The only other thing I can seem to find comparable is the runes for the Anvil Throne Warlord, even then I think that those runes seem to be harder to get rid of than the Druid's tree.
In my experience, the runes aren't much of a target for the enemy. But, if you want, you can spend a ton of mana and dissolve it.
The tree on the other hand.. easy to destroy??
That thing is really hard to kill. 12 life, 2 armor and bassically 4 regenerate. Combine that with a druid that will try to protect it...
In the 6 games online that I was a druid, only 2 of the opponents managed to kill it. 1 player had a ton of luck and the other time I had tons of luck but he just kept going. By the time it was dead I was in a huge advantage and destroyed him. (he did it, but it had cost him too much resources)
Most players waste a ton of mana on it and give up afterwards.
Being able to cast a new one would be really unfair. And when fighting a druid, I would say, ignore the tree or try to make sure it is dead in 1 or max 2 turns.