The resilience of nature at Tanner Springs Park is a surprise for visitors–especially seeing an Osprey hunt or a duckling’s first swimming lesson. Transforming a brown field to thriving stepping stone habitat took ecological planning, diverse native plant support and ongoing adaptive management. Hidden from view is the struggle between native plants and introduced non-native invasive species. Second to habitat destruction, invasive species are the largest threat to biodiversity.
Willamette Valley native plants co-evolved for thousands of years. Introduced plants pose new competitive threats which can outcompete native plants using early germination frequencies, excessive seed production, aggressive vegetative growth, or chemical dispersal. Mono crops of invasive plants can establish which limits food and habitat for wildlife. Controlling invasive plants comes with limitations. Many of the herbicide sprays have negative side effects for wildlife, people and the environment. Hand pulling is costly, time consuming.
Invasive species cost Americans about $143 billion per year. At least 30 new potential biological invaders enter the U.S. every day.
The park is designed with three zones, each with unique ecological functions and maintenance needs. The initial park design incorporated 24 native plant species which are indicated on the landscape plan below. Of these species, 16 survived. An Operations and Maintenance Manual was prepared highlighting the maintenance and progress of the plantings.
2005-2009 Initial Maintenance
At its inception in 2005, Portland Parks & Recreation along with the contractor J.D. Walsh & Associates monitored and maintained the park for four years, a critical time to keep weed competition low.
Native plant growth adage:
First year they sleep, second year they creep, third year they leap.
The ensuing years local friends groups emerged intermittently to partner with PP&R to maintain the park. Adaptive management necessitated replanting new species where others failed. During this period, areas of the park became overcome by invasive weeds and native plants diminished significantly.
In 2015 a new Friends of Tanner Springs stewardship group initiated a partnership with PP&R to restore and enhance the park. A field study by a Hoyt Arboretum botanist found remnant areas of native plantings reduced to 15%. It was determined that despite the additional time to hand weed it was a safer alternative for volunteers and pollinators than herbicide spraying.
Zone 1 Oak Savanna & Turf
Planting beds: Two Oregon White Oak (Quercus garryana) trees were healthy while one is struggling. Four Red Alder trees (Alnus rubra) are healthy and thriving. The initial understory plantings of Oregon Grape (Mahonia repens) under the Oregon White Oaks was replaced with Manzanita, Ceanothus and California Fuchsia (Epilobium canum) to infuse flowering plants in the planting bed.
Turf: Perennial Ryegrass (Lolium perenne) needed reseeding, edging and weeding; the edging had over-run the decorative cobble stones throughout the park.
Invasive plants: Planting beds and turf have Perennial Ryegrass (Lolium perenne), Annual Blue Grass (Poa annua), Crab Grass (Digitaria sanguinalis), and Colonial Bent Grass (Agrostis capillaris), Common Yarrow (Achillea millefolium), Common Dandelion (Taraxacum officinale)
Zone 2 Upland Prairie
Prairie: One Red Alder tree (Alnus rubra) is healthy while the second expired. Some prairie natives remained from the initial planting: Meadow Checker Mallow (Sidalcea campestris), Sitka Brome (Bromus sitchensis), Common Camas (Camassia quamash), Great Camas (Camassia leichtlinii), Meadow Barley (Hordeum brachyantherum).
Invasive plants: The majority of the prairie was 75% invasives–Velvet Grass (Holcus lanatus), Tall Fescue (Festuca arndinacea), Ripgut (Bromus diandrus), Orchard Grass (Dactylis glomerata), Tall Oat Grass (Arrhenatherum elatius), Dutch White Clover (Trifolium repens), Canada Thistle (Cirsium arvense), Bull Thistle (Cirsium vulgare), False Dandelion (Hypochaeris radicata), Black Medic (Medicago lupulina), Deadnettle (Lamium purpureum), Common Vetch (Vicia sativa).
Zone 3 lower biotope & Pond
Lower biotope: The biotope soil mixture designed to filter rainwater became a xeric zone. Adaptive management called for changing the plant pallet to more xeric selections from southern and eastern Oregon. Gum Plant (Grendilia integrifolia), Shiny-Leaf Spirea Spiraea (betulifolia var. lucida) healthy and abundant. The emergent zone became primarily Slough Sedge (Carex Obnupta).
Invasive plants: False Dandelion (Hypochaeris radicata), Black Medic (Medicago lupulina), Canada Thistle (Cirsium arvense).
Plant Diversity & Restoration
To restore diversity back into the habitat Friends and PP&R have planted and set seed for as many as nineteen new oak-prairie species.
400 plants added to the park 1st year (2015)
196 plants added to the park 2nd year (2016)
260+ plants added to the park including an Oregon White Oak tree (2017)
856 Total plants added to the park
71 species of plants in total (56 species of prairie plants; 15 species of aquatic plants)
19 species are new species to add more diversity