Agriculture Reference
In-Depth Information
non-native grasses and forbs. In the absence of human interference this is not
normally a threat because canopy closure generally excludes non-native aliens
from the site. However, under repeated fires and other disturbances non-native
annuals invade and contribute to increased fire frequency that feeds back into
enhanced alien invasion (see
Chapter 12
). As this occurs, the postfire endemic
annual floras and their seedbank are lost from the site, apparently from competi-
tive exclusion with the more aggressive aliens. As this invasion process proceeds
there is reason for concern over regional loss of these postfire endemics. These
observations in California suggest the hypothesis that the absence of such a
postfire endemic flora in the Mediterranean Basin may in part be an artifact of
human disturbance history, which is 50 times longer than in California and has
resulted in only 5% of the original vegetation still persisting (
Table 11.1
).
During early postfire succession in California chaparral there are interactions
between precipitation, diversity and time since fire. Peak diversity is typically in the
first year or two and largely comprises annual species but it is also sensitive to annual
variations in precipitation. Diversity declines during early succession, but even as late
as 5 yrs postfire, high precipitation events can again trigger rapid increases in annual
species diversity, almost to levels in the immediate postfire year (Keeley
et al.
2005a
,
2006b
). However, this later seral community is very different from the immediate
postfire community as it does not include the postfire endemic annuals, and 30-50%
of the species may be ones not present on the site in the immediate postfire years.
These “colonizers” are mostly not ruderal species that have invaded from outside the
burned perimeter, but rather species that were present within the burned area
immediately after fire and have expanded their populations in subsequent postfire
years. This mass effect (Shmida &Wilson
1985
) results in populations of new species
spilling over into sample plots where they were previously absent, although they
typically comprise a very small proportion of the total cover. Thus, if we consider the
total postfire flora over the first 5 yrs, community-scale diversity would be substan-
tially richer than reported in
Table 11.2
. Such temporal niche separation of species
has been reported in Australian shrublands as well (Fox
1995
). Eventually, as the
shrub canopy returns over 5-10 yrs, species diversity plummets regardless of rainfall.
Community-scale plant diversity in California shrublands is a multifaceted
problem (Keeley
et al.
2005c
) and involves: (1) Life history specialization to the
temporal heterogeneity in resources created by a predictable fire cycle, (2) species-
specific responses to characteristics of disturbance events, in particular fire sever-
ity, (3) niche specialization on different features of the landscape that vary
between growth forms, (4) growth forms that further subdivide resources along
other axes such as soil characteristics, phenology, etc., (5) species-specific differ-
ences in response to annual fluctuations in resource availability tied to the critical
role of precipitation in these semi-arid landscapes, and (6) mass effects due to
metapopulation dynamics whereby fluctuations in disturbances and resource
availability result in occasional localized expansion of highly successful core
species' populations. During early succession the importance of these factors
may change rapidly such that similar diversity levels in the immediate postfire
