Forest health monitoring at Mississippi National River and Recreation Area: 2022 field season
Authors: Sanders Suzanne , Kirschbaum Jessica 
Published: November 2023

The Mississippi National River and Recreation area (MISS), situated along a 116 km stretch of the Mississippi River through the Minneapolis and St. Paul urban corridor, encompasses ~21,800 ha of public and private land. In 2022, the Great Lakes Inventory and Monitoring Network (GLKN) resampled permanent forest monitoring sites in the park, marking the second assessment of these sites, which were established and initially sampled in 2011. The goal of this long-term monitoring project is to provides managers with routine updates on which to base management decisions; these data can also be used to tease apart impacts and elucidate causal agents when novel problems or situations arise.

We initiated a comprehensive forest monitoring program at MISS in 2011, establishing 33 sites at that time. High water levels during our sampling window that year precluded sampling on many of our planned sites while on others, water levels had only recently subsided. Here, the full complement of herbs had not yet emerged. In 2022, we resampled existing sites and established additional locations, bringing the total to 50. Sampled and derived metrics included trees (density and basal area of live trees, seedlings, and snags (i.e., standing dead trees)), understory (herb and shrub frequency), browse (bite marks on woody species and presence and height of herbaceous species), and taxa richness. We classified sites into four broad forest types using the newer (2022) dataset, resulting in two upland types (upland rich, upland disturbed) and two floodplain types (box elder-dominated and silver maple-dominated). Because of sampling difficulties in 2011, we are only comparing tree, sapling, and snag data between years.

At upland rich sites, overall tree (? 2.5 cm diameter at breast height [DBH]) density declined 22%, while that for just the small sapling component (? 2.5 cm, < 5 cm DBH) fell 41%. Species experiencing notable losses include basswood (Tilia americana L.), elm (Ulmus L.), bitternut hickory (Carya cordiformis (Wangenh.) K. Koch), and red oak (Quercus rubra L.). All three resampled sites are located in Spring Lake Park Reserve and subjected to high white-tailed deer (Odocoileus virginianus Zimm.) browse pressure.

We sampled seven sites in upland disturbed forests, where overall tree density fell 17% from 778 ? 215 trees/ha to 648 ? 72 trees/ha, largely due to declines in elm, ash (Fraxinus sp. L.), and hackberry (Celtis occidentalis L.). While changes in black cherry (Prunus serotina Ehrh.) mirrored this pattern in diameter classes above 5 cm, density of saplings increased 12-fold, largely due to a swamping effect from one site, possibly in response to buckthorn (Rhamnus cathartica L.) removal.

In the nine box elder-dominated sites, overall tree density declined from 635 ? 47 in 2011 to 500 ? 58 trees/ha in 2022, mainly reflecting changes in box elder (Acer negundo L.), elm, and silver maple (Acer saccharinum L.). In these sites, density of large (? 30 cm DBH) snags increased from 2.5 ? 1.6 to 11.1 ? 4.4 snags/ha. In silver maple-dominated floodplain forests, tree density in the 12 sites fell from 421 ? 63 to 291 ? 23 trees/ha, with declines observed in all five dominant species. Sapling density was low in these sites, falling from 62.6 ? 36 in 2011 to 23.6 ? 11 saplings/ha in 2022.

Our observations likely reflect both deer browse and alteration of the flow regime by river impoundment. At upland sites, deer browse is impeding regeneration of all major upland species: red oak, bitternut hickory, basswood, and elm. While browse is also occurring in floodplain sites, prolonged inundation may play a larger role in regeneration failure here. Saplings of silver maple, box elder, cottonwood, elm, and hackberry all have some degree of susceptibility to inundation, ranging from moderate tolerance to completely intolerant. The Mississippi River experienced flooding in 2014, 2017, and again in 2019 when flood stage was exceeded for a record number of days in St. Paul. Sapling decline at floodplain sites is likely a direct result of this.

Forest management within the park should focus both on invasive species control and floodplain reforestation. Several sites with heavy invasive weed species are in areas where leveraging local volunteers for removal projects may be possible. Floodplain reforestation requires a dual approach of research and active management. Research is needed to determine preferred propagule types and planting stock, as well as the most effective ways to control invasives, especially reed canarygrass (Phalaris arundinacea L.). Active floodplain reforestation can alleviate many of the issues we found here, although this is expensive, limited in scope, and carries with it a great deal of uncertainty. Nonetheless, projects undertaken at a small scale can provide lessons to managers, based on which aspects were successful and which were not.

Many of the park forests at MISS are nearing an inflection point and are at risk of becoming irreversibly altered if countermeasures are not undertaken in the near future. At this point, steps taken to promote ecosystem integrity are likely to be less costly and more effective than those which may be needed after further ecosystem decline. The river system through the Twin Cities metro area provides numerous services, both ecological and otherwise. As the need to act is becoming a pressing issue, it is incumbent on land managers to recognize this, and address it.