I just planted this Yoshino cherry tree late last year. But I have been wondering if I didn't bury it deep enough. Should I pile up more compost around the trunk where the red line is?
I think the answer is probably yes. Your red line seems to be at the place where the graft was made. Add soil/compost up to the level of the graft. It would benefit the tree to have a mulch of 2"/3" on top of this but be sure to keep the mulch away from the bark of the tree. The mulch will help keep the roots cool in summer, suppress weeds and retain moisture in the ground. Really soak the soil around the tree before applying the mulch unless it has just poured with rain. [A mulch applied on top of very dry soil can prevent subsequent water ingress as it effectively 'caps' the area.]
Many trees are grafted with the tree we want, [in this case Prunus yedoensis]; for flowers, leaf colour etc, onto a hardy/vigorous rootstock like Prunus avium. Whether the graft is buried or not can make a difference to the suckers that might then come from the rootstock. Generally, if the rootstock is below ground it can lead to suckers growing from the rootstock, which need to be cut off nice and cleanly, [and close to the trunk], as they are unwanted. Over time as the grafted ornamental tree grows and becomes dominant over the rootstock, the number of suckers will decrease.
Whenever I plant, I tend to dig a slightly deeper hole than I need, place the plant or tree in the hole and start to backfill. As I do this I jiggle the plant/tree as settle the soil around the roots/rootball, tugging it slightly so I can adjust the planting to get it just right. When I have got it wrong, I have been known to dig the plant up and start again to get it right. :-)
Thanks and good luck.
No, don't add compost or other similar material to the height of the red line (red line is actually the place of your graft, the place where the rootstock and the scion meet).
By doing this, you will risk that your tree develops roots from the scion (the part above the graft), which would defy the purpose of having a graft. In addition, any change of soil level around tree roots is a shock to the tree (for example, oxygen will not penetrate to the depths of the soil as before, and it is needed for shallow roots to function as they should).
A thin layer (up to 1.5 cm, or half of an inch) of compost would, though, be beneficial.