Alien Species and Fire - Fire in Mediterranean Ecosystems - page 332

Box 12.1 Two Grass Fire Cycles

A very valuable contribution to understanding invasive plant mechanisms is
the notion that grasses can alter fire regimes in ways that promote their further
invasion (D'Antonio & Vitousek 1992 ; D'Antonio 2000 ). This grass fire cycle is
not a single model but we can distinguish two very different grass fire invasion
processes that alter fire regimes in profoundly different ways. One is the
perennial grass invasion of fire-sensitive woodlands and forests and another
is the annual grass invasion of fire-adapted shrublands ( Fig. B12.1.1) .
The grass invasion of fire-sensitive woodlands is dependent on elevating fire
intensity and killing woody plants that regenerate poorly after fire. This usually
involves perennial grasses because the higher biomass of these aggressive grasses
is required to generate sufficient fire intensity to promote canopy gaps, and the
underground vegetative structures of perennial grasses protect them during high-
intensity fires, as well as providing a bud source for regeneration. An example of
this invasion process is the fire-driven C 4 grass invasion of tropical woodlands
(Mack & D'Antonio 1998 ). In temperate regions the invasion of the C 3 bamboo-
like grass Arundo donax also fits this model (Coffman et al. 2010 ).
This strategy is ineffective in fire-adapted shrublands because these
shrubs are resilient to high fire intensity; nonetheless, some shrublands are

(a)

Ecotone

Fire-
promoting
C4
grasses

Fire-sensitive
forest

Grassland Forest

Trees killed and don't
resprout and lack
seedling recruitment

Perennial
grass
productivity
leads to
high intensity
fires

Grass thatch
ignites
and spreads
fire

Grass
rhizomes and culms
survive and resprout

Fig. B12.1.1 Grass fire cycles: (a) invasion of fire-sensitive forests by increasing fire
intensity and (b) invasion of fire-adapted shrublands by increasing frequency and
decreasing fire intensity.

Continued