The Distribution of Snow Trillium.

If we look at distribution of Trillium nivale, what patterns do we see? Happen before I posted about distribution patterns in Ohio, but what about in other states?

Above is the county range map for snow trillium from the USDA website. You can see T. nivale is only in the Midwest US and in a small area in Appalachia. The state with the most counties is Illinois, but some states only have one or two counties. There are no snow trillum in east Iowa, or in central Wisconsin, or in most of Michigan (or east Ohio, but I wrote about that before). Also, there is that big bow across Illinois, the weird diagonal in Indiana, the two or three metapopulations in Appalachia, and some other patterns.

We know most times snow trillium lives on limestone or dolomite talus, so does this map and a map for potential karst (calciferous cave) overlap?

Wow. Okay, this seems not random! You can see how in Ohio snow trillium is only in karst regions. The only counties in Michigan are along the limestone part of the Michigan Shield. The diagonal counties in Indiana follows the edge of a karst region. There is no karst in central Wisconsin, so the snow trillium counties are along the edges. And the Appalachian populations are only in areas with limestone bedrock outcrops. We can see how the western extent is limited by moisture; and maybe we can guess that the south extent is limited by temperature. But what about the northern extent? And what about the bow across Illinois, and those counties in west Minnesota? And how did snow trillium come to the very disjunct area in central Michigan?

If we overlay the county records on the extent of the glacial maximums, we can see that most populations are close to the margin of the Wisconsinan glacial maximum. Happen they are not perfect, but you can really see how this is true in Indiana and Ohio. We can also see that there were possible refugia from glaciers in the whole Quarternary (2.5 million years) in Missouri, the driftless region of Wisconsin, southern Indiana, Adams County in Ohio, and all of the Appalachian metapopulation. Trillium seeds are ant dispersed so happen they move slowly. And if in most areas they are moving along river corridors where bedrock is exposed, as I think happen in Ohio, then happen they can only move where the glacial till is cut. If we compare this map with the above karst map, we can also see there is not really any limestone bedrock in northern Wisconsin and most of Minnesota. So happen the northern limit is maybe from no calciferous bedrock and the slow movement keeps them out from the Upper Peninsula yet.

These questions continue:

1. Why are there all the populations in central Illinois where there is no karst topography (and seem no limestone bedrock)?
2. Why are there populations in west Minnesota and Iowa where there is no karst?
3. How did the disjunct Michigan populations get to completely glaciated Michigan with no karst from Ohio to the shield and no populations in northwest Ohio?


First, Illinois. People describe their habitat as on river banks and bluffs. If we look at the bedrock of Illinois, we can see most that area in the center the state is Pennsylvanian shale and sandstone, and only in west part of the state we see Mississippian limestone in surface bedrock. So happen we can think all the counties in that west part the state, and in the north where there is karst potential, are bedrocks where snow trillium can live. But what about all those counties in the middle of Pennsylvanian shale and sandstone?

When we look at the map in the link, we think, okay, there is not a lot limestone in Illinois. But...if we look at the strata map:

We see, yes, those surface rocks are siltstone, shale, and sandstone, BUT, under that is many many MANY feet perfect limestone layers. How close the surface this limestone? Not really far; close enough the rivers can cut into it. If we map the counties with snow trillium against the close to surface limestone:

That is not random. And more, happen you can see you never see records down in the south, where the sandstone and shale layers are very deep. So, happen no karst, but more than enough limestone in river valleys.

Second, Minnesota and Iowa. For Iowa we can see most all the surface bedrock in the west part of the state is Cretaceous sandstone or shale. But, we can see from the larger river valleys in the Bedrock map that Mississippian and Pennsylvanian limestone are close the surface here. So happen we can guess these are also on river cut slopes and bluffs. And also these should include populations in Lincoln County, North Dakota around the Big Sioux River.

Minnesota is weird.

According to Minnesota Wildflowers, there are populations on limestone river slopes in the southeast part of the state, where you think you find snow trillium because there is karst potential there. But the biggest populations are not there, they are in the prairie regions. On alluvial floodplain soils.




What.

This is not normal for snow trillium. In Ohio we only see them on limestone and dolomite talus and gravel. In Pennsylvania and West Virginia they are on outcrops. In Indiana they are on limestone outcrops. In Illinois they are on limestone bluffs and river slopes. This pattern is most everywhere we see snow trillium, but in southwest Minnesota, in the Minnesota River Prairie, happen they are in floodplain forests. I don't know if this is a normal thing for them that happen they do less now less than they did long time ago, or happen this is adventive habitat. But, most of the Minnesota River Prairie soils are "gray calcareous till", maybe from Ordovician dolomite pushed from bedrock to the East? I really don't know. It is alkaline enough for the snow trillium.

The last question is, how snow trillium come to central Michigan? The counties where you find snow trillium here are in a ring around the Michigan Basin. You can see that limestone ring on the karst map above. But the most close populations to these are 170 miles to the south in Indiana, or similar distance across Lake Michigan in Wisconsin. The Michigan populations are in the Grand River valley, and the North Indiana populations are in the Wabash River valley, and there is a continental divide (but not high) between them. There are no glacial refugia in Michigan, it was all covered with ice 30 thousand years ago.

There are really only two possible things. First, maybe there was a corridor 20 thousand years ago when the glaciers were receding, and the snow trillium followed it North. With time the intermediate populations went away. Happen this is a vicariance explanation. Second, if there was no corridor, it must long distance dispersal. By what? Bird?

But I think we can say some things: Snow trillium is always close to rivers, most times on slopes or talus or bluffs, rarely in alluvial soils of floodplains, where they can grow if the soil is alkaline enough. And because most refugia are close to large rivers, the Ohio River, Mississippi, Missouri, it was not difficult for them to disperse along limestone and dolomite layers up river when the glaciers receeded. Because their populations are really scattered, it say to me they were more widespread before and many those intermediate populations extripated when the hydrology or soil changed, or they had too much shade under thick canopy. I think before, because snow trillium is very small and it bloom in cold weather and have short life cycle, maybe it is similar alpine plants, but I have no more information for tell me that.