Portland has volcanic/flood deposit soil and a mild climate so a lot of stuff will grow here (but we’re north enough to limit the season, so the Wilamette Valley specializes more in growing seeds and starts)

One thing though it lacks wildlife to nibble sprouts away or hard frosts so the main thing weeding out tree branches are strong windstorms from when ocean and interior air fronts sweep down the gorge and meet

So trees are given to get too bushy with tons of stubby branching while more elegant stuff gets torn off