Agriculture Reference
In-Depth Information
An indication that fire has not been an important ecological disturbance factor
in some parts of central Chile is the widespread occurrence of fire-sensitive
arborescent cacti such as
Echinopsis chilensis
on dry equator-facing slopes in
matorral communities. In contrast, in the North American California/Baja
California MTC region, fire has apparently eliminated this growth form of cactus
from shrublands and restricted it to steep canyons where it cannot be reached by
the frequent fires in the region.
Frequent fires on the matorral landscape since the arrival of Euro-Chileans
have undoubtedly had some impact on the structure and composition of matorral
communities. There is no question that matorral stands in proximity to urban
areas exhibit much more open structure in a mosaic of shrubland and non-native
annual grassland. As with other MTC shrublands, fires in matorral communities
burn with varying intensities, often leaving a patchy distribution of lightly burned
stems and intense fire areas with little aboveground biomass remaining. It is
reasonable to assume that intense fires offer a relative advantage to vigorously
resprouting species over colonizing species, while low-intensity fires would
not favor either group (Segura
et al.
1998
). The limiting factor for shrub
seedling establishment after fire may be more an effect of seed availability
due to lack of seed dormancy. When seedlings do occur after fire they survive
better and grow faster than in areas cleared by hand or in competition with herb
growth (Holmgren
et al.
2000b
). Without more detailed and extensive studies of
fire in matorral ecosystems it is difficult to fully understand causality in impacts
on shrub establishment and growth given the confounding effects of four centuries
of wood gathering for charcoal production and intensive browsing by domestic
goats.
We hypothesize that fire was a factor in Chile through much of the Miocene
and has been lost for only a few million years since the Andes reached near
their current height at the end of the Miocene (see
Chapter 10
). This hypothesis
leads to the conclusion that the presence of fire-adaptive traits such as ligno-
tubers in contemporary matorral are relictual traits that have persisted in the
absence of fire until the late Pleistocene arrival of humans reintroduced fire
(Armesto
et al.
2010
). The virtual lack of fire in the region for the last few
million years accounts for the lack of fire-dependent reproduction in either
woody or herbaceous species as delaying reproduction would be rapidly
selected against in the absence of fire. This hypothesis answers the quandary
posed by Fuentes
et al.
(
1994
) of how to account for fire-adapted traits in the
absence of a contemporary lightning regime. They hypothesized that this was
the result of volcanic activity that has provided a reliable ignition source for
matorral fires (Fuentes & Espinosa
1986
). However, it seems doubtful this
would have been predictable enough in time and space to drive trait evolution,
particularly in most of the MTC region. Another putative source of natural
ignitions are fires thought by some to be spontaneously ignited as drops of
sticky nectar on
Puya
leaves create a lens that focuses solar heat, though this
has never been rigorously tested.
