Download Pollen Analysis

Non-marine paleoclimate records
Pollen Data
Basis for Pollen Studies
• Pollen grains can
directly identify plant
species.
• Pollen is widely
present and
abundantly produced.
• Pollen grains are
resistant to decay.
Alnus
Pollen Studies
• Palynology is the study of pollen
• Pollen grains can be found in caves, lake
sediments, soils, peat deposits, marine
sediments, glacial deposits.
• Pollen data provides information of changes
in vegetation, climate, and human
disturbance of terrestrial ecosystems.
Pollen Production
• Pollen production is inversely proportional to the
probability of fertilization.
– Authogamous plants < Entomophilous < Anemophilous
(self-fertilizing)
(insect-fertilizing) (wind-dispersed)
• Pollen can be produced during different seasons by
different plants.
• How representative are pollen grains of species
distribution and abundance?
Genus
Pollen
Production
Grain/flower
Rate of Fall
(cm/sec)
Pinus
>15 million
2
Picea
200,000
9
Abies
100
40
Poacea
90,000
10
Betula
10,000
1
Quercus
9,000
5
15
8
Acer
8,000
4
Tilia
20,000
18
Fagus
Pollen Production
• Pollen production is speciesspecific. Whereas some plants
can produce 70,000 grains per
anther, others produce up to 100
grains per anther.
• The pollen record is biased
towards wind-pollinated plants
(all gymnosperms and most
angiosperms) because these
plants need to produce vast
quantities of pollen.
Pollen Production
• Anemophilous plants
(wind-pollinated) produce
light, aerodynamically
shaped pollen.
• Pollen deposition depends
on grain shape and weight,
wind velocity, wind
direction, canopy cover
Pollen dispersal
• Travel distance is inversely proportional to
pollen-grain size.
• Pollen grains are filtered as they move
through the canopy.
• Some light pollen grains can be transported
long-distances in the upper atmosphere.
• In general, pollen from low-standing plants
have low probability of dispersal.
Pollen dispersal
• Meteorological conditions control pollen
dispersal.
Pollen and vegetation
Ponds: local vegetation
Lakes: regional vegetation
Pollen percentages in surface samples from eastern Canada.
Pollen and Vegetation
• Surface pollen composition is different than
vegetation composition and abundance.
• Pollen composition at a given point within an
ecosystem is fairly consistent.
• But, pollen compositions at different sites within
the same ecosystem are slightly different.
• However, the difference between pollen
composition among sites in different ecosystems is
far greater than the difference between sites within
a single ecosystem.
Pollen Analysis
• Sediments are collected
• Pollen grains are isolated
from the sediment matrix
via chemical treatments.
• Isolated pollen grains are
mounted onto a glass
slide, and they are
identified and quantified
under a microscope.
Pollen Analysis
• Pollen counts in each
slide are reported in
percentages of the total
pollen count (excluding
wetland or rare species).
• Changes in the % of one
species are interpreted to
reflect a similar change in
the composition of
vegetation.
Pollen Analysis
• Problem: % of pollen
counts could give
unrealistic information
of vegetation
composition if, for
instance, a plant
species is replaced by
an abundantly pollenproducing plant.
X-axis: % pollen grains
Pollen Analysis
• To circumvent biases associated with pollen
production, one could use pollen flux density
values (pollen grain/yr-cm2).
• However, accurate and numerous dates are
needed. Because this is rare, pollen fluxes are
not used very often.
Pollen Interpretation
• Pollen diagrams are
usually divided into
zones to facilitate
interpretations.
• Changes in pollen
composition are
interpreted to indicate
changes in climate or
human disturbance.
St. Paul, MN (Matsch, 1976).
Pollen Interpretation
• Two general interpretative approaches exist:
– Individualistic Approach: Past environmental
conditions are reconstructed on the basis of
present-day ecology and environmental
tolerance and optima of a plant species.
– Assemblage Approach: Past environmental
conditions are reconstructed on the basis
modern plant associations in climate/
biogeographical regions.
Pollen Interpretation
• Individualistic approach
(Midwest):
• Decline in hardwood species
(beech, maple, oak, hickory)
and/or increase in conifer
species (spruce, fir, pine)
indicates cooling.
• Increase in ash and/or elm
indicates wet environments.
• Decline in trees and increase in
grasses indicates drier
conditions.
Pollen record for Money Creek and Pine Creek in southeastern Minnesota
(Baker et al., 2002).
Pollen Interpretation
• Assemblage Approach:
• Modern Analog Technique (MAT): Central
assumption: If two assemblages contain a similar
mixture of pollen grains, then the communities that
produced those assemblages must also have been
similar.
• Another assumption: Plant composition in an
ecosystem is at equilibrium with the environment.
Pollen Interpretation
• MAT: relies on the dissimilarities of modern
and fossil assemblages.
• To determine if they are the same, the
threshold of relatedness is based on
statistical comparisons between paired
populations within and between ecosystems.
Pollen Interpretation
• Hierarchical Analog Approach: Based on
functional analogs rather than on plant
associations.
• First level of analyses: similar to MAT
• Second level: based on plant functional
types.
• Third level: based on plant life forms.
Williams, 2003.
Williams, 2003.
Pollen Interpretation
• Transfer Functions: A polynomial equation
is fitted to observed pollen data.
• Not often used because these functions
assume that only one parameter controls the
distribution of vegetation (pollen).
• Particularly problematic for ecosystems
with no modern analog.
Macrofossils
• Plant remains (leaves, fruits,
flowers, roots, etc) are preserved
in some particular environments
(e.g., swamps).
• Advantage: species can readily
be identified, provides good
radiocarbon dates.
• Disadvantage: Few locations are
available and modern analogs
are difficult to established.
Tree-line (Ecotone)
• Gradual transition from mature dense forest
through open, discontinuous woodland, to isolated
trees or grasslands.
• Artic Ecotone: Boreal forest/tundra
–
–
–
–
Plant macrofossils are collected from soils.
Soil type changes as vegetation changes.
Tree line coincides with July temperature
Tree line also coincides with mean summer position of
the Arctic front.
• Caveats of the record:
• Trees invades grasslands at a faster pace
than grasses colonize forests.
• Trees live longer than grasses, but grasses
grow faster than trees.
• Fire frequency affects tree line.
• Invasion of new species.
• Records are usually incomplete.
• Are modern tree lines at equilibrium?
• What is the lag time?