Could pawpaw restore the subcanopy
where deer are overpopulated?

ABOVE: Connie Barlow took these photos November 2021 while planting pawpaw seeds onto the forested slopes 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.
HYPOTHESIS: Might the foreign honeysuckle 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?
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..."
• VIDEO: "Helping Subcanopy Trees Migrate" -
50 minutes - published November 2021
 |
|
"Helping Subcanopy Trees Migrate" features two subcanopy species of the eastern USA. Pawpaw, while having a long north-south reach in its historic range, can benefit from "assisted range expansion" northward. Florida Torreya is an endangered glacial relict for which citizens, including Connie, have done what the official recovery program implementers have been unwilling to do restore its health by nothing more difficult than planting seeds well to the north.
Indigenous values are advocated as well as the "natural history" style of observation and interpretation.
|
• WIKIPEDIA