There are some portable solar panels that can charge up an iphone with a few hours. If you have that an external battery pack that should be sufficient - assuming you are somewhere with good seasonal sun coverage.
There has been alot of advancements in kinetic charging. I mentioned in another post the Ampy Move device. Its basically an spare battery pack thats integrated with some new innovative kinetic energy generation tech that a team of Northwestern University PHD engineers developed. Very portable and they claim that an hour of activity (e.g., running / hiking) can generate 1 hour of smartphone use (they use the iphone 5c as the benchmark device). That's really good! They claim on their blog that by just shaking it for 10-15 minutes you can generate enough charge to power a dead iphone to ring up an uber. I imagine the same ideas apply for when you're out in the woods.