I think the flowers are there to address these two problems -- though that's an odd choice, because normally the flowers would change much faster than the tree.
ETA: 100,000 time range like the Onkalo waste repository in Finland: http://www.cnn.com/2010/WORLD/europe/11/12/finland.nuclear.w...
If humans or our near relations were to land on a planet in a distant galaxy and come across any symbols they could even take a wild guess at, their reaction should be: 'We've been here before.'
Correspondingly, taking aliens into consideration is the epitome of misdirected 'inclusiveness.' Puts me in mind of recent dispute in the feminist community, in which use of the word 'vagina' in a non-reproductive sense has become an issue becauese transgender women are excluded. (No cite, not going to google 'transgender vagina.' Thinking it was an Utne reader story.)
Gist, I assume: Even a human idiot should be able to figure out these symbols. Fair enough, and much discussed above/below. (And a fun design problem.)
But, other than an alien landing first in the 'forbidden zone' and accidently unleashing population-ending poison on whatever was left of life on Earth (and it's a stretch to even suggest credencene here), no proposal with 'aliens' in its text should be taken seriously.
Less than seriously, but I didn't open this can of sandworms, suppose aliens consider our radioactive material:
Tasty? A sexual aid? Unimaginably beautiful in some audio/visual/X-sensory spectrum we can't even imagine?
Don't bury it, advertise it!
An Earth rich in gamma radiation and alpha particles might be the celestial promised land, and certain aliens might gratefully resurrect our dead and take them through the pearly gates to their planet of toxic gold, frankincense and myrrh.
But, yeah, nuclear electricity could save the day for the next generation, so we need to figure out these niggling details.
It passes the alien test: if humans were exploring an alien world, and came across those pictures, but they were in circles instead of boxes and showed aliens instead of humans, and the trees were instead some plant that grew there, and the panels were shown right-to-left instead of top-to-bottom, and the radioactive symbol was replaced with some other symbol, I think a thoughtful human would still understand.