The grasping branches is, at base, a straightforward weapon: 7 evocation. Call up 7 shifts of power, roll to control. However, on the control roll, you're spending a fate point to invoke the scene aspect "nearby tree" - for a +2 bonus on your control. You could, of course, cast the same spell without the invoke, if you had a high enough discipline + control specializations + focus item bonuses to go without.
As to sponsor debt: Maybe the thunderstorm per se isn't a semi-aware entity, but it's still got a (largely destructive) agenda to it. Maybe the compel comes immediately in the form of not letting the player accept a concession that doesn't involve killing the targetted foe. Maybe the compel comes in the form of a lightning strike taking out power to the PCs home. Maybe it comes a few minutes later when the PC is trying to cast something and there's just too much running water around. Or maybe it doesn't show up until, sometime in the middle of next game session, the local Thunderbird spirit asks the PC to pay off the favor...
Similar things go with ley-lines; there should be a definite agenda to it - for example, a ley line following an active fault might have an agenda of motion, with compels relating to being unable to *stop* moving when you needed to, or compels involving the earth softening into inconvenient quicksand... If it helps you, make up a face for the place; a spirit tied to the location's power, with its own goals and desires (like
Demonreach wanting intruders to just go away
).