Environmental Engineering Reference
In-Depth Information
Table 20.1
Change in fynbos ecosystem structure and function with
Acacia saligna
invasion indicating key disturbance
variables and disturbance response variables.
Transition
state (see
Figure 20.3)
Description of invasion impacts
Key disturbance
variables
Key response
variables
1a
Low-moderately dense
Acacia
stand (0-50%
canopy cover)
- Individual
Acacia
trees establish; high annual
production of persistent seeds (10,000 m
−
2
canopy)
- Seeds disperse several metres from tree by
wind; up to 5 m by ants that bury them safe
from rodent seed predation
- Rapid
Acacia
soil seed bank accumulation
under adults
- Adaptation to fi re: adults resprout and
hard-coated seeds germinate following heat
pulse; massive recruitment, densifi cation and
coalescence of
Acacia
post-fi re
(Richardson and Kluge 2008)
Increased invader
biomass
High invader seed
production
Altered community
composition (but still
relatively intact fynbos
community)
1b
Moderate-closed
Acacia
stand (50-100%)
- Dense (
Increased invader
biomass
Increased
competition for
resources
Altered soil nutrient
availability
Altered microbial
systems
Suppression of native
species
Altered community
composition (but intact
fynbos soil seed bank
community)
Altered community
structure
Declines in species
richness
75% canopy cover)
Acacia
stand
alters plant community composition and
structure (shading and other effects)
- Dense
Acacia
stand alters faunal community,
granivory and dispersal
-
Acacias
grow taller than fynbos creating more
biomass and leaf litter
- Nitrogen-fi xing ability results in higher total
nitrogen in litter and soil, increased nitrifi cation
and nutrient-cycling rates
(Holmes and Cowling 1997b), (Musil 1993),
(Yelenik
et al
. 2004)
>
2
- Fires under hot, dry conditions result in
high-severity fi res (in both standing and felled
Acacia
) that kill buried seeds and volatilize soil
organic material; recruitment of deeper buried
seed promoted, especially
Acacia
and other
myrmecochores
- Indigenous seed banks decline with each fi re
cycle under dense
Acacia
owing to mortality,
germination and lack of replenishment; seed
bank decline particularly acute in lowland
fynbos (following one fi re cycle of dense
Acacia
)
(Holmes and Cowling 1997a), (Holmes 2002)
Increased fi re
intensity
Dominance of
Acacia
seedlings
High seed mortality
Depleted native seed
bank
Decrease of native
functional groups
3
-
Acacia
dominates ecosystem with only a few
weedy, or nitrophilous species co-occurring in
the early post-fi re period; increased nitrogen
promotes competitive species such as alien
annual grasses and
Ehrharta calycina
(Yelenik
et al
. 2004)
Increased soil
nitrogen levels
Enhanced growth
of other alien
species (or
weedy
indigenous
species)
Loss of all but weedy
herbaceous functional
group