Dangerous Doppelgangers of Native Trees
Oh, the horror! Controlling invasive plants can be grisly work, and in the heat of hunting down and hacking away at victims of land management, mistaken identity can result in tragedy. I’ve lost count of the times I’ve heard landowners say that they’ve committed ghastly acts against trees and shrubs that they thought were invasive pests, but were actually lovely native trees. To avoid macabre mix-ups in plant identification, let’s take a look at a few notable invasive look-alikes that haunt eastern forests.
Ailanthus, Sumac, or Walnut?
If you’ve ever struggled to tell the difference between tree-of-heaven (Ailanthus altissima) and sumac (Rhus spp), don’t worry; you aren’t alone! I think I’ve spoken with an equal amount of people who get this wrong as those who get it right. Both have leaves and bark that look somewhat similar from a distance, and both have the tendency to grow clonally from roots. And another native tree, black walnut (Juglans nigra), can look similar too, especially when young. But we can look closely to see diagnostic differences in all three species year-round.
Let’s start with the leaves, which mix the most people up. Sumac, Ailanthus, and walnut all have pinnately compound leaves, with a high number of leaflets per leaf (up to 14 leaflets for walnut, 31 for sumac, and 41 for Ailanthus!). The key difference lies in the leaflet margin, or edge. Sumac (both smooth and staghorn, Rhus glabra and typhina respectively) and walnut have serrated leaves, while Ailanthus leaves are smooth, with the exception of a few teeth near the leaflet base. Upon closer inspection, those teeth each host a little wart, which is a scent gland. Crushing up leaf tissue, or young stems, will emit a distinctive smell akin to burnt peanut butter. You’ll know what I mean when you take a whiff! This scent is a great way to confirm that you have an Ailanthus over most of the year, but can be very faint or absent in stems over the winter.
Beyond leaves, bark differences between the three trees are notable and easy to see from a distance. Ailanthus and sumacs have thin, smooth bark that gets increasingly punctuated with features as the individual grows wider. Sumacs have orange-tinted, horizontally oblong lenticels (gas exchange pores) that grow into lines with time. As sumacs grow, the bark around the lenticels gets rougher and begins to look more blistered, eventually yielding vertical cracks in the papery bark of old specimens. Ailanthus lenticels are a light tan color and are diamond-shaped on young stems. As the tree grows, the lenticels develop into vertical striations that resemble stretch marks in human skin. Black walnut has very different bark than both; it has papery vertical strips when young that develop into long intersecting ridges and furrows.
Finally, reproductive structures also offer another easy way to distinguish between these three trees. Black walnuts have, well, walnuts. The green-shelled nuts develop from small wind-pollinated spike flowers, and the dangling male catkins are easy to spot in the spring. Both smooth and staghorn sumac have conical inflorescences that develop into fuzzy, scarlet fruit that can persist into winter. And Ailanthus has yellowish flowers that develop into messy, asymmetrical piles of pink samaras. Each samara is an undivided membrane which holds the seed in the center. Looking for these reproductive structures in the late summer and autumn is a great way to tell which species you have from a distance.
There is only one mulberry species that is native to North America: the lovely red mulberry (Morus rubra). It’s a shade-tolerant, medium-sized tree that is usually found in mesic or bottomland woods and forest edges, where it produces a juicy, sweet fruit that is enjoyed by humans and wildlife alike.
White mulberry (Morus alba), on the other hand, is introduced and quite invasive. Here in heavily cultivated Lancaster County, Pennsylvania, M. alba is the only mulberry I regularly see, which makes sense given its legacy in the area. For around a century, from the mid-1700s to mid-1800s, silk production was attempted in the U.S., and silkworms need white mulberry to feed on. Supposedly Morus alba was planted en masse to support the silkworms, but the industry never really proved to be profitable stateside. Legend has it that there was a boom in silkworm operations following some minor successes in the 1820s, until harsh springs in the 1840s killed enough caterpillars to deflate enthusiasm for the industry. White mulberry wasn’t defeated so easily, however. It has persisted in our landscape as an invasive species and will hybridize with red mulberry, reducing the vigor and utility of our native mulberry populations. And making it harder to tell which mulberry you’re looking at!
Distinguishing between the two mulberries is more challenging than our last example of invasive doppelgangers, but is possible with a few simple tricks. For one, the leaves of the two species look similar but have diagnostic differences. Red mulberry leaves are 4-10 inches long, are dull in color, rough-feeling on top, and hairy beneath (though it can be hard to feel with your fingers- instead try a trick my college dendrology professor taught me and see if the leaf will stick to a cotton garment as evidence of its hairs). They also have finely serrated leaf margins and minor veins that are nearly as apparent as major veins. White mulberry, on the other hand, has leaves that are 3-4 inches long, sometimes glossy on top, smooth on top, and with few or no hairs below. Their leaf margins are toothed rather than serrated, and the main veins are much larger and more visible than minor veins.
Red mulberry buds are larger, have thick black stripes around scale edges, and tend to sit at an angle to the stem, while white mulberry buds sit in the center of the stem and have faint stripes on scales. Red mulberry bark is a grayish brown color, while white mulberry bark has an orange tint at all ages. Finally the fruit of the two mulberries is markedly different. I mentioned that red mulberry has sweet, juicy fruits. They are several times as long as they are wide and are typically held singly and scattered around the tree, and are a dark purple color when ripe. White mulberry fruit are quite tasteless, are not elongated, are typically borne in clusters, and can be white, red, or purple when ripe.
For excellent photos of these differences and more, check out this resource by Purdue University Extension.
Raspberry, or Wineberry?
The raspberry genus, Rubus, is not only ubiquitous in eastern landscapes, but is incredibly important for wildlife and forest health. Rubus thickets will spring up quickly in sunny areas, creating excellent cover for wildlife to hide from predators or inclement weather in, and perfect spots in which to build nests or brood young. They are also important nectar sources for pollinators and the berries feed scores of animal species (Homo sapiens included, of course). The brambles can also provide safety for tree seedlings which would otherwise be browsed by deer, which helps a young forest grow into a mature one.
Of the dozen or so Rubus species that we have here in the east, one is an invasive pest: wineberry, Rubus phoenicolasius. The plant was introduced to the US in 1890 for food production and is still used as breeding stock for cultivated raspberry varieties, though it is destructive in natural ecosystems. At first glance it looks like many of our other raspberry species; it’s trifoliate with a raggedly serrated leaf margin, has arching stems, and produces a big, juicy, bright red fruit that looks like a red raspberry and tastes fantastic. It’s actually my favorite-tasting wild Rubus, but is a harmful invader and should still be managed against (just maybe pick all the berries before you kill it).
Wineberry is very easy to distinguish from its native Rubus relatives, though. It has bright red stems that look like they’re jam packed (pun intended) with prickles. Wineberry does have straight prickles along its stems, but most of the protuberances are actually glandular hairs that secrete a slightly sticky substance which serve to discourage herbivory. The hairs also coat the calyx (outermost flower tissue which protects reproductive organs) before they’re ripe.
Our final example of a dastardly doppelganger is Norway maple, Acer platanoides. It was introduced as an ornamental tree, and in my experience is still planted by well-meaning homeowners who think that they are planting our native sugar maple, Acer saccharum. Norway maple seeds prolifically, is very tolerant of shade, releases allelopathic chemicals which hamper the regeneration of other species, and isn’t attacked by insects, fungi, or viruses as much as its native relatives are, allowing it to dominate even forested areas.
Before we get into sugar maple and Norway maple, let’s briefly touch on red maple, which can sometimes throw people off too. Red maple (Acer rubrum) leaves only have 3 dominant lobes, which you can remember because “red” only has 3 letters. On that note, “sugar” has 5 letters and sugar maple has 5 main lobes, and “Norway” has 6 letters and 5-7 lobes. To me, telling the difference between Norway and sugar maple leaves can still be challenging. But there’s a great hint in Norway maple’s name: “platanoides” means “like plane tree”, aka sycamore! Norway maple leaves are wide and squat, and their lower lobes are not as defined as sugar maple lobes, just like its sycamore namesake. Additionally, Norway maple leaves stay greener for longer and turn yellow when they do change, rather than the spectacular reds and oranges of A. ruburm and saccharum. Another great trick is to break the petiole; if a white sap oozes out, you’re looking at a Norway maple.
If you don’t have leaves to look at, don’t fret! Buds of sugar maple are quite sharp, while buds of both red and Norway maple are blunt. Also, the samaras of all native maples are held at a downward angle, while Norway maple samaras are held out to the side. Maple bark can be tricky too. To me, Norway maple bark looks like a darker, tidier version of boxelder (Acer negundo) bark, where ridges and furrows are held tightly to the trunk. Mature red and sugar maples have bark that develops into cracked vertical plates. To distinguish between red and sugar maple bark, try to break off a plate with your fingernail. If the plate is too thick to easily break, you likely have a sweet Acer saccharum!
Why care whether a plant is native or invasive?
Invasive plants are more than just weedy pests. They can bring a functioning ecosystem to its knees within a few years by completely dominating the area and thereby reducing the diversity of young and old native plants, all while providing little or no utility to wildlife. Take a look at any invasive plant’s leaf sometime and compare it to a native analog’s. The invader’s leaf will almost always have remarkably less insect damage, meaning not much is eating the non-native plant. But insects are the glue holding our animal food webs together. And those animal food webs have direct impacts on the plant community. For example, a study in Virginia observed that blue jays cached (buried for later) 54% of all acorns that were produced in that stand. Our natural systems are great at adapting to change, but can only handle so much degradation. Invaded woods don’t quite function as woods, and in the face of climate change we need as much high-quality forest habitat as possible to moderate dramatic fluctuations in weather and serve as a source of sustainable foods and materials. So get out there and work on removing invasive plants wherever you can! Just don’t be fooled by the lurking look-alikes of our wonderful native trees!