between positive and negative interactions among savanna trees: Competition, dispersal, and protection from fire

Bacelar, F.S.; Calabrese, J.M.; Hernandez-Garcia, E.
Ecological Complexity 17, 140-148 (2014)

Savannas are characterized by a discontinuous tree layer
superimposed on a continuous layer of grass. Identifying the
mechanisms that facilitate this tree-grass coexistence has
remained a persistent challenge in ecology and is known as the
``savanna problem". In this work, we propose a model that
combines a previous savanna model (Calabrese et al., 2010),
which includes competitive interactions among trees and
dispersal, with the Drossel-Schwabl forest fire model,
therefore representing fire in a spatially explicit manner. The
model is used to explore how the pattern of fire-spread,
coupled with an explicit, fire-vulnerable tree life stage,
affects tree density and spatial pattern. Tree density depends
strongly on both fire frequency and tree-tree competition
although the fire frequency, which induces indirect
interactions between trees and between trees and grass, appears
to be the crucial factor controlling the tree-extinction
transition in which the savanna becomes grassland. Depending on
parameters, adult trees may arrange in different regular or
clumped patterns, the later of two different types (compact or
open). Cluster-size distributions have fat tails but clean
power-law behavior is only attained in specific cases.


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