Non-marine paleoclimate recordsPollen Data

Basis for Pollen StudiesPollen grains can directly identify plant species.Pollen is widely present and abundantly produced.Pollen grains are resistant to decay.

Alnus

Pollen StudiesPalynology is the study of pollenPollen 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 ProductionPollen 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?

Pollen ProductionPollen 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 ProductionAnemophilous plants (wind-pollinated) produce light, aerodynamically shaped pollen.Pollen deposition depends on grain shape and weight, wind velocity, wind direction, canopy cover

Pollen dispersalTravel 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 dispersalMeteorological conditions control pollen dispersal.

Pollen and vegetationPonds: local vegetationLakes: regional vegetation

Pollen percentages in surface samples from eastern Canada.

Pollen and VegetationSurface 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 AnalysisSediments 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 AnalysisPollen 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 AnalysisProblem: % 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 grainsY-axis: Time

Pollen AnalysisTo 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 InterpretationPollen 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 InterpretationTwo 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 InterpretationIndividualistic 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 InterpretationAssemblage 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 InterpretationMAT: 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 InterpretationHierarchical Analog Approach: Based on functional analogs rather than on plant associations. First level of analyses: similar to MATSecond level: based on plant functional types.Third level: based on plant life forms.

Williams, 2003.

Williams, 2003.

Pollen InterpretationTransfer 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.

MacrofossilsPlant 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/tundraPlant macrofossils are collected from soils.Soil type changes as vegetation changes.Tree line coincides with July temperatureTree 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?