Agriculture Reference
In-Depth Information
Evergreen Oak Woodlands
On moister sites where human impact has been relatively low such as rugged
landscapes and steep mountains, dense evergreen sclerophyllous oak woodlands
occur. Previous uses were related to charcoal production, gathering forest
products and livestock grazing. On previously cleared landscapes following land
abandonment oak forests may recolonize (Pons & Pausas
2006
,
2007
). The most
abundant oak tree is
Quercus ilex
known as holm oak (Roda`
et al.
1999
), but
others include
Q. calliprinos
or Palestine oak in the eastern basin and
Q. suber
or
cork oak on carbonate-free soils in the western basin (Aronson
et al.
2009
). All of
these oaks have the capacity to resprout after fire from either basal buds or
epicormically along the stem (see
Chapter 3
). Because of this strong resprouting
capacity, repeated coppicing for wood harvesting has been carried out since
ancient times and continues on a diminished scale as an important oak woodland
management method. The result has been widespread replacement of single-
stemmed trees with multiple-stemmed trees, and many current oak woodlands
still show the characteristic clusters of trunks from recent or older coppicing. Such
coppicing also alters the understory light regime and species composition in these
communities. Pollarding (i.e. cutting the tree branches) has been traditionally used
in oak woodlands for gathering firewood and improving acorn production to
fatten hogs.
Because of their characteristic density, with high vertical and horizontal fuel
continuity, oak woodlands usually burn as crown fires. The thick and insulating
bark of
Q. suber
protects the stem buds and this species resprouts epicormically
(see
Fig. 3.3a
) after fire (Pausas
1997
; Pausas
et al.
2009
; Catry
et al.
2010
).
Furthermore, in contrast to other oaks, this cork oak possesses underground
dormant buds on a lignotuber (Molinas & Verdaguer
1993
). Although cork oak
is not very combustible itself, it grows among flammable grasses and shrublands
that promote crown fires. Postfire recovery is positively related to tree diameter
and bark thickness, such that small diameter (
<
12 cm) trees generally resprout
only from basal buds (Pausas
1997
). Variations in bark thickness are due not only
to tree size or age but also to the number of years since the last bark-stripping
harvest for cork production (Aronson
et al.
2009
). In fact, bark stripping clearly
increases the susceptibility of this species to fire, and failure to resprout has been
observed in old, recurrently stripped trees (Moreira
et al.
2007
). The fact that cork
oak can quickly regenerate after fire from stem buds gives this species a competi-
tive advantage over other coexisting woody plants. For instance, a mixed cork oak
and pine (maritime pine
Pinus pinaster
or stone pine
P. pinea
) forest has tradition-
ally been encouraged for production of both cork from the oak and wood or pine
nuts from the pine. However, the pines are sensitive to repeated fires and under
such conditions these forests may convert to monospecific cork oak woodlands, a
process often facilitated by the cork industry.
The understory of evergreen oak woodlands may be quite dense, particularly on
moist sites, comprising most of the same shrubs and lianas that appear in maquis
