Taller trees are in the Linear Arboretum’s future

Lidar is a newer technology that from space can measure the height of a tree here on earth. Researchers at Portland State University have published an online map of Portland that shows  the heights of all trees over 50 feet. Called Canopy Analytics, the map let’s people sort trees by height and whether they are in the public right-of-way or on private land, or are evergreen or deciduous.

Explore the tool here.

Using the map, we see that the tallest trees in the Ainsworth median between Martin Luther King, Jr., Boulevard and Fernhill Park are only 53 feet tall. The 31 blocks along Ainsworth form the Linear Arboretum. Considering the median has ample room for even the largest approved street trees, why are there no taller trees there?

The answer lies in the past. In the post-World War II years Norway maples were chosen to be planted in the median because of their ability to survive in tough urban environments. Survive they did, although advancing age is now thinning their ranks. Norway maple’s typical mature height is only 40 to 50 feet. Only two of the more than 100 Norway maples planted in the median have grown taller than 50 feet. One just west of NE 16th is 51 feet tall and the tallest of all at 53 feet is just east of NE 28th.

<< Pictured at left, Norway maples in the Arboretum median.

The most common tree in the Linear Arboretum is Norway maple, a tree that usually grows 40 to 50 feet. The tallest currently is 53 feet. As the trees are old and declining, they are not expected to grow any taller.

In the 1980s an attempt was made to diversify the median using three different tree species. One of the three – Raywood ash (Fraxinus angustifolia ‘Raywood’) – usually doesn’t get any taller than Norway maple.

A second species – European beech (Fagus sylvatica) is generally taller than Norway maple, with 50 to 60 feet being an average mature height. In good soils, however, some specimens can reach 80 feet or more. Unfortunately, 80 percent of the 20 European beech that were planted were killed by sunscald on their thin-barked trunks and one was knocked over by an errant pickup truck.

The most successful of the three replacement species have been the northern red oaks (Quercus rubra). Planted in two waves, the tallest from the 1980s planting is now 53 feet tall. It is a magnificent tree just west of NE 33rd Avenue. Northern red oaks typically reach 60 to 75 feet tall at maturity. They can grow much taller in favorable conditions.  The tallest known is a 160-foot tree in Tennessee.

Pictured at right, northern red oak in fall >>

Since the 1980s a number of Northern red oaks (Quercus rubra) have been added to the Linear Arboretum. This tall-growing tree has already become as tall as the tallest Norway maples it is replacing and promises to grow even taller.

Why does height matter? Because the larger the tree the larger the amount of environmental services it can provide. So the planting of taller trees should improve the amount of shade, pollution filtering, noise buffering and rainfall capture for residents living along Ainsworth. 

Since 2005 attempts have been made to further diversify the Linear Arboretum. Many of the species chosen should grow taller than the Norway maples and Raywood ashes they are replacing. One example is the Kentucky coffee tree. Two male clones of this tree from eastern North America have been planted in the Arboretum. ‘Espresso’ will reach at least 50 feet or more. ‘True North’, which was selected in Minnesota, is expected to be even higher, up to 70’. The median’s lone Japanese pagoda tree (Styphnolobium japonicum) could get close to that.

A wide variety of oaks has enriched the species mix in the Ainsworth median. Most are species that will grow to 60 feet or more at maturity. Forest Green Hungarian oak (Q. frainetto ‘Schmidt’) is a European species known to reach 60 to 80 feet tall. Oregon white oak (Q. garryana) can attain 75 feet or more. Bur oaks from the Midwest (Q. macrocarpa) soar 70 to 80 feet tall. Willow oaks (Q. phellos) have been known to grow as tall as 90 to 100 feet, with national champions above 120’. Oregon State University cites the height range of the evergreen holly oak (Q. ilex) as 60 to 100 feet, although other references put the upper range closer to 70 to 80 feet.

Some species are so new to cultivation that it’s not certain how tall they might get. The first evergreen silverleaf oaks (Q. hypoleucoides) to be planted in Portland only went in the ground in the 1990s. While wild trees in their dry, rocky mountain homeland in Arizona might only reach 30 feet in height, the species has proven a fast grower in the wetter climate and better soils of Portland. Some silverleaf oaks here have already reached 35 feet less than 20 years after planting. There are signs they could equal some of the tallest recorded heights for the species, in the 60 to 65-foot range.

The right-of-way planting strips on the north and south sides of Ainsworth street are also part of the Linear Arboretum. Not surprisingly the north planting strip is home to the Arboretum’s tallest trees, since there are no overhead powerlines on that side of the street. Here we find a few remnant silver birches (Betula pendula) on the blocks east of NE 33rd Avenue. These range from 58 to 61 feet tall.

<<Pictured at left, a set of Acer Saccharum

Some of the tallest trees in the Linear Arboretum are in this quartet of maples, one of which is above 70 feet.

Most evergreen magnolias in Portland are a modest-sized, winter-hardy southern magnolia clone (Magnolia grandiflora ‘Edith Bogue’). However, the north planting strips boasts a much taller southern magnolia at 2721 NE Ainsworth that checks in at 50 feet. A few blocks away at 2099 NE Ainsworth is the tallest conifer in the Linear Arboretum – a 50-foot Scots pine (Pinus sylvestris). Taller still but sitting  outside the Arboretum in a front yard at 3269 NE Ainsworth is an Atlas cedar (Cedrus atlantica) towering to 118 feet.

Two native bigleaf maples (Acer macrophylla) just east of NE 23rd are a respectable 60 and 64 feet tall. Two other maples just west of NE 17th are 68 and 73 feet tall respectively. The tallest maple in the Linear Arboretum grows at 251 NE Ainsworth. It measures 75 feet tall, making it the second tallest tree in the collection.

The tallest tree of all in the Linear Arboretum is one of six sweetgums (Liquidambar styraciflua) growing between NE 32nd and NE 28th. Their average height is 64 feet. The tallest, a tree at 3207 NE Ainsworth, soars to 77 feet. These sweetgums may grow taller still, as they are still relatively young. Mature trees in the eastern U.S., where sweetgum is native, are reported in the 80 to 120 foot range!

The tallest trees on average in the Linear Arboretum are sweetgums (Liquidambar styraciflua) like this one near NE 37th.