Yes, compete over was also the term I settled on at the end of that paragraph. It should also be noted that while plants - and beings in general - require magical energy to function in the Prophecy, it is actually an extremely small amount of it that is passively expended, so even though one only has so much magical energy to begin with (as is the case with tarken), one can live for decades without really noticing much of a decrease of energy without ever replenishing it. Plants consume even less energy than most other beings (or maybe it would be more accurate to say that they consume it far more slowly, since certain plants do live long enough to spend an amount of magical energy as large or larger than what is required for such as humans), but each reserve a portion of ambient energy for themselves to constitute an individual soul-structure, even though the energy that makes up that structure is exchanged at a constant rate with its surroundings. An area may feel almost entirely void of ambient energy despite being very lush with vegetation, only for it to seem absolutely brimming with energy after one burned down all the plants (remember how dense the ambient energy was in the Anaxim Forest before its destruction? After being burned, that area would be downright unhealthy to even pass through fleetingly due to the sheer enormity of ambient energy if that energy hadn't naturally started to disperse once the plants that had drawn it there were destroyed).

And the impossibility of causing decay externally is why I expressed myself the way I did ("to the extent that such a thing was possible"), since what can be magically accomplished at the current level of understanding will be at most to replicate or approximate the effect of decay. Not that such a thing would make much sense to begin with, though, since at the current level of understanding people don't realize that decay doesn't happen on its own in the first place and thus they probably wouldn't even try to find alternate ways to reproduce the effect...
I could try to weave some far-fetched explanation, but the fact of the matter is that the novel as it is now is relatively outdated compared to how the lore has evolved, particularly in regards to the Laws of Magic, since at the time I wrote it I had only established the first one. I didn't think there was anything in it that downright violated the laws, but it appears that I was mistaken.