I have had a lifelong interest in identifying the trees of our region, especially those that produce edible nuts and fruits. One species that eluded me for more than two decades was the native red mulberry. Finally, in the summer of 2012, by following herbarium collection records from the University of Wisconsin–Madison, I found it growing in southwestern Wisconsin. Over the next two summers I searched for more specimens everywhere I traveled and studied them until I could recognize red mulberries with ease.
I had a hunch that I might be able to locate this species in Minnesota, where it had not been documented in 94 years. In summer 2014 I was driving along state Highway 26 in southeastern Minnesota, just south of Brownsville, where an unusually large leaf on a small roadside tree caught my eye. I parked my car at the nearest safe pull-off and walked back to investigate. There, growing at the base of a steep slope beneath a limestone bluff, I found four fine saplings with heart-shaped leaves, resembling those of basswood but broader and larger. I recognized these as belonging to red mulberry (Morus rubra) and collected one twig for an herbarium specimen. I also took several photos.
With enormous, overlapping leaves arranged in canopy-like layers on wide, spreading branches, red mulberry is strikingly elegant. It bears the largest simple leaves of any tree native to Minnesota (up to 14 inches long). This native tree had not been documented in Minnesota since 1920, when botanists collected a specimen a few miles north of the Iowa border on a wooded hillside near the Mississippi, about five miles south of the place where I found it growing.
When I got home, I contacted botanist Welby Smith of the Department of Natural Resources to report my find. Smith was skeptical: Botanists had been watching for this tree for years, and confusion over its identity has been widespread.
A few weeks later, after he had visited the site, Smith was delighted to confirm the find. I subsequently located and documented several additional trees nearby.
Delicious, Confusing Fruit.
Red mulberry is native to most of the eastern United States, common in the South, but becoming scarce in the northern third. Minnesota is at the northwestern edge of its range. A medium-sized tree, it typically has a crooked trunk and spreading crown. The branches are sparse and the twigs are rather thick. The heart-shaped leaves are deeply lobed on seedlings, but saplings have fewer lobes, and mature trees generally have unlobed leaves.
The blackberry-like fruit, ripening from mid-June to early July, makes it obvious that the tree is a mulberry. Catbirds, robins, cedar waxwings, orioles, and many other birds eat the fruit. So do gray squirrels, raccoons, opossums, jumping mice, and black bears. Deer, rabbits, and woodchucks eat the leaves. And beavers favor the bark. Native Americans not only ate the delicious fruit, but also wove with the bark. Early European settlers used the berries for pies, preserves, juice, jelly, wine, and fresh eating. They also cut the rot-resistant wood for fence posts.
Unfortunately for the native red mulberry, European settlers also planted nonnative white mulberry (Morus alba) to raise silkworms, which feed exclusively on mulberry leaves. The silk industry failed, but the Asian mulberries took hold and multiplied, becoming far more common in Minnesota than our native species.
The common names for these two related species have led to a pervasive misconception that the two can be distinguished by the color of their fruits. The fruit of both species is initially white. As Smith points out in Trees and Shrubs of Minnesota, the ripe fruit of white mulberry can range from white to purple-black. In fact, the vast majority of naturalized white mulberry trees have dark fruit: In summer surveys of several hundred trees in Minnesota and Wisconsin, I found less than 5 percent of white mulberries had white or pink fruit.
Dark-fruited white mulberries are often misidentified as "red mulberry," sometimes even by foresters, naturalists, and botanists. The mistake appears in many field guides and on several major tree identification websites. Madhav Nepal, a mulberry researcher from South Dakota State University, found that a significant portion of the specimens submitted as Morus rubra to herbaria at Midwestern universities were actually Morus alba. This identity confusion reached a point of crisis in 2009. Four researchers from Murray State University in Kentucky, believing that dark-fruited white mulberries were Morus rubra, encountered the real Morus rubra. Confounded by their find, they wrote a paper describing it as a species new to science. Dendrologists familiar with red mulberry quickly disputed and rejected the claim.
Despite the confusion regarding red and white mulberries, the two trees appear entirely dissimilar at a glance. White mulberry is a dense, bushy tree with small, glossy, and irregularly lobed leaves. The red mulberry grows taller and has widespread branches. Lobed leaves, so common on white mulberry, are largely confined to seedlings and saplings with red mulberry.
Another Mistaken ID.
So how did this red mulberry tree in Minnesota remain undetected for nearly a century when it grows alongside a major highway, plainly visible to thousands of motorists every summer? Red mulberry has a strong resemblance to basswood (Tilia Americana) with its broad, nearly heart-shaped leaves with gently tapered tips. The appearance is so similar that, in the words of Smith, red mulberry can be "visible but not recognizable, unless the eyes are freshly attuned to it." Only upon close inspection are the differences obvious—basswood leaves are lopsided and not hairy, while red mulberry leaves are symmetrical and hairy underneath. (White mulberry leaves have hairy patches along major veins.)
Smith believes that red mulberry is the rarest tree in Minnesota. A small population of a few dozen individuals grows on hills within a half mile of the Mississippi floodplain between Reno and Brownsville. Undocumented populations could possibly exist elsewhere in Houston County, as well as in Winona, Wabasha, Goodhue, and Fillmore counties. Although associated with river courses, red mulberry is not a tree of regularly flooded lowlands. The tree's favored habitat seems to be hardwood forests on slopes beneath limestone bluffs. It also turns up at forest edges atop limestone bluffs.
Researchers in Ontario, Canada, where Morus rubra is classified as an endangered species, have identified two primary threats to the tree's long-term survival: loss of habitat and hybridization. The wooded slopes of southeastern Minnesota have remained relatively stable in recent years, and considerable acreage is publicly owned, so habitat loss is perhaps less concerning there. But "genetic swamping" by the much more common white mulberry is a real threat. This occurs when the flowers of an uncommon species are disproportionately pollinated by an abundant relative, so that each successive generation contains more hybrids and fewer genetically pure plants. Eventually, the gene pool of the more successful species can completely absorb the population of its less abundant neighbor. This is already occurring in some northern populations of red mulberry.
Hopefully, a strategy to preserve the red mulberry will emerge to ensure that our great-grandchildren will enjoy this tree's magnificent leaves and sweet fruit.