Ask most people what the word “pollen” brings to mind, and you’ll probably hear “hay fever” or “allergies.” But ask a climate scientist the same question, and you’ll have opened a doorway into the past and uncovered the history of how our forests have grown and changed over the past thousands of years.
The pollen in our air each spring and summer eventually finds its way back to earth. When it lands on a pond or body of water, much of it sinks to the bottom. This happens year after year, forming layers on the bottom of our lakes, ponds, and bogs, where the oldest pollen and sediments are on the bottom and the most recent on top. Pollen is resistant to decomposition, and, in the low-oxygen environment of lake bottoms and bogs, it is preserved, as are some larger plant parts, which help in dating each layer.
Taking core samples of these underwater sediments, scientists can identify specific species of pollen and create a history of which plants were growing in our area and when. Here in Vermont and New Hampshire, climate scientists have assembled a complete record of the last 14,000 years or so of our local forest history by analyzing this layer-cake of historical pollen.
Reasonably precise dates can be determined by measuring the amount of radioactive carbon found in the layers of sediment containing the old pollen. The method is called carbon 14 dating, which relies on the radioactive decay of carbon 14 (C14), a small amount of which is present in every living thing. Carbon 14 is a radioactive isotope of carbon that is continually being formed in the upper atmosphere and incorporated into all living things as they respire.
The ratio of C14 to C12 (the “regular,” common isotope of carbon) in living things is constant. But when a living thing dies, it stops taking in new carbon, and the amount of C14 it contains begins to gradually decline as the radioactive isotope decays into other elements. The longer something has been dead, the less C14 it contains. Scientists know the rate at which C14 decays, and so they are able to take the organic matter found with the pollen, measure its ratio of C14 to C12, and roughly determine how long ago it was deposited.
This technique is effective back to about 50,000 years ago, at which point not enough C14 remains in organic matter to be sampled. But that’s more than enough time to date the pollen in Vermont and New Hampshire, the oldest of which dates from the end of the most recent ice age, a mere 14,000 years ago.
So, what have these techniques – sorting and identifying pollen, and taking C14 dates – revealed about our forest over time? The pollen tells a story of how the range of distribution of each tree species has shifted through the years in response to changing environments. As the climate has warmed and cooled, the trees native to our forest have responded – sometimes thriving, sometimes declining. Species that today grow together did not always, suggesting that each species responds to a complicated set of environmental conditions.
The histories of spruce and fir provide an interesting example. Spruce was among the first trees to arrive here after the ice sheets melted, and its pollen shows up in cores from the White Mountains that are about 12,000 year old. It peaks in abundance around 11,000 years ago, then declines and becomes rare until only the past 2,000 years or so. Spruce pollen is scarce during the period of maximum warmth – about 5,000 years ago – and returns with the relatively recent cooling of the past few thousand years. Fir, in contrast, becomes common about 1,000 years after spruce and persists at densities similar to the present. The spruce-fir forest of our New England mountains, then, is a “temporary” phenomenon of only the past few thousand years. Changes in climate have exerted a large effect on spruce, but not on its present-day cohort, fir.
This ought to give us pause when we stop to consider that our climate is currently warming at a faster rate than at any time since the end of the last ice age. These changing conditions will favor some species over others, although not necessarily in predictable ways. Climate scientists estimate that the earth is expected to warm 3-10 degrees Fahrenheit over the next century. If this seems like a somewhat abstract concept to you, consider this: the last time it was that warm around here, there was no spruce in our forests.