In The Lord of the Rings: The Two Towers (2002 movie), Treebeard says:
I always like going South, somehow it feels like going downhill.
Is there a difference in elevation or something that can justify Treebeard's feelings, or is this just some random feeling he feels? Saying something like this sounds very strange unless it is somehow rooted in reality in one way or another.
It is stated by Peter Jackson in the commentary track of The Two Towers Extended Edition that John Rhys-Davies would often throw in ad-libbed lines that sounded quite nice. This is an example of one.
Peter: The line at the end of this scene was an ad-lib of John’s, when he says, “I always like going down south, it feels like going downhill,” which was just something he threw into the end of the recording session that we did; but he often comes up with those really nice little lines that…
The Two Towers Extended Edition Commentary Disc 2 - Transcript
However to answer your question of whether his statement had any basis:
If you rewatch the film, you will see that Fangorn forest seems to be on a bit of a slope leading down to Isengard. (Video below). However, that was not why Treebeard said what he did, as @Verdan says, it was merely a joke made by Treebeard given that South is at the bottom of the map.
From the video, at 00:43 you can see that behind Treebeard, where he has just been walking from, there seems to be hills and mountains sloping upwards. At 00:36 you can see that Saruman is down below where they are now, therefore there was some evidence to what Treebeard said.
The Ents go on a bit of an adventure, climbing up ridges and down into valleys, Isengard is indeed in a valley (Nan Curunír, the Valley of Saruman) and the Ents did in fact end climbing down into the Valley.
“At last they stood upon the summit, and looked down into a dark pit: the great cleft at the end of the mountains: Nan Curunír, the Valley of Saruman.”
The Two Towers: Book III, Chapter 9 - Flotsam and Jetsam
Classically, south-facing gardens get the most light (in northern hemisphere gardens, that is). Treebeard is referring to being drawn by the increased sunlight coming from that direction.
Update from the comments: the effect known as phototropism (thank you Edlothiad) has plants growing in the direction of their light source. So the Ents would seem to be naturally inclined to head South, the direction of the greatest amount of light.
“For a photosynthetic organism, facing towards light may result in receiving more energy (especially if they have better / more light processing surface in the front). In which case, going toward tight will result in less net loss of energy per time, which may feel similar to using less energy going downhill. – Misha Rosnach” (thank you Misha Rosnach for clarifying the point I was trying to make, down in the comments).
For Treebeard, it would be a relief to follow his natural inclination to move southward, feeling like one were going downhill, rather than struggling against his natural urges, which would feel like going uphill.
John Rhys-Davies, the actor who lent his voice to Treebeard, has gone on record stating that the line was actually an ad-lib. He just thought it would be the strange, almost non-sensical sort of thing that Treebeard might say.
Is it really?
Short answer:
No, it is probably not — not even in the Peter Jackson version of Arda.
Peter: The line at the end of this scene was an ad-lib of John’s, when he says, “I always like going down south, it feels like going downhill,” which was just something he threw into the end of the recording session that we did; but he often comes up with those really nice little lines that… We can always try to find places for them in the film.
The Two Towers Extended Edition Commentary Disc 2 - Transcript
To put it another way:
An ad-lib was made by John Rhys–Davies at some time while he was recording lines written in the script. Then, during editing of the movie — when raw video and audio are trimmed, sequenced, combined, and post–processed, — the editors and producers were looking for somewhere to insert that particular line. Alas, however, we don't know how much time they spent deciding that, nor their exact considerations made during that decision.
So, we need to make an educated guess as to what happened.
Well, we lack much of the situational information which could be gained if we were either present at the aforementioned session or had access to the unedited audio recorded thereat.
I can best figure that it was a technique actors will use to help themselves embody a character. I never learned a proper name for it — probably a Method thing, for those who are curious, — but it consists of saying lines which were never written but which emerge from your understanding of a character. We could call it ‘emulation’. If it is indeed Method, then it involves an actor inventing character from means other than script analysis or the like. Voice actors do it occasionally during warm–ups. It has other uses too, but that's enough about that.
Of course, John could've said the whole thing in jest or in a moment of silliness.
Either way, it seems that he was attempting to emulate lines like these:
But if I had seen you, before I heard your voices — I liked them: nice little voices; they reminded me of something I cannot remember — if I had seen you before I heard you, I should have just trodden on you, taking you for little Orcs, and found out my mistake afterwards.
I can see and hear (and smell and feel) a great deal from this, from this, from this a-lalla-lalla-rumba-kamanda-lind-or-burúmë. Excuse me: that is a part of my name for it; I do not know what the word is in the outside languages: you know, the thing we are on, where I stand and look out on find mornings, and think about the Sun, and the grass beyond the wood, and the horses, and the clouds, and the unfolding of the world. What is going on? What is Gandalf up to?
Both are quotes from Treebeard in The Lord of the Rings, book 3, chapter 4.
Let's analyze the line. It begins by saying
I always like going south
which tells me that this is something Treebeard has, and did when speaking, felt or believed. In other places than when he mentioned it the scene.
Again, although we lack context for the line originally, Peter Jackson on the commentary told us that it was not purposed for the scene in which it was placed: it was simply something that John Rhys–Davies said and which Peter, Philippa, or Fran wanted to insert into a scene as if it were ‘additional dialogue recording’.
It continues with
Somehow,
Which is to say that Treebeard has not discovered how exactly it does, but simply that it does.
Finally:
it feels going downhill
What would an Ent feel while going downhill? Unfortunately, I know of nothing which would give us much information there.
Because I know of nothing which would imply that John Rhys–Davies was an eminent scholar of Tolkienology, I cannot suppose otherwise than to say that the line was mostly gibberish.
Meaning no discredit to John, of course. If anything, the blame lies on the editor who inserted that line into the finished movie. (Of course, that's my personal take on all this.)
So, to conclude this answer:
Is there a difference in elevation or something that can justify Treebeard's feelings, or is this just some random feeling he feels?
Probably the latter. The rendition of Treebeard in the Peter Jackson version of the story seems to imply that he is rather — shall we say, senile.
I mean, maybe it was included so as to help explain why Treebeard would believe Merry's rather ludicrous request that he carry them south at all.
Let's look at the map of Middle-earth, paying particular attention to the Misty Mountains:
Being them a mountain range, walking away from them means, basically by definition, also going downhill.
It is clear that the Misty Mountains run from North-East to South-West. So if you are on their right side (where Fangorn is placed), going North and West means going uphill, and going South and East means going downhill1.
Of course, going directly South-East means losing altitude more quickly, but going directly South equally brings you on a lower terrain position.
We must also consider that very probably Treebeard did't mean South as a precise direction as determined by a compass, but in a broader sense to indicate a generic Southwards direction.
This terrain conformation is depicted also in the maps from Barbara Strachey's Journeys of Frodo, where to contour lines clearly show the morphology of the terrain:
1. Granted, Fangorn is placed on a part of the Misty Mountains that has an almost North-South direction, but we can consider the general course of the whole range to be valid even here.
