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.