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.

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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

Fagus 15 8

Acer 8,000 4

Tilia 20,000 18

Pollen Production

• Pollen production is species-specific. 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 pollen-producing plant.

X-axis: % pollen grains

Y-a

xis:

Tim

e

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.