In the context of the phrase "decimated" as it might relate to the end of big trees, the most accurate meaning would be:
D. partly destroyed
The term "decimated" originally referred to the Roman practice of killing one in every ten soldiers as a punishment, but it has evolved to mean causing significant destruction or loss. In this context, it likely implies that a substantial portion of the big trees has been harmed or removed, rather than complete annihilation or displacement.