Since quite a few now also mention agro use within forests: that’s an interesting added value. But the original thing I meant is a very widespread light forest type, that we have in mountains, that are quite often not used by anyone. While I enjoy more uses mentioned in the forest department, I hope this does not sink under water 
There is wood:density=, and was an idea of canopy_density=. The latter more obviously differentiates between number of trees and crown coverage, which can be further specified as cover or closure, being affected by height and other factors. The former needs to be compared with basal area.
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I do see a need for discrimination between classical wood with closed canopy, open wood and grassland with scattered trees not only in mountain areas but also in the subtropics (tree savannah) and in human mangaged areas like wood pasture.
I’m very reluctant for introducing new first level tags, I rather prefer adding information by second level tags. First level tag should come from predominant classification, here natural=wood or landuse=forest, second level tag can be something like scattered_trees=yes or wood:type=tree_savannah. It may even be useful to have a natural=grassland together with e.g. scattered_trees=yes when there is less then a quarter tree coverage but more than a few trees.
There is of course a considerable ambiguity between scattered trees and closed canopy, but that is unavoidable when trying to bring the real world into a classifying scheme and is more or less common to all tagging.
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Generally, unspecific adjectives like “light” or “soft” or (as suggested recently) “demanding” don’t make for good tags. Everyone has their own definition and it tends to just encourage endless rounds of iterative wikifiddling.
If you can find a reasonably common noun that defines what you mean, that would help.
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I have been wondering if “light forest” might be causing confusion through translation.
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Agreed, as mentioned before, and I also like the idea of “scattered_trees=yes” for this issue.
natural=wood / natural=grassland / natural=scrub as well as landuse=meadow could all go together with scattered_trees=yes to come as close as possible to the real OTG situation. A simple and pragmatical solution imho.
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I agree with the fact people interpret such adjectives themselves or won’t use them at all, since it confuses them more than it helps. Difficult to do by just words. You could in theory specify this with a density of “X to Y trees per square meter”, but nobody will use this, really.
The “light forest” is something that clearly has forest character. It’s not a meadow with a few trees, there are still too many to map them individually. That’s the best guide I could come up with, if we want to stay simple.
I’d rather go with examples/images as an aid in addition. Especially in the mountains, this will all be couchmapped by aerial fotos, since a lot of the areas are not even accessible. That’s why I think examples of aerial imagery are somewhat important as well as ground photos.
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FWIW there is a documented tag for meadow orchards specifically:
Here a nice example in the High Tatra near Zakopane, roughly here. Pinus mugo scrub with scattered Willows, Larch and Spruce. I would think the trees survive until there is a large avalanche, which may happen every few years (a similar situation exists on the N slopes of the Feldberg)
The “light forest” is something that clearly has forest character.
by soil composition? Animals living there? Microbes? Humidity?
There are various definitions for wood, the FAO (UN) says tree crowns must cover at least 10% of the area and size must be at least 5000 square meters, which seems a reasonable lower end for both properties
Yes, and there is also landuse=meadow & meadow=meadow_orchard depending on the predominant use.
In this case a differentiation is even more difficult because predominant use can only be achieved by OTG knowledge.
I suspect this diagram from Ellenberg Vegetation Mitteleuropa mit den Alpen (6th edition, p. 684) may clarify. I assume you are referring to what the diagram calls “lichter Wald”.
Note that, somewhat to my surprise both Baumgrenze and Waldgrenze are given as standard terms in my English Dictionary of Ecology, which also use Krummholz and Kampfzone for the area between these two lines.
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I’m inclined to try to introduce a new top-level tag (knowing it would be a hard sell to the community). See the above image of an African tree savanna – those areas are vast and should get a dedicated rendering. And apart from dehesas and alpine forests, I believe similar landscapes exist in other parts of the world. The tagging problem is currently worked around by not mapping almost any land cover in Africa. 
These can be tagged with natural=grassland + grassland=savanna according to the wiki. New top level tags are always somewhat of a horror for data consumers. (And conversely, if software support for a tag is miniscule, there is little chance that it is widely adopted.)
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If, as wikipedia asserts, savanna covers 20% of the world’s land surface it surely deserves a separate natural tag. However, the other options discussed, of having tags for canopy cover/closure and tree density, would apply equally well to this as a distinct natural tag, or if it was desired to map it as a subclass of natural=grassland or natural=wood. (sorry 'cos I was searching using a terminal “h” I didn’t see the grassland tag)
Most of our terminology for natural features, and particularly vegetation, is inherently boreo-temperate. Tagging is not even particularly good for types of vegetation encountered around the Mediterranean.
It may well be worth creating a separate thread for discussing savanna.
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A useful picture. It’s a bit more difficult to decide for the borders of these areas on an airfoto, but that’s something we would have to live with. It is still better, than when it’s not existing, though. But I see the implementation seems to be quite variable in ideas.
An example image of flyover photography in Switzerland:
The Red line surrounds dense(r), normal forest (I was a bit sluggish … the “south” has a bit of light forest included). The blue line surrounds a light(er) forest. Way to many trees to mark individually, but very clearly spaced much more with an alpinegrassland substrate they stand on. This spearation can be done fairly clearly … which I find such a tag or subtag (and also distinct rendering/border in ID editor etc) very useful.
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FWIW I did look at the usage of that tag in UK/IE - a few normal orchards, and a few normal meadows.
It may be used more consistently elsewhere, but wasn’t much use to me.
Somewhat related:
In orienteering there is a thing called runnability which usually describes how dense the vegetation is
https://wiki.openstreetmap.org/wiki/IOF_mapping
While density of trees does not equal runnability, maybe it may inspire you somehow. Also, forests may have a directional runnability - easier to traverse in one direction than the other.
An example: see how the forest color is white, light green and darker green showing runnability.
So if new tagging is to be invented, maybe it should not be an boolean yes/no solution, but have a gradation of density?
Here, grown planted wood (we call it Hochwald, all trees same age, ~40+) might be very runnable/walkable, yet the canopy is dense. To capture such, two gradients might be warranted? Canopy-density, value as percentage from aerial, and underbrush-density? Values for the latter, in order of traversabilty, not height of vegetation: fallen_needles|blueberry|bushes|blackberry or some such?
Ecologists tend to talk about the field layer (one of several layers in a wood: ground, field, undercanopy, canopy, emergent). The classic bluebell wood in Britain has no field layer, but many woods have a thick field layer (bracken, brambles etc). This is usually captured in vegetation classifications where the various plants in ground and field layers discriminate between different kinds of wood with the same dominant trees, but I have long thought it might be appropriate to have a distinct tag for these. Use cases: pretty woods (bluebells etc); runnability etc.
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