Land use in southern Michigan offers ideal habitat for uncontrollable population growth by deer: Our patchwork of small woodlots and forested river edges amidst farm fields, large rural properties with lawns and gardens, and abundant roads provide edge habitats favored by deer. Dwellings and people are interspersed such that opportunities even for bow-hunting (much less rifles) are difficult to arrange. As well, once the sound of rifles begins, deer can be expected to retreat to hunting-free safe zones that they have experienced in the past.
Overall, so long as traditional carnivores are missing and hunting is not a year-round activity, winter starvation, disease, and collisions with cars are what controls the deer population. Meanwhile, the native palatable plants are all but lost and the deer-resistant non-natives take over.
Most worrisome for foresters is that canopy trees will not be able to replace themselves wherever deer are overpopulated: their seedlings and saplings are killed by perpetual browsing.
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LEFT: Invasive Amur honeysuckle, Lonicera maackii from Asia is now a dominant subcanopy woody tree/shrub in many recovering forested zones in southern Michigan.
This photo, taken in early spring, along the Huron River near Ypsilanti, shows a dense patch of the mature honeysuckle.
The shade is now too dense for the fallen canopy tree in the background to be replaced by native canopy trees. |
According to scholarly papers, there are only three native subcanopy trees in Michigan that can, to a large extent, grow and maintain despite overpopulated deer: PAWPAW, striped maple, and American hornbeam.
RESOURCES:
• News article, 2023, "Few good options for shrinking Michigan's problem deer herds". EXCERPT: "... Beyond risk of crashes and disease, deer can destroy forest ecosystems, suburban hosta gardens and farmers' crops. One study found that in Presque Isle County alone, deer consumed a tenth of farmers' soybeans. In overbrowsed forests, saplings disappear and mature trees have a manicured look from deer nibbling branches as high as their necks can reach. That eliminates habitat for ground-nesting songbirds and other animals..."
• "Impacts of White-Tailed Deer Overabundance
in Forest Ecosystems: An Overview", 2008, by Thomas J. Rawinski, USFS. EXCERPTS: "Deer have successfully exploited the human-altered environment, feeding in agricultural fields, orchards, roadsides, lawns, and gardens... The forest floor is presently dominated by unpalatable plant species, such as black huckleberry, Japanese barberry, Pennsylvania sedge, and various woodland grasses. As trees mature and die, or topple over during storms, gaps in the canopy become larger and more numerous. There are no young trees to fill the gaps.... Some plants resist deer herbivory, owing to chemical or morphological defenses or low digestible content. Among tree species, black cherry would be classified as fairly resistant. Horsley and others (2003) demonstrated a trend toward black cherry dominance in forests impacted by deer. At Rock Creek Park in Washington, D.C., Rossell and others (2007) found all major woody species to be impacted by deer, with the exception of American beech and spicebush. At very high population densities deer will ultimately browse black cherry, American beech, spicebush, mountain laurel, and many other less palatable species....Native herb populations are often decimated by deer, but some species are consistently avoided. Examples [of native species avoided by deer] include white snakeroot, black bugbane, mayapple, blue cohosh, Pennsylvania sedge, and eastern hayscented fern. A forest understory dominated by deer-resistant species is usually diagnostic of a forest afflicted by overly abundant deer. Foresters are especially concerned that eastern hayscented fern has become too abundant in some areas, preventing or impeding tree seedling establishment (Horsley and others 2003)."
• "Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks", 2023, by Kathryn M. Miller et al., Ecological Applications. EXCERPT: "... Many of these browse-tolerant species, like paw paw (Asimina triloba), American holly (Ilex opaca), and American hornbeam (Carpinus caroliniana), are short-stature trees that are unable to grow to current canopy height..."
• "Compounding human stressors cause major regeneration debt in over half of eastern US forests", 2019, by Kathryn M. Miller and Brian J. McGill, Journal of Applied Ecology EXCERPT: "... For example, several low canopy, browse-resistant species (e.g. striped maple and pawpaw) have shown increases in areas with high deer densities, which can further suppress regeneration of canopy species and greatly alter forest structure (Kain et al., 2011; Rossell et al., 2005)."
• Anecdotal report by Laurie Potteiger, West Virginia Native Plant Society: "Does anyone have experience removing/controlling pawpaw trees? We won't get to planting trees or shrubs until this fall or next year and in the meantime they have/are taking over. What should I do in the meantime? More background: I live in an area (Jefferson County, near Harpers Ferry at about 700' elevation) with an extremely high deer population. The deer eat virtually every other native tree and shrub. Even spicebush. There are virtually no saplings of trees other than pawpaw, although the overstory contains tulip poplars, maples, oaks, hickories, etc. The seedlings for those other native species don't get more than an inch or two high before the deer eat them. We have an acre of mostly wooded land (on the top of the ridge and downslope facing NW). Pawpaw are already dominant in some areas, and popping up where they are not yet dominant, but grow way faster than anything else. We have been planting some native forbs, and have planted a few shrubs and trees, but have to cage or fence everything I plant to protect them for the deer. (When I find a tiny tree seedling other than pawpaw I try to cage it before the deer get to it.) The goal is to eventually put an 8' deer fence around a significant portion of the property rather than having all the individual-plant cages and the multiple deer exclosures, which still leave most of the area unprotected. Also: most of my outdoor time in spring the last two or three years has been pulling garlic mustard (along with several other invasive non-native plants). But making significant progress. I've already started cutting down some of the young [pawpaw] trees (leaving a stump so I know where they were) but I've read that only stimulates more growth. And I've read painting stumps with glyphosate in fall doesn't really help kill the tree the way it does with many other trees and shrubs.
What to do?"
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Appreciating Asimen: Prevents Invasion by Exotic Plants
JAPANESE STILTGRASS: In 2017, Michigan DNR reported first detection of invasive Japanese Stiltgrass in this state. Resembling a small bamboo, Microstegium vimineum was spotted in Washtenaw County. Wikipedia reports that it arrived first in Tennessee, as packing material for porcelain from China, in 1919.
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Japanese stiltgrass is similar to and often grows along with the North American grass Leersia virginica.
A silver stripe running down the center of its leaf (PHOTO LEFT) is diagnostic of this non-native grass.
Also, Leersia flowers one to two months earlier than the stiltgrass, which flowers in late summer.
Connie Barlow reports seeing dense patches of a similar-looking grass while walking the pawpaw patch at Draper-Houston. But she quickly learned by observation that if such a grassy patch is present, there will be no pawpaw stems there.
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Fortunately, a 2004 report confirmed that in Tennessee, "Dense M. vimineum cover stopped abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy.... Whereas overstory tree canopy apparently facilitates the establishment of this shade-tolerant grass, the interaction of overstory canopy with midstory canopy interferes with M. vimineum by reducing the availability of sunflecks at the ground layer." Ref: "Light limitation creates patchy distribution of an invasive grass in eastern deciduous forests", by Patrice Cole and Jake Weltzin, 2005, in Biological Invasions.
OTHER EXOTICS: Once established, the dense summer shade beneath a pawpaw patch might preclude many weedy exotics from establishing. But Connie Barlow was not able to find any such papers published in journals. Instead, she launched a field experiment in 2023 to learn this:
Could introduced pawpaw overtop and shade out two subcanopy woody exotics: AMUR HONEYSUCKLE and ASIAN PRIVET?
Advance to a lengthy webpage that ongoingly documents the outcomes and what is learned via this ongoing experiment:
• "Pawpaw Ecological Survey in Michigan: Part 3 - Wild-Planting Experiments"
Toward Renewing and Expanding Reciprocity: Helping Pawpaw Walk
Whether one is attempting to help pawpaw expand at and northward of its historically "native" range, or is experimenting with planting pawpaw to recapture the river bottomland forest from exotic Amur honeysuckle and privet, western science and natural history are important to consult.
If possible, finding participants and advisors grounded in Traditional Ecological Knowledge (TEK) would also be of help. Botanist Robin Wall Kimmerer offers encouragement in "Restoration and Reciprocity: The Contributions of Traditional Ecological Knowledge," chapter 18 in Human Dimensions of Ecological Restoration: Integrating Science, Nature, and Culture, 2011, edited by Dave Egan et al.
"The idea of reciprocity with land is fundamental to many indigenous belief systems. Indeed, such beliefs serve as the foundation for what have been described as cultures of gratitude. In such cultures, people have a responsibility not only to be grateful for the gifts provided by Mother Earth, they are also responsible for playing a positive and active role in the well-being of the land. They are called not to be passive consumers, but to sustain the land that sustains them. Responsibilities to the more-than-human world are simultaneously material and spiritual, and, in fact, the two are inseparable. Ecological restoration can be viewed as an act of reciprocity, where humans exercise their care-giving responsibility for ecosystems."
• "HELPING PAWPAW WALK" is a variation of the phrase Robin Wall Kimmerer has used when addressing the need to engage in (what is usually called) "assisted migration" of plants that cannot move fast enough (via wind, water, animals) to track ongoing climate change. She calls it "helping forests walk." More information and references are available in the below paragraph of the WIKIPEDIA page Assisted migration of forests in North America:
• REVITALIZING HUMAN RECIPROCITY WITH THE FRUITS OF ASIMEN. There is little debate that humans in North America have been crucial dispersal agents for helping Asimen track the warming climate northward as the glaciers retreated. Whether or not seeds were intentionally selected, carried, and planted, it is indisputable that,
"As the ice retreated from its last southward advance, which peaked about twenty thousand years ago, four of the five above-mentioned anachronistic trees of eastern and central North America would have been helped to reclaim former territory by newly arriving humans. Pawpaw and persimmon fruits would have been carried back to camp, their seeds removed or spit out at the time of eating." — Connie Barlow, 2001, "Anachronistic Fruits and the Ghosts Who Haunt Them", Arnoldia
• WIKIPEDIA "Asimina triloba": "The natural distribution of the common pawpaw in North America, prior to the ice ages and lasting until roughly 10,000 years ago, was done by certain megafauna until they became extinct during the Quaternary extinction event. After the arrival of humans and the subsequent extinction of megafauna that were distributing A. triloba, the probable distribution of these large fruit-bearing plants has been by humans."
• "Ecological niche modelling and phylogeography reveal range shifts of pawpaw, a North American understorey tree", by Graham E Wyatt et al., 12 January 2021, in Journal of Biogeography.
ABSTRACT: Predictions of species' responses to accelerating global climate change require an understanding of historical range shifts. However, large-scale phylogeographical studies of Eastern North American understorey plant taxa are relatively scarce. Here we employ ecological niche modelling and genetic analyses for inference of optimal pawpaw habitat in the past and future.... Models suggest that 22,000 YBP A. triloba was restricted to two major refugia in narrow bands of mostly now-submerged habitat and possibly several small inland refugia. Molecular data are consistent suggesting that the eastern refugia expanded to give rise to the eastern cluster which is characterized by higher genetic diversity. The Gulf of Mexico refugium likely gave rise to populations in the western cluster, which is characterized by lower genetic diversity.
22,000 years ago
Year 2070 projected range
ABOVE: Green signifies the geographic range of the most suitable habitats past and future. LEFT: Refuges for pawpaw during last glacial maximum, 22,000 YBP (low sea level at that time enabled retreat to coastal areas now underwater). RIGHT: Projected ideal range in 2070 (using IPCC high emissions scenario).
"Given the low fruit-set observed in populations at the northern edge of the current range (Wyatt, unpublished data) and limited vagility of contemporary seed dispersal vectors (gravity and small mammals), A. triloba is unlikely to expand its range rapidly and track environmental changes in lockstep.... Although our models indicate that optimal habitat will expand significantly, between the possible loss of southern populations and environmental change that outpaces the rate of dispersal and colonization at the northern edge, the realized distribution of A. triloba may diminish by 2070."
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Could pawpaw restore the subcanopy
where deer are overpopulated?
HYPOTHESIS: Might the foreign honeysuckle, Lonicera maackii, express so invasively because overpopulated deer eat virtually all of the seedlings of our native trees that attempt to grow? Yet, because pawpaw is poisonous to deer, might our native subcanopy tree be able to effectively compete with (possibly even wrest control from) the now-dominant subcanopy invasive?
FIELD EXPERIMENT UNDERWAY: Connie Barlow began testing this hypothesis in 2023 by planting pawpaw seeds into weeded patches of bottomland along the Huron River. Advance to a lengthy webpage that ongoingly documents the outcomes and what is learned via this ongoing experiment:
• "Pawpaw Ecological Survey in Michigan: Part 3 - Wild-Planting Experiments"
ABOVE: Connie Barlow took these photos November 2021 while planting pawpaw seeds into the forested bottomland along the Huron River, where it flows through collapsed industrial landscapes of Ypsilanti, Michigan. All green leaves in these photos are of a non-native honeysuckle: Lonicera maackii. This exotic Amur honeysuckle is the dominant subcanopy woody plant in these photos. Notice the distinct longitudinal-ridged bark of the foreground stem in the left photo and the opposite leaves with red berries in the right. Uniquely, here in Michigan this invasive deciduous plant retains bright green leaves that simply fall off in December without browning or yellowing. Additionally, first to leaf out in the spring, this honeysuckle shades out our spring-flowering herbs. (Pawpaw leafs out late and thus offers spring herbs good sunlight beneath.)
Note: The technical term for this ability is EXTENDED LEAF PHENOLOGY. A 2023 article by Michael Anderson in American Scientist provides an excellent discussion — not only about why this trait can be extremely competitive but also why Asian temperate habitats fostered it while the northeastern United States did not: "Is Garlic Mustard an Invader or an Opportunist?"
He wrote, "The potential advantages provided by extended leaf phenology appear to be great enough that a high proportion of other invasive plant species in North America seem to possess some form of it."
A 2016 review paper by two Ohio researchers details the variety of competitive advantages of Amur honeysuckle over native plants in eastern North America and confirms that even the most successful combinations of stem cutting followed by herbicide application are not capable of killing all the individuals. Therefore, it appears that once Amur honeysuckle has established, management interventions will need to continue into the future. But this, in turn, opens the possibility that concurrent introduction of our native pawpaw may be able to reduce opportunities for this invasive to replenish and expand — without requiring further use of herbicides. This prospect is amplified by the fact (reported in the 2016 paper) that deer do browse on the leaves of Amur honeysuckle. This would explain why only privet dominates at heights below the browse level of deer (see the photo directly below). Seedlings and saplings of Amur honeysuckle are indeed rare in the bottomland.
SCIENTIFIC REPORTS OF PAWPAW THRIVING WHERE DEER ARE OVERPOPULATED:
• "In recent decades, naturalists have noted the expansion of pawpaw from well-drained, lowland habitats into drier, upland forests. This phenomenon appears to be driven, at least in part, by patterns of deer browse. Deer find pawpaw foliage unpalatable and, therefore, avoid browsing pawpaw seedlings and saplings. Instead, they preferentially browse species such as spicebush (Lindera benzoin), oaks (Quercus spp.), red maple (Acer rubrum), and blackgum (Nyssa sylvatica).... This deer behavior benefits pawpaw in two ways. First, small pawpaws don’t need to allocate energy to recovering from browse, and instead can put that energy towards growth and reproduction. Second, frequent deer browse on sapling and shrub species preferred by deer suppresses the growth of these species, clearing the way for pawpaw. As a result, we might expect to see pawpaw becoming more common in forest understories that are heavily impacted by deer browse (which describes most NCR forests).... Another potential contributor to the success of pawpaw is the suppression of fires that were an important part of the disturbance regime in many eastern forests before European settlement. Pawpaw are not strongly fire-adapted (unlike other common canopy dominates, such as oaks), and they likely benefit from the lack of fire in contemporary forests." National Park Service webpage: "Pawpaw: Small Tree, Big Impact".
• "Pawpaw (Asimina triloba L.) may act as a recalcitrant layer. Pawpaw utilizes annonaceous acetogenins as chemical defenses against herbivores (Ratnayake et al. 1992; Harborne 2001; Arnason & Bernards 2010). While deer will browse other, less-palatable woody vegetation when more-palatable stems are gone, deer avoid browsing pawpaw plants (Wakeland & Swihart 2009; Slater & Anderson 2014). Pawpaw's unpalatable quality, coupled with its shade tolerance (Battaglia & Sharitz 2006) and its vegetative reproduction strategy (Hosaka et al. 2016), may allow pawpaw to form a recalcitrant layer. Slater & Anderson (2014) found that deer browsing led to a dense pawpaw understory as a result of decades of intensive deer browsing. Other studies have suggested that pawpaw may limit canopy tree regeneration (Shotola et al. 1992; Shelton et al. 2014). 2016, "Tree Regeneration in a Southwestern Indiana Forest: Implications of Long-Term Browsing by Deer", by Cris G. Hochwender et al., Proceedings of the Indiana Academy of Science.
• "Indiana state parks historically did not allow hunting. But by the 1990s, white-tailed deer populations in parks had swelled to such size that many species of native wildflowers such as trillium and lilies largely disappeared, replaced by wild ginger and exotic species such as garlic mustard and Japanese stiltgrass, plants not favored by deer. Oak and ash tree seedlings gave way to highly deer-resistant or unpalatable trees such as pawpaw." 2014, "Hunting gives deer-damaged forests in state parks a shot at recovery".
• "Although white-tailed deer are generalist herbivores, they can have significant effects on species composition and abundance of forest trees, especially when deer densities are high and most plant species are heavily browsed but a few are selectively avoided as browse. We evaluated effects of selective deer browsing on tree species abundance in an old-growth mesic/wet-mesic forest in central Illinois.... The study site has high deer density (75 deer km2) during winter months, and initial observations indicated that pawpaw (Asimina triloba (L.) Dunal) was strongly avoided as browse. Pawpaw density increased in the seedling stratum between the two sample periods. However, nearly all other tree species declined in density.
"... Historically, pawpaw was limited to wet areas, because its abundance is diminished by fire. Frequent or intense burns, which were common on upland mesic sites dominated by shade-intolerant, fire-resistant oaks and hickories (Carya spp.), caused local reductions of pawpaw (Larimore et al. 2003; Holzmueller et al. 2009). Currently, pawpaw is still largely restricted to moist, shady environments (Adams and Anderson 1980; Anderson and Mitsch 2003), at least within the northern portion of the species range (Lagrange and Tramer 1985). However, a sharp reduction in frequency of fire disturbance following European settlement (Abrams 1992; Nowacki and Abrams 2008)
has resulted in a decline of shade-intolerant, preferred-browse species (Quercus spp.)
(Strole and Anderson 1992) and increased abundances of fire-sensitive, shade-tolerant tree species (Adams and Anderson 1980; Hartnett and Krofta 1989; Nowacki and Abrams 2008).
"... These changing conditions coupled with high deer densities, which reduce seedling abundance of most species, increase the likelihood that pawpaw may eventually dominate forest understories over a widespread area. Thus, its spread and establishment may become an issue of concern for existing and future forest communities." 2014, "Intensive Selective Deer Browsing Favors Success of Asimina triloba (Paw Paw) a Native Tree Species", Mitchell A. Slater and Roger C. Anderson, Natural Areas Journal.
ABOVE: In contrast to the invasive Amur honeysuckle, a pawpaw subcanopy welcomes native spring ephemeral herbs. (Photos by Connie Barlow, 2 May 2021, Draper-Houston Meadows Preserve.)
WHERE TO PLANT PAWPAW SEEDS FOR FOREST HEALTH AND REWILDING: Pawpaw likely has a much greater habitat capability than just the strips of sloping and raised areas near a river. Rivers likely became the dominant means for pawpaw seed dispersal when the megafauna disappeared — and even moreso when black bears retreated as Euro-American colonizers arrived and began farming and grazing operations.
Below is a natural history observation about a wild, fruit-producing pawpaw patch in an upland forest in southwestern Virginia. The email came from Sharon Mohney, sent on 17 December 2021:
"... This year was my best pawpaw season so far, and I missed probably half of them due to having to go out of town. You'd think a lot of wildlife would eat them, but they tend to just rot on the ground if I don't get them, although wasps and bees get some of them, and I found a box turtle eating one once. It's surprising to me that the deer, bear, possums, and raccoons around here don't scoop up all the pawpaws before I get a chance at them, but they don't.
In recent years I've noticed a lot of pawpaws showing up on higher drier sites on our place. I didn't think much about it until I noticed that my "Flora of Virginia" phone app also makes note of this development: "In recent decades, this species has expanded into dry-mesic or even dry upland habitats and increased in abundance in many areas. The reasons for these changes are not clear but could include fire exclusion and the plant's unpalatability to white-tailed deer (and the deer's selective browsing on competition.)"
I've had good luck planting pawpaw seeds soon after I clean the fruit. I understand they don't do well if allowed to dry..."
ABOVE LEFT: Range map of Asimina triloba, pawpaw (USDA, Little 1971).
ABOVE RIGHT: Unripe pawpaw fruit still hanging along the Saline River of southeastern Michigan, 6 October 2023. This is at its northernmost range. Wild patches of the clonal pawpaw species are typically found along or upslope from rivers.
MAPS SOURCE: 
