Solution: The Lorax
Answer: SEEDS

Written by Ethan

By "Mastermind" in the title, it should be clear that each of the 4 forests represents a mastermind puzzle, with the 5 letter words across being the guesses. However, this does not seem to be a normal mastermind, as while letters in the same column are always the same color (Y's in "Useful"), the same letter may be colored differently depending on what column it is (ex T in "Vehicle" is colored white in col 1, yellow in col 3, and blue in col 5). This indicates that position is not important to the mastermind.

Next, the abundance of blue tiles hints that they are the "incorrect" color. Perhaps the easiest break-in is in "Vehicle", where the white letters spell TR??K, which is TRUCK, a vehicle. Trying this same process with other colored letters doesn't yield anything substantial. We then try to see how these letters in TRUCK could have been formed by the other colorings. By the hint of "shifted once" in the title, we can see that the yellow letters indicate that a letter has been caesar shifted -1, or the letter before, while the pink letters indicate that a letter has been caesar shifted +1, or the letter after. In "Vehicle", in column 1, the S (in yellow) is one letter before T. In column 4, D is one letter after C. This mechanic can be applied to the rest of the 4 forests to get AFTER (a time relation), NIFTY (meaning useful), RIVER (a body of water), and TRUCK (a vehicle).

Repeat this process one more time by placing these 4 words in the tufts of thet trees, creating a "meta" mastermind, to get SEEDS, how to save the Truffula trees.

Author’s Notes

Almost all of the puzzle is just figuring out the mechanic. There's pretty much no "grind" stage - once you get what's happening, the puzzle is essentially solved. It's certainly a different style than some of the other puzzles in this hunt (cough, Where the Wild Things Are), but not a bad thing, in my opinion.

The abundance of white letters helps confirm you're on the right track, but unless you actually understand the shifting mechanic, you're not solving the puzzle as there are no white spaces in the tops of the trees.