Posts Tagged ‘pawpaw’

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.

References:

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

http://www.arboretum.harvard.edu