Many trees like their roots far away from anything saturated, which is the bottom of the pot. Two in particular, pines and azaleas. And in muddling about the Western bonsai world I’ve been haunted by the number of pines planted in very shallow containers.
‘Rules’ are slippery things, as anything in bonsai has exceptions. So consider this a ‘slippery-rule’, a ‘you might want to consider’, and not necessarily a ‘darn it you’d better do it or get swatted with a bamboo chopstick’ sort of offering—-but please, in general, get your conifers, particularly pines, in deeper pots, and your deciduous might go in shallower ones because you can get away with it horticulturally.
Now the disclaimers. Breathe in. (That’s for me).
You can certainly plant your pine in a shallow pot (loud thwack of a chopstick on my fingers), but consider mounding it. Mounding and getting live moss established will help greatly in drying out the root ball the way a pine wants it. Mounding helps drain excess water, and moss prevents excessive drying of the surface, which might sound like an oxymoron but it works.
The happy zone of any tree is fairly specific. The top often dries out a lot. The bottom might stay too wet. And the pine likes the middle zone. So most pots for pines should be deeper to broaden that happy zone. Or, if you mound a tree by 3″ above the lip in a 3″ pot, then you’ve essentially given the tree 6″ of growing space and a fairly broad happy zone. Shallow pots for conifer bunjin, clumps, and forests are good examples where mounding is appropriate and often used.
Without an erosion free and water retentive surface, however, like live moss, mounding is unlikely to be a happy equation. Search for ‘moss’ on the search field in the upper right of my blog, and you should find some moss articles there and how to establish it. I think I wrote a few.
Happy potting! Or resting, if you’re in the Southern Hemisphere.