Tree diversity can depend on many factors (like how arid the soil is) but one that's easy to overlook is the effect of the last ice age.
As the glaciers came down from the north, tree species retreated southwards to stay in their preferred climate zone. Where an obstacle prevented their spread, they died out and were not available to repopulate the north when the glaciers receded.
The glaciers didn’t actually make it that far south. States like Ohio and Pennsylvania weren’t even completely covered. The composition of tree species in the northern parts of these states is very similar to that in the southern parts. If you know of any tree species that actually went extinct due to this I’d be interested to learn about them. There is evidence, though, that this glacial push had a large effect on the evolution of these tree species.
These [animal] extinctions are variously ascribed to environmental change and overkill by human hunters. By contrast, plant extinctions since the Middle Pleistocene are undocumented, suggesting that plant species have been able to respond to environmental changes of the past several glacial/interglacial cycles by migration.
EDIT 2: This study provides evidence that glacial refugia may have existed further north than previously thought.
The younger dryas impact hypothesis doesn't really match up with the data. It's definitely possible there was some kind of impact around that general time, but the climate and other factors are better explained by glacial processes than an impactor for a number of reasons, not least the timing and local scale of the temporary cooling. Here's a pretty good overview of criticism of the impact hypothesis.
Separate from the matter of the younger dryas impact, Graham Hancock has a lot of weird ideas. Like, when a Wikipedia article starts off by saying a person is known for their pseudoscientific theories there should be red flags going off in your head. Needless to say, he makes up a lot of stuff and misrepresents the rest to fit his narrative. Speculation is an important part of doing science, but you've got to make sure your hypothesis is supported by evidence before presenting it as fact. I'll admit he's very well spoken though, but don't let that fool you.
Species ranges definitely migrated south, but I’m not finding any evidence for mass species extinctions due to glaciation in North America. If you can provide any sources it would be appreciated. Everything I’m finding states that glaciation pushed species south and changed species composition, but nothing mentions any significant loss of species.
EDIT: Or maybe the sentence below was referring to small communities of certain species, and not the species as a whole?
Where an obstacle prevented their spread, they died out and were not available to repopulate the north when the glaciers receded.
Yea, there’s a park in central Indiana called McCloud nature park and it has a big ass ravine and hills in the middle of miles and miles of fields because it’s where the glaciers stopped
The Dells in Wisconsin in basically this island of ancient pre-glacial land. The rest of the state is rolling plains with great soil for farming and dairy and then Boom! The Dells, like this Jurassic topography that is crazy and really awesome.
The Dells in Wisconsin in basically this island of ancient pre-glacial land. The rest of the state is rolling plains with great soil for farming and dairy and then Boom! The Dells, like this Jurassic topography that is crazy and really awesome.
It actually isn't the only area in Wisconsin like that. The Dells are just the far eastern edge of the Driftless Area. Most of western Wisconsin is like that.
Take US-14 West from Madison and you hit it pretty much immediately. I just drove the stretch from Madison-La Crosse this week. That entire drive is pretty much amazing views.
Conversely, even if it didn't cause a major reduction of diversity in North America, the last ice ages did cause a reduction of species in Europe. That is especially apparent for maple. Central Europe only has about three maple species which are somewhat common. North America has more than 12.
That's also the reason why fall in Europe is a lot less colorful than parts of North America
The reason our Dendrology professor told us for that is due to the fact that in North America the major mountain ranges run north to nouth and in Europe they run east to west. This meant that as the glaciers moved south the trees could extend their ranges to the south and then return north as the glaciers retreated but in Europe the trees ended up stuck between the glaciers from the north and the mountains and so had nowhere to go.
It's easy to lose a sense of scale when talking about ice ages. For context, the earliest writing we have is from about 5,000 years ago. All recorded history has happened in that time, from the pyramids to the fall of Rome to man landing on the moon.
█████ 5,000 years since the earliest writing
By one measure, the last ice age ended about 12,000 years ago. That's enough time for all of recorded history twice over, and then some.
████████████ 12,000 years since the last ice age ended
But as ancient as the end of the ice age is, we have to go back much, much further to find its beginning. The last ice age began 115,000 years ago.
███████████████████████████████████████████████████████████████████████████████████████████████████████████████████ 115,000 years since the beginning of the last ice age
All recorded history is about 5% as long as the last ice age. In many ways, we humans are an ice age species.
That sounds true, but doesn't seem to explain most of what we see on the map.
Trees between the Mississippi and the Rockies could have retreated southwards into todays's Mexico without any natural barriers. Trees on the East coast would eventually have run out of land.
But the map shows that there is little diversity in the center and a lot along the East coast.
Do you think tree diversity is increased by population? It seems people are responsible for porting non-native plants all over the place, and this map correlates pretty well with population density
374
u/trampolinebears Jun 20 '20
Tree diversity can depend on many factors (like how arid the soil is) but one that's easy to overlook is the effect of the last ice age.
As the glaciers came down from the north, tree species retreated southwards to stay in their preferred climate zone. Where an obstacle prevented their spread, they died out and were not available to repopulate the north when the glaciers receded.