Agriculture Reference
In-Depth Information
mortality, because none of these conifer species have the capacity to resprout,
either from the base or epicormically. These fire-caused gaps play an important
role in both the maintenance of community and landscape diversity of flora and
fauna but also in the regeneration of forest trees (Keeley & Stephenson
2000
;
Keeley & van Mantgem
2008
).
Tree Regeneration
None of the dominants in these forests have serotinous cones, nor do they
accumulate dormant soil seedbanks. Regeneration is heavily concentrated in open
gaps created by patches of high-intensity burning. Species such as
Pinus ponderosa
are dependent on gaps for seedling recruitment, with high light at the soil surface
and absence of litter, both of which are needed for successful establishment
(Fowells
1965
). Other species such as
Abies concolor
are more opportunistic, as
they readily recruit seedlings in the understory of unburned forests, but also
exhibit massive recruitment on burned sites (Tappeiner & Helms
1971
; Mutch &
Parsons
1998
; North
et al.
2005a
; van Mantgem
et al.
2006
).
Size of fire-caused gaps plays an important role in tree regeneration. Seedling
recruitment is dependent on parent seed trees dispersing seeds into the gaps; thus
as gap size increases the seed rain declines (McDonald
1980
; Greene & Johnson
2000
). Gaps on the order of 0.01-10 ha generally have successful recruitment, but
much larger gaps pose a problem for regeneration. In general the very smallest
gaps have abundant seed fall but seedling growth rate may be inhibited by
shading, whereas larger gaps sometimes favor higher seedling growth rates if seeds
can reach them (McDonald & Abbott
1994
).
Although mineral soil and high light environments characteristic of gaps play
key roles in seedling establishment, another feature of gaps that enhances seedling
recruitment is that they retain higher soil moisture longer into the summer
drought (Ziemer
1964
). Another factor critical to recruitment is the vulnerability
of young trees to even low intensity surface fires. A key attribute of gaps is the
absence of overstory trees, and thus a slower rate of surface fuel accumulation and
a greater probability of subsequent fires missing them until saplings reach suffi-
cient size and bark thickness to survive fires (Keeley & Stephenson
2000
). This
pattern is reflected in the fact that most fire-scar dendrochronology studies show
that the interval between establishment to the first fire scar is nearly always longer
than subsequent fire intervals.
Unlike crown fire shrublands where many dominant species recruit in a pulse of
seedlings from dormant soil-stored seedbanks, none of the forest tree dominants
exhibit this trait. Forest regeneration is dependent on parent seed trees surviving
fire, which disperse seeds into fire-generated gaps. All of the dominant tree species
have periodic masting cycles;
Abies concolor
commonly produce massive seed
crops on a 2-3 yr cycle and
Pinus ponderosa
and other species have a longer
periodicity (Fowells
1965
; Krannitz & Duralia
2004
). As a consequence, recruit-
ment is largely a function of the coincidence of fire and a masting cycle (Keeley &
van Mantgem
2008
; see
Fig. 3.6
). In addition, successful recruitment will often
