Nice case study btw. I also really liked The Serengeti Rules, which describes these studies.

Just wondering, is the "nutriënts per individual" a different nutriënt source than the grass population?

Yes, it's a more general model. I would introduce it after using the other model.

I've used something resembling the second model this year, which worked really well. I like the first model, but my students really struggle with the term concentration gradiënt.

Ah, never thought of the internal surface. I'm not a plant expert, is there a lot of variation in the internal surface area?

Nice! I usually use surface area of the stomata instead of the total surface of the leaf, although they are of course correlated.