Discussion: Mapping forest

In the orienteering map, the forest is white. But I have confused in what is the edge of the forest ? The edge is the trunk of tree ? or the end of leaves of tree ? or others ?

Good question. And not as trivial as some might suspect. On O maps, keep in mind that it is all about runnability: how fast can a runner cover the ground in question? In some situations, runnability might be compromised far beyond the trunk of the tree. :-)

But normally, the quality of the vegetation outside the forest would extend all the way to the tree trunk line itself.

I would say it the other way around. The edge of the forest is the edge of the tree canopy.

I learned as a kid the distinction between white and yellow, which technically have the same runability, also as a function of visibility (and shade changing clear visibility).
So as a fun way to remember, we learned: if sunshine on a sunny day can reach your shoes (mowed grass, bare land, etc.) it is bright yellow; if the sunshine can reach you but not your shoes it is light yellow (usually tall grass), and if the sun can not reach you, but you can see far then you're in white. We further differentiated the same way for green stripes and full green: if it is in the way of your feet but not you eyes, then it is striped.
The canopy extension also comes into play when you have sprint maps with single large trees having a white area around a single green circle symbol on a big yellow lawn.
Tree depictions on Sprint maps in Kell Sønnichsen's Sprint Mapping guide

There isn't a single answer that is going to work for all maps.
On a 1:10000 map, the few meters of difference that you are thinking about is meaningless, and you should think more about how it is going to be drawn in a clear way. At that scale the size of the symbols on the map represent much more area on paper than the object is in the real world, so positioning of boundaries and points is more about relative alignment and clear readability and meaning at speed rather than sub millimeter accuracy.

On 1:4000 sprint maps it starts to make more of a difference, but again you are pushing lots of things around to make room for the wide black lines for walls and fences, and it is more important the the veg boundaries line up with the man made features in a meaningful way than that they are positioned in an exact way.

I think about the answers to a lot of these questions in terms of how the drafting will look, instead of thinking of the map as a engineering survey. Everything is approximate if it makes the map make more sense to the runner.

Most maps I run on tend to use the canopy, meaning single trees in a paddock are mapped as white (in the shape of the canopy) rather than green dot or circle.

I agree there is no single right or preferable answer.
However I think the principle of " let the ground vegetation (or under-story) dictate" will best address most situations.
This is where the eyes are, as well as the runnability distinctions.
Map the change. If no change, then canopy,
This principle applies to the mapping of most area features, whether they be blue, green, yellow, brown, grey, or ...

...and not all canopies are born equal. High wispy canopies can be ignored, for mapping as area or point features, while low lush canopies indeed function as understory, running and visibility-wise.

A translation for Australia.
if sunshine on a sunny day can reach your shoes it is bright yellow or white;
if the sunshine can reach you but not your shoes it is light yellow or white,
and if the sun can not reach you, but you can see far then you're in another country.

Most of the comments I have seen here make sense for sprint maps but not so much for forest/ISOM:

Degrees of green has in theory nothing to do with forest/not forest, just what the average running speed and visibility is, but in a forest this is determined by the ground/ground cover and vegetation.

At the edge of a forest it is extremely common to have dense green for a 1-3 meters, where all the trees are trying to place their branches/leaves so as to get maximum sun, but once you pass that boundary the forest behind it has almost no understory and should be mapped as white.

ISOM has lots of rules for the minimum sizes of various vegetation objects, selected so as to make the resulting map as readable as possible, but this means that you cannot map individual trees or copses, except as special vegetation symbols.