Pretty straight forward Weaken question here. Let's break the argument down:

Conclusion: Harvesting the chanterelle mushrooms seriously hurts the Douglas firs.

Evidence: There is a symbiotic relationship between the trees and the fungi. The mushroom's filaments provide water and nutrients for the trees.

There are a couple of assumptions present in this argument which you should be able to identify:

(1) The nutrients and water are critical for the trees survival.
(2) The trees don't have another adequate source of nutrients and water.

Knowing our job on Weaken questions is to find the answer choice which makes the conclusion less likely to be true (and that it typically involves attacking an identified assumption) we have some pretty strong footing here. In essence, we're looking for the answer that says, "No, the trees aren't as likely to be in trouble".

(D) Does exactly that. If the harvesting of chanterelles actually results in stimulating chanterelle growth, then harvesting the mushrooms doesn't stop the flow of water and nutrients to the tree. In fact, it actually provides a situation in which we may increase the amount of water and nutrients available to the tree.

Admittedly, harvesting the mushrooms may result in a short term problem for the trees, but remember: our job on weaken questions isn't to disprove the argument. We need only make the argument less likely to be true (less likely the trees are in serious danger).

Choices (A),(B), and (C) are all irrelevant. They should leave you asking yourself "who cares?" Trends in harvesting, other trees, and other mushrooms are outside the scope of this argument.

Choice (E) is exactly the opposite of what we're looking for: it strengthens the conclusion. If young Douglas firs DIE without these nutrients, then surely they ARE in serious danger when we harvest the mushrooms. We should have expected at least one wrong answer to strengthen this argument; opposite answers are very common wrong answers to Strengthen/Weaken questions.

All in all, very straightforward. On Strengthen/Weaken get the conclusion down clearly; without it, you cannot do your job.

IMO - D...nice explanation given above.

Good Question.

IMO D

Pretty Straight forward!!!!!

I'd say D but I'm afraid more because of my intuition than any logic