Answer: A partially shaded western exposure can be difficult due to the heat of the afternoon sun that does reach the plant, and when the shade is provided by trees sometimes there is an overly dry soil due to competing tree roots. Knowing this, it may take some experimenting and careful attention during the establishment phase to find a plant that can do well for you in that location. A Japanese maple, with its requirements for an evenly moist yet well drained, humusy, acid soil and shelter from hot sun as well as wind -- although a lovely tree -- would most likely find that kind of setting difficult.
You might consider Hydrangea paniculata, a large shrub which can be trained into a tree form if desired. This plant blooms on new growth of the season and so can be pruned in early spring as needed to control or direct its growth.
Another possibility might be the sweet autumn clematis, a vine that can be trained onto an attractive pillar or trellis. This would give the vertical visual effect you need without taking excessive width. This plant is very hardy and also blooms on new growth of the season so can be pruned as needed in early spring to train it. Although it is a clematis with fragrant white blooms in the fall, it is not a "fussy" plant (as some of its relatives can be) once established.
Another possibility is the shrub dogwood, Cornus sericea, an adaptable plant appreciated for its colorful twigs. This plant can be a bit rangy and needs to be trimmed back in late winter/early spring to produce the best twig coloration, but it is another one to consider. The variegated foliage types can be particularly ornamental in some settings. This shrub tolerates a wide range of growing conditions.
If the location is not overly dry, you might enjoy Virginia Sweetspire or Itea virginica. This plant is a summer bloomer and provides good fall color, and there are dwarf selections avilable that would fit the space you have.
I hope I have given you some ideas, your local county extension may also have some suggestions.
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