Signs of Fall 10: PA Pawpaw Patch

Photo by D. Sillman

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About three miles down the Roaring Run Trail is a dense copse of slender trees that stretch their long, thin branches and their dark, shiny green leaves out over the path. If you stop to look at them you can see the boundaries of the patch just up the terraced hillside and over to the edge of a fast running creek. If you then turn around you can also see that a few of these small trees (or, according to some authorities, large shrubs) are growing on the river side of the path, too, right down to the abrupt drop-off of the shoreline.

I can’t even begin to calculate how many times I have ridden past this cluster of trees over the years without noticing how different they were. A good friend had to point them out to me one afternoon when we were on a group ride down the trail.

These are pawpaw trees (let’s not call them shrubs even though they don’t show up in my dendrology or silvics books), and they are growing in a proverbial pawpaw patch!

Photo by D. Sillman

Pawpaws (Asimina trilobal) have a southern U.S. cachet to them, but they are naturally found all across the eastern half of the United States as far west as eastern-most Kansas and Oklahoma and as far north as southern Ontario. They are not terribly common across their wide, natural range and tend to grow in single species patches usually in relatively wet, bottom-land soils surrounded by mixes of other deciduous tree species. A quick glance at a few of the local names for the pawpaw gives you a sense of their broad distribution. The are called prairie bananas, Indiana bananas, Kansas bananas, hillbilly bananas, Michigan bananas, Missouri bananas and Ozark bananas (and more). The name “pawpaw” probably came from early European settlers who thought that it was the same species as the tropical papaya tree (which is also commonly called, to the great confusion of Internet searches, the “pawpaw”).

Pawpaws tend to grow in clonal patches that have formed via root sprouts from a single, or small group, of parental trees. The copse of pawpaws along the Roaring Run trail, then, may actually be the above-ground growth of a single individual.  That individual may have been planted here decades ago as a part of a surface mine rehabilitation project. The patch is growing in what looks to me like terraced mining spoil.

Young pawpaw seedlings are very sensitive to sunlight and must grow in the shade. Seedlings from root sprouts under mature pawpaw trees, then, would be self-shaded by the taller, older clones and would readily grow in an expanding, but sustainable dynamic and, eventually, form a large clonal patch.

Pawpaw flower, Photo by Kenraiz, Wikimedia Commons

The fruit of the pawpaw is one of the species’ great features. It is the largest fruit produced by a native North American tree and was highly valued by both Native Americans and European settlers. The fruit begins to form in the early spring, reddish purple flowers after they have been pollinated by a variety of flies (especially blow flies) and beetles (especially carrion beetles). That the pawpaw’s dominate pollinating insects are carcass decomposers gives you an idea of what the scent of its flower is like (it is delicately described as a “fetid” odor).

It is also said, possibly to explain the observation that wild pawpaw trees produce very few fruits, that pawpaws are unable to self-fertilize. That means that NONE of the clonal sprouts of a pawpaw patch can produce pollen that can fertilize any of the other clonal flowers! For a patch to produce fruit (and seed), it must have different genetic individuals close by that can exchange their pollen via their flies and beetles.

Pawpaw fruit. Photo by S. Bauer, USDA, Public Domain

The fruit starts out green in color and grows rapidly. By early autumn the fruits, which are really large berries, can be 2 to 6 inches long and 1 to 3 inches wide and weigh over one pound. The weight of the fruit pulls the slender upper branches of the pawpaw trees downward. When ripe the fruits turn yellow-green to brown and fall to the ground where, unless they are gathered up by humans, they are eaten by raccoons, opossums, squirrels and bears. The very large seeds of the pawpaw, though, are not easily swallowed by any of these animals and are, therefore, not readily transported in the animals intestines. The pawpaw, like the Osage orange and a number of other temperate and tropical tree species, are thought to have evolved symbiotic, seed dispersal relationships with much larger animals (like giant ground sloths and mastodons) which went extinct (probably due to human activity) in the late Pleistocene. These tree species, then, have no natural way to disperse their seeds and have consequently retreated into smaller and smaller actual growth ranges or patches (see Signs of Summer 11, August 11, 2016). Possibly in response to this aborted seed dispersion, the pawpaw now relies primarily on root sprouting and clonal reproduction (and clonal patch formation) for its propagation.

The pawpaw fruit has a very intense flavor variously described as a combination of a mango and banana with the texture of a custardy pudding. Ripe fruit must be handled very carefully to avoid bruising and damage. The fruits also ferment very rapidly and so must be eaten quickly or be frozen for preservation.  The spotty nature of fruit production and the fragility and lability of the fruit make pawpaws a difficult plant to grow in agroecosystems. Also, the need to grow the pawpaw seedlings in the shade for their initial growth years makes commercial orchard management difficult.

Photo by DE. Sillman

The pawpaw produces a number of secondary chemicals which act as protections against insect damage and vertebrate herbivory.  A major consequence of this is that pawpaw leaves, twigs and fruit are not readily eaten by white-tailed deer. In places with high numbers of deer, which readily consume most other tree species, pawpaw patches tend to expand. Possibly in our present day, “deer dominated” eastern U.S. forests, pawpaw trees will assume a greater and greater presence.

One specific secondary chemical (acetogenin) synthesized by pawpaw trees plays an important role in the lifecycle of a coevolved insect (the zebra swallowtail butterfly (Protographium marcellus)). The zebra swallowtail preferentially or possibly exclusively lays its eggs on pawpaw trees. The larvae (the caterpillars) that hatch from these eggs then feed exclusively on pawpaw leaves up until they metamorphose into adult butterflies. This exclusive feeding on pawpaw leaves causes the bodies of the caterpillars and also the bodies of the adult zebra swallowtails to be rich in the metabolic by-products of acetogenin. These chemicals, then, make both the caterpillars and the adult butterflies toxic to most potential predators. This plant-insect relationship is similar to the frequently described monarch butterfly and milkweed symbiosis that is so vital to the life cycle of the monarch.

So as the old folk song says (leaving out some of the repetition):

Where is dear little Nellie?

Way down yonder in the pawpaw patch!

Pickin’ up pawpaws and puttin’ in her pocket!

Pickin’ up pawpaws and puttin’ in her pocket!

Enjoy!