Changing patterns in the Holocene pollen record of
northeastern North America: A mapped summary - Bernabo and Webb 1976
J. Christopher Bernabo
Grad student at
Brown University
First student of
Thompson Webb III
Thompson Webb III
Professor Emeritus
at Brown University Department of Geosciences
PhD in atmospheric
sciences at University of Wisconsin
Adviser to J.
Christopher Bernabo, Jack Williams
John (Jack) W. Williams (blurb writer)
Professor at University of
Wisconsin-Madison Department of Geography
PhD Brown University Department of
Geosciences
Fourteenth student of Thompson Webb
III (before Jennifer Bravo and after Dr. Benjamin Felzer)
Unique for its time: studies spatial vegetation shifts
instead of merely temporal shifts at a single site
Uses Holocene pollen records across the Midwest, east coast,
and southern Canada to study species geographic shifts throughout the Holocene
Chronological summary of vegetation shifts:
11,000-10,000 BP (warming)
Spruce becomes less abundant
Oak and herbs replace spruce in the
west
Oak replaces pine in the southeast
10,000 - 9,000 BP (warming)
Spruce replaced by pine
Oak replaced by pine in southeast
9,000-8,000 BP (warming)
Not as much change but similar
trends to before
8,000-7,000 BP (Laurentide ice sheet collapses)
Spruce disappears from southern
range
Herbs expand east
7,000-4,000 BP
Gradual trends
Herbs decrease in the west
Oak and spruce decrease in their
southern ranges
4,000-2,000 BP
Spruce increased in Manitoba
First southward movement of boreal forest
during Holocene
Oak reduces in the southeast
Herbs reduce in the west
Pines reduce somewhat sporadically
2,000-500 BP
Few changes, mostly small and
localized
500 BP - present (European settlement)
Pine and oak decreased
Herbs increased
Likely due to deforestation and
settlement
Figure 2 shows that the largest changes in spruce, pine, and
oak abundances occurred in the early Holocene between 11,000 and 7,000 BP.
Species shifted northward with glacial retreat. Changes ramped up again in the
past thousand years, especially in the herbs. A sign of deforestation due to
European settlement.
Figure 21 shows spruce migration northward, especially in
the first 3,000-4,000 years of the Holocene
Figure 21 shows a northward shift of the
conifer-hardwood/deciduous forest ecotone during the early Holocene. This
tracks with shifts of the boreal forest and with expansion of birch, maple, beech,
and hemlock (hardwood forest stables).
Discussion Questions:
1) Will species and ecosystem shifts of the same magnitude be
mirrored today in response to climate change?
2) Could you observe similar range and abundance shifts with
your research system? Does necessary data exist for your study system?
3) North-south shifts can be attributed to changing
temperatures, but what factors may have contributed to the east-west shift observed
in herbs?
Comments
1. This question about species and ecosystem shifts for plants is an important focus for a lot of researchers right now. At the height of the holocene warming period I believe that temperatures were 1-2 degrees C warmer than they are today. Under most climate change models, we are predicting a similar increase in temperatures unless there are big changes in global greenhouse gas emissions. The big difference here is time. While holocene warming occurred over thousands of years, our predicted warming will occur over the next ~100 years. Plants shift their range slowly, which means a lot of plant species will struggle to deal with such rapid temperature shifts. While I think some species/groups will see large shifts, other groups will probably have greater mortality and range reductions instead of shifts.
2. Similar range shifts occurred for Pinon-Juniper woodlands during the holocene. PJ woodlands shifted north and their overall range size increased. See: Richard F. Miller and Peter E. Wigand (1994) ‘Holocene Changes in Semiarid Pinyon-Juniper Woodlands’, BioScience, 44(7), p. 465. doi: 10.2307/1312298.
3. I would expect precipitation and glaciation to drive east-west range shifts in herbs.
I am a little confused by figure 4. The bottom panels are comparing herb pollen to pre-settlement vegetation and modern land use. Is the bottom right panel showing pre-settlement vegetation with modern land use (vertical stripes) on top?
If the comparisons for oak, spruce and pine used tree inventory data (post-settlement), why didn't they just use a map of only current land use/vegetation?
2. The prevalence of macrofossils is much lower than the prevalence of microfossils such as pollen, due mainly to their relative abundance in life. I would not expect to be able to make the same conclusions for small mammals on a large regional scale, at least not with the same resolution and clarity that the pollen record provides. But maybe gaps in preservation can be predicted by studying taphonomic processes. I’m curious to see what others that are more familiar with paleobiology know about this.
I have a question about how pollen data is presented: does the variation in abundance of pollen at each time point signal anything other than the depositional environment? It sounds like the authors don’t consider variation of amount important as long as the influx of pollen is above a certain threshold (p.4 under Pollen cores)
1) I wonder if anyone has made recent range maps like this with modern distributions of plants or other taxa to document rapid anthropogenic climate change?
2) The large carnivore/scavenger bird groups that I work with from a slightly earlier (late Pleistocene) time period experienced many extinctions, similarly to mammal communities at the time. The species that survived into the Holocene likely experienced shifts in foraging behavior and diet that allowed them to survive. The direct effects of climate change on bird ranges are difficult to see because like Toni said, macrofossils often aren't as well preserved and there isn't really enough data to determine the ranges of these species from fossils.
3) A similar pattern of expansion of grasslands with simultaneous range contraction in forests and vice versa can also be observed over tens of thousands of years in southern Africa, which is thought to explain speciation patterns of some African vertebrates. Precipitation and temperature would be driving forces behind these vegetation shifts.
For Fig. 4 I think you're right, Jessie, that the modern land use is superimposed on earlier vegetation distributions. I think the authors are trying to show that areas with heavy agricultural use post-human settlement had a reduction in oak pollen, but it's not well explained in the text.
Toni, to answer your question, abundance data is generally very difficult to gather from fossils. Also, pollen cores can mostly only be collected from lakes because of the way they are deposited, so the data you get is presence/absence and an estimate of diversity for whatever pollen was blowing around in the area during that time period.
In response to comments made about the velocity of anthropogenic climate change versus that in the past ... I suggest you investigate the warming that occurred at the end of the Younger Dryas cold episode, some 11.5 ka.
we can talk about this more. Velocity of the response is an important question!
2. Ponderosa pine will likely increase in higher elevations and decrease in lower elevations over the next several decades. Fire will likely remove much if not all of the mix conifer forests in upper elevations and be replaced by ponderosa pine while lower ponderosa pine forests have the same thing happening to them in lower elevations for shrublands/pj woodlands. In the context of this paper, I would expect shrub and Pinion Juniper pollen to generally increase, ponderosa pine pollen to stay comparable, while other mix conifer species drastically decrease. This has been seen in western forests already and has been projected to continue to happen in models that look at future changes in forested landscapes across the US.
3. I expect that the herb dominance was controlled by disturbance patterns, predominantly fire that could happen at lower return intervals with denser surface fuels if the climate had dry periods encouraging fire to occur. As the glaciers retreated, I would assume that regional climate patterns also changed that would have impacted fire, wind, and biotic disturbances that these trees experienced.
On side notes, I found it interesting how, although the general patterns agree between modern pollen and forests landscapes, there were pretty big differences in specific areas of where pines and oaks dominate in specific parts of the map. I know that these were handdrawn maps, but it still seems like the variation could call into question some of the smaller, more nuanced patterns that were found. I would also be curious to have these records matched with charcoal records of the areas as well to see if there were fire patterns that could describe some of the shifts in vegetation, which could explain the expansion of the BAFT pollen group.
- I tried checking out the North American Pollen Database-- curious about the features and availability and was disappointed (assuming I was looking in the right place). It's not very user-friendly and contains many broken links. I wanted to look at possible data for the Southwest and figure out if there was enough to understand ecotone shifts in the last 11,000 years.
- When they discussed the 5 groups, they addressed that they excluded Cyperaceae because of the "localized vegetational conditions at the site of deposition" -- does this mean that for these types of studies we must exclude small, localized vegetation types? And/or we cannot consider groups that have specific, non-uniform dispersal and/or deposition?
- Has anyone done studies on past and present Pinyon pollen in the Southwest?
- Is there still a lack of definition when it comes to "ecologically significant features such as ecotones"? Has this been rectified in the last 40 years? Do the authors ever define a "steep gradient" for isopolls?
- I'm curious about the distribution of pollen and its relationship to vegetation cover-- I would have liked to see a graph looking at this relationship and potential variation in these relationships.
These questions are great!
1. I was thinking about this too-- what does this look like today? I agree with all the above thoughts about rapid climate change occurring now and in the future. My qualm is about the extreme habitat loss and fragmentation. I think this introduces an incredible confounding variable that will both influence pollen dispersal and deposition. I'm wondering how you would control for this or factor it in.
2. I often work with closely related, congeneric species and based on this paper, I'm not sure fossil pollen would be useful given that pollen types are lumped into large groups.
3. I agree with the east-west comments above and I'm wondering what role 1) animal migration and 2) wind play in east-west shifts in vegetation.