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Mercer Slough Nature Park — November 2001

By Fred Weinmann

A wetland walk with many habitats: disturbed wetlands, farmed wetlands (blueberries), natural wetlands, open water habitats and upland forests. Undisturbed portions of the site (mostly beyond the bridge) are wet and interesting throughout the year because the water levels stay quite high as they are controlled by the Ballard Locks.

The site and its trails can be accessed from either the west or east by road and up the middle by water. Perhaps the most convenient approach is from the South Bellevue Park and Ride so I will describe the hike from there. From the SE corner of the P&R walk past the domestic hedgerow on a bark trail and down the wide and obvious path. The trail quickly passes into shade under several species of willows. This is a good opportunity to begin your review of willow identification based on twigs and a few remaining leaves. Find the three common species: Salix lucida (=lasiandra) (Pacific or gland willow; there are wonderful example of old growth trees having diameters exceeding 12 inches), S. scouleriana (Scouler's willow) S. sitchensis (Sitka willow). Later during the hike you will encounter Salix piperi (Piper?s willow) and possibly S. geyeriana (Geyer's willow), but the latter has not been confirmed.

After struggling through willow taxonomy (which may take a while) continue through a mixture of disturbed habitats with Spiraea douglasii (Douglas spiraea) Phalaris arundinacea (reed canary grass), Scirpus microcarpus (small-fruited bulrush), Equisetum telmateia (giant horsetail) and a wide variety of both native and non-native species which reflect the dry/wet/disturbed nature of the area. On the left will be large patches of the late-blooming/non-native/noxious Polygonum cuspidatum/sachalinense (Japanese/giant knotweed). I slash this name because I have just learned from botanist, Peter Zika that most of the populations we see are probably a hybrid of these two species with leaves intermediate in size and exhibiting other technical characteristics that are intermediate.

Continue on the trail eventually crossing the bridge over Mercer Slough. Spend a minute looking for late blooming Myosotis scorpioides (water forget-me-not) or possibly a few remaining blossoms of the Impatiens ecalcarata on the slough margins and Potamogeton spp. (pondweeds) in the open water. After the bridge continue to the end of the boardwalk to a trail fork. Keep left to make a clockwise loop (following the sign which says Bellefields Loop) through several different communities representing examples of forested wetlands. Many fine examples of Populus balsamifera (=trichocarpa) (black cottonwood), Rhamnus purshiana (cascara) and Fraxinus latifolia (Oregon ash); these are joined by the ubiquitous Alnus rubra (red alder) and a wide variety of shrubs and herbs. The dominant white-barked birch of the area is European silver birch (Betula pendula). Note the drooping branches and deeply cut black V?s at the juncture where the large limbs join the trunk. This is the only non-native wetland tree that is common in our area and is the most often seen birch of wetlands in and around Seattle and suburbia. There are two (often unnoticed) native birches along the trail. One of these is Betula occidentalis (water birch) generally thought of as an eastern Washington species but is native (or naturalized?) in the Mercer Slough Natural Area. The second is Betula pumila (swamp, scrub or peat bog birch; also known as B. glandulosa var. hallii). This species usually grows as a small shrub, but along this trail are examples up to 15 foot tall with a main trunk up to five inches in diameter. To tell these species apart, look for the pubescent and glandular twigs of water birch and the glandular but hairless twigs of swamp birch. They both grow in moist to very wet habitats.

As you continue a clockwise route you will pass through areas which are obviously boggy in nature as the trail quakes beneath your feet. Watch for our native crabapple, Malus/Pyrus fusca, huge lady ferns, Athyrium filix-femina and equally large skunk cabbage (Lysichiton americanus) as well as many other species which typify our wetlands. In an obviously boggy site, take time to examine the soils. They are a fine example of a peat soil? bright brownish/red with undecomposed plant fibers.

At the next trail fork, keep left to take the upland route or straight ahead for the wetland route. The wet route could be named skunk cabbage lane. Either way leads shortly to a stream crossing and an open grassy area, the old archery range. At the far end of the range make a choice [admiring the large Thuja plicata (western redcedar) tree with DBH of 52 inches] while deciding. Keep right to complete the Bellefields Loop, then back across the bridge the way you came. Keep left and up the hill on the gravel trail for a longer loop back to the P&R. This trail will continue south to I-90, turn right and continue west across a large wetland and eventually a foot bridge across Mercer Slough. Keep right on the blacktop trail and pass by a wetland mitigation pond with Osprey pole (unused as of last inspection); continue north on trail along Bellevue Way to P&R.

When not studying willows or puzzling over birches or admiring the large examples of native trees, or digging in the soils, look around for over 100 other species of native plants and over 30 non-natives which have naturalized in the Park. Some species that bloom into late fall include Tiarella trifoliata (foamflower), Veronica americana (American speedwell), Bidens cernua (nodding beggar's tick) and Geum macrophyllum (big-leaved avens). If the birds haven't finished them off there will be berries on several species (Lonicera involucrata, Sambucus racemosa, Viburnum opulus, Sorbus aucuparia, etc.) and nice fall colors on the leaves still remaining on the birches, maples, ash and cottonwood.

To get there: Take I-90 to the Bellevue Way exit. Continue north for about a mile to the South Bellevue P&R. For closest access park in the SE corner of the lot.

Updated: July 3, 2016
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