Agriculture Reference
In-Depth Information
If one takes a climate-centric view of species' distribution patterns then it would
appear that
P. ponderosa
is widely distributed under quite different climate condi-
tions as a result of ecological sorting processes, and evolutionary adaptation is not
a major factor determining its current distribution (e.g. Ackerly
2004a
). An
alternative hypothesis is that the most relevant selective factors have not changed,
in particular cold winters and seasonal soil moisture deficit, as well as suscepti-
bility to a surface fire regime - factors common throughout the current range of
ponderosa pine and likely the selective factors responsible for the origin of this
species.
This hypothesis of course does not rule out the possibility of continuing trait
selection in response to subtle differences in climate (Sorensen
et al.
2001
) and fire
regime (Parker
1987
). There is evidence of evolutionary fine tuning within regions
at the microhabitat scale of different slope faces, elevation and soil types (Linhart
1988
; Zhang & Cregg
2005
).
Despite the widespread distribution of tree dominants outside the MTC, the
assemblage of species in Californian conifer forests, both in terms of codominants
and understory species are relatively unique. For example, in ponderosa forests
codominants include endemics to the California Floristic Province such as
Quer-
cus kelloggii
and
Calocedrus decurrens
. The understory also includes endemics in
all main growth forms - shrubs, herbaceous perennials and annuals - although
many of the genera may be shared across different regions (Steele
1988
). Unders-
tory species within California forests are distinctly different from similar forest
types outside California; however, within California these species do not exhibit
high fidelity to particular forest types (Mellmann-Brown & Barbour
1995
),
The mixed conifer forest exhibits a similar pattern in that the assemblage in
California contains many unique elements, not the least of which is the giant
sequoia,
Sequoidendron giganteum
. There is also much intraregional variation
based on substrate characteristics (Abella & Covington
2006
). Thus, while the
distribution of tree dominants may be tied more to fire regime than to climate,
there appears to be a clear MTC signal in the current assemblages.
Fire Management
Native American Management
When Spaniards first settled the California coastline late in the eighteenth
century they inherited a landscape that Native Americans had long managed
with fire. However, compared with other MTC regions, the New World land-
scapes in California and Chile have had the shortest period of human land
management. The oldest reliable archeological records are about 15 000 yrs
before present (BP). North America and South America together have
relatively few sites in this range and there is some contention about their
validity. Considering the number of scientists searching for such sites it seems
