Archive for the ‘botany’ Category

Pleistocene Watermelon (Citrullus lanatus)

August 3, 2019

During July and August I always have watermelon in my refrigerator.  After I run 3.3 miles in 95 degree F heat, nothing quenches my thirst better than a crisp cold slice of watermelon.  Primitive people living in the deserts of North Africa ~5000 years ago discovered the same usefulness of this species, but the story of the watermelon began long before that.

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Wild citron melons in South Africa.  They are a different species than the sweet watermelon we commonly know, but the type specimen (the first described by biologists during the 18th century) of Citrullus lanatus was mistakenly described from a specimen of this species, C. caffer.  Scientists didn’t discover this mistake until a DNA test of the museum specimen was conducted a few years ago.

A recent study of watermelon genetics estimated the citrullus genus (which includes all watermelons) first diverged from the rest of the Cucurbitidae family about 11 million years ago.  This family also includes bottle gourds, squash, and many other plants. There are 6 species of watermelon, and they originally occurred in Africa.  A single species of the 6 spread to India and Australia during pre-history, and man may or may not have been a factor in the distribution of this species.  The sweet watermelon (Citrullus lanatus) diverged from a sister species within this genus about 3 million years ago.  All members of the Citrullus genus are desert vines that survive arid conditions, but thrive and produce heavy yields following occasional downpours.  The abundant seeds are consumed by megafauna such as elephants and rhinos and spread across the landscape in their dung.  Today, elephants often destroy grain crops interplanted with cultivated watermelons when they seek out this delicacy.

The oldest known remains of sweet watermelons were found at a 5,000 year old settlement in Libya.  Harry Paris believes desert people in North Africa began cultivating watermelons as a portable source of water, rather than as a food source, though the seeds are edible and nutritious.  The first watermelons were green-fleshed, watery, and bland or even bitter.  Early farmers selected for sweeter fruit, and the gene for red flesh is paired with an increased sugar content.  Soon ancient mariners started carrying watermelons on their ships as a source of potable water and spread them throughout the warmer parts of the world.  Christopher Columbus brought watermelons to North America in 1493, and the Indians planted them everywhere they would grow.  William Bartram enjoyed a feast of oranges and watermelons when he visited Florida in the fall of 1775.

There are over 1200 varieties of cultivated watermelon.  I’ve successfully grown 2–Georgia Rattlesnake and Florida Black Diamond.  I used to pull out all the stops using fertilizer and special mulching tarp and was able to grow 35 pounders.  This year I bought a cheap $1.50 pack of Congo watermelon seeds and put minimal effort into the project and was justifiably rewarded with zero melons.

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Georgia Rattlesnake.  I successfully grew 35 pounders years ago.

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Black Diamond.  I successfully grew these 1 year too.

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My late grandfather grew Crimson Sweet watermelons.

Among the more interesting varieties of watermelon are the Carolina Cross and the Christmas King.  The Carolina Cross was bred for its prodigious size and holds the record for biggest watermelon ever grown at 351 pounds.  This could be served at baseball games and fairs.  The Christmas King is less sweet than most watermelons but if kept in a cold place will still be edible in late December.  How is that for an extended season?

Most watermelons sold in supermarkets today are seedless–a recent development.  Farmers began growing seedless watermelons during the 1990’s.  To make a seedless watermelon farmers chemically induce a change in the chromosome number in 1 of the parents.  When they backcross the 2 melons with different chromosome numbers it results in a seedless (or sterile) offspring in the same way crossing a horse with a donkey results in a sterile mule.  Modern seedless watermelons are of consistent quality and are just as sweet, crisp, and aromatic as the old time varieties.

Watermelons are mostly sugar water, but they do contain Vitamin C, a type of beta-carotene known as lycopene, and potassium.

The muskmelon (Cucomis melo) is not closely related to the watermelon but shares a common history.  It also was originally cultivated in the North African desert as a portable source of water.  Later, famers over many generations improved the quality of the fruit by selecting the sweeter individuals for seed.  Most people call muskmelons “cantaloupe.”  However, cantaloupe specifically refers to just 1 variety of muskmelon first cultivated in Western France.


Chomicki, G.; and S. Renner

“Watermelon Origins Solved with Molecular Phylogenetics including Linnean Material: another example of Museomics”

New Phytologist Trust October 2014

Paris, Harry

“Origin of the Dessert Watermelon, Citrullus lanatus

Acta Horticulture March 2017


Pleistocene Mimosas

July 6, 2019

A remnant of the local original landscape grows on a 4′ by 4′ area of my front yard near the road.  It is known as sensitive briar (Mimosa microphilia), and it attracts numerous bees and other pollinators.  Sensitive briar resembles its close relative, sunshine mimosa (M. strigillosa), but differs in that its vine is covered with thorns.  These mimosas, native to southeastern North America, prefer to grow in prairies, ravines, savannahs, and open woods; and they do well on sandy or rocky soils.  I live in a sandhill area located on the fall line between the piedmont and the coastal plain.  The sandy soils originated during the Eocene when this region was a coast line and the fall line was a sandy beach.  Today, the co-dominant trees are loblolly pine and sand laurel oak.  Before humans repressed fires, it seems likely this area was dominated by longleaf pine with some sand laurel oak.  Longleaf pine is slow-growing and dependent upon fire for germination, and it has largely been replaced in my neighborhood by faster growing, less fire-dependent loblolly pine.  Also in the absence of fire, sand laurel oak forms dense stands on vacant lots when formerly they would be thermally pruned and grow farther apart.  I hypothesize the original environment was open woodland or even savannah, dominated by the aforementioned trees and with a great variety of flowers and grasses growing in the open spaces between the trees.  Other species in my neighborhood that might be remnants of this original environment include yellow cottony aster, a type of small daisy, prickly pear, ground cherry, blackberry, lowbush blueberry, false foxglove, and passion flower.

Sensitive briar growing in my front yard by the road.

The mimosa genus includes about 400 species of herbs, shrubs and trees within the legume family.  The most commonly noticed mimosa in southeastern North America is Albizia julibrisson,  a native from Asia and the Middle East.  It was introduced to Charleston, South Carolina in 1783 and has spread like crazy all across the region.  They are found in open areas alongside roads and are now considered an invasive species.  I think they are more beneficial than detrimental.  Their flowers feed the bees and butterflies, and their roots, like those of all legumes, help fix nitrogen in the soil.  They are probably more beneficial than many of the native plants they outcompete.  They produce numerous bean pods that readily germinate in road side ditches.

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Mimosa trees are widespread and common in disturbed areas of the southeastern United States.

The mimosa genus is very old with a world wide distribution.  They likely originated before the continents drifted apart.  During the Pleistocene sensitive briar flourished in the open spaces that resulted from rapid climate change and megafauna foraging.  However, there is no pollen or fossil evidence of this species from that age.  Nevertheless, I’m sure they were common in some areas and just perchance left little evidence of their previous existence.

Plants in the mimosa genus are capable of rapid movement.  Sensitive briar closes its leaves when being touched.  Scientists think this response helps protect them from foraging herbivores, but they are vulnerable to at least 1 predator–the mimosa webworm (Homodaula anisocentra).  The larva of this species wraps mimosa leaves inside a protective webbing which prevents the leaves from closing completely.  They then feed upon the leaves before their transformation into their adult stage.

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Mimosa webworm.

Pleistocene Mulberries (Morus sp.)

April 28, 2019

Ordering fruit trees from a catalogue or internet site is a dodgy endeavor.  The trees are overpriced with an added cost of shipping, and in my experience I’ve learned the trees produce poor quality fruit, if they even survive long enough to bear.  I’ve had much better luck transplanting trees from local nurseries.  I have 26 year old grape vines and blueberry bushes almost as old that I purchased from local nurseries, and I’d still probably have a great fig tree, if plumbers didn’t have to dig a new drain field for my septic tank.  Peach trees I’ve grown from seed are much stronger and produce much better fruit than any I ever purchased through the mail.  A mulberry tree growing by the side of my house is an example of mail order disappointment.  The tree is thriving and flowers every spring but it produces no fruit.  This puzzled me until I finally figured out why.  The tree died back the first year I bought it, but it regrew from the stump.  Nurseries sell mulberry trees with male and female branches.  (Male flowers produce no fruit.)  Unfortunately, the part that grew back on my tree is all male.

Male flower on my mulberry tree.  Female flowers have a more round shape.  All the flowers on my mulberry are male, much to my disappointment.  Click to enlarge the photo.

Mulberries belong to an ancient family that has existed since at least the mid-Cretaceous, and dinosaurs likely dispersed the seeds of the fruit from this family in their feces.  The Moraceae family includes mulberries, figs, Osage orange, jack fruit, and bread fruit.  The species of native mulberry common and widespread in eastern North America is Morus rubra.  The range map of this species shows some affinity in its northern limit with the ghost boundary of the Laurentide Glacier.  (See https://markgelbart.wordpress.com/2017/03/30/the-ghost-boundary-of-the-last-glacial-maximum-ice-margin/ ) However, like many other species of trees M. rubra successfully recolonized some territory that became deglaciated following the last Ice Age.  It’s likely red mulberry grew in mixed forests all the way to the glacial boundary during Ice Ages but became much more common during warmer wetter interstadials and interglacials.  Mulberry trees prefer early to mid successional woodlands where they can get plenty of sunlight.  Hence, they are a pioneer species dispersed in bird droppings.  Disturbed plant communities caused by rapid climate change and megafauna foraging were common during the Pleistocene and so were mulberries, though their pollen is rarely detected in core samples.

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Range map of the red mulberry.  It has been widely transplanted outside its range.  Avian dispersal helped this species recolonize deglaciated regions following the end of the last Ice Age.

Birds love mulberries, and avian dispersal explains how mulberries recolonized deglaciated regions when so many other species did not.  A study of 1 backyard mulberry tree in Arkansas counted 32 species of birds feeding on the fruit.  Cedar waxwings and robins made up 77% of the individual birds visiting the tree.  Other birds feeding on the fruit from this tree included mockingbirds, finches, catbirds, eastern kingbirds, and warblers. The fruit appeals to bird species that normally prefer insects.

Mulberries were especially common around Indian villages during Colonial times and earlier.  William Bartram mentioned M. rubra  at least 20 times in his Travels. Mulberries ripen over a 3 week period during late May and early June and were an important earl summer fruit for the Indians.  They can be dried and dried mulberries are a staple in Afghanistan.  European settlers brought white mulberries (M. alba) to North America, hoping to start a silk industry.  For 4 thousand years the Chinese have been raising the silk worm moth (Bombyx mora), a species no longer found in the wild.  The silk worm moth larva feed upon mulberry leaves and produce silk for their cocoon.  The silk industry collapsed in North America long ago and now they grow wild, often hybridizing with native mulberries.

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Silkworm moth larva feeding on mulberry leaves.

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Red mulberry fruit.  Despite the name, they aren’t ripe until they turn black.

Mulberries are very sweet and nutritious.  They are high in Vitamin C and iron and also a good source of potassium, Vitamin K, Vitamin E, and fiber.  They are rich in cholesterol-reducing and cancer-fighting anti-oxidants.  They make good desserts as well, but they do have 1 drawback.  The stem grows well into the fruit and is difficult to remove.


Jackson, J; and R. Kanin

“Avian Frugivory in a Fruiting Mulberry Tree (Morus rubra) in Arkansas”

Journal of Arkansas Academy of Science 2016

Piedmont Plant Species Bartram First Encountered at the Augusta Shoals

July 22, 2018

I’ve read all or parts of Bartram’s Travels hundreds of times, but whenever I re-read it I always find something new that fascinates me.  William Bartram journeyed from Savannah, Georgia to Augusta during 1773, and in his book he describes the flora of the maritime forests, lower coastal plain, and upper coastal plain.  The descriptions are so packed with information I didn’t notice until recently a small paragraph about some piedmont plant species he first encountered alongside the shoals of Augusta.  He refers to this spot as a cataracts.  Several important Indian trails converged here because the shoals afforded a shallow crossing.  Augusta developed as an Indian trading village because of these shoals.  Bartram describes Augusta as a small village that reaches all the way to the “cataracts,” and it was surrounded by “gay lawns and green meadows.”  Augusta is on the edge of the hill country, and species that prefer higher elevations begin to occur here.  Bartram arrived in May when all of these species were in full bloom.  He listed Rhododendron ferruginumPhiladelphus inodorus, Malva, and Pancratium fluitans.   I haven’t visited the shoals in a while, but I don’t recall seeing any of these species next to the shoals.  They’ve been eliminated from the immediate vicinity, though the first 3 are commonly planted as ornamentals in people’s yards.  Bartram wrote Pancratium fluitans inhabited every rocky islet on the shoals.  (The common name of this species is rocky shoals spider lily.  It’s modern scientific name has been changed to Hymenocallis coronaria.)  Unfortunately, today there are just 50 populations of this species left because reservoirs inundate their favored habitat.  The natural beauty of rocky shoals has diminished since Bartam saw them.

Scenes around Augusta, Georgia - Savannah River shoals - Stock Image

Augusta shoals.  The lock was built 100 years after Bartram saw it.

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Rhododendron ferrugineum is a common ornamental plant in Augusta.  It grew wild near the Augusta shoals.

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Scentless mockorange is also commonly planted as an ornamental but wild populations grew near the shoals.

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Common mallow is a non native species that was already widespread in Augusta by 1773.  This is probably the Malva species Bartram mentions.  There is a native species of mallow–Carolina mallow (Modiola caroliniana), however Bartram described the mallow he saw as blue, and this is the wrong color for Carolina mallow.

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Rocky Shoals spider lily.  Only 50 populations of this species still exist.  Most have been wiped out by reservoir creation.  In Bartram’s day they inhabited every rocky islet on the Augusta shoals.

Pleistocene Peaches (Prunus kunningensis =P. persica)

July 17, 2018

The ancestor of the modern cultivated peach (P. persica) depended upon megafauna for dispersal and is now extinct in the wild.  Asian elephants and primates such as macaques and early humans ate the fruit and distributed the seeds throughout the environment, but without these species P. persica disappeared from the wild, and now only exists in cultivated fruit orchards tended by modern humans.  Peaches originated in China, and peach seeds dating to 2.5 million years BP have been found there.  Scientists designated these ancient peaches as a unique species they refer to as P. kunningensis, but they admit there is no real difference between this species and P. persica.  Peaches have been cultivated in China for at least 7500 years where evidence of early peach cultivation has been found in the Yangtze River Valley.  Farmers began grafting varieties with larger fruit to pit ratios on to root stocks of other peach trees then.

Peach cultivation spread from China to Persia (today known as Iran) and from there to Europe.  Early Spanish explorers brought the fruit to southeastern North America during the 1500’s, and John Lawson found peach trees thriving in Indian villages when he explored and settled in North Carolina between 1700-1711. (See: https://markgelbart.wordpress.com/2012/07/27/john-lawsons-voyage-to-carolina-1700-1711/ )  The quality of some of the fruit was so much better than European peaches Lawson mistakenly thought some varieties originated in North America.  The real reason American peaches were better than the European fruit was because the climate in southeastern North America was similar to their land of origin–China.  Lawson planted peaches in his orchard, and he had 1 freestone yellow nectarine tree that produced 15-20 bushels every year, unless there was a late spring frost. (A nectarine is simply a smooth-skinned variety of peach.)  He claimed peach trees planted from seed bore fruit in 2-3 years, and the fruit from the offspring was the same as from the parent.  Peaches were so abundant he fed the excess fruit and corn to the hogs, resulting in sweet pork. He also made vinegar from peaches.

Lawson’s account of raising a peach orchard from seeds fascinated me.  Most fruit varieties are mutants grafted on to rootstocks because most wild trees produce fruit of inferior quality.  I researched online and found a discrepancy.  Some agreed with Lawson and claimed fruit from peach seeds produced fruit similar to their parent, but a study written by an horticulturalist from LSU found that fruit grown from seed was usually inferior.  So I conducted my own experiment.  I noticed peach trees often germinated in my compost pile.  I took these seedlings and transplanted them in my yard.  Now, 5 years later I have 4 trees that are bearing heavily.  1 tree produces freestone peaches during the last 2 weeks of June.  All the peaches on this tree were infested with plum curculio larva.  Plum curculio is a beetle that damages all kinds of fruit.  However, I cut away the worm-infested, bird-pecked parts and tasted the fruit.  (Birds get a double treat from my peaches–fruit and protein.)  For an early ripening variety it is a good peach.  The other 3 trees have fruit that ripens throughout July.  1 of them was partially infested with plum curculio, but the fruit is excellent.  Another tree produces very large peaches that are as good as the best farmer’s market peaches.  The 4th tree produces small, bitter, heart-shaped peaches that in appearance resemble the most common variety grown in Georgia and South Carolina–the red globe peach.  I’ve concluded Lawson was mostly right, and the LSU study was wrong.  75% of my peach seeds produced good quality fruit.

Late June peaches from a tree I planted from seed in my yard.  The fruit from this tree had an 100% plum curculio infestation rate.  They were still edible, if I cut away the damaged part.  The quality was good for an early season peach.

This tree produces large luscious peaches. None were insect-damaged but some cracked open because of rain, even though the soil in my yard is sandy and well-drained.  This was the best-tasting peach I ate all season.

This tree is a bit of a natural dwarf.  The fruit is small and bitter.  I’ve read thinning out the fruit may have improved the quality.  I might try that next year.

My experience with this peach tree growing experiment has taught me a few things.  Spraying insecticide doesn’t work.  Rain washes the insecticide off, and the insects just return.  The peaches with no insect damage were growing in open sunlight with no undergrowth.  I hypothesize shade and undergrowth shelters insects from predators and harsh sunlight.  I can’t do anything about reducing the shade over my other 2 peach trees, but I will try harder to control the Virginia creeper.  This vine is tenacious, but I think a thicker layer of mulch might suppress it.  Peaches need more nitrogen than other fruits–more evidence they evolved in plots rich in megafauna manure.  My fastest growing peaches just happen to be growing over the drain field to my septic tank.

The Georgia extension office recommends 58 varieties of peaches, but they don’t even list 3 of my favorite varieties–Indian blood cling, Oregold, and Halehaven.  Peaches stay in storage for just 2 weeks, so many varieties that have different ripening schedules have been developed to extend the season.  They recommend 1 late April variety, 12 May varieties, 16 June varieties, 23 July varieties, and 6 August varieties.  The mid-season free-stone peaches are the best-tasting.  There are also white peaches.  These are sweeter and more aromatic, and in my opinion taste like a completely different fruit.  After my difficult experience however, I recommend other fruit for the casual home gardener in Augusta, Georgia.  Blueberries, figs, muscadine grapes, and even apples are much easier to grow than peaches here, even though Georgia is known as the peach state.


Su Tao; et. al.

“Peaches Precede Humans: Fossil Evidence from Southwest China”

Scientific Reports 2015

Tallow Plum (Ximenia americana) and Pre-historic Rafting Events

June 3, 2018

I posted the below photo on the Florida Flora and Systematics Facebook page, and the 2 plants in the picture were identified within about 5 minutes.  I saw this shrub and flower growing at Manatee State Park in Florida, while I was in the sunshine state visiting my sister and mother who recently moved there.  I joined that Facebook group because I am not as familiar with plants found in Florida as I am with Georgia’s flora.  I’ve read about tallow plum, but it was a big help for someone to help me identify it.

The flower in the foreground is Chapman’s pea (Chapmannia floridanus); the scrub bush in the background is tallow plum.

Tallow plum is in the Olalaceae family which includes olive, ash, and privet.  It produces an edible, waxy, sour fruit; and the tree reaches an height of 18 feet.  The tough plant thrives on sandy soils and can even grow on beaches, perhaps explaining its wide geographical distribution.  This species is found throughout most of Florida as well as the Caribbean, Central and South America, Africa, and Australia.  I wondered how it attained such a vast geographic distribution, but when I researched the species on google, I found no scientific studies delving into this mystery, and I learned no genomic wide studies of the Olalaceae family have been conducted yet.  I couldn’t even find any speculative discussions of its range, so the ancient history of this species has been overlooked.

I hypothesize tropical storms disbursed this species on rafts of vegetation to different continents near the equator.  Hurricanes can wash plant material far out to sea, and when it lands on a different continent, surviving flora and fauna can then colonize new territory.  (Animals often cling to these rafts of vegetation.) This hypothesis has also been proposed to explain how monkeys and rodents originating from Africa colonized South America, and it is the commonly accepted explanation for how anole lizards conquered Caribbean Islands and southeastern North America.  The ability of tallow plum to grow on sandy soils helped them set roots on beaches when they made landfall, following severe storms and currents that carried them halfway around the world.  Uprooted plants must have been able to survive for weeks while floating on the ocean before reaching land where perhaps waves or river currents reburied the roots in soil.  Some soil likely clung to the floating uprooted plants, and timely rains helped keep the plants alive.

The only fossil site with specimens of tallow plant is of Pliocene-age, and it is found in Africa.  The site is estimated to be between 4.3 million years BP-3.8 million years BP.  The only other species of tallow plum (X. caffra) is also found in Africa.  It seems likely Africa is the continent of origin for tallow plums.  Geneticists could shed light on the evolutionary history and distribution of tallow plum, if they ever look at its genome.


Journey of the Pawpaw Seeds

April 29, 2018

Dinosaurs probably ate pawpaws.  The large size of the fruit likely evolved to attract hungry dinosaurs that then deposited the still viable seeds in piles of dung. Scientists identified fossil pawpaw leaves from both late Cretaceous and Eocene age strata.  Though many  Cretaceous plant genera didn’t survive the K-T impact that rubbed out the dinosaurs, pawpaws along with fig, sycamore, and laurel did.  Pawpaws profusely produce sucker roots, and this habit likely allowed them to regrow after the cometary impact incinerated above-ground vegetation.  The species of pawpaw that grew during the Eocene was Asimina eocenica.  Fossil leaves, or rather impressions, of this species have been found in Texas, Colorado, and Wyoming.  Pawpaws no longer range into the latter 2 states, but, of course, climate was much warmer during the Eocene.

There are 7 species of pawpaws.  The most common is the eastern pawpaw (A. triloba) which ranges throughout most of Eastern and Midwestern North America and is found in 26 states.  All other species of pawpaws occur either in south Georgia, Alabama, and Florida or are found exclusively in Florida.  The list of other pawpaw species includes slimleaf pawpaw (A. longifolia), big flower pawpaw (A. obovata), small flower pawpaw (A. parviflora), dwarf pawpaw (A. pygmea), netted pawpaw (A. reticulata), and 4 petal pawpaw (A. tetramera).  The great number of species found in Florida might suggest this may have been a center of evolutionary origin, but pawpaws first evolved before Florida even existed. Instead, pawpaws likely speciated in this region when high interglacial sea levels isolated various populations.  Pawpaws are the only genera within the Annonacea family that can survive in temperate climates.  All other species within this family such as custard apple, cherimoya, and sweetsop are restricted to tropical regions.  The larva of the zebra swallowtail butterfly (Protographium marcellus) feeds exclusively on pawpaw leaves, and it also is a rare temperate member of a mostly tropical subfamily.  There are 150 species of Graphinii butterlies, and the zebra swallowtail is 1 of only 2 species that occur in temperate climates.  There were probably more species in the Annonacea family and Graphinni subfamily that occurred in North America during the Miocene when climate was more tropical to subtroptical, but pawpaw and zebra swallowtails were the only ones that evolved the ability to survive frosts.

Scientists believe the present day patchy distribution of pawpaws indicates it depended upon now extinct megafauna for dispersal.  Beasts such as mastodons and ground sloths ate the large fruit and deposited the seeds throughout the environment.  Today, pawpaws mostly grow as an understory tree in shady bottomland forests, but they produce more fruit in sunny locations.  It seems probable that in the past mastodons would carry the fruit away from the shade in their alimentary tracts and defecate the seeds in a sunnier spot.  In a recent experiment horses and Asian elephants refused to eat pawpaws.  However, mastodons were not the same species as the Asian elephant, and they co-existed with pawpaws (an exceptionally nutritious food source) for millions of years.  Despite the results of this experiment, I believe mastodons ate pawpaws.  Genetic studies suggest pawpaws still have high genetic diversity because not enough time has passed since the extinction of their megafaunal dispersers.  Native Americans cultivated orchards of pawpaws, and  Iroquois Indians may have spread the fruit to its northernmost range limits.

Pawpaw fruit.  I grew saplings from these seeds.  The fruit is very sweet, has the texture of a cooked sweet potato, and exudes a tropical aroma.

Pawpaws are not well known today because they are difficult to market and cultivate.  The fruit has a shelf life of 2 weeks, so it can’t be warehoused, though it can be frozen.  Experimental breeding may eventually prolong the shelf life, however.  Wild strawberries have a shelf life of 1 day, but cultivated strawberries have been bred to last at least a week.  Pawpaws grow slowly and have low pollination rates and thus low fruit production.  Bees don’t pollinate pawpaw flowers.  Instead, the red flesh color of the fruit attracts scavenging beetles, fruit flies, and blow flies.  Flies and beetles are less efficient pollinators than bees.  The center of modern day pawpaw cultivation is in Kentucky.  Many new varieties are being developed and perhaps soon, they will be found in grocery stores or farmer’s markets.  All but 2 of the 60 varieties grown during the early 19th century have been completely lost as the fruit became forgotten by urban and suburban spawn of the rural folks who grew them.

A nice lady from Indiana sent me some pawpaw fruit almost 6 years ago.  I was able to germinate 5 saplings from 30 seeds.  The saplings are growing slowly and the top trunk of 2 of them broke off last fall.  I’m not sure why this happened. I don’t see zebra swallowtails in my neighborhood, but some insect, perhaps another species of swallowtail larva, has been eating the leaves.  I doubt I’ll ever get fruit from these trees.

The best of my 5 year old pawpaw trees.  I’m not going to transplant it.  Thick roots are already growing through holes in the bottom of the pot.

Compare the above 5 year old pawpaw sapling with this 4 year old peach tree that I also planted from seed in my yard.  The peach tree is 12 feet tall and loaded with fruit.  Click to enlarge.


Berrry, Ed

“The Lower Eocene Floras of Southeastern North America”

U.S. Geological Professional Paper 91 1916

Lu, Li; J. Lowe, and S. Crabtree

“Genetic Variation in Pawpaw Cultivation Using Marosatellite Analysis”

Journal of the American Society of Horticulture 2011

Homaza, Jose

“The Pawpaw, a Forgotten North American Fruit Tree


The Ghost Boundary of the Last Glacial Maximum Ice Margin

March 30, 2017

The southern margin of the Laurentide Ice Sheet is still evident today in the range maps of at least 19 species of eastern trees.  During the most recent Ice Age about 20,000 years ago glaciers advanced to their farthest extent, a time period known as the Last Glacial Maximum (LGM). This giant sheet of ice pushed boulders, obliterated forests, and even blocked and bent major rivers.  After the glacier began retreating many species of plants colonized the nearby deglaciated territory, but 19 species of trees never advanced and remain locked in the same ranges within which they probably occurred during the Ice Age.    Though the ice margin is long gone it still marks the northern limit of these trees, serving as a kind of ghost boundary.

A majority of paleoecologists long thought a boreal forest consisting of spruce and northern species of pine existed for hundreds of miles south of the ice sheet during the LGM. Studies of fossil pollen abundance deceptively support this belief.  Spruce pollen dating to the LGM predominates in sites as far south as north Georgia.  But spruce and pine trees produce more pollen than oak and other hardwood species. Moreover, spruce and pine pollen is more resistant to decay, so oak pollen is likely underrepresented in samples.  The range maps of the below listed 19 species suggests they occurred all the way up to the ice margin during the LGM.  The forest that existed immediately south of the ice sheet during the Ice Age was probably a strange mix of spruce and hardwood trees not found in any extant natural community.  Sweet gum, post oak, and river birch grew side by side with white spruce and fir.    Scientists refer to this unusual plant composition as “non analogue” communities.  Farther south, spruce pollen excavated from fossil sites likely originated from the extinct temperate species–Critchfield’s spruce.

Below is the list of 19 species whose northern range limit reaches the ghost boundary of the Laurentide Glacier.  This list is from the below referenced study.

shortleaf pine (Pinus echinata)

Virginia pine (P. virginiana)

pitch pine (P. rigida)

sourwood (Oxydendrum arboreum)

willow oak (Quercus phyllos)

southern red oak (Q. falcata)

swamp chestnut oak (Q. michauxii)

overcup oak (Q. lyrata)

post oak (Q. stellata)

yellow buckeye (Aescules flava)

river birch (Betula nigra)

persimmon (Diospyros virginiana)

pecan (Carya illinoinensis)

pumpkin ash (Fraxinus profunda)

sugarberry (Celtis laevigata)

winged elm (Ulmus alata)

sweetgum (Liquidamber styraciflua)

white basswood (Tilia heterophylla)

black locust (Robinia pseudoacacia)

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Map of the Laurentide Ice Sheet.  The northern limits of at least 19 species of eastern trees coincides with the ice margin of this glacier where it advanced during the Last Glacial Maximum about 20,000 years ago.

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Range map of shortleaf pine.   The northern limit of this species closely coincides with where the southern lobe of the Laurentide Glacier advanced.

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Sourwood range map.

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Willow oak range map.  Note how it grows to southern New Jersey, just short of where the ice sheet advanced.

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Yellow buckeye range map.

Liquidambar styraciflua range map 4.png

Sweetgum range map.

When I was researching this blog article, I discovered a 20th species that might be added to this list.  Blackjack oak (Q. marilandica) is a small scrubby oak that grows on poor sandy soils.  The northern limit of this species range also is nearly identical with the ghost boundary of the Laurentide Glacier.  However, there are 2 small disjunct populations of this species in southern Michigan.  Unlike the other 19 species on this list blackjack oak must have temporarily colonized deglaciated territory, thriving on poor gravelly soils recently scoured by glaciers.  After soil improved other species outcompeted blackjack oak over most of the Midwest, but it remains in locally favorable habitat


Loehle, C; and H. Iltis

“The Pleistocene Biogeography of Eastern North America: A Nonmigration Scenario for Deciduous Forest”

U.S. Department of Energy Technical Report 1998


Pleistocene Pecans (Carya illinoinensis)

November 20, 2016

The pecan tree is 1 of 17 species of hickory trees.  Hickories are native to North America and Asia and formerly occurred in Europe, but Ice Ages, beginning about 2.5 million years ago, wiped them out there.  European mountains have an east to west orientation, while American mountains are oriented north to south.  Hickories prefer temperate climates, and the east-west mountains blocked their retreat in Europe during glacial expansions.  This explains why hickories and so many other tree species survived Ice Ages in North America but not in Europe.

Evidence of fossil pollen grains suggests hickory trees grew alongside dinosaurs during the late Cretaceous, though the oldest fossil hickory nut dates to about 34 million years ago.  Most early hickory species had thin shells, but they evolved thicker shells about 38 million years ago in response to the evolution of tree squirrels.  Squirrels love the nutrient rich nuts, so hickories evolved nuts with thicker shells, and the squirrels in turn evolved greater gnawing power.  Evolution is a constant struggle.  However, pecans retained the thinner shells of their early ancestors.  This puzzled me because it seems as if squirrels would have eliminated all hickory species with thinner shells because they were easier to exploit.  I wondered if pecans were a recent species, cultivated and spread by Native Americans.  I’ve concluded however, based on certain lines of evidence, that pecans are an ancient species of hickory, not a recently evolved species manipulated by man.

Wild Pecan Tree

Wild pecan tree.

{The native range of Carya illinoensis}

Native range of wild pecan trees.  Man has greatly expanded this range by planting pecan orchards.  Georgia is now the leading producer of pecans, though they are not native to this state.  Pecans need longer growing seasons than other species of hickory because their nuts mature later.

Genetic studies determined pecans have a large genetic diversity within populations.  If pecans descended from human cultivation, they would have low genetic diversity because they would descend from a small population initially cultivated by man.  I was also mistaken in considering squirrels the only major predator of hickory nuts.  The pecan weevil ( Curculio caryae ) infests all species of hickories, and pecan trees growing in mixed stands with other hickory species have an advantage over their cousins.  Pecans mature later in the season than other species of hickory.  The pecan weevil hatches and emerges in August and will infest whichever hickories have developed kernels.  Because thick shelled hickories mature before thin-shelled pecans, the pecan weevil will infest them first and go through their life cycle without ever infesting the thin-shelled pecan.  Weevils will wait for pecans to mature, if no other hickory trees are available.  So though thin shelled pecans may suffer heavier squirrel predation, they are less likely to have their nuts destroyed by weevils, if they grow near other species of hickory.

Pecan weevil larvae in nut

Pecan weevil larva.  Pecans mature later than hickories.  Though squirrels favor pecans over hickories, the later maturing pecans are less likely to be attacked by pecan weevils in mixed forests, giving pecans an advantage over other hickory species.

Pecans are native to river bottomland terraces where they grow in forests dominated by sycamore, sweetgum, and elm.  Other subdominants in these terrace forests include water oak, box elder, silver maple, cottonwood, green ash, hackberry, and other hickory trees. Pawpaw, bamboo cane, pokeweed, grape vine, poison ivy, and green brier make up the thick undergrowth of bottomland forests.

Pecans hybridize with 5 other species of hickory.  The nuts produced by wild pecans and hybrids vary in quality.  Most are smaller and have somewhat thicker shells than cultivated varieties of pecans, and some even have high amounts of bitter tannins–all part of their ongoing evolutionary war with squirrels and weevils.  Human cultivation of pecans on a large scale began circa 1900.  Though Georgia isn’t part of the pecan’s native range, the state is the leading producer, and there is a large demand in China and India, resulting in high prices at the grocery store for the nuts.

Most cultivated varieties of pecans don’t mature before the first killing frost occurs in Midwestern states.  A few small early maturing varieties can produce in the Midwest as can the hican–an hybrid cross between pecan and shellbark hickory.

Hican 5-nut sample prior to cracking. US quarters shown for size comparison.












Hican nuts–an hybrid cross between a pecan and a hickory.  They produce earlier maturing nuts  for northern locations that have frosts before pecans can fully develop.

The southern Mississippi River Valley has long served as a refuge for pecans and other hickory trees during glacial expansion cycles.  Here, they grew in mixed Ice Age forests with spruce, beech, walnut, and oak.  Pecans expanded their range north up the Mississippi River Valley following the end of the last Ice Age.

The oldest written recipe for pecan pie dates to 1886.  It was a custard pie made with sugar, eggs, and butter.  (Sugar/custard pies originated during the Middle Ages.)  Pecan pies became more popular in the 1930s when some unnamed employee of Karo syrup invented a recipe for a pecan pie using syrup as well as sugar.  I prefer my pecan pie made with maple-flavored corn syrup.

Pecan Pie

Pecan pie with whiskey maple cream sauce.  I like my pecan pies made with maple flavored corn syrup.


The Ecology and Evolution of Live Oaks (Quercus subsection virentes sp.)

September 2, 2016

The southern live oak (Quercus virginiana) is my favorite species of tree.  I love the sprawling canopy, usually covered in silvery-gray Spanish moss, and the way they grow in shady groves that cool off their native habitat–the hot sea islands and lowlands of southeastern North America.  The southern live oak is specially adapted to grow on the edge of maritime forests where their sturdy limbs can reach over adjacent fresh or saltwater marshes.  Before Europeans modified the environment, most live oaks occurred on the edges of watery habitats.  Unless a natural gap occurred within the forest, the limbs of most individual trees sprawled in one direction over water, enabling them to capture sunlight that other species of trees couldn’t reach.  But man cleared the original maritime forests and replanted live oaks in groves where the limbs of widely spaced trees could sprawl in all directions.  The limbs evolved to withstand strong sea winds, and when Europeans first colonized the region they were quick to make use of the solid wood for ship-building.  Today, the tree is planted as an ornamental, and its abundant acorn production feeds wild hog, deer, squirrel, and other wildlife.

There are 7 species of live oaks including the southern, the sand (Q. geminata), the dwarf or runner (Q. minima), the Cuban (Q. sagraeana), the Texas (Q. fusiformis), the Baja (Q. bradegeei), and the encina or Central American (Q. oleoides).  All of these species grow at low elevations in well drained soils.  They are evergreen trees that require cross pollination.  Live oaks will occasionally hybridize with other species of white oaks and with other species of live oaks in ranges where they overlap.  The genetic evidence suggests white oaks first evolved 28 million years ago, and the live oak group (a subsection within the white oak family) diverged from other white oaks 11 million years ago.

The Seven Sisters live oak in Mandeville, Louisiana is the largest and oldest known live oak in the world. It is estimated to be 1500 years old.

Sand Live Oak (Quercus geminata) in a stand of open pine savannah.  It is more fire tolerant than Quercus virginiana.

Dwarf Live Oak shown as a small group growing and expanding naturally via underground stems/roots

Dwarf live oak (Q. minima).  This species is fire dependent.  Its root system is much larger than the shrub itself.

The ranges of southern, sand, and dwarf live oak overlap; but they don’t hybridize often because they flower at different times and they prefer different habitats.  The southern live oak is a large tree that is fire intolerant.  This species can only grow successfully in environments where it is protected from fire such as islands surrounded by salt marsh and inlets or hardwood hammocks surrounded by swamp.  The sand live oak is fire tolerant, and it can grow in pine savannahs subject to frequent fire.  The runner live oak is a small shrub that is fire dependent.  It has an extensive root system underground and will re-sprout after a ground fire, but it can’t grow in shade.

Scientists believe live oaks evolved in southern North America because most species are frost tolerant.  The encina or Central American live oak lost this frost tolerance after it colonized the tropics.  The genetic evidence suggests the Cuban live oak is derived from the Central American species, but scientists don’t know how the ancestor of the Cuban species made it to the island.  Acorns likely rafted across the Caribbean protected from the salt water in a clump of vegetation.

The genetic evidence suggests the ancestor of the Baja live oak was formerly much more abundant, but the opening of the Sea of Cortez isolated this species.  It shared a common ancestor with the closely related Texas live oak.


Cavender-Bares, J.; et. al.

“Phylogeny and Biogeography of the American Live Oaks (Quercus subsection virentes); a Genomic and Population Genetics Approach”

Molecular Ecology 24 2015