The resilience of nature at Tanner Springs Park is a surprise for visitors–especially those who enjoy seeing an Osprey hunt or a duckling’s first swimming lesson. The transformation from a brown field to a thriving stepping stone habitat for wildlife emerged from ecological planning, diverse native plant support and ongoing adaptive management. Hidden from view is the struggle between native plants and animals with introduced invasive species. Second to habitat destruction, invasive species are the largest threat to biodiversity. At Tanner Springs Park both plant and animal invasive species are a dilemma. Here is a glimpse of how using adaptive management Portland Parks & Recreation maintains the park’s habitats.
Challenges of Introduced Exotic Fish
The wetland system of recirculating runnels and settling pond was initially designed to function as a natural pool–self filtering through its own biological organisms and plants. Maintenance on a natural pool consists of periodic vacuuming and plant maintenance. Despite educational information, the design was undermined by the addition of exotic fish by the public. Periodic vacuuming by PP&R ceased due to the addition of fish and an ultra violet water treatment system was added to reduce the increased algae and bacteria. The addition of carp pose habitat problems for Tanner Springs Park as well as for other natural area parks when eggs are dispersed from migrating waterfowl. Carp are notorious for destroying the ecology of any body of water they inhabit. Ammonia from fish excretion and algae decomposition can combine to reach levels that can kill pond organisms needed to filter the pond water. The management of the pond has been a difficult process and has yet to be resolved.
Challenges of Exotic Invasive Plants
Willamette Valley native plants are adapted to each other and the conditions in this region for thousands of years. However, they are not adapted to the invasive exotics that are introduced. They can be outcompeted and overrun very quickly. When this happens, mono crops can be established, which limits food and habitat for wildlife. Their fruits and seeds are brought into an area many ways–on people, animals, contaminated soil, wind or water. Exotics have many strategies to overcome native plants–germination frequencies, excessive seed production and disbursal mechanisms, aggressive vegetative growth and chemical dispersal. There are several methods to remove invasive plants but none without limitations. Many of the herbicide sprays have negative side effects for wildlife, people and the environment. Hand pulling is costly, time consuming and must continue until all the seeds in the surrounding soils have germinated.
Invasive species cost Americans about $143 billion per year. At least 30 new potential biological invaders enter the U.S. every day.
2005 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. There is a saying about the growth behavior of native plants:
First year they sleep, second year they creep, third year they leap.
The park is designed with three zones, each with very specific ecological functions and maintenance needs. Controlling exotic invasive plants and animals presented problems from the park’s beginning. The initial park design incorporated 24 native plant species which are indicated on the landscape plan below. Of these species, 16 survived. Over the four years, an Operations and Maintenance Manual was prepared highlighting the maintenance and progress of the plantings.
In the ensuing six years, local friends groups emerged intermittently to work with PP&R to help with park maintenance and habitat restoration. 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.
2015 Assessment and Maintenance
In 2015 the current Friends of Tanner Springs volunteer group began partnering with PP&R to restore and enhance the native plant community that had collapsed in several areas of the park. It was determined that hand-pulling was safer for volunteers and pollinators than herbicide spraying despite the additional time frame needed to reduce weeds. An assessment was done with the help of a botanist from Hoyt Arboretum to determine the remnant areas of native plantings and their health.
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