Triumph of the Natives?

Our house is next to an outlot with a field bordered on three sides by woods and streams. When we moved in about nine years ago, a dense thicket of wineberry (Rubus phoenicolasius), an East Asian raspberry, occupied one side of the field, but most of it was covered in Japanese stiltgrass (Microstegium vimineum: Poaceae) with occasional mile-a-minute vines (Persicaria perfoliata: Polygonaceae) crawling over stumps and logs. There seemed to be little hope for any native plants to re-establish themselves, due, in part, to an unholy alliance between stiltgrass and too many deer. Deer don't eat stiltgrass (nor will anything else, it seems) and the deer seem to protect the grass by eating native competitors, including seedlings of trees. So the prospect of any natural succession toward something less tragic seemed unlikely. Even unnatural succession, aided by pulling the grass, mowing or applying herbicides, would have to go on for years to exhaust the extensive seed bank already in the soil. Short of a herd of herbivorous pygmy godzillas (godzillae?), the Japanese stiltgrass seems to have won. Go, go, Godzilla. [For those of you who aren't keeping score, this is yet another allusion to a Blue Oyster Cult song.]

Japanese stiltgrass

However, there were some interesting changes over the years.  A sycamore sapling, apparently too deep in the wineberry thicket for deer to bother with, and a few common milkweeds (Asclepias syriaca: Apocynaceae) established themselves among the wineberries. 

Milkweed flowers and pods with nymphal milkweed bugs (Oncopeltis fasciatus: Lygaeidae)

After a few more years, the wineberry thicket simply vanished, despite abundant opportunities for reseeding from adjacent thickets, and was replaced by the milkweed, a native plant that is as close to deer proof as we are likely to get.  By the time this had happened, the sycamore had reached a size at which it was largely immune to deer. The milkweed continues to advance across the field at a remarkable rate. Stiltgrass (at lower densities) and creeping charlie (Glechoma heeracea: Lamiaceae), a vine in the mint family, still form a sort of low understory but the grass is no longer the dominant player.

Outlot Field. A dense population of milkweed (center left) surrounds a young sycamore tree. The rest is Japanese stiltgrass, but the milkweed is advancing yearly to the right. The milkweed area was once a wineberry thicket.

Some other interesting plants have started bringing up the rear of the milkweeds. The first ones I noticed were purple passion flowers or maypops (Passiflora incarnata). The family Passifloraceae is most diverse in the tropical New World. It is an amazing flower that seems out of place in temperate Maryland. Three stigmas on top, five stamens (look like shower heads); a filamentous dance floor (coronal filaments) with nectaries in the center; white petals at the bottom. It also has a very powerful scent, a little spicy and almost sickly sweet; the odor will fill a room rapidly.

Purple passion flower and leaves of passion vine

The "passion" in the name came from the conquistadors, who thought the coronal rays of the flower resembled the crown of thorns worn by Christ at the crucifixion, the final act in his period of suffering or “passion”. I suppose if people can see Jesus in toast and Cheetos, they can see a crown of thorns in the sexual organs of a plant.  [By the way, passion fruit is my favorite flavor of Chobani yogurt. So stop asking.] Ironically, most parts of the plant, including the nectar, have a sedative rather than impassioning effect on both insects and people; it increases the consumer's level of the inhibitory neurotransmitter GABA (gamma acetyl butyric acid). This also accounts for the “drunken bee” phenomenon, wherein bees on the flower seem to stagger around or simply “sleep” with their faces stuck in the nectary.  I suppose this is a way for the plant to maximize the deposition of pollen on the pollinator, but who knows really? See a drunken carpenter bee below and in the YouTube video.

"Drunken" carpenter bee (Xylocopa sp.) on a passion flower, and my favorite yogurt, because, like The Ramones, I wanna be sedated.

Before leaving the topic of passion flowers and bees, I should mention a second species of passion flower in Maryland and points south and west. The yellow passion flower Passiflora lutea is a smaller more anemic-looking version of its purple cousin. There is an unusual and uncommon halictid bee, Anthemurgus passiflorae, the only member of its genus, that provisions its brood solely with the pollen from P. lutea. In contrast, the plant has many pollinators and only the bee is prepared to commit. The bee does not appear to occur in Maryland.

Yellow passion flower. Photo by Dr. Bill Shear taken recently in southern Virginia and used with his blessing.

A clump of Allegheny monkeyflowers (Mimulus ringens, Phrymaceae) turned up for the first time this year. It’s named for a supposed similarity to the face of a monkey (…duh), although I don’t really see it myself when my eyes are open. I would, however, accept “undergrad-in-my-biology-class” flower, given its vacant, slack-jawed expression. Imagine if the flower were looking at a smart phone or laptop screen during lecture and you’ll see what I’m talking about. Monkeys are engaged, enthusiastic and curious creatures.

Allegheny monkeyflowers

Back to the never-ending war on stiltgrass. My anecdotal observations suggest that there may be conditions under which natives may triumph or at least put up a good fight. Again, the transitions I observed were unaided by humans … no spraying, weeding, burning, etc. The key may be in breaking the compact between deer and stiltgrass. Plants that provide a persistent physical barrier to deer may allow the re-establishment of natives within the thicket, even if the thicket itself is composed of an invasive plant. This could allow trees to reach a size where they are no longer browsed by deer and thus begin to shade out the maternal thicket, reduce the density of stiltgrass and initiate a process of quasi-renaturalization or reforestation. In contrast to raspberries, milkweed does not represent a significant physical barrier to deer, but a large, dense population may offer a “needle in the haystack” sort of protection for other natives, like Passiflora and Mimulus. Organized planting of the major players might accelerate this process, thus giving natives the upper hand and reducing the need for persistent human intervention with herbicides. There are other examples of this process around my house. The only places I see trees actually making it to a reasonable height are those surrounded by wineberries or mixed wineberry and black raspberry thickets. There is one thicket protecting another sycamore and another protecting a hickory. 

Sycamore sapling in wineberry thicket

In short, there may be an organic approach in the fight against at least some invasive plants through manipulated succession. Research needed, I guess.