Thursday, February 05, 2009

Cherries and Other Cherries

Road Edge Thicket of Choke Cherry

Everybody loves cherries. Good to eat, pretty to look at when they bloom - especially the fancy oriental varieties. Its a commercial crop that brings high prices to the orchardists of the American West. A staple of our diet, they add flavor to martinis, chocolates and soda pop.

The sweet cherry is derived from Prunus avium, the bird or fruit cherry, a widespread Eurasian species. Here in the Pacific Northwest we have two native species, choke cherry (Prunus virginianum demissa) and bitter cherry (Prunus emarginata mollis)., Both are sun-loving pioneer species, one of which grows large enough to survive in forests, at least until the conifers get large. Unfortunately our cherries are as good tasting as their names imply, meaning don't even try.

Pictured above is a thicket of choke cherries, the smaller of our native cherries. Lining a road and enjoying the sunshine, this suckering tree-shrub does not extend into the dark shaded North-slope forest behind. The larger bitter cherry, Prunus virginiana demissa can pioneer on streambanks and come up thick after burns or logging. As the other trees overtop the cherry, it dies and soon falls to the forest floor, the bark outlasting the wood.

Fruit and Choke Cherries side by side

Here the wide branching deciduous tree down the hill behind the smaller choke cherries is a naturalized bird cherry. It seems this Eurasian import finds our area to its liking. With home and commercial orchards providing bird-dispersed seed sources, as well as hikers chomping on cherries along the trail, this should come as no surprise.

Like any introduced plant that is able to naturally propagate itself and thrive in our climate, the bird cherry has the potential to alter the habitat it finds itself in. Some introduced plants, like the laurel daphne (Daphne laureola), survives well but will never seriously compete with our much taller growing native shrubs. Others, like ivy or Himalayan blackberries, can dominate an area at the expense of most of the native flora. How does the bird cherry measure up on a scale of "invasiveness"?

The next photo shows its potential on a site favorable to it.

Bird Cherry Thicket, Little Woodrose Nature Park, Tualatin

The picture was taken in Little Woodrose Nature Park in Tualatin, Oregon. This site is unusual in having a very warm sandy soil, and that extra underground heat is very encouraging to bird cherry.

Every stem visible in this photo is a bird cherry. The young seedlings are very shade tolerant, unlike our native cherries, and can hold their own indefinitely under a mature forest while slowly shading out most or all of its competition, both shrubs and trees. Young shaded trees often take on what the forester calls a wolf form, spreading out more that up, so that it more quickly blocks the light from the plants around it. Open grown trees shoot up quickly, and then begin to spread.

Bird Cherry, Cook Park

The invasive bird cherry can develop a massive broad crown. Under forest conditions it forms a strong central leader like this one but in more open conditions the leader can branch strongly and the tree becomes much more spreading, as in the following picture. Seedlings in deep shade can form thickets, especially in warm or disturbed soils.

On cooler, forested habitats the bird cherry tends to grow scattered throughout the stands, with the thickets seen on warmer soils rare or absent. It is not as large as our native bigleaf maple, but it is more shade tolerant, giving it a competitive advantage. It keeps lower limbs longer than most native trees.

Bird Cherry, Little Woodrose Nature Park

The potential range of bird cherry is not fully known. Its been found naturalizing from Vancouver Island on down into California. In its native haunts it grows from the mountains of North Africa to Scandinavia, Turkey, Ukraine, Poland and east to Iran. This would suggest wide adaptability. A good guess would predict a range from California well into British Columbia, and east into the Ponderosa pine belt, where late frosts would limit its ability to reproduce. In the mountains it likely could spread into the lower snow belt, 3,000 to 3,500 feet in Northern Oregon. All this is an estimate and will need to be verified by observation.

Cherry Thicket Forming, Cook Park, Tigard

Management Issues

Large older bird cherries can send up seedlings readily if land is disturbed or cleared around them, as is the case with this one in the bottomlands of Cook Park, Tigard. All the smaller upright stems in the picture above are young bird cherry seedlings. Park development provided the opportunity and the invasive bird cherry took advantage. Thank you very much!

Bird Cherry adaptation in select natural areas in the Portland area:
Little Woodrose Nature Park, Tualatin - dominates understory with many large older trees, mid-sized thickets, and thickets of saplings.
Cook Park, Tigard - Scattered older trees with some continued reproduction in various habitats
Mount Talbert, Clackamas - Many large older and mid-sized trees with some continued but scattered reproduction
Cooks Butte, Lake Oswego - Spread limited in this park.
West Linn Wilderness Park - Very few present, with limited new saplings.
Powell Butte, Portland - Many trees of all age classes, but prejudicial management has begun here.

So bird or fruit cherry is a highly invasive species already widely established in our area. It comes with the potential to alter extensively most land up to moderate elevations, and to completely dominate warm soil areas to the potential exclusion of most native trees and shrubs. It is much more common than native cherries.

Ivy and Bird Cherry Equals Lost Native Habitat

One of the problems in management of cherries is lack of awareness of the situation on the ground. It is widely assumed that cherries are native and to be left alone. This is fine when true, but managers need to be aware of what they have in their natural areas - which means that the ability to identify the three types of cherry met in our area is a crucial management tool.

The heavier root system of the invasive cherry is often a help in identification. The natives are lighter and smaller in all respects and will not show this much root. Bird cherry has thick, brittle stems; the native light, flexible stems. All three species are able to form thickets, and the bird cherry does this both by seedling reproduction and root suckering, and the natives do so by extensive suckering with only a few seedlings.

Heavy Bole of the Bird Cherry

This bird cherry trunk is about the maximum size of the native bitter cherry. However, this bird cherry is still a youngster, and reports indicate a potential trunk size up to five feet across. It is a rapid grower and may reach that size in a century. Some of our biggest examples are farmyard orchard trees of that age.

Suckering Choke Cherries

This choke cherry patch contrasts with the heavy-boled bird cherry, and is also smaller and lighter than the bitter cherries here in the valley.

Maple and Bird Cherry Leaves on the Forest Floor

The leaves of the bird cherry are twice the length of the native species and sport deeper teeth along the edges. The bird cherry also will take on some red fall color, which is absent with the native cherries.

The first active management of this invasive I observed is at Camassia Natural Area in West Linn. There The Nature Conservancy put this cherry to use lining the trails. Bird cherry removal is underway at Powell Butte in Portland. The chain saw is the preferred tool. Long term vigilance is crucial, as young seedlings can be missed and new ones will come along. Resprouting from low-cut stumps should be limited, and cherries as a group do not resist decay once cut.

The bird cherry Prunus avium is a habitat-altering invasive. It is able to form thickets in sun or shade to the detriment of native trees and shrubs. Unlike most larger trees, its shade tolerance and retention of branches in forest conditions allows it to compete with not just trees but smaller shrubs as well. It spreads easily from seed produced by both naturalized and orchard trees. Effective forest management in parks and other natural areas should include removal of this tree.

Click here to view more pictures and a tabular presentation of cherry characteristics used to distinguish the locally occurring species.

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