I have played with caster Shaman, and tested Entangling Roots damage.
When naked (no equipment), my roots deal ~100 damage per tick to the dummy.
When I put items that specifically add spell damage, hovever, these items seem super-effective.
Isadora’s Gravechill adds about 200 damage, when combined with Shaman passive, that adds +2 cold damage per point.
The Invoker’s Static Touch add about 200 (per ring) as well, combined with passive that adds lightning damage.
And druid passive that adds physical damage doesn’t do much. Seems like a broken interaction between Entangling Roots and “spell damage” in some way.
Interestingly, attunmenent, “increased damage”, “increased cold damage”, and “increased lightning damage” seem to add very little to this inflated damage.
But “increased spell damage” seems to add a lot.
Entangling Roots also has passive that is both worded strangely and works strangely. It adds “25% increased spell damage per point”. This sounds strange because damage over time should not be affected by spell damage. Well, the Roots description says “added damage applied at 35% effectiveness”, maybe this is why. But still, seems strange because it works as a multiplier. 4 points (+100% incr. spell damage) double damage of the spell, 8 points triple the damage.
I didn’t test Tornado that much, but it seems to be suspiciously super-effective as well.