Agriculture Reference
In-Depth Information
weather. Historically lightning-ignited fires likely originated upslope in conifer-
dominated forests and during the twentieth century these fires have been success-
fully suppressed at a small size (see Coniferous Forest section below).
Postfire Community Response
Chaparral and sage scrub shrublands are highly resilient to fires at intervals of
30-150
yrs. Within this range these communities return to prefire functional states
rapidly with little or no loss of species. The process has been described as “auto-
successional” because all components of the prefire state are present after fire and
colonization plays a limited role in returning these systems to their prefire state
(Hanes
1971
). In other words, plant regeneration is almost entirely endogenous,
involving dormant seedbanks that germinate after fire and resprouting from per-
sistent roots and basal lignotubers. As a consequence, the spatial extent of shrub-
land area that burns has relatively little impact on success of vegetative recovery.
Species diversity is substantially higher in the first postfire year (see
Chapter 11
),
unless there is a significant rainfall deficit, in which case diversity may peak in the
second postfire year (Keeley
et al.
2005c
). Diversity declines in later years although
this trend may be reversed by very high rainfall in early seral stages (Keeley
et al.
2005a
).
Detailed postfire studies have shown that at a community scale (0.1 ha), roughly
90%of the cover in the fifth year after fire was from species present in the first postfire
year, and because of the timing of fires and phenology of dispersal it is certain all of
this is due to endogenous regeneration (Keeley
et al.
2005a
,
2006b
). Of particular
interest though is the observation that this represented only about 50%of the species
richness present in year 1. Thus, it would appear that diversity in early seral stage
shrublands is strongly affected by colonization. However, essentially all species
present in the fifth postfire year were present immediately after fire somewhere within
the fire perimeter; thus, rather than colonization of the burned site, this pattern is
more an example of mass action effects (Shmida & Wilson
1985
). In other words
many species had limited populations immediately after fire and these populations
expanded throughout the burned area during the first five years after fire.
As the shrub canopy returns, herbaceous species persist largely as dormant
seedbanks. The intensity of herbivory, competition and resource limitations are
primary factors that limit herb growth in these mature stands (Swank & Oechel
1991
). These factors have selected for deeply dormant seeds that persist in the soil
seedbank until triggered to germinate by fire (Keeley & Fotheringham
2000
).
þ
Ephemeral Postfire Flora
The one feature that separates California shrublands from all other North Ameri-
can ecosystems is the unique and highly diverse postfire flora comprising many
species endemic to only postfire environments (see
Fig. 11.2c
)
. The brief pulses of
resources present after fire are utilized by many short-lived species that vary in
