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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?