How the Study of Plants revealed the Variability of Climate

The news of the resuscitated "Ice Age plant", regenerated from 30.000 year old tissue conserved in permafrost, is an intriguing discovery that will help to better understand the evolution of ecosystems during one of the most dramatic epochs of earth history - the Pleistocene-Holocene transition some 10.000 years ago, characterized by strong climatic oscillations from glacial to interglacial conditions.

Already the study of fossil plants played an important role in the reconstruction of the environment and the climate of this period.  The German explorer Alexander von Humboldt (1769-1859) was one of the first naturalists to scientifically discuss the distribution of plants related to environmental factors like temperature, rainfall, latitude, height and soil - one of the most famous results of this work is the depiction of the vegetation belts of the Andes.

Fig.1. Vegetation zonation in the Andes, from the "Berghaus-Atlas", a supplement to Humboldt´s work "Kosmos", published in 1645-1862.

Based on these observations it became clear that the fossil remains of plants can be used to reconstruct the environmental factors during the lifetime of these plants.
In 1876 the Norwegian botanist Axel Gudbrand Blytt (1843-1898) published a book on his observations about the distribution of the Ice Age flora on the Scandinavian Peninsula. He recognized various plant communities - defined as Arctic, Subarctic, Boreal, Atlantic, Subboreal and Subatlantic - and suggested, based also on layers of peat where he observed the same plant communities in a certain order, that these communities were the results of various climate driven migrations waves on the peninsula. For Blytt especially the change of dry and wet periods controlled the distribution and migration of plants. These supposed changes could also explain the discoveries of layers with tree stumps in peats all over Europe, grown there when the climate was more favourable for trees. 

Fig.2. Blytt´s map of the distribution of plant communities in Norway, from BLYTT 1876.

The Swedish geologist Lennart von Post (1884-1951) used particular plant remains to infer past climatic oscillations. Flowering plants produce pollen grains covered by a chemically very stable substance named Sporopollenin, therefore pollen grains usually are well preserved in soils and sediments. The structures, like spikes or pores, on the surface of pollen grains are species-specific and can be used to determine from which plant-species the pollen was produced. Von Post counted and identified many hundred of pollen grains found in specific depth-intervals of sediment cores and plotted the relative percentage of every species in a diagram. He found that there was a succession of different plants; cold periods during an Ice Age were dominated by pollen from trees adapted to cold and humid conditions, like birch or pine. During warmer periods the pollen of these species disappeared and new tree species, like oak and fir, appeared in the pollen diagram. When the landscape finally became occupied by humans the amount of tree pollen decreases, as the forest is replaced with fields of grass or crop and the pollen of these plants dominate in the sediment core. 

The study of pollen, or palynology, is still today an important tool that helps to reconstruct local environment and climatic changes, the stratigraphy of recent deposits, human impact on the landscape, the rise and fall of civilizations and is even used to solve criminal cases.

Bibliography:

BLYTT, A. (1876): Essay on the Immigration of the Norwegian Flora during Alternating Rainy and Dry Periods. Alb. Cammermeyer, Christiana, Oslo, Norway: 89
HILGEN, F.J. (2010): Astronomical dating in the 19th century. Earth-Science Reviews 98: 65-80
POST, V. L. (1944): The Prospect for Pollen Analysis in the Study of the Earth´s Climate History. New Phytologist Vol. 45: 198-203