Unknown tree in my garden, not looking so good. What is the tree and is it salvageable?

Question

So, this is Western Europe in May. It has been a dark and wet up to now. Currently we got some clear skies but it is cold.

Now I have this tree:

It is planted in soil that is mostly clay-ish and quite wet. Although last summer was exceedingly dry and hot for three months to the point where the grass was starting to die off and the soil started to crack.

The leaves do not look healthy, and some look straightforward undeveloped:

Although at the top of the tree there is a branch with nice green leaves (you can see it in the first photo, too):

I'm not even sure what tree it is, but the white-edged leaves are a normal feature if I remember well. It looks a bit like a Cornus Alba, if a Cornus Alba were a tree, not a shrub (dammit Jim, I'm a IT guy, not a botanist). But then it would be in the wrong kind of soil altogether.

Here is a close-up of the trunk. Maybe it's fungus. Other trees in the garden are also subject to lichen, except for the birch. It's shady and wet around here.

Answer 1

This plant looks a lot like Acer Negundo variety Flamingo. The variegation is normal but sometimes branches will revert to type which is plain green leaves. Once that happens they tend to take over as all green leaves photosynthesize more efficiently than multi colour ones.
I recommend getting an arborist in to remove that branch before it dominates and becomes the central leader.

The lichen is natural and does not harm the tree.

Now the funny thing is that in North America this tree is known as the Manitoba Maple or Box elder. It does have the advantage of being tough and growing fast but if you found one growing in your neighbour's lawn you would point out what a pest it is and recommend removing it as fast as possible. In some people in spring the pollen will be an allergen.

Growers won't let a species with a few problems go and now we have many cultivars of which you have one. The species is now considered invasive in Central Europe and Australia.

Your experience with this tree may be different. Homeowner's in Canada here usually notice the truly amazing amount of seedlings growing just about everywhere and that after cutting it down the tree sprouts up again. Chipping the stump down a few feet resolves that issue.