A natural history notebook and joint project of the Missouri Botanical Garden, the Ecological Health Network, and the Restoration Ecology Lab at Virginia Tech.
By Andrew Kaul, a Restoration Ecology Post-doc in the Center for Conservation and Sustainable Development at the Missouri Botanical Garden. His new, open-access paper in Ecological Solutions and Evidence is available here.
Throughout most of the eastern United States, oak woodlands were once a widespread and dominant ecosystem. These woodlands experienced periodic fires, which prevented woody trees and shrubs from growing so densely that the overstory canopy became closed. The partly open canopy allowed light to reach the ground, supporting a diverse community of herbaceous plants including wildflowers, grasses, and sedges. However, over the past two centuries, human induced changes including fire suppression, invasion by non-native shrubs, and other factors have caused most woodlands to become overgrown, and lose much of the diversity of plant species in the herbaceous ground layer.
Research on how to manage and restore these woodlands has shown that cutting down some trees to thin out the woodland, as well as removing non-native shrubs, and reintroducing periodic fires, are all strategies that help improve the quality of these habitats. However, even after employing all of these management strategies, many desirable plant species may still not return on their own. Ecosystem restoration often involves re-introducing plant species as a seed mix distributed over a cleared area, and this method can be very effective for grassland and savanna habitats that contain few trees. Restoring wildflowers and grasses in wooded areas with the addition of a seed mix could drastically improve the diversity and quality of the herbaceous community, but this approach has not been experimentally studied, and little is known about how to select the right species for re-introduction this way.
To address these knowledge gaps, scientists and land managers at the Missouri Botanical Garden started an experiment at the Shaw Nature Reserve in 2016, where highly diverse seed mixes of native plants were added to a degraded woodland undergoing active restoration. Throughout late 2016 and much of 2017, crews of managers and volunteer land stewards worked to thin the canopy by removing less desirable tree species, especially the aggressively fast growing native conifer, Eastern Redcedar (Juniperus virginiana). After thinning the canopy, crews used a combination of mechanical removal and herbicides to control the dense non-native shrubs. Fire was reintroduced through controlled burns starting in late 2017.
Large Eastern Redcedars dominate a degraded woodland at Shaw Nature Reserve in Gray Summit MO. The understory is overgrown with non-native woody species including bush honeysuckle (Lonicera maackii), privet (Ligustrum obtusifolium), and wintercreeper (Euonymus fortunei). The lack of recent fire has led to a build up in leaf litter, and native herbaceous species are mostly absent. Photo: CCSD & SNR staff.
After the woodland was thinned, in January of 2018, we added seed mixes to three different management units, along a gradient of lower/wetter to higher/drier parts of this landscape. The seed mixes contained between 79 and 93 species, and all of the seed was collected from plants growing at the nature reserve. In order to track how these seed additions influenced the establishment of the herbaceous community, we collected data on the composition of the plant communities in areas that received seed and areas that did not. We sampled the plant community in 2017 before the seed additions and in the following two years, 2018-2019.
Top: Woodland under management at Shaw Nature Reserve in March of 2017, after selective thinning of trees to open up the canopy and removal of most woody shrubs. Leaves of some persistent bush honeysuckle can be seen. Bottom: Same woodland in June of 2019 after the addition of a seed mix in 2018. Photo: CCSD & SNR staff.
In both the seeded and non-seeded woodlands, the effect of management actions was very clear and positive, since both the number and cover of herbaceous species dramatically increased from the sample in 2017 to later sample dates. This is consistent with previous research showing that thinning the canopy, removing shrubs, and reintroducing fire promote restoration of herbaceous plants.
We also found substantial benefits from reintroducing species with seed mixes. The areas that received seed had about 10 more plant species present within a one square meter area, than the areas that did not get seed. We were also interested in the quality of the kinds of species that were establishing based on coefficients of conservatism, which denote how sensitive species are to human disturbances. We found that areas with seed added, contained fewer plants that were weedy ruderals, and more that were conservative and generally found only in high-quality intact habitat. Interestingly, areas that got seed additions were also more dominated by grasses and the areas that did not receive seed, although less rich in species, tended to have more abundant wildflowers (forbs). Specifically, common grasses that were sown at high rates tended to dominate areas that received seed additions, including river oats (Chasmanthium latifolium), hairy woodland brome (Bromus pubescens), and bottlebrush grass (Elymus hystrix). The restored areas that did not get a seed addition were dominated by ruderal (low conservatism) forbs such as jumpseed (Persicaria virginiana), white snakeroot (Ageratina altissima), and common yellow woodsorrel (Oxalis stricta).
A representative area that had seed added (top) and an area that did not (bottom) in June of 2019. In the non-seeded area, a large patch of the weedy native composite, giant ragweed (Ambrosia trifida), can be seen in the foreground. Photo: CCSD & SNR staff.
Our final goal was to examine the recruitment success of the over 100 different plant species that we added as seeds, to see if there were patterns in which kinds of species tended to establish best. Perhaps surprisingly, over half of the species we added were never detected in vegetation samples. These species might not have been sown into favorable conditions, or potentially, the quality of the seed might have been poor, since it came from wild populations and the seeds might not have been viable or mature. Still, some seeds may be dormant for many years, and more added species may break dormancy and recruit later. Among the species that did establish from added seeds, we found that recruitment was much higher for species that were sown at higher rates, suggesting that some species might have benefitted from a higher seeding rate. Both grasses and forbs tended to recruit well when sown at high rates, but the 25 sedge species we added had little or no recruitment success.
Based on our results, future research on woodland restoration should address why sedges are difficult to restore and methods to remedy this deficit. Additionally, it will be interesting to track the development of these herbaceous communities into the future, to examine how sown and unsown areas resist re-invasion by shrubs while they are continually managed with periodic burns. Our seed mixes dramatically improved the diversity and floristic quality of the herb layer in this woodland, however many species did not recruit, and key functional groups including sedges and forbs were underrepresented in their abundance. Future research should investigate what ratios of functional groups in seed mixes produce the best restoration outcomes, since conventions established for grassland restoration may not be the best approaches for restoring herbaceous species under a tree canopy. If you are interested in learning about this project in greater depth, the paper is freely accessible here. If you have any questions, feel free to contact Andrew (akaul@mobot.org).
Leighton Reid is an assistant professor of ecological restoration in the School of Plant and Environmental Sciences at Virginia Tech. Ryan Klopf is the Mountain Region supervisor and natural areas science coordinator for the Virginia Natural Heritage Program. They describe a new research project that aims to understand how an important restoration tool impacts the population dynamics of federally threatened small whorled pogonia orchids. This project has an open PhD position available to start in January 2023; details can be found at the end of this post.
Deep in the heart of Virginia’s Shenandoah Valley, nestled against the western edge of the Blue Ridge Mountains, two clusters of small, green orchids grow in the dappled sunlight of a woodland understory. The orchids are small whorled pogonias (Isotria medeoloides) – a rare species that is considered threatened by the United States government because its population is declining so quickly that it could become endangered in the foreseeable future. We have monitored these populations for the past two summers, keeping tabs on every individual, to learn how this species is affected by one of the most important restoration tools in North America – prescribed fire.
A small whorled pogonia orchid with two flowers at Mount Joy Pond Natural Area Preserve. Photo: Lindsay Caplan.
Small whorled pogonia
As their name implies, small whorled pogonias are small (≤25 cm) and whorled (their leaves radiate outward from the stem). This species is a member of the Pogonieae, an orchid tribe that includes species in Asia and eastern North America. Its closest relative is the large whorled pogonia (I. verticillata) which sometimes grows alongside small whorled pogonia, but is distinguished by its purplish stem (small whorled pogonia has a whitish green, glaucous stem).
Small whorled pogonia (left) with a whitish, glaucous stem compared to large whorled pogonia (right) with a purplish stem base. Photos: Sara Klopf (left) & JL Reid (right).
Small whorled pogonias emerge from the leaf litter in late spring and in some years produce one or two solitary greenish yellow flowers, particularly when plants are exposed to more sunlight. Their flowers do not require any help with pollination; they produce the same amount of seed whether they are cross-pollinated or pollinate themselves.
The seeds themselves are tiny – like vanilla seeds, which are in the same orchid sub-family (Vanilloideae). The parent plant (which is usually both a mother and a father) provides almost no resources at all to its offspring. Each seed’s fate is closely linked to whether or not it finds a mycorrhizal fungus in the Russulaceae family to help it acquire the resources that it needs to survive and grow. In a typical relationship between plants and mycorrhizal fungus, the fungus scours the soil for nutrients like nitrogen and phosphorus and provides them to the plant in return for energy in the form of carbohydrates, which the plant produces through photosynthesis.
A developing fruit on a small whorled pogonia orchid at Mount Joy Pond Natural Area Preserve in June 2022. Photo: Andres Cunningham.
Fire and water at Mount Joy Pond
The story of this research project begins about 80 years ago, in a DuPont chemical plant in Waynesboro, Virginia. In the 1930s-1950s, the DuPont facility used mercury to produce rayon – a synthetic, silk-like fiber. Some of the mercury escaped from the plant and leaked into the South River – a tributary of the Shenandoah River. Mercury is a neurotoxin, and in the environment it can accumulate to dangerous levels in animals that are higher on the food chain, like fish. For many years, people living along the South River have been warned about the poor water quality and advised not to eat the fish.
In 2016, DuPont reached a $50 million USD settlement with the United States Department of Justice, the Department of the Interior, and the Commonwealth of Virginia to restore habitat for wildlife in the South River watershed, enhance water quality, and improve recreational areas. This settlement represented one of the largest environmental damage settlements in United States history.
Some of the DuPont settlement money was allocated to the Virginia Natural Heritage Program, a division of the Virginia Department of Conservation and Recreation that uses science-based conservation to protect Virginia’s plants and animals. Specifically, funds were provided to allow the Virginia Natural Heritage Program to protect and restore woodland habitat surrounding a unique wetland at the Mount Joy Pond Natural Area Preserve in Augusta County.
Briefly, Mount Joy Pond is a Shenandoah Valley Sinkhole community; that is, it is a groundwater-controlled wetland that floods intermittently when water percolates up through underlying carbonate rocks and then floods over the top of a clay lens perched in a layer of soil derived from the overlying sedimentary rocks. When this happens, the water becomes trapped, like water in a saucer. This unique situation creates wetland habitats which have persisted for the past 15,000 years and contain numerous rare and disjunct species, including the globally rare Virginia sneezeweed (Helenium virginicum). There are several dozen Shenandoah Sinkhole ponds, but only a handful of them are protected.
Virginia sneezeweed, an endemic species in the southeastern United States with disjunct populations in Virginia’s Shenandoah Valley sinkhole ponds and in a similar wetland situation in the Ozark Mountains of southern Missouri. Photo: JL Reid.
In the past, Mount Joy Pond filled with water every few years, but in recent decades it has filled up less and less often. To restore the wetland’s hydrology, the Virginia Natural Heritage Program set out to thin the surrounding forest and re-introduce fire to prevent fire intolerant trees, such as red maple, from regenerating. This may sound counterintuitive to some, but the logic is this:
Each tree is like a drinking straw sucking water out of the ground and releasing it into the air via transpiration. If there are a lot of trees, the groundwater may stay too low to fill up the pond.
Fire used to be much more common in the Shenandoah Valley. Prior to European colonization, Indigenous People burned the landscape and maintained much of it as savanna and open woodland – ecosystem types that have fewer trees than present day forests.
By removing some trees and reintroducing a regular fire cycle, land managers at Mount Joy Pond Natural Area Preserve can restore an open woodland and raise the groundwater level, causing the pond to flood more often.
The Virginia Natural Heritage Program began to implement this restoration project in 2017, and the first thinning operations and burn were a success. In the years since, the groundwater level appears to have gone up, suggesting that the hydrological restoration plan is working.
Small whorled pogonia discovery
In the first spring after that first fire, a botanist was surveying the burned woods near the pond and found something unexpected – a small population of small whorled pogonia orchids, which had not been seen previously in the preserve despite extensive surveying by the Virginia Natural Heritage Program’s inventory team. Were the orchids there all along and nobody noticed them? Maybe. Or maybe the fire helped the orchid population emerge after years of suppression in the dense leaf litter in the shady understory.
Our team uses a grid sweep survey to search for new small whorled pogonia individuals in June 2022. Photo: JL Reid.
The story became more complicated later that summer when a more intensive search turned up a second population of small whorled pogonia orchids on the preserve – this one in an area that had not been burned.
The immediate consequence of discovering the new pogonia populations was that the United States Fish and Wildlife Service expressed concerns that future fire management might be detrimental to this threatened species. Nobody had studied how small whorled pogonia responds to fire, and there was a chance that burning could damage the population, even if it was good for the nearby pond’s hydrology. Of course, there was also a chance that not burning could damage the population. With fire, inaction is still an action.
To help settle the issue, the United States Fish and Wildlife Service agreed to sponsor a PhD student to study the small whorled pogonias at Mount Joy Pond and figure out how their population dynamics are impacted by prescribed fire.
Lindsay Caplan and Jimmy Francis monitor a population of small whorled pogonias at Mount Joy Pond Natural Area Preserve in June 2022. Photo: JL Reid.
Effects of prescribed fire on small whorled pogonia orchids
The main goal of our ongoing research is to understand how prescribed fire impacts small whorled pogonias. To do this, we will map and monitor the two subpopulations and the woodland plant communities in which they live. Over the next two years, one of the two subpopulations will be burned during a winter or early spring prescribed fire, and we will continue monitoring to document changes in plant vigor, reproduction, and population size. We will pay special attention to the light environment, which seems to be important for small whorled pogonia reproduction, and to the diversity and composition of soil fungi, which are important for small whorled pogonia emergence. We will also conduct annual surveys of the entire reserve to search for additional populations.
Ethan Dunn uses a canopy imager to measure canopy cover, photosynthetically active radiation, and leaf area index over a tiny small whorled pogonia individual in July 2021. Photo: JL Reid.
This project is just beginning. To date, we have monitored the two populations for two growing seasons (2021, 2022). There is still much work to be done. One of the next steps will be to produce an accurate map of each plant’s location, which will require centimeter-level precision using high-quality GPS equipment under a forest canopy.
We are currently seeking a PhD student to lead this research project starting in January 2023. A description of this opportunity is below. This project is an excellent opportunity for a student to develop expertise in ecological restoration and threatened species conservation from both a scientific perspective and an on-the-ground land management perspective.
Ultimately, the results of from this study will inform management of natural areas and small whorled pogonia restoration projects throughout the species’ wide range – from Ontario to Georgia.
Andrew Kaul is a Restoration Ecology Post-doc in the Center for Conservation and Sustainable Development working with Matthew Albrecht at the Missouri Botanical Garden, and Michael Barash is a junior Biology major at Washington University in St. Louis. Here they describe Michael’s undergraduate research on commercial native seed availability for woodland restoration.
One of the largest barriers to restoration of degraded terrestrial habitats is availability of seed for use in reintroduction of desirable native plant species. Over the past few decades, the industry of native plant seed production has grown rapidly, but most native species in the US (and globally) are still not commercially available, and there can be strong biases in which types of species tend to be selected by seed producers.
In ecological parlance, a “species pool” represents all of the species which can colonize and occupy a certain region. Not all of the species in a regional species pool are available commercially, which is how many restoration practitioners acquire seeds, so the subset of species pool that contains only the species that are commercially available in a given region is sometimes called the “restoration species pool”.
For most ecosystems around the world, it is not well documented what proportion of the species pool is commercially available, and why these species have been selected for commercial trade. The few studies that have been conducted on commercial seed availability for restoration have found consistently that herbaceous (rather than woody) and rare species (rather than common ones) are less likely to be available, and there are strong taxonomic biases in which plant families are more represented. In the US, these studies have focused on open-canopy habitats with few trees, such as grasslands, rather than on more closed-canopy systems like woodlands and forests.
An open rocky glade (left), and a glade to woodland transition (middle), a woodland understory (right; Shaw Nature Reserve, Gray summit MO).
To address this information gap, we assessed the capacity of the native seed industry to support ecological restoration across terrestrial habitats in the Ozark region of the midcontinent USA. The use of seed additions to accelerate recovery of plant diversity in Ozark woodlands and forests is not well studied, and little information is available on how to best select species for reintroduction from seed. The specific goals of this project were to:
1) Identify the species pool of native herbaceous (non-woody) vascular plants appropriate for restoration of glades, woodlands, and forests in the Ozark Highlands;
2) Define the restoration species pool by identifying which of these species are commercially available;
3) Quantify biases in this restoration species pool with respect to growth form, rarity, habitat affinity, and a few important functional traits;
4) Identify candidate species which are not available from seed vendors, but should be a priority for seed production due to their importance for Ozark habitats.
The spatial scope of this study is the Ozark Highlands, level III ecoregion 39, which covers all of Southern Missouri, as well as parts of Northern Arkansas, and NE Oklahoma.
We began this project by developing a targeted species list of 1,178 herbaceous species native to upland habitats in the Ozark region, based on existing datasets from the Ecological Checklist of the Missouri Flora, the Flora of Missouri, and the Biota of North America Program (BONAP).
We predicted there would be selection, implicit or explicitly, by seed producers for species based on their growth form, conservatism score wetness rating, rarity, and functional traits. Each species’ physiognomy (growth form), conservatism score (that is, sensitivity to disturbance), and wetness ratings (a type of habitat affinity) were included in the Missouri Checklist. For each species in our pool, we compiled data on two measures of rarity including a qualitative measure – the official Missouri State conservation ranking, and a quantitative measure – the range size of the species in the US, as measured by the number counties in the nation where there have been recorded occurrences in the BONAP database. We compiled trait data on height of adult plants, and bloom timing and duration from species descriptions in the Flora of Missouri. For each grass species, we compiled data on photosynthetic pathway from published literature.
We made predictions that species with certain growth strategies, traits, range sizes, and habitat preferences would be under- or over-represented in the pool of species produced by seed vendors. We predicted that compared to other growth forms, perennial forbs would be over-represented in the restoration species pool because the aesthetic value of restoration projects is often a high priority, and perennial forbs with their big flowers, are “showier” and will return year after year. Similarly, taller species, and those with a longer bloom period may be selected preferentially because their blooms are more noticeable. We expected that species which have a smaller range or are not abundant in sites where they do occur are less likely to be in demand by restoration practitioners, so are less likely to be commercially available. Based on this pattern we expected species with a lower conservatism score, larger range size, and higher conservation rank (less concern for conservation) to be more commonly produced by seed vendors.
We predicted that species with a larger range such as pale purple coneflower (Echinacea pallida; map on the left), would be more likely to be available from at least one producer than species with a smaller range, such as the related yellow coneflower (Echinacea paradoxa; right). Maps are from BONAP, and light green areas denote counties where the species has been reported.
The cottage seed industry for prairie plants has grown especially rapidly in recent years, so we expected that species which are generally found in more open habitats like glades, prairies, and savannas, would more likely to have been selected by at least one producer than the species which occur mostly in shady habitats like woodlands and forests. Similarly, since open habitats tend to have drier soils than shaded ones, we predicted there may be a bias toward species with a higher (drier) wetness rating. Many grass species that grow in open habitats have evolved a more efficient way of conducting photosynthesis under hot sunny conditions. There are fewer of these “warm-season” grasses than “cool-season” ones, but we predict that proportionally more warm-season grasses will be commercially available, because they are common in the prairie seed market.
Inflorescences of big bluestem (Andropogon gerardii), and Indian grass (Sorghastrum nutans) can be seen at this glade to woodland transition at Victoria Glades Conservation Area in Hillsboro, MO. These warm-season grasses are dominant in many prairies and common in glades, but generally do not occur under the canopy of wooded areas.
In order to test these predictions, we needed to compile information on which species in our pool are available from seed vendors. We identified ten seed vendors that are likely potential sources of seed materials for species native to the Ozark Highlands. These include five seed vendors within Missouri, four large regional seed vendors located in Iowa, Minnesota, and Kentucky, and one very large seed vendor that produces seed for regions all across all the US. We were able to get information on which species each vendor produces from their website, or if they did not have a website, then through personal communication. Many vendors sell a combination of seeds and potted plants, with most species only being available in one form or the other. For this study, we were only interested in seed products because restoration of herbaceous communities through seed additions is the most common and affordable approach.
Based on preliminary analyses, we found that 501 (43%) species were commercially available from at least one vendor. We found the strongest trends supporting the prediction that species differ in their likelihood of commercial availability based on physiognomy or “growth form”. Perennial species were twice as likely to be available as shorter-lived annual or biennial species, and as predicted, forbs were better represented in seed vendor catalogues than grasses or sedges.
We predicted that more common species would be better represented in the restoration species pool and our results somewhat support this prediction. Conservatism scores are assigned to species by expert botanists in each region, so they reflect how rare and how disturbance tolerant species are within local areas. In the US, these scores are often assigned at the state level. In order to avoid over-interpreting these designations, we binned scores into three groups including ruderal (0-3), matrix (4-6), and conservative (7-10) for use in our analysis. We found that “matrix” species with middling conservatism scores were more likely to be available than conservative or ruderal species. This may be because ruderal species can be somewhat weedy and may be expected to recruit into restored areas as volunteers. And on the other hand, highly conservative species may be difficult to grow for seed production, or have a small range, and thus limited restoration potential or demand. The state of Missouri has designations for the conservation concern of all native species. We found that species classified as “vulnerable” (S3), “imperiled” (S2), or “critically imperiled” (S1) were less likely to be available from seed vendors, as species classified as “secure” (S5) or “apparently secure” (S4). And finally, as predicted, we found that species with larger ranges are more likely to be commercially available.
We expected species which mostly occur in open habitats with little tree cover to be more likely to be commercially available. We classified each species as belonging to one of three habitat affinity groups, being an open habitat specialist, closed habit specialist, or a generalist. We found no bias in species availability based on habitat affinity or based on the wetness rating for Missouri. Based on the prediction that the prairie-focused seed market would promote availability of warm-season grasses, we thought they would have greater proportional representation in the seed market, but we also did not find evidence for that prediction. Warm and cool season grasses were equally likely to be available, with about a third of all species belonging to each group being available.
While we did not find that species with affinity to open habitats were more likely available from at least one producer than species from closed habitats, we did notice that the species which were sold by the most producers tended to be “prairie species” like butterfly milkweed (Asclepias tuberosa; left), which was available from 9 of the 10 vendors we surveyed, or stiff goldenerod (Solidago rigida; right), which was available from 8 vendors.
Traits of species may also contribute to seed vendors’ interest in propagating them. We found evidence that within perennial wildflowers (forbs), species with a taller maximum height are more likely to be available. We also predicted that species with a longer potential bloom period would be better represented in the seed market, but surprisingly our data shows a negative relationship, where species that can bloom for many months are less represented in the restoration species pool. This pattern may be driven by differences between functional groups or plant families and deserves further investigation.
The final goal of this project was to identify candidate species to recommend to seed producers as valuable for restoration potential. We identified such species based on the highly detailed descriptions provided in a keystone reference for this region, Paul Nelson’s The Terrestrial Natural Communities of Missouri (2005). This book describes the geologic, climatic, and natural features of natural community types in Missouri. We only considered habitats within the broad designations of forests, woodlands, savannas, prairies, and glades, and we narrowed our focus to only habitat types that occur within the Ozark Ecoregion. For each of these 37 Ozark habitats, this reference provides lists of plant species that are “dominant”, “characteristic”, or “restricted” to that habitat. We propose that a good starting place in assessing the capacity of the native seed industry to support ecological restoration across terrestrial habitats in the Ozark region is to examine whether all of the “dominant” plant species in habitats within the Ozarks are available from vendors. Of the 120 species identified by Nelson as “dominant” in Ozark habitats, 80 of them (66%) were commercially available. This is encouraging, since it is higher than the overall availability rate of 43%, however there are still 40 species which would be difficult for restoration practitioners to acquire without hand collecting from wild populations. This highlights how biases in the restoration species pool could potentially make assembling a high-quality seed mix more difficult, if the species for sale represent those which are easiest to cultivate, rather than being the ones which have the most biological significance to restoration.
Birdfoot violet (Viola pedata) is classified as a dominant species for dry sandstone woodlands and is common on dolomite glades. Fortunately, we found it is commercially available from two vendors. Two other violets, wood violet (Viola palmata), and arrowleaf violet (Viola sagittata) are dominant in other Ozark habitats, but are not available from any of the vendors we surveyed.
Here, we are only scratching the surface in terms of identifying ways in which the seed production industry may inadvertently be biasing the restoration species pool and consequently the diversity and composition of restored plant communities. In the future we recommend continued collaboration between seed producers, restoration practitioners, and conservation scientists, to identify the limitations of available seed stocks and better align supply and demand for native seeds. Most seed vendors do not label products at taxonomic designations below the species level. However, conservation goals are sometimes identified for subspecies or varieties. The extent to which these taxa are commercially available is difficult to assess. Additionally, many restoration projects call for seed from a local provenance, but obtaining information on ecotypes of native seed lots from vendors can be difficult. While nearly 40% of our species pool for restoration projects in the Ozark Highlands are commercially available, the proportion of those species that are available from an Ozark ecotype is likely much lower.
We are currently preparing this project for publication. If you are interested in learning more, or have any questions, feel free to email Andrew (akaul@mobot.org).
In a state whose flora has been studied for hundreds of years, grassland conservation and restoration are still hindered by a need for better understanding of basic plant ecology and systematics. Leighton Reid, Jordan Coscia, Jared Gorrell, and Bert Harris contributed to this post.
All ecologists deal with puzzling groups of plants. In eastern North America, sedges (genus Carex) and panic grasses (genus Dichanthelium) are notorious for having many species with similar characteristics. In Central America, tree seedlings in the avocado family (Lauraceae) can be tricky to separate.
Sometimes we also encounter oddballs – plant species that it’s hard to see where they fit into the contemporary landscape.
Torrey’s mountain mint (Pycnanthemum torreyi) is a bit of both – an oddball species whose relationships to other mountain mints is not yet worked out.
Late-season aspect of Torrey’s mountain mint. Photo credit: B. Harris.
Like others in its genus, Torrey’s mountain mint is an aromatic herb that grows (mostly) in more-or-less open areas. Its crushed leaves have a delightful minty smell. In summer, it produces clusters of small, white flowers that are visited by a variety of pollinators.
Unlike some other mountain mints, Torrey’s is also rare. NatureServe ranks it as a G2, meaning that it is imperiled throughout its range – which extends sporadically from New Hampshire to Kansas.
Virginia has more Torrey’s mountain mint populations than the other states. The Flora of Virginia describes its habit as “dry, rocky, or sandy woodlands and clearings.” In some places, like the Piedmont, it occurs mainly on basic soils, whereas in other places, like the Coastal Plain, it lives in sandy, acidic soils. In the mountains it has been found also in limestone seepages.
An oddball species
While restoring natural areas in Chicagoland in the 1980s, Stephen Packard described some of the plants he saw as “oddball species”. Species like purple milkweed (Aslcepias purparescens) and cream gentian (Gentiana alba) grew neither in closed forest nor in open prairie, so where did they belong? These species preferred intermediate levels of light, such as would be found beneath a spreading burr oak. Packard’s observation that these species preferred savanna conditions sparked his realization that savanna had once been a frequent component of the Chicago landscape.
Matthew Albrecht has considered a similar possibility in Tennessee for Pyne’s ground plum (Astragalus bibullatus). This species grows on so-called cedar glades around Nashville, but it does not grow right in the middle. It prefers the edges where there is intermediate light. This suggests that these cedar glades may once have had softer edges that tapered slowly from exposed, rocky glade into open woodland. With modern fire suppression, these edges have become hard; many glades are now bordered by dense forests of eastern red cedar.
Could our own Pycnanthemum torreyi fall into the same category? An “oddball species” with a preferred niche that is neither full sun nor full shade? In our fieldwork on the northern Virginia Piedmont, we encountered several populations of Torrey’s mountain mint, all of which were growing in edgy sites, like powerline right of ways, or the edge of an old apple orchard.
A small population of Torrey’s mountain mint grows along one edge of this field near the forest edge, not in the open center of the field. Is this typical of this species’ preferred light environment?
Last summer, one of us (Leighton) tested P. torreyi’s habitat affinities inadvertently and with a very small sample size. He planted three seedlings in his small, Blacksburg, Virginia yard – one in an exposed spot on the south side of the house and two in a partially-shaded spot on the north side of the house. The plant in the more open, southerly spot grew okay, but it was somewhat stunted – like a spider plant that has been left out in the sun. Its stem and leaves grew short and tough. In contrast, the two plants on the north side of the house grew full and spread out, both flowering and fruiting in their first season. They also remained green late into the season, even after nearby P. tenuifolium and P. incanum had senesced. If this was a species desirous of full sun, shouldn’t it be doing better in the exposed position in the back by the parking lot?
Two Torrey’s mountain mints growing well and flowering in partial shade on the north side of JL Reid’s home in Blacksburg, Virginia.
Clearly Leighton’s sample is way too small to draw any conclusions, but it does make us wonder if Torrey’s mountain mint prefers and intermediate level of light, such as would be found in a savanna or an open woodland. These disturbance-dependent habitats were once widespread but are excluded today in much of the eastern United States. Maybe Torrey’s mountain mint is an oddball species whose habitat preferences will eventually lead us to design new restoration targets in Virginia, but we’ll have to study its ecology in a bit more detail first.
A “Problematic Species”
The Flora of Virginia also highlights that Torrey’s mountain mint is a “problematic species”, whose interpretation is “confounded by its similarity to Pycnanthemum verticillatum and its hybridization with other species.”
During our fieldwork in 2020, we were able to positively identify all of the individuals that we encountered, differentiating P. torreyi from P. verticillatum by characteristics of their flowers and leaves. Still, the possibility that Torrey’s mountain mint is not a well-differentiated species is troubling. Several landowners in our area are conserving open habitat in part because this rare species occurs there, so it would be nice to know if it is a good species.
I asked Gary Fleming, a Vegetation Ecologist for the Virginia Natural Heritage Program, for his thoughts. “Well, the entire genus Pycnanthemum is a bit problematic!” Gary wrote me in an email. He explained that the problem is that nobody has studied this genus using molecular phylogenetics, that is, using DNA to reconstruct the evolutionary relationships between species. As a result, our understanding of how species in this genus relate to each other is pretty fuzzy.
“Personally, I think P. torreyi is a good species,” Gary continued, “Over the years, I’ve observed it in numerous places state-wide and it appears to be morphologically very consistent.”
In a state whose flora has been studied for hundreds of years, apparently the Pycnanthumum nut has not yet been cracked. Hopefully some enterprising botanist will take this up soon (and maybe Packera while they’re at it).
Torrey’s mountain mint flowers. Photo credit: JL Reid.
Note: Part of this blog post represents a USDA NIFA Hatch project.
Natural History of Ecological Restoration 