Has anybody some suggestions for cropping & warping the image in order to express the dark mood of this trail?
Well, for starters, soft, diffuse lighting on an overcast day with a blown-out high-key sky does not really scream 'dark' or 'moody' to me. Consider taking this shot on a day with shadows, in the early morning or late afternoon, when they will not be too harsh, but will nevertheless break up the vertical lines of the trees with diagonal interest. Unless you can get a cloudy, polarized sky, crop it ou for the most part.
Try a lower camera angle relative to the trees--right now it feels like a vista point, which is generally thought of as inspiring, bbecause the camera is higher than some of the trees. Try shooting from a low vantage slightly upwards to give the sense that the trees are looming over the viewer--perspective distortion could even help here by making the trees look like they are closing in above the viewer's vantage point.
Lisa's suggestions on black and white and contrast are exactly what you should do, but keep in mind that you will not get great contrast effects on a negative that is so lacking in contrast anyways--you want deep shadow and rich, true blacks to evoke a dark feeling.