The wood-betony (Pedicularis canadensis) plants colonizing the lightly vegetated sandy ground left behind by construction of a septic drain field years ago appear to be simple opportunists. But this species isn’t simply exploiting an opening here and an opening there. Each rosette of feathery green leaves is paving the way for more of its own kind, and more than a few other species as well.
Wood-betony—which is found in dry to mesic prairies, savannas, and forests throughout much of Wisconsin—is a hemiparasite. It contains chlorophyll and can live as an autotroph, like other photosynthetic organisms, or it can extract organic carbon, nitrogen, and additional useful chemicals from other plant species. Some hemiparasites are very choosy, relying on one or a few host species. Others are generalists. Plant taxonomist Martin Piehl discovered that wood-betony can tap at least 80 different plant species in 35 families and occasionally latches onto other wood-betony plants as well. A random selection of victims you might recognize include Achillea millefolium, Allium cernuum, Aster laevis, Equisetum arvense, Rhus hirta, Rudbeckia hirta, and Trillium grandiflorum. Wood-betony accomplishes this by means of haustoria that develop along lateral roots and establish connections with the host plant’s roots. This occurs very early during seedling development, and water from hosts is likely essential for survival of young wood-betony growing in dry sandy soil—something to keep in mind if you scatter its seeds in your prairie, savanna, or even your perennial garden.
What does wood-betony acquire from this relationship? There are the basic products of photosynthesis. There are minerals. There’s water. And there are alkaloids. Alkaloids? No one has looked at Pedicularis canadensis, but chemical analysis of tissue from five other Pedicularis species consistently revealed the presence of alkaloids also found in the host plant’s roots. This could be a general feature of the genus and raises interesting ecological questions. Alkaloids exhibit potent biological effects—often deadly—and can protect plants from insects or other herbivores, including humans. Can hemiparasites commandeer alkaloids for their own protection? Biologist Lynn Adler demonstrated just this by growing Texas paintbrush (Castilleja indivisa), a southern relative of our local Castilleja species, alongside “bitter” and “sweet” varieties of lupine. Texas paintbrush plants parasitizing “bitter” lupines, which contain high levels of alkaloids, were far less prone to insect damage and produced more viable seed.
What are the implications for preserving hemiparasitic plant species, especially generalists? For each species, it isn’t just a matter of securing a host, but identifying the appropriate host—or combination of hosts—to ensure the plant is prepared to resist the local array of insect and mammalian (e.g., deer) pests. There’s also a serious lesson here for herbalists. The value or safety of a plant could depend on where it is growing. For example, though wood-betony is classified as a medicinal plant, you probably don’t want to consume wood-betony growing near Senecio vulgaris; this introduced host’s roots contain potent alkaloids that cause hepatic necrosis.
Wood-betony also acquires an indirect benefit from its relationship with host plants. By weakening the competition—by diverting resources—this generalist can suppresses the growth of other plants and opens up the canopy for itself and its comrades. Ecologists noticed a tendency for wood-betony to grow alongside relatively shorter populations of certain grasses and forbs, but there was the question of which arrives first, the wood-betony or the shorter competitors. It could simply be soil conditions. But research by Andrew Hedberg and others of Illinois State University clearly showed wood-betony reduces the above-ground growth of plants such as tall goldenrod (Solidago canadensis) and big bluestem (Andropogon gerardii). Not all species studied were affected, but it is clear wood-betony can shift the balance of power on a patch of prairie sod.
This is where the hemiparasite starts contributing to and building a richer prairie or savanna plant community. By opening the canopy for itself, wood-betony inadvertently opens the canopy for others and promotes biodiversity. The researchers who demonstrated that wood-betony can suppress competition also discovered a positive correlation between wood-betony and species diversity. Three years earlier, Wisconsin DNR researcher Richard Henderson—based on observation and experiment—had suggested that wood-betony is one of several important keystone species that preserve Midwest prairie diversity. Introduction of the seed into a small established planting was followed by a substantial decline of dominant grasses and the appearance of fourteen native forbs. Richard Henderson pointed out that other factors might have contributed, but his call for further study deserves repeating. The right mix of plant species—as far as restoration and conservation are concerned—can set the stage for success or failure.
Wood-betony might exert influence on a plant community by another, distinctly different and a bit more distant, means. The biologist Terence Laverty showed that mayapple (Podophyllum peltatum) fruit set and seed set were higher for colonies less than 25 meters—versus 50 meters—from flowering wood-betony. Mayapple doesn’t produce nectar, though most colonies are self-incompatible clones and rely on native nectar-seeking bumble bees for cross-pollination. Wood-betony, however, produces bountiful nectar and bumble bees love it. The result is a four-fold increase in the number of bumble bee visits to mayapple growing near wood-betony colonies and a corresponding 22-79% increase in fruit set and 27-42% increase in seed set. Whether mayapple benefits from other magnet species requires further research. And whether wood-betony promotes pollination of other plant species, especially host species, also requires further research.
What are the implications for restoration and conservation? Again, the right mix of plant species—including appropriate magnet species and those that rely on them—could set the stage for success or failure.
Wood-betony—a hemiparasite that might derive alkaloids from its hosts, can suppress the growth of some species, encourages the growth of others, and enhances pollination for a species that relies on bumble bees but has nothing to offer them—shines a light on a few of the nuances of preserving ecological communities. The loss of a species that prevents other species from overwhelming a community is essentially loss of the entire community. This is important to keep in mind when deciding where to focus our efforts.
Learning about the factors that contribute to building an ecological community is reward enough to justify looking for subtle variations across the landscape. However, I also have to wonder whether restoration ecologists can exploit some the described natural processes. I’m always alert for ways to work with nature, ways to temporarily harness a natural process to achieve a goal. Are there circumstances where wood-betony, or other parasites and hemiparasites, might serve on the front line against infestations of aggressive exotic plants? Richard Henderson’s simple experiment revealed there are powerful tools available if we’ll invest time in looking for them. This is where the citizen scientist can play an important role, especially given the difficulty of funding botanical and ecological research. As you enjoy the natural world, look for variations in plant communities that suggest one species is preventing another from overwhelming the whole. Do you see greater fruit and seed set when certain species appear together? Share your observations with others. Test your hypotheses. You could help find an affordable alternative to the expensive weapons—in terms of labor, machinery, toxic chemicals, and energy—currently deployed against the onslaught of invasive exotic plants.
Adler, LS. 2000. Alkaloid uptake increases fitness in a hemiparasitic plant via reduced herbivory and increased pollination. The American Naturalist 156(1): 92-99.
Hedberg, AM, VA Borowicz, and JE Armstrong. 2005. Interactions between a hemiparasitic plant, Pedicularis canadensis L. (Orobanchaceae), and members of a tallgrass prairie community. Journal of the Torrey Botanical Society 132(3): 401-410.
Henderson, RA. 2002. Are there keystone plant species driving midwest prairie diversity. In: Foré, S (ed.), Proceedings of the 18th North American Prairie Conference, Truman State University Press, Kirksville, MO.
Laverty, TM. 1992. Plant interactions for pollinator visits: a test of the magnet species effect. Oecologia 89(4): 502-508.
Piehl, MA. 1963. Mode of attachment, haustorium structure, and hosts of Pedicularis canadensis. American Journal of Botany 50(10): 978-985.
Schneider, MJ, and FR Stermitz. 1990. Uptake of host plant alkaloids by root parasitic Pedicularis species. Phytochemistry 29(6): 1811-1814.