Are Ants and Trees in a Relationship? Urban Ecology Gossip on Grinnell's Campus

Matias Bambi

As human urbanization increases, and wildlife habitat destruction along with it, the spaces that urban landscapes create are critical to understand for species conservation. Fragmentation and loss of native vegetation are two factors associated with habitat destruction that are known to reduce arthropod species richness and increase replacement of native arthropod species with exotic/ invasive ones (Hashimoto et al., 1994; Gibb and Hochuli, 2002; McKinney, 2006). However, isolated trees in urban landscapes can serve as habitat refuges for ant species and other arthropods; examining population dynamics between ants and trees in urban settings may provide insight to how urban planners can best incorporate species conservation into their designs.

Yasuda and Koike investigated how trees contributed to ant and other arthropod abundance and diversity in the urban landscape of Matsudo City, Japan. More specifically, they examined how host tree arrangement, size, and species, as well as surrounding land abundances, influenced arthropod abundance and variety. When in the field, these entomologists searched for arthropods primarily on the trunks of the trees as tree bark plays an important role as egg-laying sites and pathways to canopy resting sites for some arthropod species (Moeed and Meads, 1983; Nicolai, 1986; Hanula and Franzreb, 1998; Majer et al., 2003). Yasuda and Koike found that tree species had the greatest influence on arthropod abundance, with tree size also having a significant effect and arrangement (cluster, linear, or isolated) having little effect (Yasuda & Koike, 2009). The authors note, however, that this differs from other studies, which have found that tree arrangement was an important factor in arthropod richness and abundance (Majer and Delabie, 1999; Ozanne et al., 2000; Reyes-López et al., 2003; Gove and Majer, 2006). Yasuda and Koike assert that to increase arthropod diversity and abundance, it is important to include tree species that attract arthropods in urban landscapes. Their research highlights the importance of investigating arthropod-tree population dynamics in order to better understand ecological community structures in urban planning.

A map of Matsudo City from Yasuda & Koike 2009 (left) and a map of Grinnell's Campus (right). https://bit.ly/2YTgnai

In their paper, Yasuda and Koike discuss the importance of tree selection when considering arthropod abundance and diversity. This is a concept that we can apply to our urban landscaping here in Grinnell. The derecho in 2020 devasted many trees on campus, and its aftermath provides us with an opportunity to think about how trees we select to plant might affect other species on campus. With this in mind, my lab partners, Tianyu and Will, and I seek to determine how ant abundance and ant species abundance is affected by host tree species and tree arrangement on the Grinnell College campus. To do this, we have travelled around North campus to observe what species of ants (and their abundance) are found on given tree species’ lower trunks, and how tree arrangement (clumped, linear, or isolated) might affect ant community structure (see pictures below). Through our study, we hope to provide better insight into the population dynamics between ant and tree species on campus. By examining this relationship, our findings could provide insight about important community structures, which could be incorporated into future tree-planting projects here in Grinnell.

        Two entomologists from our lab group observing ants and distance between trees in Grinnell's North campus.        

 

Works Cited:

Gibb, H., Hochuli, D.F., 2002. Habitat fragmentation in an urban environment: large and small fragments support different arthropod assemblages. Biological Conservation 106, 91–100

Gove, A.D., Majer, J.D., 2006. Do isolated trees encourage arboreal ant foraging at ground-level? Quantification of ant activity and the influences of season, in Veracruz, Mexico. Agriculture, Ecosystems and Environment 113, 272–276.

Hanula, J.L., Franzreb, K., 1998. Source, distribution and abundance of macroarthropods on the bark of longleaf pine: potential prey of the red-cockaded woodpecker. Forest Ecology and Management 102, 89–102.

Hashimoto, Y., Kamihogi, A., Hattori, T., 1994. A study on the conservation of fragmented forest as inhabitant arthropods in the newtown, using ant-biodiversity for indicator as arthropods-biodiversity. Journal of the Japanese Institute of Landscape Architects 57, 223–228 (in Japanese).

Majer, J.D., Delabie, J.H.C., 1999. Impact of tree isolation on arboreal and ground ant communities in cleared pasture in the Atlantic rain forest region of Bahia, Brazil. Insectes sociaux 46, 281–290.

Majer, J.D., Recher, H.F., Graham, R., Gupta, R., 2003. Trunk invertebrate faunas of Western Australian forests and woodlands: influence of tree species and season. Austral Ecology 28, 629–641.

McKinney, M.L., 2006. Urbanization as a major cause of biotic homogenization. Biological Conservation 127, 247–260

Moeed, A., Meads, M.J., 1983. Invertebrate fauna of four tree species in Orongorongo Valley, New Zealand, as revealed by trunk traps. New Zealand Journal of Ecology 6, 39–53

Nicolai, V., 1986. The bark of trees: thermal properties, microclimate and fauna. Oecologia 69, 148–160

Ozanne, C.M.P., Speight, M.R., Hambler, C., Evans, H.F., 2000. Isolated trees and forest patches: patterns in canopy arthropod abundance and diversity in Pinus sylvestris (Scots Pine). Forest Ecology and Management 137, 53–63.

Reyes-López, J., Ruiz, N., Fernández-Haeger, J., 2003. Community structure of groundants: the role of single trees in a Mediterranean pastureland. Acta Oecologica 24, 195–202.

Yasuda, Mika, and Fumito Koike, 2009. The Contribution of the Bark of Isolated Trees as Habitat for Ants in an Urban Landscape. Landscape and Urban Planning, 276-281.

Are our Cities Polluted? Just ask Lichens

 Urban environments are everywhere. It is estimated that almost three percent of the earth’s land is urbanized, and that number is predicted to rise significantly in the near future. Gao et al., 2020 predict that by 2100, the total area that is urbanized will rise by a factor of 1.9-5.9. This is incredibly significant for the Earth’s species, because urban environments are different from natural environments [citation needed]. In a paper by Coffee & Fahrig (2012), the authors attempt to separate the impact of three of these changes brought on by urbanization—loss of habitat (which they measure using tree cover), vehicle pollution (measured by the number of roads in the area and bus traffic [Fig. 1]), and moisture levels in the outer bark of trees—on Lichens in the city of Ottawa. Lichens have been proposed as an indicator of air pollution in cities because they receive their nutrients from the air, so harmful pollutants are easily taken up by the lichen and quickly reach toxic levels. However, Coffee & Fahrig wanted to understand whether the effect of air pollution was all it took to cause poor lichen abundance and diversity, or whether these other effects of urbanization were also connected. 

Fig. 1: Mapping air pollution in the city of London. Notice how pollution maps really strongly with major roads, showing the importance of vehicle pollution. From https://gsttcharity-uk.shorthandstories.com/thecausesofairpollutionininnercities/index.html

To separate the impact of each of the three effects of urbanization is challenging, as it really isn’t possible to experimentally “block” the effects of two effects in order to isolate one effect of the urban landscape. Instead, the authors used a “mensurative” experiment, where data are collected in situ and they separate the effects using statistical techniques when analyzing the data. By choosing sites carefully, they can tease apart the impact of each predictor on their response variables, species richness and cover (how much of each tree had lichen).

Their experiment showed that air pollution did have a significant effect on the cover of lichens, but not on species richness (Fig. 2). They found that species richness was positively correlated with colonization sources, especially when looking at large spatial scales. Thus, they conclude that lichen cover could serve a purpose as an indicator of air pollution in cities. However, they point out that if managers in urban spaces are attempting to increase species richness of lichens, they would do better to increase tree cover than simply attempting to lower vehicle emissions.

Fig. 2: Correlations between species richness and cover at different spatial scales with (a) vehicle pollution (b) colonization sources. Adapted from Coffee & Fahrig (2012)

This experiment is very relevant to my group’s urban ecology project in Grinnell. In our project, we want to measure how different factors (distance to roads/parking lots and distance to the nearest tree), affect several measures of lichen populations growing on lampposts (lichen abundance [measured using presence/absence and percent cover], species richness, and the largest lichen in each of five sections [a potential correlate of population longevity/health]). To do this, we have sampled lichen on each lamppost contained within South campus. One thing that will be interesting to see is whether the effects that these authors have shown will be present on a much finer scale within South campus. Additionally, we hope to explore colonization sources more closely by analyzing the direction of the nearest tree and determining whether it correlates with areas of higher lichen density on lampposts. If that were the case, we could have a much better understanding of how lichen colonize the lampposts, which could help with lichen management.

References:

Coffey, H. M., & Fahrig, L. (2012). Relative effects of vehicle pollution, moisture and colonization sources on urban lichens. Journal of Applied Ecology, 49(6), 1467-1474.

Gao, J., & O’Neill, B. C. (2020). Mapping global urban land for the 21st century with data-driven simulations and Shared Socioeconomic Pathways. Nature communications, 11(1), 1-12.

What ants are in our pants?!?… Oh wait, on campus trees: Ant Community Composition and Abundance on Campus Trees

                                                                                                                                             Will Bebee

             As the world's human population continues to climb and industries expand, urbanization is becoming more of a threat than ever to biodiversity. The shift from natural habitat to concrete, buildings, and roads leads to negative impacts for many organisms including, habitat loss and fragmentation, alterations in microclimates, and contamination due to pollution. However, many organisms are very adaptive, and ecology and conservation biology do not rest as the lush green grasses and forests transform to gray concrete jungles. As Uno, Cotton, and Philpott (2010) note, species generally inhabit four types of habitats in urban spaces; built habitat (i.e., buildings and pavement), managed vegetation, unmanaged green spaces (i.e., vacant lots and farmland), and natural leftover vegetation. Understanding which species and how many of them occupy these habitats along with how they utilize the urban space will be essential for conservation management and planning.

Arthropods, specifically ants, are a good model organism for assessing biodiversity in urban landscapes (Uno, Cotton, and Philpott 2010). To begin with, they play an essential role in the food web of the environment and provide multiple ecological services acting as decomposers and nutrient cyclers.  Their short generation time is also advantageous for observing the dynamics of urban ecology as they can quickly respond to environmental change. Also, different ant species are affected uniquely depending on the habitat structure like leaf litter, habitat disturbance, vegetation structure, plant species richness, etc. Additionally, the presence of dominant competitors and exotic species can influence ant community structure. There are mixed results on how habitat loss and fragmentation impact ant richness within urban spaces. However, there is a consensus that ant species composition is affected when comparing urban habitats versus natural rural areas (Uno, Cotton, and Philpott 2010).

In this study, Uno, Cotton, and Philpott (2010) asked how ant abundance, richness, and species composition change depending on the urban habitat and whether ant biodiversity and species composition correlate with specific habitat characteristics. To perform this study, they surveyed ants in three urban green spaces: community gardens, vacant lots, and forest remnants in Detroit, Michigan, and Toledo, Ohio. Ant sampling methods utilized tuna baits at “baiting stations” approximately in the center of each habitat. They inspected the baits 30 min. after placement, where the authors assessed the number of ant species and their abundance. They also measured 24 habitat and vegetation characteristics to explore potential effects on ant richness and abundance (Table 1). This involved measuring the size of the habitat, estimating the percent of the area covered with concrete, buildings, bare ground, grass or herbs, and shrubs, and counting the number of trees. Different habitat characteristics were assessed at varying spatial scales (100x100 m plots, 20x20 m plots, and 1x1 m plots).

The authors found that forests displayed the highest ant richness (26) compared to vacant lots (20) and gardens (14) (Fig. 1). They contribute these findings to two potential causes, habitat and landscape factors, and competitive interactions (Uno, Cotton, and Philpott 2010). Regarding habitat and landscape factors, the authors found habitat type significantly influences ant community composition, abundance being lowest in forests. This is largely due to site characteristics differing in the three habitats. In general, forests had more woody plants and leaf litter, and less concrete than gardens and vacant lots. Vacant lots contained taller herbaceous vegetation while gardens consisted of higher forb richness and more bare ground. Interestingly, the differences in the vegetation structure and factors of the habitats significantly influenced ant species composition, but not ant richness. Additionally, the size of the habitat may affect ant community structures as dispersal limitation may be more prevalent in larger habitats. In terms of competitive interactions, the authors found the abundance of exotic ant species negatively influences ant richness in gardens and vacant lots, impacting native ant richness in urban spaces. This is likely because, in urban habitats, the most successful ants are generalist and opportunistic species, dominant competitors, and form sizeable, aggressive colonies. Therefore, the introduction of an exotic species that is more competitive (i.e., Tetramorium. caespitum in this paper) can lead to the local displacement of native ant species.

Figure 1: Rarefaction curves for ant species richness observed in urban forests, community gardens, and vacant lots sampled in Detroit, MI (b), Toledo, OH (c), and across both cities (a). Thin lines show upper and lower 95% confidence intervals for symbols of the same color.

This research article is relevant to my group’s urban ecology project in Grinnell, Iowa, since we are asking how different managed tree species on campus (native vs. non-native) act as habitats for ants and impact ant community structure and abundance. Different tree species may be utilized as a habitat by specific ant species depending on whether there is a mutualistic relationship between the two or whether the ants require a specific tree species for foraging needs. Furthermore, due to the mixing of native and non-native tree species on campus, it will be interesting to observe whether native trees are compromised primarily of native species of ants in higher abundance than non-native trees. Additionally, identifying whether any exotic ant species are present on campus and how their competitive interactions with native species impact the species composition and abundance of ants on the trees will be valuable information for the potential addition of management procedures. The distance between trees may also provide valuable information by impacting the dispersal patterns and abilities of the ants to colonize some trees.

 

References

Uno, S., Cotton, J., & Philpott, S. M. (2010). Diversity, abundance, and species composition of ants in urban green spaces. Urban Ecosystems, 13(4), 425-441

The Localized Influence of Fire on Historic Spruce Forest Death

The study of paleobiology provides ecologists the opportunity to see into the past how previous ecosystems in a place were influenced by environmental changes. Not only does paleobiology allow us to see into the past, but also to anticipate how future species and ecosystems will react to the rapidly changing climate today. The relationship between warming climate trends and the transition from boreal species to ones that are better suited for the new climate conditions. In the eastern United States, this shift was characterized by boreal taxa moving primarily northward where climate conditions better suited their environmental needs. However, the impact of fire on this transition is not well studied, so Jensen et al. (2021) set out to investigate the role of both fire and climate in the shift from Picea (spruce) conifer forests to broadleaf deciduous and Pinus (Pine) woodlands from the late Pleistocene to the early Holocene in the southern Great Lakes Region (Fig. 1). 

Vegetation History of the southern Great Lakes Region

The analysis of cores collected from five different kettle lakes across the southern Great Lakes region (Fig. 1) provided pollen and charcoal samples to inform the researchers about the vegetation history of each site. In their research, the first step Jensen et al. (2021) took was to define four different temporal zones based on patterns of species pollen abundance at the sites (Fig. 2). The first zone, Zone A, begins at the record onset, 19-17.5 kyr BP, to 15.8-14 kyr was dominated by Picea (spruce), with some Pinus (pine) and cold tolerant deciduous taxa including Alnus (Alders), Salix (Willows), and Betula (Birches). Transitioning into Zone B (15.8-14 to 13.3–11.6 kyr BP), a marked decline in Picea, resulting in mixed deciduous forests and woodlands, composed of waning populations of Picea and new populations of deciduous hardwood trees. Zone C (13.3–11.6 to 12–10.5 kyr BP) notably occurred during the Younger Dryas, in which all sites saw an increase in Pinus and decrease in deciduous hardwoods. During this time, some sites also saw a second Picea peak where it recovered, but this was not a universal phenomenon at all five sites. Finally, Zone D (12–10.5 to 8.8–7.8 kyr BP) had a sharp decline in Pinus and Picea (where second peaks occurred) as well as a steep  rise of temperate deciduous forests at all of the sites. Quercus (Oak) species showed a significant rise in abundance in this zone, indicating the presence of oak savannas, similar to those restored at Conard Environmental Research Area (CERA) here in Iowa.

So what’s the deal with fire?

Jensen et al. (2021) used their construction of the vegetative history at each site to identify patterns of vegetation change coinciding with signs of fire at that time. The results of combining charcoal analysis and Picea vegetative history divided the sites into two distinct patterns of Picea vegetation change. The first pattern, termed “decline and return”, was observed at Stotzel-Leis and Silver Lake sites. It is characterized by abrupt declines in Picea that coincided with periods of enhanced fire activity (Fig. 3). After that decline, deciduous forest primarily replaced Picea, with some return of Picea and Pinus during Zone C (Fig. 2). In this case, fire significantly accelerated the process of Picea replacement. The second pattern, “stair step” was observed at Appleman Lake, Bonnett Lake, and Triangle Lake Bog sites. It is distinguished by the decline in Picea through several abrupt events, however the relationship between Picea declines and fire activity is less clear (Fig. 3). Picea was primarily replaced by Pinus after the decline, and deciduous forests later on (Fig. 2). These two patterns indicate that the effects of fire vary significantly at the local level. Of the five sites studied, the two that were significantly impacted by fire had different patterns of replacement after significant fire events, pointing to fire significantly affecting what species colonize disturbed areas. 

Why Should We Care?

The rapidly changing climate due to global warming is creating new environmental circumstances that species have little time to adapt to. During the late-glacial period, the warming climate after the Last Glacial Maximum provides scientists an opportunity to see how species reacted to that warming period. Spruce forests and other boreal species habitat zones are farther North now, and may fare similarly as the global temperature continues to rise. Changing patterns of fire frequency are also associated with climate change. Jensen et al. addresses how fire has interacted with climate to produce different ecological outcomes, which can inform how areas with frequent fires will be impacted differently. 

Reference

Jensen, A. M., Fastovich, D., Watson, B. I., Gill, J. L., Jackson, S. T., Russell, J. M., ... & 

Williams, J. W. (2021). More than one way to kill a spruce forest: The role of fire and climate in the late‐glacial termination of spruce woodlands across the southern Great Lakes. Journal of Ecology, 109(1), 459-477.

The Driftless Area: Is There A Future Without Knowing The Past?

    There is a common misconception that most of the beauty and allure of the Midwest landscape lays in the awe of the Great Lakes. Well, this idea is likely fueled by the fact that most Midwest states form what is known as the “Corn Belt”. In 2007, the Midwest consisted of 20,360,396 hectares of corn and 14,277,472 hectares of soybean, while the total area of agricultural land spans well over 127 million acres. Flat and homogenous landscapes of corn and soy are quite often what comes to mind when most Americans conceptualize the imagery of the Midwest. Though this may be a predominantly true narrative, there are many relatively small ecoregions that constitute the ecological diversity of the Midwestern landscape, one of which is a distinct region referred to as the Driftless Area. Extending across 4 state borders, this region is constructed by Southwestern Wisconsin, Southeastern Minnesota, Northeast Iowa, and a very small portion of Northwestern Illinois. It is known to have been skipped over by glacial drift at the end of the last Ice Age, which saved this relatively small region from the flattening effects of glaciation notable on the surrounding landscape. Diverse and karst topography allows for the watersheds to run cold with groundwater-dominant streams that support a world-class trout fishery. To the enticement of an angler’s solitude, the extent of national publicity that this region has received is strikingly low considering that the Mississippi corridor that flows through the Driftless Area is home to over 40% of waterfowl in the US, more than 300 bird species, and 260 species of fish. Not to mention that it functions as a massive recreational resource to more than 3 million people annually (more than Yellowstone), which produces a $6.6 billion annual recreational/tourism economy.

   Consequently, agricultural and cattle operation land expansion is undoubtedly a leading factor in ecological degradation and poses the largest threat to the ecological integrity of this region, which doesn’t align with the simultaneously high ecological and ecotourist economic value previously mentioned. Many ecological restoration efforts are actively combating degradation. In hopes to protect what is left and recreate an ecologically diverse and healthy future, it is vital that we consider the impact of perspective in this reconstruction. This may go without saying, but its worth reiterating so I’ll say it here for emphasis, “in order to know where we are going, first, we must know where we have been”.

    So, let's take a look at the past. The Shea et al. (2014) study attempts to reconstruct pre-Euro colonial vegetative composition of the Driftless Area. They bluntly recognize the critical role of “reference conditions and the development of knowledge regarding the processes that contributed to development of those conditions and their variability across landscapes” (Shea et al., 2014). Public Land Survey records were used to collect mid-1800’s land condition data. The data was analyzed in two intersectional groupings:

1. Vegetation cover and structure
• i.e. whether the composition of tree cover and species resulted in the existence of:
1. Prairie (No trees)
2. Savanna (Few trees)
3. Open Forest (Open canopy; substantial sunlight penetration to ground)
4. Closed Forest (Closed canopy; very little sunlight penetration to ground)
2. Statistical analysis of cover class associations with environmental factors, specifically: 
1. Soil Texture (silt, sand, or clay)
2. Topographic Roughness (diversity in elevation of the landscape)
3. Distance From Waterway

The cover classes are important for determining the demographic composition of the vegetative distribution to determine a relative biome type throughout the region. Complimentary to these data is the importance of understanding the conditions which led to the formation of these species compositions.

After collecting the data, Shea et al. found that pre-Euro colonial Driftless Area was predominantly comprised of oak savanna with 72.6% of all recorded trees being bur oak (Quercus macrocarpa), white oak (Q. alba), and black oak (Q. velutina) and 69.4% of the area was dominated by savannas. Importantly, they also found that their work similarly reflects other larger scale statewide assessments in other studies.

As a result of their ecologically distinct small-sale study Shea et al. was able to reconstruct a more precise representation of the tree cover and species composition of the area.  In alignment with greater precision was their ability to distinguish narrower assessments of environmental factors leading to the resulted vegetative composition. For example, this precision exposed that the prevalence of oak and the diverse oak cover classes, and being distinctly associated with savannas suggest a critical presence of low intensity fire in pre-Euro colonial eras as an Indigenous ecological management technique. Relevantly, they propose that variance of fire intensity and frequency has been known to have direct impacts on tree species composition; exemplified here by the absence of fire in oak savanna leading to mesophication. 

    This leads to comparatively reference present-day land cover in the Driftless Area. We know that today ~47% of the Driftless Area is agricultural land, 13% is developed, and 34% is forested, mostly in small fragments of closed forest. So, understanding that this study, as the first of its kind for this region, is intended to further inform management decisions is vital to recognize as a tool for future restoration efforts in the Driftless Area. The largest public concern and strongest political draw would undoubtedly be trout stream management and restoration. Shea et al. has already given us a outlook at potential future impacts by suggesting that hackberry (Celtis occidentalis), boxelder (Acer negundo), and honeylocust (Gleditsia triacanthos) are likely to increase in dominance. Strong efforts have been made to remove encroaching riparian boxelder communities and replace with prairie grasses, and more recently prairie cord grass strips have been planted as riparian buffers. This one area of interest where I could see Shea et al. offering a more critical look at restorative management. Although open to further interpretation, it is vital to critically and collectively inform future restoration. 

Work Cited

- Shea, M. E., Schulte, L. A., & Palik, B. J. (2014). Reconstructing vegetation past: pre-Euro-American vegetation for the midwest driftless area, USA. Ecological Restoration, 32(4), 417-433.

Image Sources

- https://www.extension.iastate.edu/news/driftless-region-beef-conference-jan-24-25

- https://www.mnopedia.org/thing/oak-savanna

- Shea, M. E., Schulte, L. A., & Palik, B. J. (2014). Reconstructing vegetation past: pre-Euro-American vegetation for the midwest driftless area, USA. Ecological Restoration, 32(4), 417-433.

Mysteries in the Sediment: Piecing Together the History of Zizania palustris

 A woman stands on the edge of a canoe, whipping a bawa'iganaakoog against the tall stalks of manoomin (Zizania palustris or northern wild rice). Most of the grains fall onto the base of the boat, but some are lost to the estuary. The hydrophilic seeds sink into the mud and grow into next year’s crop. A man sits in the back of the canoe, moving the pair forward along the water’s edge. Later, the rice will be roasted over a fire and danced over in a ceremony. The Ojibwe tribe’s relationship with Zizania palustris dates back to their arrival when a prophecy foretold their new home where plants grow on water. Since European settlement, they’ve adjusted cultural practices with the ever-evolving landscape marked by settler disturbance. Paleo-ecologists are piecing together evidence from resilient populations of native species like this. Herbarium specimen like Henry Conard’s collections of Zizania palustris in the Grinnell College Herbarium are some of the limited accesses we have to these historical landmarks (Figure 1). 

Figure 1: Henry Conard’s collections of Zizania plaustris in the Grinnell College Herbarium. Left: 1923, Swan Lake; Right: 1922, Rice Pond, Jasper County, IA

In 2015, Nurse et al published a deep analysis of the shifting populations of Zizania palustris on the heavily trafficked St. Louis River Estuary, a waterway leading to Lake Superior between Duluth, MN and Superior, WI, since Euro-American settlement 250 years ago. For the last two centuries, economically driven industries like timber, paper and travel have held precedent over preserving natural landmarks. By the mid-1800s, excessive logging in the region caused massive soil erosion. The completion of the Duluth ship canal in 1871 and growing number of mills scattered along the river compounded the soil disturbance, adding pollution and removing viable habitat. Although the state of Minnesota declared the area contaminated in 1928, the Clean Water Act was not passed until 1972, mandating that sewage deposited in the river must be treated. 

 

Evidence of these events still exist in the estuary’s sediment. The researchers studied pollen and phytoliths from soil cores at five separate locations to determine the population shifts of Zizania palustris and reconstruct a vegetation history of the region (Figure 2).

 Figure 2: Left: Common species on the St. Louis River Estuary found in sediment collections. Right: Map of collection sites used by Nurse et al.

Poaceae pollen grains are notoriously hard to identify. The authors mention Calamagrostis canadensis (Bluejoint grass),Phragmites americanus (American reed), and Glyceria canadensis (Rattlesnake manna grass) as species easily confused with the study subject (Figure 2). The team used four herbarium specimens and eight fresh samples of Zizania palustris to develop a key for verifying pollen samples. They listed a psilate surface visible under light microscopy and polar-to-equatorial ratio of 1.2 as the best indicators of Zizania palustris pollen (Figure 3). 

 

The pollen analysis was paired with an investigation of remanent silica particles, known as phytoliths, to verify results amid sediment redistribution. The team identified inflorescence bracts as a reliable source of rondels, an identifiable morphotype of phytoliths from Zizania palustris. When the rice falls from the plant into the water, the inorganic silica from the inflorescence bracts remains in the sediment. Unfortunately, phytoliths from northern wild rice are commonly found in the exact location of a historical stand and there was no precise record for North of Clough and Pokegama Bay (Figure 3). 

 Figure 3. Left: Micrographs of phytolith morphotypes from the St. Louis River Estuary. Box A in the top right shows the rondels from Zizania palustris inflorescence bracts. Right: SEM images of pollen differentiate species (A) Calamagrostis canadensis: scabrate texture and spherical shape (B) Zizania palustris: psilate surface and polar-equatorial ratio of 1.2  

Despite mass erosion, the dual methods proved effective. The researchers found pollen in all five core samples. Sediment cores from the North Bay, Billings Park and Minnesota Point had inconstant amounts of phytoliths from Zizania palustris dating back to the 18^th century, although it did not appear in the samples from North of Clough and Pokegama Bay (Figure 2). 

 

The authors pieced together the vegetation shifts from Zizania palustris data, microcharcoal remnants, sediment accumulation and paleo-environmental evidence from other species. Evidence of increased sediment accumulation, increased microcharcoal and decreasing pine stands in the mid-1800s coincided with the arrival of settlers. The presence of Ambrosia (ragweed) pollen in the samples furthered the evidence of disturbance. As species like Pinus disappeared, pollen from hardwood trees like Betula (birch), Fraxinus (ash), and Quercus (oak) expanded. Sediment accumulation from the dying forests ran into the estuary, lowering water levels and pressuring the population of Zizania palustris. This effect was mirrored in the further development of dams and shipping canals coupled with natural events like droughts and wildfires. 

 

Environmental impacts caused by settlers reached all corners of the Midwest. The Nurse et al paper demonstrates effective species identification strategies for populations marked by mass disturbance and highlights the biological and cultural resistance of Zizania palustris. Northern wild rice stands still exist across the Midwest today and documentation of these populations is necessary to track its vitality into the future. 

Andrea M. Nurse, Euan D. Reavie, Jammi L. Ladwig, Chad L. Yost, Pollen and phytolith paleoecology in the St. Louis River Estuary, Minnesota, USA, with special consideration of Zizania palustris L., Review of Palaeobotany and Palynology, Volume 246, 2017, Pages 216-231, ISSN 0034-6667, https://doi.org/10.1016/j.revpalbo.2017.07.003.

 

Lost in the Sands of Time: Attempting to Construct Climate and Vegetation Narratives in the NWSP

 

One of the challenges researchers face in reconstructing vegetation history is understanding why vegetation changes occur. Climate changes can be inferred from evidence of vegetation changes such as fossil pollen records, but these records do not prove that climate changes alone cause vegetation to change and cannot serve as independent climate proxies (Calcote et al. 2020). Finding alternative reference points for climate would be especially useful in the Upper Midwest, where changes in forest structure are well documented but drivers of vegetative change are not entirely understood (Calcote et al. 2020). In their study, Calcote et al. compare lake level data from sediment cores against fossil pollen records to determine if lake sediment can serve as a valid independent proxy for climate and moisture levels in the Northern Wisconsin Sand Plain. 

 

 

Figure 1a. Mesic and xeric tree species found at Cheney Lake. Images courtesy Peter M. Dziuk. 

 

Figure 1b.  Cheney Lake (the white lines represent past shorelines) (Calcote et al. 2020).

           

Figure 2. Fossil pollen data from one of the sediment cores (from Calcote et al. 2020). 

 

 

Figure 3. LOI and magnetic susceptibility data from one of the sediment cores (from Calcote et al. 2020).  

The Northern Wisconsin Sand Plain (NWSP) is an area in Wisconsin that features forests with a mixture of coniferous and broadleaf trees growing in poor, sandy soils. The vegetation makeup of the NWSP responds to changes in moisture, with xeric species such as jack pine (Pinus banksiana) and red pine (Pinus resinosa) increasing during drier conditions and mesic species such as white pine (Pinus strobus) and oak (Quercus) increasing during wetter conditions (Figure 1a). Vegetation varies with soil richness and abundance of fire according to location, with fire having an important stabilizing effect on jack pine populations, as Bjorn Larson writes in this blog post. 

The authors took sediment cores from the research site, Cheney Lake, and analyzed fossil pollen of mesic and xeric species along charcoal data (Figure 2). To determine the suitability of Cheney Lake for climate records, they estimated the depth of the lake and its response to climate changes in the 20th century from photographs and past shorelines (Figure 1b). They then estimated lake level over time based on percent organic matter (through LOI), magnetic susceptibility, and 14C dating of sediment cores (Figure 3). Sediments dating from 7200 cal. years BPA to 2010 CE were studied; sections of sediment core were divided into zones based on composition and appearance, as shown in Figure 3.  

The authors found that the lake level and pollen records did not match as closely as expected. Between 3300 and 1500 cal. yr BP, for example, the lake levels (and therefore moisture levels) at Cheney Lake were high, but the percentage of fossil pollen from mesic species did not increase, and fire frequency did not decrease as expected; there was also not as much organic matter in the sediment as expected. A drought beginning around 1500 cal. yr BP led to decreased lake levels, but xeric species’ pollen percentages did not increase and fire frequency did not increase.  The authors theorize that a shift from mesic species to jack pine did not occur during the 1500 cal. yr B.P. drought at Cheney Lake because jack pines were already established there due to a lack of fire breaks. The most significant correlation at Cheney lake was a slight increase in white pine pollen at 500 cal. yr BP, when moisture increased.

Interestingly, at other locations such as Ferry Lake (the lake in Bjorn Larson’s blog post), vegetation followed the climate trends at Cheney Lake, with jack and red pine pollen percentages increasing and oak pollen percentages decreasing during drier periods. These differences between the locations could be due to differing soil composition or presence of fire breaks. Here, oak pollen percentages did not recover even after moisture levels increased after the 1500 cal. yr BP drought, which also suggests that jack pine populations remain stable after they have been established. Records of lake level in sediment were not reliable predictors of vegetation change at Cheney Lake, but they matched regional trends in climate, growing drier during the Medieval Climate Anomaly and growing wetter during the Little Ice Age. The drought at 1500 cal. yr B.P. was previously undocumented. 

The study may have not been able to construct a cohesive narrative about vegetation history out of sediment records, but it did something just as valuable: it showed that lake level changes do not consistently correlate with vegetation changes, and that local variations can greatly affect vegetation composition. It also brought up new questions—the authors introduce theories, but it is not clear why the vegetation at Cheney Lake did not match the vegetation at other sites in the NWSP. As our understanding of vegetation history develops, we will learn how to better use proxies for climate to understand the drivers of vegetative change.  

Works Cited

Calcote, R., Nevala-Plagemann, C., Lynch, E. A., & Hotchkiss, S. C. (2021). Late-Holocene climate changes linked to ecosystem shifts in the Northwest Wisconsin Sand Plain, USA. The Holocene, 31(3), 409-420. 

Image sources (for Figure 1a)

    https://www.minnesotawildflowers.info/tree/jack-pine 

    https://www.minnesotawildflowers.info/tree/northern-pin-oak

    https://www.minnesotawildflowers.info/tree/red-pine

    https://www.minnesotawildflowers.info/tree/white-pine