Turn to the foresters, saying: "This tree is only 4 years old. Have you heard of any effects which could cause that? There is clearly something more than a small aura that is odd about this area. Have you seen anything else that might help tell what it is? Before I start randomly poking the ground?"
One of them shrugs, "Nothing normal. You the magus, so magical stuff is really your area, not ours."
The other one adds, "Every stuff else around here looker normal enough. Even thee birds they don't look so special. Just many of 'em, is all. Many chicks as well, looks like. Usual, birds lik' that, they lay two, maybe four eggs. But from her', looks like more of that then 'em." Both men are speaking Low German with a thick Saxon accent.
(If I am understanding the descriptions, something around here is producing Creo vim.)
Look around for the water source that waters the tree, and probably also provides most of the water the birds drink. Have the grogs keep an eye open for any other animals, as they may have grown surprisingly large. I will try to look in the crevasse first, as that is the most likely place. If I find a water source, look around for anything unusual to indicate where the vis coming from (I need to determine if it is in the water or if there is some source that is being absorbed / dissolved by the water.)
(As far as I can tell, there is no spell for the situation "somewhere here is a vis source" other than taking each possible source and testing it. And if the tree produces berries once a season that have vis, but they have been eaten for summer, I don't know any way to tell that. Just checking that I am not missing some extra approaches. And yes, I understand I may be barking up the wrong tree.)
(A Creo source of vis being the cause is certainly a possibility.)
There doesn't seem to be any obvious water source, aside from the moisture gathering in the notch from condensation and occasional rain. Certainly no stream or river in there.
(The easiest general way to detect vis source is to detect the vis they produce. But, depending on the source, the vis can evaporate (or be consumed) relatively fast, so you are correct that there isn't any one specific spell to locate a source. Some only manifest themselves when specific conditions are met -- time, location, planetary alignments, etc.)
(On the other hand, an InVi spell using a sensory target can be quite useful in finding vis amongst a target-rich environment. You could "smell" the vis, or "hear" it, or "see" its emanations. These sensory targets would help you locate the vis, although depending on the sense being used and the environment, each sense can be more or less effective. See ArM5 pp.113-114 for using magical sense in Intellego spells.)
(If I am reading this right, giving myself a sense makes the range personal, the target the one from the description, and the duration would seem to be Concentration. Assuming I have that right:)
I will give myself temporary sight to see Vis, to determine if there is a visible source of Vis. Base 1, +1 for concentration, +4 for sense vision is a base of 10. Intellego 15, Vim 5, Stamina 2, for 22 plus minor aura and stress die. If that shows nothing try hearing (one magnitude easier, but may be able to be heard through obstacles that block sight, harder to pin down the direction.)
Herman casts his first spell and spends a few minutes scanning the area for raw vis but does not see anything special. (Die roll of 9 when casting the first spell.)
So he casts his second spell to try and detect the presence of vis using his hearing. (Die roll of 6.) With all the noise of the birds flying around and their numerous chicks chirping for food, Herman has trouble hearing much else at this distance. When he tries to move closer to the tree, he isn't sure but thinks he can detect a very low buzzing sound somewhere in there. Not raw vis, exactly, but there seems to be something magical going on indeed. But the hearing spell isn't very accurate to try to locate the source, amongst all the noise in the area.
Take a good look around the base of the tree, and then try to get a decent look into the crevasse behind the tree. (It sounds like whatever is the cause is at the core of the tree, and cutting it down seems very much the wrong answer. So I want to check alternatives before I try to find out more about the tree.)
The base of the tree is very much obscured by fallen debris -- small branches and dried up cedar needles, as well as small rocks that broke off from the sides of the notch. Not to mention plenty of bird droppings coating much of the debris. The tree grew up so much, so fast, that it almost completely blocks off the notch. It takes one of the grogs a few minutes to clear off one side enough so that he can move to the other side of the trunk and deeper into the crevasse. He reports that there isn't much space there, before moving out so that you can explore it youself if you want to.
Yes, I will step in to examine the crevasse. (Do we have any kind of portable oil lamp and flint and steel in case it is too dark in there? Creo Imagem / Creo Ignem are not my strengths.)
A flame might be dangerous in there, with so much dried cedar needles and small branches. The grog cleared things a bit, but any spark falling down here might cause a fire to break out, despite there being squishy spots that caused Herman's shoes to become quite wet.
None of the grogs brought any closed lamps. One could go back to the covenant and get one, but that would take a couple of hours at least.
I'll have to hope that there is enough light to see by, I will go in and look around. If it gets to the point where I can't see more, and I have not seen anything particularly noteworthy, I will try the sight and hearing Vis spells.
He searches the base of the tree for a while, but the only unusual thing that he finds is a old shard of pottery that shows a colored glazing on the outside.
Does the crevasse itself go back, or is it just a shallow cut into the rock-face? If it goes back, try to look back in there.
Otherwise, I am going to have to go for the target 15 variation on the intellego herbam spell (base 5 for full knowledge, +1 for touch, +1 for concetration; use Intellego 15 + Herbam 5 + Stamina 2 + Artes 1 + Philosophae 1 + Aura + stress die, need a 6 or better net; I am assuming that doing this as momentary would be a waste as I can not get any information in a moment.)
The cut is a bit deeper, but not by much. All told, about 5 paces deep. It is almost completely filled with the cedar's branches, so one can't see much when looking up.
Herman successfully cast his spell, but learns little else. The tree is just a mundane tree, grown too large for its age through the influence of some magic.
Standing in the crevasse, trying listening for vis (as above). If it sounds like it comes from the tree, wait until the birds settle in for the night, and try it then, while standing in the crevasse to keep directions clear. There has to be a source of magic / vis somewhere, both to produce the aura and the apparent vis effect. (It seemed likely that the tree had grown around something, but I figured the full knowledge spell would have told me that.) (At this point, I am looking for enough information to make a useful report to the covenant. I had hoped to get some Vis for myself, but I am doubtful of that.)
It takes Herman a few minutes of using a new casting of his magic-hearing spell to determine where the buzzing sound seems to come from. Although it is clearer here close to the tree, it doesn't really seem to be stronger. It is only when he moves a bit deeper into the crevice that Herman realizes that the sound is a bit stronger -- not before him, but rather above him!
Okay, up the side of the rock, lets see what we can find.
How?
I presume from your question taht the face in the immediate vicinity is too steep to climb? Look around for some other way. This may involve climbing up some of the other gentler slopes to see if we can at least see a way that is within our climbing skills. I can use Eye of the treacherous terrain to detect terrain that may be more dangerous than it appears. While I would like to get above the tree on this rock face, I will settle for getting higher up on something with a line of sight.