Pawpaw fruits — the largest native fruits in North America — have become popular among foragers and foodies alike, with their custard-like texture and a sweet flavor often described as a cross between a mango and a banana.
Pawpaws are the state fruit tree of Missouri, and they tend to grow in dense clumps. They can reproduce clonally, meaning pawpaws can spread through their roots, much like aspen trees. Once one adult pawpaw gets established, it is likely to spread underground and send up lots of stems nearby. This trick of producing copies of itself is what ends up making a pawpaw “patch,” as memorialized in American folk song.
The patch is a good place for pawpaws but nothing much else, according to new research from Washington University in St. Louis. Pawpaw trees tend to choke out woody bushes and flowering plants nearby, exerting a haphazard kind of pressure on would-be neighbors.
“Pawpaws are leafy agents of chaos,” said Anna Wassel, a graduate student in biology in Arts & Sciences at WashU and first author of a new study on pawpaws in the journal Ecosphere. “Basically, we discovered that pawpaws create a habitat where the rules of which species can win in competition and persist are more random than when pawpaws aren’t there.”
Wassel’s team — dubbed the Pawpaw Patrol — investigated the effect of pawpaw on herbaceous community composition at Tyson Research Center, WashU’s environmental field station, located near Eureka, Mo.
“Previous studies have focused on how dominant tree species affect the number of plant species in the forest understory at smaller spatial scales, leaving open the question of how and why they affect variation in species composition at larger spatial scales,” said Jonathan Myers, a professor of biology in Arts & Sciences and co-author of the new study. “This study is among the first to explore how the presence of a locally dominant tree affects spatial variation in understory plant composition through non-selective or selective processes of community assembly.”
Wassel measured the abundance of all plant species in 50 1×1 meter plots both inside and outside pawpaw patches at Tyson. Then she and Myers ran a number of statistical tests to determine whether the presence of pawpaws influenced which woody and herbaceous plants species grew inside the patch, out of the overall pool of all local plants that possibly could grow there.
At larger spatial scales, the scientists found that the composition of herbaceous plant species was not only more variable inside than outside of pawpaw patches, but also more random inside pawpaw patches. At smaller spatial scales, local plant species diversity and total stem densities (community size) were significantly lower inside than outside of pawpaw patches.
Focus on the Ozarks
At Tyson Research Center, the forest dynamics plot is part of the Forest Global Earth Observatory (ForestGEO). In this plot, all trees and shrubs with stems larger than 1 cm in diameter are tagged, identified, mapped and measured every five years.
The Tyson plot is the only site in this research network located in the Ozark ecoregion, according to Jonathan Myers, a professor of biology in Arts & Sciences, who has managed it for more than 14 years.
Additional monitoring includes surveys of birds, mammals, ticks, fungi, decomposition, microclimate, soil and plant chemistry, and seed production and dispersal.
For herbaceous species, median local diversity was 49% lower inside than outside pawpaw patches. Similar patterns were observed for the total understory community (woody and herbaceous species combined, 29% lower inside the patches).
Several factors could help explain the biodiversity patterns that Wassel and Myers observed.
First, pawpaws are strong competitors for light. “It’s hard not to love their big, floppy, tropical-looking leaves,” Wassel said. But all those loveable leaves mean shady conditions for shorter understory plants.
Another possible factor is high pawpaw stem densities and clonal growth in pawpaw patches, which may increase belowground competition for soil nutrients and water.
Or maybe it’s just that white-tailed deer at Tyson don’t like to eat pawpaws. If deer avoid pawpaws, then they may be selectively browsing through and munching all the other little green plants that try to get started in the patch.
Land managers can draw their own individual lessons from these findings, Wassel said.
“If you are monitoring a plot of land with the aim to encourage the growth of understory species, unfortunately, pawpaws are not your friends,” she said. “They will choke out diversity and whatever diversity persists is at the unpredictable whim of pawpaws’ shade.”
But others may just want to enjoy the largest edible tree fruit in the United States. “Then you want to encourage as many genetically distinct patches as possible so that cross-pollination can occur,” Wassel said.
“The presence of pawpaws is never inherently good or bad, it’s just natural,” she said. “Nature’s goals are never as linear as ours.”
Funding: This research was funded in part by National Science Foundation grants DEB 1557094 and DEB 2240431.
