The 'woods o' Germany' is quite possibly a reference to the fact that impoverished/cleared Highlanders desperate for any employment, or 'king's shillinged', often ended up in Highland and other regiments that went over to the Continent fighting for the Hanoverians' allies, eg the 87th and 88th in Hessen, Germany, in the Seven Years' War (after Culloden)- more here from a tri-lingual Scotophile German scholar:
https://sites.google.com/site/acsailognanron/saighdearan-gaidhealach-ann-an-hesse-1759-62---highland-soldiers-in-hesse-1759-62---hochlandschottische-soldaten-in-hessen-1759-62.
Others were there as mercenaries, often on opposing sides, in the Napoleonic Wars too.
The soldiering /Clearances theme is a recurring one in songs, eg, I will go, I will go.

The conifers theory is just possible too, if taken poetically, as the trees would not have been planted till long after the Clearances. Though I don't really see 'the woods o' Germany' meaning the same as 'German trees'/'trees from Germany'. To me the reference sounds like death in exile.

I would discount the WW1 and WW2 interpretations as too modern.

A reminder of the song itself: http://www.youtube.com/watch?v=HvNL81LxhOA