Forest:Ancient:Nelson Lake

The pollen diagram is from Nelson Lake, Kane County Illinois. It is one of the longest, most complete late-glacial and Holocene (the last 11,500 years) records for Illinois. Today it is the location of Nelson Lake Marsh Nature Preserve, just 35 miles west of downtown Chicago.

How the diagram tells the story
The y or vertical axis of this diagram (the labels going down the left side), show the age of the pollen samples, from 17,000 years old at the bottom. Today's pollen samples are at the top of the diagram (near the zero).

The x or horizontal axis of this diagram has percentage (numbers going from left to right). Each plant species has a set of percentages. This axis measures what percentage of all the pollen found in one core section belonged to each species.

The names of the plants from which pollen was found are located across the top of the diagram.

Put these all together and we can interpret the data, marked in purple ink: (about 17,000 years ago)

Sedge pollen was found to make up 35 per cent of all samples 17,000 years ago.
If you continue across with each species of plant at 17,000 years ago (bottom of the diagram) you will see that spruce trees (in blue) accounted for 25 per cent of all pollen counted.
Add up sedge and spruce and you have 60 per cent of all the pollen.
All the rest of the plant pollen found adds up to the other 40 per cent to make 100 per cent. There was grass and ragweed in the herbaceous layer, and a few of all the other tree types (in green) listed grew in that area, too.

We don't know exactly where each type of tree grew because the winds carried the pollen from them into the lake. However, the diagram helps you can make a mental picture of that forest. You can see a sparse spruce forest and an herbaceous layer of sedges growing under the trees. That is what the landscape around Nelson Lake looked 17,000 years ago. This type of vegetation is called tundra, probably similar to what exists at the northern extent of the forests in Canada today.

Spruce forest
This diagram also shows that about 14,000 years ago,

the spruce forest became more closed (more trees growing close together - up to 50 per cent of all pollen counted.
At the same time, sedge cover decreased (to less than 25 per cent), and ash appeared in the forest.

Spruce trees do not grow under the same climatic conditions as ash trees do today. Ash prefers warmer, wetter conditions that spruce. Because we know the climatic preferences of these trees, we can deduce that the climate was changing from cooler and drier to warmer and wetter by looking at the diagram. During this interval, in fact, the climate was changing from glacial to interglacial, just the conditions deduced from the diagram. The summers were cooler but the winters were warmer, allowing for the presence of ash in the community.

The forest is in transition
Spruce began to decline. By about 12,000 years ago tree species that would be dominant in a different type of forest were starting to spread. Balsam fir, aspen, and birch (green ink in diagram), appeared as the climate became warmer and wetter.

Deciduous forest
Ash peaked (at 25 per cent in the diagram) between 12,000 and 11,000 years ago, when elm and oak (orange ink in diagram) were increasing. By 11,000 years ago, the spruce was gone, and deciduous forest with elm, ash, alder, ironwood, and oak took over. Ash and elm, which today grow on poorly drained soils, indicate high rainfall during this period.

Prairie openings in the forest
After 11,000 years ago, oak became more abundant (up to 40 per cent of total), and hickory appeared. Elm and ash declined, but elm remained important in the forest until about 6500 years ago. Increasing grass and ragweed (yellow ink in diagram) after 11,000 years ago indicate the local development of prairie openings, but forest prevailed over much of the landscape until about 6500 years ago.

Prairie developed
After this time, the tall-grass prairie with scattered groves of oak developed. The development of prairie indicates a trend towards drier climate during the Holocene (our current interglacial). Nelson Lake does not show it so well, but other sites in the region indicate the driest interval was from about 6500-3500 years ago.

The rise of today's landscape
Perhaps you noticed that the diagram was organized to reflect that the species that grew at Nelson Lake during the oldest time period (on the left of the diagram) were slowly replaced by species farther and farther to the right (green to orange to yellow inks). Species that were barely able to survive in the cold glacial climate now grow abundantly. The evergreen and other cold climate trees that grew well during the Ice Age disappeared as the climate warmed.

After 3500 years ago, the essentially modern mosaic of prairie and groves developed. The big increase in grass at the top of Nelson Lake is from aquatic grasses, perhaps wild rice, which became common as the lake filled with sediment and a large wetland developed around Nelson Lake.

Arrival of European Settlers
The uppermost portion of the pollen diagram shows an increase in ragweed about 200 years ago. This coincides with the time of European settlement. As the settlers plowed the prairie sod and cultivated crops, ragweed populations exploded and native grasses and other prairie plants declined. During this time, many forests were removed as well, for fuel, building materials and agriculture.

For more practice in reading this diagram, see the Pollen Graph Activity.