Don't know enough to answer your question, but from beginner experience can say that the baby thujas I've planted have done best in partial shade. They like water but not saturated 'lake' soil. Mine have also liked fertilizing with slow-release and especially wormcasting. Mine took about 2 1/2 years before they started really growing; they need to get established with frequent watering. They like to be misted frequently.

All my trees have done better in partial shade, even the eucalypti. The sun is just too scorching burning relentlessly hot here for babies.

You are lucky to get more cloud cover which also helps baby trees.

The young thujas are more scrawny than mature well-watered ones.

Some grow more finely branched, narrow crowns due to something about the local environment in some places. Along Hogan Avenue in/near Gresham, OR this behavior is apparently so extreme propagations from there have been made and dispersed as a cultivar 'Hogan'. (More recently Stanley & Sons have made other selections ('Brick' etc.) from the vicinity and put them on the market. As with at least some stock sold as 'Hogan' the variant growth is not fully maintained outside of the original environment.)

Others are genuine habit variants with a genetic basis. Those producing growth of this type appear to have been called hedging cedars in the Lower Mainland of BC. Old pseudo-species names like 'Atrovirens', 'Excelsa' and 'Virescens' are used for such, along with more recently coined ones including 'Green Sport'.

An interesting and unusual rare one is the unfortunately named 'Colvos Hybrid', a seedling selection of pure Thuja plicata with an arrowhead shape and almost monstrously thick shoots. The original seedling grew slowly and did not seem unusually bothered by the gloom produced by a Monterey pine that soon overtopped it.

Check with Colvos Creek nursery, Vashon, WA for availability.

Sounds interesting! I'll look into the Colvos selection. Thanks for the answers, both of you.

There is a different form growing along the Coal Creek Parkway south of Bellevue that is narrower than the type. I don't know if it's genetic or cultural. Near Mutual Materials. I grew a cutting once, but had to cut it down because it was in 'the wrong place'.

Such occur in various locations. I've probably been asked about those in and near Woodinville, WA more than once (seedlings line the freeway in some sections, like a hedge). It's apparently environmental. In much of UK it appears all cypress family conifers produce the Hogan cedar habit. Perhaps the most dramatic common example is Leyland cypress, the same clones looking nearly like columnar Italian cypresses or level-branched wide-spreading Monterey cypresses depending on where they have been growing. Tags on plants often show the columnar habit, the photos used perhaps having been taken in England.

the tree most likely grows lankier and longer in the shade by nature and denser and more compact in the sun. the same is true of sequoias.