Tintin -if you are pushing film, you are going to lose shadow detail, because you are underexposing it. 100-400 is only 2 stops, but it is quite on the edge. Given the contrasty light in the trees shot, and that the meter will be underexposing anyway as it is backlit, you are going to have to sacrifice shadow detail unless you use a speed increasing developer.
Pushing in a non-speed increasing developer will only stretch the contrast - more development = higher highlight tones. So, yes, you will get a more contrasty result.
This is not too much of an issue these days as most people scan negs, which gives much greater control over contrast. If you were printing these, I would imagine much of the shadow detail here would be unprintable without some crafty darkroom workarounds.