Pawpaw — Wild Plant Culture

My first experience with pawpaws was a memorable one...

As a novice botanist, I was inventorying species along a linear transect in a forested section of the Sourlands. As I neared a Cattail Brook, I veered instinctively to the left through a gap in the thorny, thickety woods. Some force had just compelled me to turn off my chosen path and seek out this new area.

A field of diabase boulders sloped to the stream. Bladdernut shrubs grew in tall woody tufts between boulders, with lindens and sugar maples overhead.

This particular spot was new to me, and my eyes were drawn to a low, leafy mound, chest-high, as round as tall. Its long, wide foliage draped around its many woody stems, creating the impression of some frondy, tropical shelter.

I had two initial thoughts as to its identity. Hickory proved wrong almost immediately, as the leaves were not compound. Spicebush didn't fit either, the foliage was simply too large. I examined an oddly naked terminal bud, and reached into my pack for a trees book.

For whatever reason, I had a hand-me-down "Little Golden Book" of trees in my bag, and, excitedly flipping through, I came to rest on a page with an illustration that matched my find. Asimina triloba, the pawpaw. An almost legendary native fruit tree, the sole member of a tropical genus to thrive in our region.

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At the time, Asimina triloba was classified by the state as S1 - State Endangered. This rank is reserved for species with less than five occurrences in the state (or where relevant, fewer than 1000 individuals).

In the summer of 2016, pawpaw was reclassified as an S2 species in New Jersey, which is often loosely spoken of as "Threatened", though the official language does not support this[1]. The state rank S2 corresponds to 6-20 occurrences, or 1001 to 3000 individuals where relevant.

New Jersey is approaching the northeastern limit of the range for Asimina triloba, with populations in Pennsylvaniaand New York (where it is ranked as "Threatened") but few or non further into New England. In the early 1980s, the not-quite-comprehensive New Jersey Wild Plants by Mary Hough lists pawpaw as "local along the Delaware River and its islands...." citing known populations in Hunterdon and Burlington Counties.

One can conjecture a few reasons for a recent, subsequent range expansion. First and foremost, pawpaw has become something of a cult favorite among native plant gardeners. Its tropical foliage and intriguing fruit give it a wild mystique and one can assume a considerable amount of deliberate planting in the past three decades. Second, global warming probably favors the northerly expansion of this species, which sometime waits until July or August to germinate in our nursery, as if it is still responding to climate cues from some southern landscape. Presumably this does not favor the species if the first frost is a month or two later. Last, pawpaw is largely deer-resistant and this confers an enormous advantage in the midst of the deer overpopulation crisis.

Was the population I had discovered at Cattail what some would term a "natural" population, arriving by non-human devices... or perhaps an "escape" from some forgotten planting? Hard to say. On the one hand, the boulder field in which I discovered what ended up being three shrubby individuals was hardly a likely planting spot. And for central New Jersey, it was in a fairly wild location, surrounded by 1,000 acres of contiguous forest. However, less than a half-mile away was an old homestead site, with remnant populations of European hops, and also a large clone of Carolina allspice. A nearby disturbed area harbored the occasionally aggressive Amur corktree, with is fluorescent yellow inner back and oddly lacking terming bud (the branches terminate in what looks a bit like Barbie's crotch). Maybe the long-gone homesteaders had fancied exotic flora and had imported pawpaw, which subsequently made its way towards the brook, following its natural proclivity for riparian areas? Perhaps, but there was no evidence of pawpaw anywhere around the old homestead.

Other New Jersey stations for Asimina triloba away from the Delaware also seem questionable in terms of "naturalness", at least if you define the term so as to exclude human-assisted dispersal (more on that later). Among those I haven't seen, there is a well-known population in Cranbury, and another one off an exit of Route 130. Word of mouth suggests a population in a powerline in Livingston, another in Teetertown Ravine, and several in Monmouth County.

A population in Trenton near a dismal semi-abandoned industrial area is right off a disused road and surrounded by Japanese knotweed, with which it was effectively competing when I saw it about seven years ago. This is right near the Delaware and could fit into the older picture of a classic, in-range natural population. Another I discovered near the Musconetcong River is perhaps an escape, as it is on the grounds of an old estate with a few planted magnolias, chestnuts, etc. Another memorable pawpaw used to poke out of a hedge by a business on the main drag (if it counts as such) in Milford, NJ, right by the river. New tenants seem to have tidied up the landscaping and I'm not sure the pawpaw is persisting.

Given the somewhat random and disjunct stations for pawpaw I've seen in New Jersey, I decided to visit some relatively nearby areas in Pennsylvania to get a more complete sense of the plant's ecology.

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About two hours to my west, the Susquehannah River swells and curves in the vicinity of Lancaster, Pennsylvania. During two separate trips, I visited three natural areas with significant populations of pawpaws.

The first, Ferncliff Nature Preserve, is listed on the National Historic Register as an old growth forest. A modest stream runs through a steep-sided clove, gathering a tributary or two before flowing under tracks and out into the Susquehannah. I recall a mesic forest characterized by hemlock, beech, and tuliptrees. Here, I saw rhododendron and pawpaw side by side, almost miming each other with multiple trunks and broad leaves reaching out for dappled light. Maple-leaf waterleaf was exceedingly common in the herb layer, and wild hydrangea and the occasional spikenard clung to outcrops of vertically upthrust grey rock over the stream.

I had driven through Amish country to get here, driving among vast cropfields in rolling hills before abruptly entering a low woody ravine as I approached the river. In the ravine, a beech tree with various graffiti including the name "Josiah" scribed into the bark reminded me I was still in Amish country.

In one spot, high on the flanking slope, a canopy gap had prompted growth of a dense grove of 15' - 20' tall pawpaws. These were fruiting abundantly, especially directly around the windthrown trunks of former canopy trees. Beneath the pawpaws was a sward of blue cohosh in fruit. Beneath the cohosh, poking out of the soil, were the little reddish root bases and scales of Dutchman's breeches. Also in the community were spicebush, white snakeroot, garlic mustard, green briar, sigzag goldenrod, and false solomon's seal.

A second site, Tucquan Glen, was even more scenic, with a cool stream spilling over giant boulders in the depths of a hemlock-lined ravine. Here too, pawpaws grew on mesic hardwood slopes but also in the purple shade of hemlocks and alongside rhododendron.

The third site, the Turkey Hill Nature Trail, boasts "[t]he world’s largest paw paw patch north of the 39th Latitude" near the northern trailhead. Here, the flora is more degraded, and pawpaws fruit along decommissioned right-of-ways over Japanese stiltgrass, mugwort, and multiflora rose. I recall pawpaw grower Larry Rossi once describing an forested hill that had been logged in prior decades, resulting in an exponential growth of pawpaw after the canopy was cleared. I wonder if this was perhaps the place, as the signs of disturbance and the pawpaw response fits the story well.

What became clear from these sites is that pawpaw is exceedingly skilled at playing the "shade game". Under a dense canopy, I encountered numerous shoots of pawpaw sometimes only a few feet high, with a few big leaves gathering dappled sun. Here, they just hang out, spreading slowly, probably for decades or longer. But in canopy gaps, whether caused by windthrow or by logging, these scattered individuals coalesce into dense groves of 20' or taller stems. It is in these groves that the pawpaws begin to fruit, in order to be spread into some other shady place and there wait for some big trees to fall.

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There is a theory about pawpaw seed dispersal I find absolutely fascinating. Pawpaws are large fruits relative to most others in our flora, ranging in size from fingerling potato to full-on Yukon Gold. The seeds are quite a mouthful too, about an inch long and vaguely bean-shaped. What manner of creature would eat one of these whole and poop the seeds out? Or, if the seeds are not intended to be animal dispersed (they do float...), why would the plant create so much succulent flesh around them?

Imagine yourself back in time with me, for a moment. 12,000 years or so will do it, though we'd have to go back before the Ice Age if we want to stay in New Jersey. We're standing beneath a giant tupelo tree, looking out at a grove of pawpaws. Before we step out to seek some fruits, we look around for other creatures that might consume pawpaws. Keep in mind that these might include:

American mastodon

woolly mammoth

Jefferson’s ground sloth

Wheatley’s ground sloth

flat-headed peccary

long-nosed peccary

Leidy’s peccary

vero tapir

Cope’s tapir

woodland muskox

Harlan’s muskox

stag-moose

fugitive deer

giant horse

giant beaver[2]

While we're uncertain which of these giant herbivores consumed pawpaws, there isno doubt that many inhabited the same landscapes as Asimina triloba. "[F]ossils closely resembling A. triloba date to the Late Miocene from New Jersey" according to pawpaw expert R. Neal Peterson[3]. That's sometime between 5.3 and 11.6 million years ago. And the majority of megafauna listed above only disappeared in the last 8,000 - 12,000 years - an evolutionary blink of an eye.

According to Dan Janzen's hypothesis of megafaunal dispersal, certain tropical plants like avocados and mangoes are "evolutionary anachronisms", persisting despite the absence of any credible faunal seed disperser. The hypothesis, hatched in Central America, was extended to North American species such as the osage orange, Kentucky coffee tree, persimmon and pawpaw.

How is it that pawpaw has persisted in the intervening millenia, lacking a giant sloth or gomphothere disperser? Some suggest that the species' clonal habit -- its ability to "reproduce" by root runners, sending up new stems, possibly for centuries -- has allowed for its persistence.

An equally compelling proposal considers a megafauna species which was a new arrival as the native Pleistocene megafauna went extinct. Humans.

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In Peterson's extensive monograph on Pawpaws, he suggests that, with the extinction of frugivorous megafauna, Asimina triloba might have been consigned "to an evolutions backwater of popukation decline, inbreeding, and genetic loss. The arrival of humans in North America towards the end of the Pleistocene probably saves A. triloba from such a fateful decline". He credits humans with spreading pawpaws north from Ice Age refugia, and cites Native American dispersal of other food and medicine species, from American lotus to Kentucky coffee tree.

* * *

Part of the status determination for rare plants, though not a well-documented aspect, involves consideration of an occurrence's "naturalness". A species occurence in its core habitat, having arrived via seed dispersed by natural forces such as wind, gravity, water, and wildlife, is more likely to be considered "natural" than a species found in someone's garden, arboretum, or home landscape.

With pawpaw, the criteria for "naturalness" becomes less clear. If one of Asimina triloba's primary dispersal agents for the last 10,000 years has been human beings, how can "naturalness" criteria demand non-human dispersal agents? Unless someone seriously proposes (though probably quietly) that Native Americans can be considered with wildlife, and Euro-Americans as somehow not part of nature, the pawpaw in my backyard might be just as natural as the classic populations on Delaware River islands and shorelines in Hunterdon and Burlington Counties - core territories for the Lenape, who also refer to themselves as "river people".

Was the population of pawpaw I found along the Cattail Brook a "natural" population? In one sense, I'll never know. That is, I'll never know how that plant arrived in that place (one of the great mysteries, regardless). In another sense, no matter what animals brought pawpaw there, the pawpaws probably spread by the same mechanism they've been using for millions of years -- delicious fruit.

[1] Leading legislation protecting "threatened" plants in New Jersey without much legal footing, unfortunately.

[2] Adapted from Roger Latham, continentalconservation.com

[3] Peterson, R. N. (1991). Pawpaw (Asimina). In: J.N. Moore and J. R. Ballington (eds). Genetic resources of temperate fruit and nut trees. Acta Hort. 290:567- 600.