GA plants, Fall 2008 GA vegetation in the past 10 million years Lecture notes for PBIO 3650: Natural History of Georgia Plants Fall 2008 Dr. Chris Peterson

GA plants, Fall 2008

GA vegetation in the past 10 million years

• Lecture notes for PBIO 3650: Natural History of Georgia Plants

• Fall 2008

• Dr. Chris Peterson

• Reading assignment:– Wharton, pp. 8-12


A long, long time ago.....• All of the major groups of organisms existed by the late Tertiary (10 mya).

The Cenozoic era is divided into Tertiary and Quaternary periods. The Quaternary began 2 mya.


Plate tectonics....

• What was the world like 10s or 100s of millions of years ago?

• Climate was different, for many reasons; even the locations of continents were different.

• Modern perspective is that continents move around on the surface of the earth, on large plates.

• Many geological features (e.g. the Appalachian Mts., the Himalayas) have been caused by such movement.

• Some vegetation patterns can be explained by plate tectonics: e.g. why is there so much similarity between forests of eastern U.S. and Japan & China?


Continental plates today


From deep time to last 10 million yr

• Several important broad changes occurred in the Tertiary period (66 – 2 mya).

• In Early Tertiary, formation of Rocky Mountains created a rain shadow in the center of North America, leading to spread of grassland species and decline of many broadleaf deciduous trees.

• Globally, temperatures gradually decreased. In SE US, overall diversity decreased, and the relative importance of trees in the oak/beech family increased.

• At ~15 mya (Tertiary period, Miocene epoch), forests of eastern North America contained genera of:

-gymnosperms: Abies (firs), Picea (spruces), Pinus (pines), Podocarpus, Tsuga (hemlocks),

-angiosperms: Alnus (alder), Betula (birches), Carya (hickories), Castanea (chestnuts), Fagus (beech), Ilex (holly), Liquidambar (sweetgum), Nyssa (black gum), Quercus (oaks), and Ulmus (elms).


Miocene (10 mya – 2 mya)• Continued cooling and more seasonal rainfall led to extinction of many

tropical species from eastern North America. Example is the Annona species below.

• Evergreen coniferous forests expanded in the north and at high elevations.

• Broadleaf deciduous forests that had been abundant in western North America disappeared between 10 and 2 mya; those in Europe by 2 mya.

• The land connection to Eurasia was severed by 10 mya, so with the deciduous forest diebacks in Europe and Western North America, the remains of early Tertiary forests were left in eastern North America and eastern Asia (e.g. China and Japan).


A southeastern U.S. close-up over time

• The past 2 million years is called the Quaternary period, and is subdivided into the Pleistocene (before 10,000 years ago), and the Holocene (from 10,000 years ago until present).

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Ice ages• At various times during the Quaternary (2 mya until present), the upper

half of both North America and Europe have been covered by ice.

• The North American ice was called the Laurentide ice sheet.

• Other parts of the world experienced glaciations to a lesser extent.


Cycles of the ice ages

• At least five glacial periods (ice ages) have occurred in the last 2 my: four most recent are the Nebraskan, Kansan, Illinoian, and Wisconsin glacials.

• Glacial/interglacial times cycle roughly every 100,000 yr. Progressive cooling for 80 – 90,000 yr, then 10,000 yr of warming. We are currently near the end of one of those 10,000 yr warm periods.


Causes of the ice ages

• Ice ages probably had multiple causes, but major factors seem to be eccentricity of Earth’s orbit, changes in the tilt of Earth’s axis (oblequity), an precession of the axis, and which combine to form ~100,000 Milankovitch cycles that describe solar radiation striking different parts of the Earth at different times of year.

• Other major factors: atmospheric gas concentrations including CO2, distribution of continents

100,000 years

41,000 years

19,000 and 23,000 years


Responses of vegetation

• Ironically, the vegetation of the full-glacial periods was apparently quite stable and predictable, while vegetation of the inter-glacial periods (such as today) were much less so.

• One interesting case study is from Watts, who reported the full-glacial vegetation of sites near Rome, GA (Bartow Co.).

• His findings showed that during the full-glacial times, Bartow County vegetation was dominated by jack pine (Pinus banksiana).

• This is interesting for two reasons: jack pine’s range today is 700 – 1000 miles north of Bartow Co.; and jack pine today does not generally mix with oaks.


Responses of vegetation

• Watts also found a low/moderate abundance of spruce (probably white spruce) in Bartow Co.

• Spruce is today distributed far to the north of GA.

• Watts also found that the herbaceous vegetation was made up of species typical in prairies today; this suggests that the forest was perhaps rather open.


How do we know ?• One of the most useful

types of evidence for Quaternary vegetation research has been the pollen core.

• Pollen that settles on still water eventually sinks and ends up in the sediment at the bottom of a pond or lake.

• If the sediment has not been disrupted, then a vertical cylinder (core) can be extracted, and examined microscopically for pollen.


Hornbeams in the last 12,000 yr

• An interesting post-glacial story exists for two closely related species that are both called hornbeam – Ostrya and Carpinus.

• During the glacial maxima, these species apparently did not retreat as far south or east as some other broadleaf spp.

• They remained spread across TN, AR, MO.

• They became much more abundant from ~11,000 BP until ~6,000 BP; making up nearly 30% of the forest in places.

• Although widespread today, they are much less abundant – clearly their peak was 6-11,000 years ago.


Migration of spruce (Picea)

• Red lines show leading edge of range of a species; white line is the southern boundary of distribution of white spruce today.

• Spruce was an aggressive pioneer, moving into tundra areas soon after the ice retreated.

• Note that while northern boundary of spruce expanded to north, the southern boundary of its range retracted, so it is today absent from its mid-Appalachian ice age refuge.

• (note, red spruce is an exception, still found Appalachians, e.g. Clingman’s Dome in GSMNP)


A slightly different look at spruce

• Some species, like spruce (Picea) have closely followed the glaciers northward, with little delay in colonized newly-deglaciated ground.

• These figures show spruce abundance as a % of total forest cover.


Migration of pine (Pinus)

• White pine held out in a refuge along the mid-Atlantic coast and in the foothills of the Appalachians.

• For a short time, white pine actually spread further north than its current range, then “retracted” its northern limits.

• Compare to previous slide: spruce and pine spread out from very different “refugia”, in different directions, and at different rates.


Migration of oak (Quercus)

• Oaks were spread through the southeast during the full glacial times.

• They expanded north fairly rapidly, especially during 11,000 – 8,000 yr ago.

• They reached approximately their current northern limit 7,000 ago.

• The advance of oaks is closest to the stereotype of a species trapped in the far south during glacial maxima, and then expanding more or less north.

• Compare this and 2 previous slides: result is a continuously-changing mix of species across eastern U.S. in last 10,000 years.

• Refugia for oaks apparently in interior of SE; they were rare on the coastal plain.


Some overview points

• Several important points about Late Quaternary vegetation of SE US:

• Much more predictable than vegetation patterns during the interglacials.

• Modern vegetation types probably have only existed for the last 5,000 yr.

• Each interglacial warm period has had different vegetation assemblages and patterns.

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Just before glacial maxima: 40,000 BP• Prior to the glacial maximum around 18,000 BP, there was a period of

slightly more moderate conditions: substantially colder than today, but warmer than at 18,000 BP.

• During this period, broadleaf deciduous trees were more abundant than at 18,000 BP, but mixed with some conifers: for example, oak/hickory/ southern pine much more extensive in the SE than later.

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Glacial maximum: 18,000 BP

• Tundra near the edge of the ice; huge areas of spruce/jack pine

• Thin strip of conifer/northern hardwoods

• Large areas of oak/hickory/southern pine

• Mesic deciduous combination existed only as pockets (not shown here)

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Initial retreat of the glaciers• 14,000 BP very similar to 18,000 BP

• Major changes by 10,000 BP: note mixed conifer/northern hardwoods

• Oak/hickory abundant in west-central

• Mixed hardwoods abundant in east-central; this is an important trend: a warm but moist period from ~ 11,000 – 6,000 BP was apparently widespread, leading to an upsurge in mesic deciduous forest species.

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Almost today

• By 5,000 BP, large expanses of mixed confer/northern hardwoods just south of the spruce/jack pine combinations.

• Mixed hardwoods have contracted somewhat.

• Southern pine zone very extensive, reflecting warm and dry conditions

• Oak/hickory/southern pine zone in today’s location along Piedmont

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Types of vegetation in the Southeast• These are from satellite

images.

• Given the unique nature of vegetation in each interglacial period, should conservation efforts focus on forest types?

Documents

GA plants, Fall 2008 GA vegetation in the past 10 million years Lecture notes for PBIO 3650: Natural History of Georgia Plants Fall 2008 Dr. Chris Peterson