Agriculture Reference
In-Depth Information
depend on a year of high precipitation as well as the seasonal timing of rain
(North
et al.
2005a
; League & Veblen
2006
). Not surprisingly, the pattern of gap
formation relative to wind currents (Gordon
1970
) and seed predation also affect
regeneration (Shearer & Schmidt
1970
).
Thus, patterns of postfire recruitment are expected to be complex due to
species-specific differences in response to the coincidence of fire, masting and
precipitation. Additionally, surface fuel type will play a significant role because
in ponderosa pine forests grass regrowth can inhibit pine regeneration (Pearson
1942
), particularly if seed fall is delayed very long after fire. In mixed conifer
forests shrub recruitment from dormant seedbanks may have a similar inhibi-
tory effect (McDonald & Fiddler
1990
), but this is often very patchy, and
sometimes shrubs have positive nurse-plant effects (Keyes
et al.
2001
,
2007
).
On top of all this, there are marked elevational differences in response to rapid
climatic changes along steep mountain slopes (van Mantgem
et al.
2006
;
Gworek
et al.
2007
).
Understory Recovery
Understory species vary with forest type and region (van Wagtendonk & Fites-
Kaufman
2006
). Most perennial herbs in the understory survive low-intensity
surface fires and resprout, but are killed by high-intensity fires (Knapp
et al.
2007
; Rocca
2009
). In ponderosa pine forests the rhizomatous subshrub
Chamae-
batia foliolosa
is persistent even after fairly high intensity fires (Rundel
et al.
1981
).
Similarly, fire intensity affects survivorship of soil seedbanks and thus the extent
of postfire annuals. Some annuals that are restricted to gaps in mature forests have
dormant seedbanks in the understory that germinate in response to fire; however,
strict pyro-endemics, which are only found on burned sites, are rare in these
forests (Keeley
et al.
2003
). Postfire recovery patterns have a deterministic com-
ponent resulting from species-specific life history components and a stochastic
component resulting from site history and timing of disturbance (Halpern
1989
)
and surface fuel load (Rocca
2009
). Persistence of understory species is dependent
on microhabitat variation in light and moisture (Naumburg & DeWald
1999
;
North
et al.
2005b
).
Several shrub species,
Ceanothus integerrimus
,
C. cordulatus
and
Arctostaphylos
patula
, are widespread throughout the conifer region in California, and establish
after fire from soil seedbanks (Kauffman & Martin
1991
). These shrubs form an
early seral stage (Knapp
et al.
2007
) and, depending on soil depth, may outcom-
pete tree regeneration for water resources (Royce & Barbour
2001
). After several
decades shrubs may be shaded out by overstory trees (Conard & Radosevich
1982
), but seedbanks appear to persist for hundreds of years (Quick
1961
).
In permanent shrub patches, shrubs such as
Quercus vaccinifolia
,
Chrysolepis
sempervirens
,
Ceanothus cordulatus
and
Ribes
spp. resprout after fire and, due to
severe substrate conditions, are not displaced by forest trees (Kauffman & Martin
1990
; Nagel & Taylor
2005
).