Agriculture Reference
In-Depth Information
forest, which do not burn as readily as fynbos. As in these other MTC regions,
obligate resprouters are nearly all broadleaf species with fleshy fruits dispersed by
birds and have forest/thicket affinities (e.g.
Rhus
,
Diospyros
,
Maytenus
,
Heeria
,
Myrsine
). They do not recruit after fire but establish between fires, often under
perches frequented by birds (Manders
et al.
1992
; Cowling
et al.
1997b
).
Fire and Community Dynamics
Which species or functional groups occur together in communities and how do
different community assemblies respond to perturbations? The answers have
generally been sought in differing abilities to compete for resources or to escape
competitors by dispersal. However in fire-dependent ecosystems, the emphasis has
been on the ability of a species to persist in the face of fires at varying intervals,
intensities and seasons and competitive structuring of communities has received
less attention. In fynbos, as in other MTC ecosystems, there have been numerous
studies on the mode of recovery from burning, survival of propagules from one
fire to the next, and fire-stimulated recruitment. The mix of species in a commu-
nity has been viewed as a function of the individual attributes of a species and
whether these would allow them to persist in a given fire regime. The implicit
assumption is that the vital attributes of a species would remain the same regard-
less of the species mix. This view has been amended since the early 1990s following
the recognition of competitive interactions between different functional groups,
particularly overstory vs. understory plants, or between different growth forms
in the understory or overstory (Cowling & Gxaba
1990
; Moore & Noble
1990
;
Keith & Bradstock
1994
; Vlok & Yeaton
1999
; Bond & Ladd
2001
; Keith
et al.
2007
). For example, a non-resprouting shrub might persist over a range of fire
frequencies but would be unable to do so in the presence of fast-growing overstory
shrubs which suppress its growth and flowering.
The great majority of fynbos species have some form of fire-stimulated
recruitment. They are also mostly very intolerant of shading. This is readily
apparent when fynbos stands are invaded by alien trees, such as northern hemi-
sphere conifers and Australian acacias and hakeas (see
Chapter 12
). Most fynbos
species die under dense alien trees while some persist but fail to flower. A small
group of species tolerate shade, mostly the obligate resprouters that are distin-
guished by their broad leaves and fleshy fruits and affinities to closed-forest
lineages, but also several graminoids, especially Cyperaceae, and some geophytes
such as
Oxalis
spp. (Holmes & Cowling
1997a
). Shading by tall native species also
suppresses the understory and can markedly influence community dynamics
(Cowling & Gxaba
1990
; Vlok & Yeaton
1999
,
2000
). Tall proteoid shrubs
dominate many fynbos communities and shade out understory plants. Most of
these overstory species are serotinous obligate seeders, killed after every fire, so
that the shading effect is cumulative, increasing with age since fire. Resprouting
graminoids that spread vegetatively after fire are also important understory
competitors, suppressing seedlings that emerge after a fire. Vlok & Yeaton
(
1999
,
2000
) have argued that, in the absence of shading by dense proteoids,
