just remember, you need to think in black and white terms.
the first scence is full of middle grey values, which allow don't do well for making great black and white images.
When converting from black and white though, those aren't middle grey values - they're originally colors. With either film (in the field) or photoshop (in digital post-production), individual colors can be controlled as to the intensity of how bright/dark they are. For example, the in the French flag (simple stripes of blue, white, and red) the blue and red would constitute these "middle grey values" for you since they aren't dark or light blues/reds and a quickie B&W conversion could likely end up showing only 1 shade of grey and white. However, with Photoshop or with film techniques you can take that blue and say that you want it to end up representing a very dark grey while you want the red to represent a very light grey and you end up having 3 tones, 1 dark grey, 1 white, and 1 light grey, that looks much more like a French flag.
Which goes to my point - the photo sucks because it's visually busy and has no clear subject, not because all the trees and foliage were the same shade of green.
To the OP - this is one of those shots that is flawed in concept and not just execution because you can't alter what's making the image bad (read on...). If the foliage wasn't there or else was very sparse, and if the walls didn't end in 20 feet, then there might've been an opportunity.
BUT - you said that you didn't dare affect the scene because it's a National Park and what with the fines etc. Oh well. Move on, there are better photos to be had. But, if you could affect the scene, what you would want to do is plant the ground with a kind of messy grass, clear the walls of most of the foliage and certainly any foliage that has fallen between the two walls, re-angle the camera slightly to the left to reduce attention on the beam, and then give the photo a subject - say, a silhouette of someone standing at the end of the tunnel with a halo of light around him caused by wireless flash, which would evoke this kind of imagery of a person of power walking down a path trying to be reclaimed by nature sort of thing. In this example, B&W would probably still be a good medium because now the photo isn't about all this green shrubbery but rather about power and nature, which are abstract ideas. As the photo stands though, the B&W conversion probably hurts the image more than it helps it since it reduces the differentiation between the various shrubbery in the photograph and the diffused light coming between it.
From a technical standpoint, I would also try to make sure that both of the walls are lit with the same intensity, instead of the sun coming over 1 wall, lighting up the other wall while leaving the first wall (the right one) in shadow. This means taking the photo at a different time of day - sunrise or sunset would probably work best.
And that's probably enough food for thought for you right now.
