Agriculture Reference
In-Depth Information
Riparian
Riparian communities comprise mixtures of hardwood trees and shrubs. Distri-
bution of riparian communities is controlled by drainage patterns as these are
restricted to stream courses and often the same plant assemblage is distributed
across a large elevational gradient (Harris
1987
; Holland & Keil
1995
; Rundel &
Sturmer
1998
). Indeed, although the taxa may change, the same genera dominate
riparian habitats far outside MTC California:
Acer
,
Alnus
,
Baccharis
,
Fraxinus
,
Platanus
,
Populus
,
Salix
,
Pluchia
and others. Almost all of these taxa are winter
deciduous, which stands out in stark contrast to the evergreen flora that domin-
ates the surrounding upland landscape. This seasonal pattern reflects the trade-
offs in carbon gain during summer when water availability is sufficient to maintain
large leaves but winter day length and temperature is insufficient to maintain a
viable carbon balance (e.g. Mooney
1972
).
The annual flooding cycles have been a far greater selective force than periodic
fires (Bendix
1997
). Essentially all woody species in these communities have the
capacity to resprout after top-kill from either flooding or fire (Davis
et al.
1988
;
Busch
1995
). In most years, regardless of the aboveground water flow, these
woody species will have access to underground water for much of the dry summer
season and thus live fuel moisture will have a dampening effect on fires. At the
level of individual plants, access to soil moisture will enhance their capacity to
survive and resprout after fire. At the community level the higher fuel moisture
will act as a heat sink, often preventing fire spread across stream courses (Dwire &
Kauffman
2003
). During extreme droughts low fuel moisture will reduce the
capacity of this vegetation to dampen fire intensity and thus result in severe crown
fires with complete crown scorch and less resprouting.
Seedling recruitment in these riparian habitats is highly influenced by annual
flooding cycles that scour substrates and select against long-term soil seed storage.
Recruitment after fire will be a function of flowering and seeding behavior of
mature trees and resprouts that survive fire. In most woody species seed dispersal
occurs in late winter and seeds must germinate quickly as viability is extremely
short, on the order of weeks (Vaghti & Greco
2007
). This recruitment pattern
occurs annually regardless of fire. Following high-intensity fires, locally available
seed sources may be lost and the regeneration delayed until seeds disperse in from
other riparian sites (Dwire & Kauffman
2003
).
Oak Savannas and Woodlands
Savannas with a grass understory are dominated by one of several oak species,
including winter deciduous
Quercus lobata
and
Q. douglasii
, evergreen
Q. agrifolia
and semi-evergreen
Q. engelmannii
(Barbour
1987
). Surface fires are carried
by herbaceous fuels, and trees survive fire because of thick bark and also deep
roots that maintain high foliage fuel moisture, which acts as a heat sink and
dampens fires. During extreme droughts fuel moisture declines, making these trees
