Laying out a transect in a silver maple forest with a groundcover of Laportea canadensis. NPS photo.
In 2011 the National Park Service's Great Lakes Inventory and Monitoring Network set up 33 long-term forest monitoring plots within the approximately 84 square miles of public and private land in the Mississippi National River and Recreation Area, through the Twin Cities. In 2022 it revisited them and added 17 more, for 50 in all.
What was measured
The program tracks the density and basal area of live trees (1 inch/2.5 cm or more in diameter at breast height), seedlings and snags (standing dead trees); how often herbs and shrubs occur; browse — bite marks on woody plants and the presence and height of herbaceous ones; and taxa richness. "Taxa" is broader than species: a decline might be in one orchid species, in all coralroot orchids (genus Corallorhiza), or in orchids as a whole — a useful term for plants, with their many subspecies and varieties.
Sites fall into four forest types, two upland and two lowland. High water in 2011 kept crews from many planned sites and delayed the growth of groundcover at others, so only trees, saplings and snags could be compared between the two years.
What changed
| Forest type | Findings |
|---|---|
| Upland rich (three sites, all in Spring Lake Park Reserve) | Elm, basswood and hackberry common but not replacing themselves well: fewer elms 6 in (15 cm) and smaller, basswood 4 in (10 cm) and smaller, and bitternut hickory under 6 in. Overall tree density down 22%, small saplings down 41%. White-tailed deer browse heavily here |
| Upland disturbed (seven sites) | Tree density down 17%. Bitternut hickory dominates the canopy, but it and other common species are struggling to regenerate; elm, ash and bitternut hickory fell in the two smallest size classes. Black cherry trees over 2 in (5 cm) declined, but its saplings increased 12-fold, largely at one site where invasive buckthorn was likely removed |
| Box elder–dominated (nine sites) | Tree density down, mostly box elder, elm and silver maple; more large standing dead trees (12 in/30 cm and up) |
| Silver maple–dominated floodplain | Tree density down, with declines in all five dominant species, and fewer saplings than in 2011 |
Saplings here are trees from 1 in (2.5 cm) up to 2 in (5 cm) in diameter. Across all sites, no cottonwoods were found smaller than 7 in (17.5 cm) in 2011 or 9 in (22.5 cm) in 2022. American elm was among the most common species, with box elder, hackberry and green ash — in line with others' reports that it ranks second or third in some upper-river floodplains — but trees over 6 in (15 cm) dropped noticeably.
Measuring a huge silver maple, higher on the trunk than usual to get above a large burl. NPS photo.
What it means
The findings match other studies of large river floodplains: none of the dominant tree species on the plots is successfully regenerating. Two forces are likely at work — deer browse and a river flow regime altered by dams.
- Uplands: deer browse is blocking regeneration of all the major species — red oak, bitternut hickory, basswood and elm.
- Floodplains: browse happens too, but prolonged flooding may matter more. Silver maple, box elder, cottonwood, elm and hackberry saplings range from moderately tolerant of flooding to not at all. The river flooded in 2014, 2017 and 2019, when St. Paul saw a record number of days above flood stage, and the floodplain sapling decline is likely a direct result.
Silver maple forest at Crosby Farm Regional Park. NPS photo.
What can be done
Forest management in the park should focus on controlling invasive species and reforesting the floodplain. That needs both research — the best ways to control invasives, especially reed canarygrass, and what to plant in their place — and active management. Replanting is expensive, limited in scope and uncertain, but small projects can show what works.
The river corridor through the Twin Cities provides many services, ecological and otherwise, and many of the park's forests are nearing a turning point beyond which they could be permanently changed. Acting now to support their health is likely to cost less, and work better, than acting after further decline.
Sources
Rewritten from the National Park Service article "Forest Health Along the Upper Mississippi, 2011–2022" (Mississippi National River and Recreation Area; public domain), based on Sanders, S., and J. Kirschbaum, 2023, Natural Resource Data Series NPS/GLKN/NRDS—2023/1400, in hubnx's own words. A caption there calls Laportea canadensis "stinging nettle"; the page gives only the scientific name.
In these publicationsMississippi National River & Recreation Area
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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