ESTABLISHMENT OF WOODLAND GROVES FOR URBAN REFORESTATION
Andy Sudbrock
INTRODUCTION
Greening the Great River Park, a private organization established
in 1995 to reforest and restore ecological functions to St. Paul's urbanized
Mississippi River Valley, is taking new and innovative approaches to urban
reforestation. Charged with planting 25,000 trees and shrubs over five years,
the Greening project is focusing on establishing woodland groves (100 to 15,000
square feet in size) with native species representative of local forest
communities. These community types include maple-basswood forest (sugar maple,
basswood), floodplain forest (silver maple, cottonwood), oak savanna (bur oak,
quaking aspen), and oak forest (northern red oak, white oak). This project is
occuring within the 1,400 acre Riverfront Business District of St. Paul,
Minnesota on the south bank of the Mississippi River (Sections 4, 5, 6, 7, 8, 9,
T. 28 N., R. 22 W., 4th PM).
The Riverfront Business District is located in
the former floodplain of the Mississippi River but most of the site has been
filled with construction debris, developed for light industrial and office
space, and is now encircled by a 500 year flood wall constructed by the US Army
Corps of Engineers. A small portion of the project area is still subject to
periodic flooding and retains characteristics of a native floodplain forest.
The intended results of creating groves of native woodlands (vs. a single tree
focus) within this urban / industrial matrix include increased habitat for
resident and migratory wildlife within the Mississippi River corridor, increased
canopy cover and landscape connectivity, increased species, structural and
habitat diversity, increased on-site nutrient cycling and soil moisture /
nutrient reserves, increased efficiency and results of soil preparation,
increased rooting zones and plant vigor, and decreased maintenance of
traditional high input landscapes.
METHODS
Establishing woodland groves
resembling local forest communities on vacant urban lots and within highly
disturbed forested areas involves more than selecting the appropriate species
and planting them. Urban soils are notorious for their poor conditions and
often hold buried surprises (Bullock and Gregory 1991, Craul 1992). Soils in
the Greening project area are highly variable from site to site, but many sites
share the common characteristics of high pH (average 8.0), very low organic
matter, and buried rubble including clay and cement bricks, limestone building
stones, and other demolition debris. Nutrient levels fluctuate widely from site
to site depending on past ground cover and lawn fertilization rates. Some sites
are contaminated with heavy metals and other hazardous wastes necessitating
greater soil remediation techniques than those outlined in this paper.
Site preparation for establishment of woodland groves includes many steps and
efficiency is increased with increasing grove size. Outlined below are the
steps and techniques devised by Greening project ecologists used for soil / site
preparation and planting of woodland groves in St. Paul's Mississippi River
valley.
- Check with appropriate agencies (Saint Paul Port Authority,
Minnesota Pollution Control Agency) to rule out contamination of site with
hazardous materials. Locate and mark all underground utilities in or near
restoration site. Adjust for conflicts as necessary.
- Visually inspect
site for potential conflicts or problems (drainage patterns, large debris).
Collect soil samples to be analyzed by the University of Minnesota soil testing
laboratory for N, P, K, pH, soluble salts and organic matter content.
- Till
restoration site to a depth of 6 to 8 inches. Hand operated or tractor mounted
rototillers are used depending on the size of the site. Inspect for and remove
any large, visible debris to minimize damage to cultivation machinery.
- Amend soil as necessary with appropriate levels of nutrients as determined by
soil analysis. Lower pH with additions of elemental sulfur or raise pH with
agricultural lime as determined by soil analysis (Harris 1983).
- Apply 2 to
4 inches of composted organic matter to soil surface and till site again to
incorporate amendments and organic matter. The Greening project uses free
composted yard waste from Ramsey County, Minnesota.
- Plant trees and shrubs
with spacing and placement resembling the appropriate model community. Tree
spacing and placement of species vary between community types and successional
stages so these should be based on approximations of spacing and placement
observed in natural model communities (e.g. 2 to 6 feet spacing for floodplain
forest seedlings vs. 10 to 30 feet spacing of larger planted stock in oak forest
groves). Water plants thoroughly.
- Cover the entire grove site with 4 to 6
inches of coarse, aged wood chips. Keep mulch away from tree and shrub trunks
to prevent excessive moisture and disease potentials.
- Return at a later
date to underplant late successional species and / or appropriate woodland
wildflowers. Timing the underplanting of shade tolerant species varies between
community types and should be based on the individual species habitat and shade
requirements. Oak forest wildflowers may be planted the year after tree
planting while the planting of spring ephemerals in a maple-basswood forest may
need to be postponed for two to three years for sufficient canopy closure.
Listed below (Table 1) are species assemblages used by the Greening project to
plant native woodland groves. True restoration of most of the project area to
presettlement conditions is both impossible and impractical as the site formerly
consisted of wetlands, wet prairies, and floodplain forest but is now highly
urbanized and no longer receives periodic flooding. Where feasible, we are
attempting to restore and enhance floodplain forest communities on the upstream
side of the flood levee. The decision to recreate groves resembling maple -
basswood forest, oak woodland, and oak savanna in the flood protected sections
of the project area was made because these forest community types were
historically found nearby. Oak savanna and open prairie communities were
dominant on the top and south facing slopes of the river bluffs and
maple-basswood and oak woodlands were found near St. Paul and on the shaded
north facing slopes of the river bluffs (Marschner 1974, Wovcha, et al. 1995).
Remnant groves of these forest, woodland, and savanna communities can still be
found near the project area though most are severely degraded and invaded by
European buckthorn ( Rhamnus catharticus ) and other alien species. Exotic
species removal from planted groves and existing forested areas will remain an
ongoing management effort. The species assemblages listed below are incomplete
and are only intended to give the reader an idea of the species and community
diversity achieved by planting woodland groves.
Table 1: Species assemblages for woodland grove establishment.
- = initial planting (bare site)
- = successional planting (after initial planting or sufficient canopy closure)
FLOODPLAIN FOREST
(In areas still receiving periodic flooding)
|
| TREES | SHRUBS | GROUNDLAYER |
|
| silver maple 1 | buttonbush 1 | |
| Acer saccharinum | Cephalanthus occidentalis | |
| hackberry 1 | bladdernut 1 | |
| Celtis occidentalis | Staphylea trifolia | |
| green ash 1 | elderberry 1 | |
| Fraxinus pennsylvanica | Sambucus canadensis | |
| box elder 1 | | |
| Acer negundo | | |
| black willow 1 | | |
| Salix nigra | | |
| cottonwood 1 | | |
| Populus deltoides | | |
>
MAPLE - BASSWOOD FOREST
(In mesic, shaded areas and within existing groves)
|
| TREES | SHRUBS | GROUNDLAYER |
|
| sugar maple 1, 2 | pagoda dogwood 1, 2 | wild geranium 2 |
| Acer saccharum | Cornus alternifolia | Geranium maculatum |
| basswood 1 | prickly gooseberry 1 | hog-peanut 2 |
| Tilia americana | ribes cynosbati | Amphicarpea bracteata |
| red oak 1 | American hazelnut 1 | Canada mayflower 2 |
| Quercus rubra | Corylus americana | Maianthemum canadense |
| green ash 1 | chokecherry 1 | big-leaved aster 2 |
| Fraxinus pennsylvanica | Prunus virginiana | Aster macrophyllus |
| black ash 1 | nannyberry 1 | columbine 1, 2 |
| Fraxinus nigra | Viburnum lentago | Aquilegia canadensis |
| ironwood 1, 2 | Juneberries 1 | false Solomon's seal 2 |
| Ostrya virginiana | Amelanchier spp. | Smilacina racemosa |
| bitternut hickory 1 | | Pennsylvania sedge 2 |
| Carya cordiformis | | Carex pensylvanica |
| black cherry 1 | | |
| Prunus serotina | | |
| butternut 1 | | |
| juglans cinerea | | |
OAK WOODLAND
(Dry and mesic species associations chosen for site suitability)
|
| TREES | SHRUBS | GROUNDLAYER |
|
| bur oak 1 | gray dogwood 1 | bracken fern 2 |
| Quercus macrocarpa | Cornus racemosa | Pteridium aquilinum |
| northern pin oak 1 | chokecherry 1 | woodland sunflower 2 |
| Q. ellipsoidalis | Prunus virginiana | Helianthus hirsutus |
| red oak 1 | American hazelnut 1 | big-leaved aster 2 |
| Q. rubra | Corylus americana | Aster macrophyllus |
| white oak 1 | Juneberries 1 | Virginia creeper 2 |
| Q. alba | Amelanchier spp. | Parthenocissus inserta |
| quaking aspen 1 | red raspberry 1 | Canada mayflower 2 |
| Populus tremuloides | Rubus strigosus | Maianthemum canadense |
| black cherry 1 | | wild geranium 2 |
| Prunus serotina | | Geranium maculatum |
| red maple 1 | | false Solomon's seal 2 |
| Acer rubrum | | Smilacina racemosa |
OAK SAVANNA
(On tough, dry sites and in association with prairie restoration areas)
|
| TREES | SHRUBS | GROUNDLAYER |
|
| bur oak 1 | chokecherry 1 | little bluestem 1 |
| Quercus macrocarpa | Prunus virginiana | Schizachyrium scoparium |
| northern pin oak 1 | smooth sumac 1 | big bluestem 1 |
| Q. ellipsoidalis | Rhus glabra | Andropogon gerardii |
| quaking aspen 1 | staghorn sumac 1 | prairie dropseed 1 |
| Populus tremuloides | Rhus typhina | Sporobolus heterolepis |
| prairie rose 1, 2 | hairy grama 1 |
| Rosa sp. | Bouteloua hirsuta |
| | leadplant 1, 2 |
| | Amorpha canascens |
| | purple prairie clover 1, 2 |
| | Petalostemon purpureum |
| | prairie sage 1, 2 |
| | Artemesia ludoviciana |
| | sky-blue aster 1, 2 |
| | Aster oolentangiensis |
| | heath aster 1, 2 |
| | A. sericeus |
| | dotted blazing star 1, 2 |
| | Liatris punctata |
| | wild lupine 1, 2 |
| | Lupinus perennis |
DISCUSSION
These techniques have been successful for the
Greening project and can be used to establish woodland groves of any size. We
are also using the grove planting strategy to reconnect existing forested
corridors and to enlarge or reconnect remnant forest patches within the
Mississippi River Valley. Larger size groves facilitate efficient site
preparation and volunteer planting events. Larger connected groves also allow
for the reintroduction and potential natural recolonization of greater numbers
of native species. Species and habitat diversity are increased and community
stability is enhanced in large forest groves (Hobbs 1988).
Woodland grove
establishment using all native species has been so far successful in the
drastically altered soils of St. Paul's Riverfront Business District. Many of
the 2,848 trees and shrubs planted in the spring and fall of 1995 were planted
in groves and first year survival exceeded ninety percent. Long term survival
of species planted in woodland groves will be monitored into the future. The
scattered distribution of some of the groves throughout the project area will
facilitate diagnosing future plant health problems as site specific or epidemic.
Other criteria used to evaluate this method for urban reforestation will
include overall vigor and growth rates of woodland grove plants, use of groves
and corridors by resident and migratory wildlife, soil pH, soil organic matter
content and nutrient levels, maintenance input levels, public acceptance and
sustained volunteer involvement.
Greening the Great River Park continues to
improve in establishing native woodland groves. Each grove planted has been
better than the last with increased species and habitat diversity, increased
community structure and horizontal stratification, larger grove sizes and
improvements in soil preparation and restoration techniques.
LITERATURE CITED
Bullock, P. and Gregory, P.J. 1991. Soils in the urban
environment / edited by Peter Bullock and Peter J. Gregory on behalf of the
British Society of Soil Science and the Nature conservancy Council. Blackwell
Scientific Publications. Oxford. 174 pp.
Craul, P.J. 1992. Urban soil in landscape design. John Wiley & Sons, Inc. New York.
396 pp.
Harris R.W. 1992. Arboriculture: Integrated management of landscape trees, shrubs, and
vines. Prentice-Hall. New Jersey. 673 pp.
Hobbs, E.R. 1988. Species richness of urban forest patches and implications for urban landscape diversity.
Landscape Ecology vol. 1 (3): 141-152.
Marschner, F.J. 1974. The original vegetation of Minnesota (map). St. Paul: U.S. Dept. of Agriculture, Forest
Service, North Central Forest Experiment Station. (Redraft of the original 1930 edition).
Wovcha, D.S., Delaney, B.C., and Nordquist, G.E. 1995. Minnesota's St. Croix River Valley and Anoka
Sandplain: A guide to native habitats. University of Minnesota Press. Minneapolis, Minnesota. 234 pp.
Return to the
Restoration & Reclamation Review Home Page