Chad's Garden: Native Trees' Glacial Retreat

During multiple glaciations, native U.S. species migrated southward. Those populations exist as islands. Some of these are pockets still exist in Mexico.

Posted: Dec. 28, 2018 4:58 PM
Updated: Dec. 29, 2018 2:54 AM

Native eastern U.S. trees migrated southward during the cold Ice Age & its multiple glaciations.  Indiana has had some amount of glacial ice cover on four occasions over the roughly 1.5 million years (as recent as around 12,000 years ago).  These massive walls of ice completely changed much of the Indiana landscape each time they were here.  The Nebraskan, Kansan, Illinois & Wisconsin glacial episodes & the cold, dry climate caused nearly all of our current native tree species to retreat southward & seek refuge in a warmer, wetter climate.  Many of our native trees went as far south as Mexico.

As the climate warmed, pieces of these populations remained in Mexico, restricted to high, more moist elevations with a bit more temperaturate climate.  So, many of our native trees in Indiana have cousins in the mountains of Mexico.  When the species were cut off, they evolved into new species, but share close, close lineage with our Hoosier counterparts.  Think of it as British coming to the United States in the 1600s & 1700s & then the U.S. is cut off from that population more & more after the Revolutionary War.  The U.S. became independent as a governing body & as a population.  We lost that British accent.

Furthermore, our own regional accents & dialects developed in the U.S. as certain groups of people were cut off & separated.  Now, you can listen to someone's accent & sort of figure out where they are from in the U.S.  Plant populations are somewhat similar.

Like the tide receeding, leaving tidal pools, the cooler, wetter climate of Mexico's higher terrain provide a good place for these species.  In many of these isolated populations, multiple cousins of our Indiana trees grow together.  Today, these pockets of populations are highly-endangered due to logging & agriculture.  With a lack of good reproductive success considering the continued cutting off of these trees in their already-limited range (preventing good gene exchange/flow in pollination) their future remains in jeopardy.  Some are protected in Mexican national forests & parks, but many are not.  Some of these rare, odd species are found as tiny populations in southwestern Texas in protected ravines of deep hollows & coves along streams.

Area glaciated in the Nebraskan, Kansan, Illinoian & Wisconsin episodes in a period around 1.5 million to 12,000 years ago (Kansan & Nebraskan glacial deposits have been found as far south as southeastern Indiana & northern Kentucky, patchy with Illinois deposits):

We have American Beech (Fagus grandifolia) in our area.

Here is the very similar Mexican Beech (Fagus mexicana) in its native forest habitat in the Mexican state of Hidalgo.

Image courtesy of Diego M.:

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Mexican Flowering Dogwood (Cornus florida urbaniana) is a very interesting tree.  It is very unique, but has been nearly non-existent in the horticultural trade due to the inaccessibility of obtaining seeds or clones from its remote habitat.  It is considered to be a subspecies of Flowering Dogwood by some & a separate species of dogwood by others.

When this Flowering Dogwood population was separated by the parent Flowering Dogwood population of the U.S., its bracts evolved into this intricate pattern upward-turning shapes that is consistent on all trees.  It is a true morphological characteristic that permeates the species & is not an edaphic or genetic fluke on a random tree. 

This close cousin of our Flowering Dogwood is actually quite resistent to many of the diseases that plague our own Hoosier woodland native Flowering Dogwood.

The tree in its habitat in the high, moist, temperate forests of eastern Mexico:

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Like our very own native Eastern Redbud, Mexican Redbud (Cercis canadensis var. mexicana) beautiful in spring.  A subspecies or completely different species to Eastern Redbud, the bloom are a bit darker in pink & ruby compared to the lavender & pink of our Hoosier redbud.  Also, the leaves are thicker & shinier.

Images courtesy of Lady Bird Johnson Wildflower Center in Texas.

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Shagbark Hickory is also found in eastern Mexico as isolated populations.

There is more & more consensus that there is a Northern Shagbark Hickory (Carya ovata) in the U.S. & a Southern Shagbark Hickory (a.k.a. Carolina Hickory or Carya ovata carolinae-septentrionalis) & that they are two separate species.  They differ morphologically & ecologically.  I planted a Southern Shagbark from Georgia about 18 years ago back home & I can tell you that our Indiana Northern Shagbark & the Southern Shagbark differ quite a bit.

The Southern Shagbark occurs in Piedmont flatwoods forests on seasonally wet, rather heavy clay soils in North & South Carolina to Georgia, Alabama & Mississippi.  It also shows a liking on limestone soils, specifically on those over the Black Belt of Mississippi & Alabama where it can be find with two rare species:  Durand Oak & Nutmeg Hickory.  I also has a liking for the limestone soils in Florida.

The Northern Shagbark is our hickory with the shaggy bark, big buds, thick,, light gray-brown twigs & big leaves & 5 leaflets.  Southern Shagbark tends to have 7 leaflets more often, the twigs are thinner & black to dark rust in color with smaller buds.  The nuts are also smaller & the bark looks more like the Shellbark Hickory than our Shagbark.  It peels off into curling, larger pieces than the thin strips of our Hoosier Shagbark.

Mexican Shagbark Hickory is similar to the Southern Shagbark (buds are more pointed with the nuts having notable sutures like Nutmeg Hickory) & is found on slopes between wetter cloud forests of magnolia, beech, etc. & drier lower elevations.

Image is courtesy of Will Cook & carolinanature.com

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Carya palmeri or Mexican Hickory is closely related to & similar to our native Indiana Bitternut Hickory (Carya cordiformis).  It even has the sulfur yellow buds like our Bitternut.  Such similarity, it is likely that these spotty populations in eastern Mexico's temperature forests are relicts of a large Bitternut Hickory population that retreated around 1.5 million years ago.

Pecan Hickory (Carya illinoiensis), which is native to river valleys as far north as southern Fountain County (but is most common in southwestern Indiana on floodplains of rivers & creeks) is also found as isolated pockets of population in Mexico.  It does not differ much morphologically

Image of Mexican Hickory & leaf is courtesy of LJ Grauke , Research Horticulturist& Curator USDA-ARS Pecan Genetics:

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Sweetgum can be found in towns & cities over our viewing area, but natural woodland populations are only found as far north about I-70 in west-central Indiana & only as far north as the Bean Blossom bottoms near Bloomington & over southeastern Indiana.  However, it is abundant in far southwestern Indiana on the flatwoods & poorly-drained glacial sediment lakebeds & on the poorly-drained Illinoian Till Plain or "crawfish flats" of southeastern Indiana.

Sporadic populations can be found in the mountains of Mexico to high elevations of Guatamala to Honduras.  Interestingly, morphologically, there is no difference in the populations.  This is a bit of an oddity considering the speciation of beech, etc. in this area.  Sweetgum is also found deep into Florida where it grows in abundance southwest of Lake Okeechobee. 

The only morphological differences are similar to that of Winged Elm (Ulmus alata).  Upland Sweetgum varies from bottomland Sweetgum in height & the winged appendages on the twigs.  Upland Winged Elm in hilly south-central Indiana is shorter with winged twigs, while the Winged Elm of southwestern Indiana's glacial lake plains lacks the wings & grows much larger.

Here it is in eastern Mexico in its native forest habitat:

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Bigleaf Magnolia is an ancient, primitive species with giant leaves & huge flowers.  It actually has three distinct subspecies in a highly-fractured native range.

The traditional Magnolia macrophylla is most common in the mountains of eastern Tennessee & far southeastern Kentucky.  Isolated populations are found near Bowling Green, Kentucky, in far southern Ohio, near Raleigh, North Carolina & in the deep hollows & coves of the hills near the Tennessee River a couple hours northwest of Florence, Alabama. 

Magnolia macrophylla subsp. macrophylla is found in a discontinuous range from Louisiana to Mississippi, Alabama to Georgia (& isolated population near Charleston, South Carolina), its look typifying the subtropical, humid, wet climate of the Deep South.

Magnolia macrophylla subsp. ashei is only found in northwestern Florida with glacial reliects like Florida Yew & Florida Torreya.

You would not think that any water & humidity-loving magnolia would be native to Mexico.

Magnolia macrophylla subsp. dealbata is also called Mexican Bigleaf Magnolia or Cloudforest Magnolia or "Mexican Cow Cucumber".  It is considered a separate species by some botanists, but this is not a universal conclusion.  Regardless, it varies slightly morphologically in the U.S. subspecies of macrophylla.

In Indiana, we have the Cucumber Magnolia that is only found in a few isolated locations in south-central Indiana & the Umbrella Magnolia, which is only found in one isolated place in scenic Crawford County in the far southern portion of south-central Indiana along the Ohio River.  This is an extension of Appalachian flora into southern Indiana.

Mexican Magnolia (Magnolia talauma mexicana) is another magnolia with near very similar relatives in the U.S. & also occurs as isolated relict populations in Mexico.  It is related to the Fraser Magnolia, which occurs in the mountains of West Virgina to Tennessee & northeast Georgia.

Magnolia schiedeana bears resemblance to Pyramid Magnolia (Magnolia pyramidata) with has a spotty range from Louisiana to South Carolina.  It is the most common in southern Alabama & over the Florida Panhandle.  M. schiendeana grows is an endangered tree of ravines in eastern Mexico, usually sloping toward the Gulf with oaks, beech, other magnolias, etc.

Here is the Mexican Bigleaf Magnolia in Mexico:

Image courtesy of Stan Sheb:

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Knowlton Hophornbeam is an endangered species of hophornbeam very similar to our Eastern Hophornbeam that is found in woods in our area.  I only has 7 small populations left in the southwestern U.S. to northern Mexico.  Here it is in southwestern Texas in a protected cove.

Image courtesy of the University of New Mexico:

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Vermont's Sugar maples?  Sugar maples at Happy Hollow?  Nope.  This is not too far from the Rio Grande in Texas!

According to the Native Plant Society of Texas:  "Evidence suggests that the maple trees that give the preserve its name are relics: remnants of a larger, more widespread population that flourished during the cooler and wetter climate of the last glacial period. Today, their distribution is limited by the relative rarity of the soils and microclimates they require to thrive."

This is Bigtooth Maple, a close relative of Sugar Maple & likely a relict of a vast Sugar Maple range during wet, colder glacial times.  This tree is found in a deep, moist canyons & ravines in deep colluvial or alluvial soils in a very spotty range from Mexico, to southwestern Texas to parts of the Southwest.  There is a strip of good population in Utah. 

Within this Sugar Maple-like species is some variability between the Mexico populations & those in Utah.  They all look like Sugar Maple, but exhibit scorch resistance (native Sugar Maple shows up in Indiana during hot, dry years).

There is also Mexican Elm (Ulmus mexicana), which is closely related to the American Elm (Ulmus alba), Carpinus tropicalis of Mexican Hornbeam, which is a relict from the American Hornbeam population (Carpinus caroliniana) & Mexican Walnut to Mexican Plum, which are closely related to the Black Walnut & American Plum. 

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