19.4 Plant Macrofossil Assemblages
Greater accuracy in interpreting the relative abundance of taxa in the fossil pollen record can be obtained by calibrating pollen accumulation values
based on influx into modem pollen traps (Whitlock,
1993; Fall, 1997). Although pollen records provide
information about changes in relative plant species
composition over time, patch or age class structure
of vegetation cannot be detected.
EXAMPLES
1. Postglacial vegetation history was examined
in the Harvard Forest, Massachusetts, at two sites
that differed in size and hence in the spatial resolution of the vegetation reconstructed from each
site (Foster andZebryk, 1993). A lO-ha swamp provided information about regional changes in vegetation, whereas a 0.006-ha hollow provided a local
vegetation history within a Tsuga canadensis forest. Postsettlement land-use activities were found
to produce the greatest changes observed in regional and local vegetation in the postglacial period.
2. Pollen analysis was an important component
of the reconstruction of change in Indiana
oak-prairie vegetation over multiple temporal
scales (Cole and Taylor, 1995). The reconstruction
confirmed a gradual successional change from pine
forest to prairie-oak savanna during the period A.D.
200 to 1000 and provided a historical context for
rapid changes now taking place.
3. Whitlock (1993) examined pollen and plant
macrofossils from lake sediments in Grand Teton
and Yellowstone National parks to determine postglacial vegetation changes in response to climate,
geology, and elevation. Modem pollen samples
were also collected from 58 sites in the region to
understand the relationship of the modem pollen
rain to present-day vegetation. Pollen spectra were
found to compare well with current vegetation
zones, although Pinus and Artemisia pollen types
were consistently overrepresented in samples.
19.4 Plant Macrofossil
Assemblages
Two sources of plant macrofossils, or plant material visible to the naked eye, are available for reconstructing vegetation through time: those present
in sediments from lakes and wetlands (Birks and
Birks, 1980) and those deposited in middens by
packrats (Neotoma species) and other small animals
(Betancourt et aI., 1990).
277
19.4.1 Plant Macrofossils in Sediments
Plant macrofossils are often analyzed in conjunction with pollen from sediment or peat cores. Segments of cores are washed through screens of various sizes and the plant material retained, such as
seeds, fruits, and leaves, is identified through comparison with herbarium material and other reference collections (Whitlock, 1993). Because of the
heterogeneous nature of macrofossil deposition in
sediments, macrofossil analysis alone cannot be
used to reconstruct the kinds of time sequences of
vegetation change that are possible using pollen
analysis (Birks and Birks, 1980). However, plant
macrofossils can be associated with chronological
frameworks established for pollen records taken
from the same sediment sample. The relatively
large size of macrofossils results in limited transport, producing a sample with a spatial resolution
within the local watershed (Birks and Birks, 1980).
Plant macrofossils and pollen provide complementary information for vegetation reconstruction. The
consideration of plant macrofossils along with
pollen records is useful because macrofossils, in
contrast to pollen, can often be identified to species
and can also confirm the local presence of a species
(Birks and Birks, 1980; Mayle and Cwynar, 1995).
In addition, some species that are infrequently represented as pollen may occur as macrofossils (e.g.,
Dryas octopetala) (Birks and Birks, 1980). Limitations of the use of plant macrofossils include their
relatively small quantities and heterogeneous distribution within sediments (Birks and Birks, 1980).
EXAMPLES
1. Mayle and Cwynar (1995) investigated the
pollen and plant macrofossil content of sediment
from six lakes in New Brunswick and Nova Scotia, Canada, to determine vegetation response to an
800-year cooling event with rapid onset and rapid
subsequent warming. Macrofossil evidence enabled Mayle and Cwynar (1995) to identify particular arctic-alpine species in the vegetation. The detection of very rapid vegetation changes (50 to 100
years), as well as lags in response for some species,
has implications for understanding vegetation response to future climate change.
2. Changes in species composition in a subalpine forest were studied over the past 8000 years
using pollen and plant macrofossil analysis of samples derived from a small bog in western Colorado
(Fall, 1997). The combination of the two methods,
along with calibration of values based on modem
pollen influx, resulted in a detailed reconstruction
Greater accuracy in interpreting the relative abundance of taxa in the fossil pollen record can be obtained by calibrating pollen accumulation values
based on influx into modem pollen traps (Whitlock,
1993; Fall, 1997). Although pollen records provide
information about changes in relative plant species
composition over time, patch or age class structure
of vegetation cannot be detected.
EXAMPLES
1. Postglacial vegetation history was examined
in the Harvard Forest, Massachusetts, at two sites
that differed in size and hence in the spatial resolution of the vegetation reconstructed from each
site (Foster andZebryk, 1993). A lO-ha swamp provided information about regional changes in vegetation, whereas a 0.006-ha hollow provided a local
vegetation history within a Tsuga canadensis forest. Postsettlement land-use activities were found
to produce the greatest changes observed in regional and local vegetation in the postglacial period.
2. Pollen analysis was an important component
of the reconstruction of change in Indiana
oak-prairie vegetation over multiple temporal
scales (Cole and Taylor, 1995). The reconstruction
confirmed a gradual successional change from pine
forest to prairie-oak savanna during the period A.D.
200 to 1000 and provided a historical context for
rapid changes now taking place.
3. Whitlock (1993) examined pollen and plant
macrofossils from lake sediments in Grand Teton
and Yellowstone National parks to determine postglacial vegetation changes in response to climate,
geology, and elevation. Modem pollen samples
were also collected from 58 sites in the region to
understand the relationship of the modem pollen
rain to present-day vegetation. Pollen spectra were
found to compare well with current vegetation
zones, although Pinus and Artemisia pollen types
were consistently overrepresented in samples.
19.4 Plant Macrofossil
Assemblages
Two sources of plant macrofossils, or plant material visible to the naked eye, are available for reconstructing vegetation through time: those present
in sediments from lakes and wetlands (Birks and
Birks, 1980) and those deposited in middens by
packrats (Neotoma species) and other small animals
(Betancourt et aI., 1990).
277
19.4.1 Plant Macrofossils in Sediments
Plant macrofossils are often analyzed in conjunction with pollen from sediment or peat cores. Segments of cores are washed through screens of various sizes and the plant material retained, such as
seeds, fruits, and leaves, is identified through comparison with herbarium material and other reference collections (Whitlock, 1993). Because of the
heterogeneous nature of macrofossil deposition in
sediments, macrofossil analysis alone cannot be
used to reconstruct the kinds of time sequences of
vegetation change that are possible using pollen
analysis (Birks and Birks, 1980). However, plant
macrofossils can be associated with chronological
frameworks established for pollen records taken
from the same sediment sample. The relatively
large size of macrofossils results in limited transport, producing a sample with a spatial resolution
within the local watershed (Birks and Birks, 1980).
Plant macrofossils and pollen provide complementary information for vegetation reconstruction. The
consideration of plant macrofossils along with
pollen records is useful because macrofossils, in
contrast to pollen, can often be identified to species
and can also confirm the local presence of a species
(Birks and Birks, 1980; Mayle and Cwynar, 1995).
In addition, some species that are infrequently represented as pollen may occur as macrofossils (e.g.,
Dryas octopetala) (Birks and Birks, 1980). Limitations of the use of plant macrofossils include their
relatively small quantities and heterogeneous distribution within sediments (Birks and Birks, 1980).
EXAMPLES
1. Mayle and Cwynar (1995) investigated the
pollen and plant macrofossil content of sediment
from six lakes in New Brunswick and Nova Scotia, Canada, to determine vegetation response to an
800-year cooling event with rapid onset and rapid
subsequent warming. Macrofossil evidence enabled Mayle and Cwynar (1995) to identify particular arctic-alpine species in the vegetation. The detection of very rapid vegetation changes (50 to 100
years), as well as lags in response for some species,
has implications for understanding vegetation response to future climate change.
2. Changes in species composition in a subalpine forest were studied over the past 8000 years
using pollen and plant macrofossil analysis of samples derived from a small bog in western Colorado
(Fall, 1997). The combination of the two methods,
along with calibration of values based on modem
pollen influx, resulted in a detailed reconstruction