Agriculture Reference
In-Depth Information
(1) Although macrofossils provide the most extensive record of past landscapes, it
is not an unbiased sampling of past environments; rather it can be highly
biased against arid land plants and thus against fire-prone vegetation. Other
than fossil sites resulting from volcanic ash, deposition of fossils usually
occurs in wetland sites, and plants growing far from water are poorly repre-
sented (MacGinitie
1969
; Spicer &Wolfe
1987
; Robinson
1989
). What portion
of the upland vegetation is represented is not known, although it has been
suggested that plants farther than 1000 m from a lowland deposition site may
not be represented (Rich
1989
). Illustrative of this is the observation that most
North American Miocene or later fossil floras from western North America
have one or more species of the water-dependent
Salix
, and commonly a half
dozen or more other wetland taxa. Thus, poor representation of arid land
plants in a fossil flora limits our ability to determine the origin of these taxa
and history of arid land vegetation and ecosystem processes such as fire. As
arid land vegetation expands there is a greater chance that lowland deposition
sites will record these taxa. Bias is strongest with macrofossils, as pollen fossils
may sample over a much wider part of the landscape. The macrofossil record
is highly biased against herbaceous taxa, except for wetland taxa such as
Typha
,
Carex
and
Equisetum
. The pollen fossil record is less biased against
growth form but heavily biased in favor of wind-pollinated taxa as well as
being a less specific taxonomic indicator, usually only to genus and sometimes
only to family. An additional bias is that arid land ecosystems may lack
suitable deposition sites and thus not leave any fossil records (e.g. a possible
example may be the lack of Miocene floras from southwestern North
America).
(2) In trying to understand the origins of the MTC plant communities, the focus
should not be based strictly on a reconstruction of events within the present-
day MTC regions (e.g. Axelrod
1973
,
1989
). Climate considerations above
suggest that semi-arid MTC may have developed in other regions and moved
during the late Tertiary into the present MTC regions (e.g. Kemp
1981
). It is
to be expected that the origins of many MTV taxa were outside the present
MTC region and those taxa migrated into present MTC regions in the late
Tertiary or more recently.
(3) Macrofossils are often used to reconstruct the paleohistory of plant commu-
nities but since they fail to include most of the herbaceous component it could
lead to erroneous conclusions about ecosystem structure. For example, con-
temporary California and Arizona chaparral are dominated by many of the
same evergreen taxa, but Arizona differs markedly from California in the
presence of C
4
bunchgrasses and duel winter and summer rain postfire floras
(Fotheringham
2009
). These two chaparral systems represent very different
plant communities, but if the only information available were macrofossil
assemblages one might conclude these were the same ecological assemblages,
or if viewed over time they might be considered representative of ecological
stasis arising from similar environments (e.g. DiMichele
et al.
2004
).
