Can climbing up a tree prevent a creature from being targeted with Magic Missile?

Question

Is climbing treated the same as movement in a combat round? Will my Magic Missile still work, with all the monsters up a tree?

It's a scenario about being attacked in a dense forest, by monsters called Boggarts who fight my wizard character. They've lost a couple of rounds, and now they’ve decided they should try and evade the wizard (me) by climbing a tree.

Boggarts are about the size of a 10-year-old according to Wikipedia, and are mischievous and evil, in turn. I don't think they'd be easily missed even halfway up a Sycamore tree.

Answer 1

Climbing costs two feet of movement for each foot travelled.

The rules for climbing state:

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb, or a swimming speed and use it to swim.

So yes, the boggarts probably could climb up a tree with little issue, up to 15 feet if they have a speed of 30 feet.

Magic missile hits automatically if you can see the target.

Magic missile states:

Each dart hits a creature of your choice that you can see within range.

No attack rolls are necessary, you just hit, as long as you can see the target.

The DM determines what level of cover a target has.

It is likely that a target up a tree may have some degree of cover, due to branches or foliage blocking your view. The rules for cover state:

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.

There are three degrees of cover. If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together.

The degrees of cover are half, three-quarters, and full. A creature with half or three-quarters cover can still be seen, so neither of these would block magic missile. However, if the DM rules that the creatures had total cover, then you would not be able to hit them with magic missile:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

So depending how the DM rules on the level of cover provided by the trees, the boggarts may or may not be able to avoid your magic missile by climbing a tree.

Answer 2

In DND 5e, the fact that the target is up a tree will not by itself defeat Magic Missile as long as the target is otherwise in range. In fact magic in general and magic missile in particular is quite good for dealing with flying opponents, though you may need to pay attention to range.

Note that I said by itself. Magic Missile, at least in the 5e version, specifically states that it hits a target "that you can see". If the foliage is sufficiently thick that you cannot see the target then you cannot cast Magic Missile or any other spell that requires you to be able to see the target.