The most common and most likely answer to this crossword clue is the 8 letter word REDWOODS
I've seen this crossword clue in The Guardian
R
E
D
W
O
O
D
S
More clues leading to the result REDWOODS
Buddy explains!

The straight part
We know that "Trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trees = redwoods.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does REDWOODS mean?
REDWOODS noun- (USDA-preferred term) The species Sequoia sempervirens.
- Any of the evergreen conifers belonging to the genus Sequoia in the wide sense.
- The wood of the species Sequoia sempervirens.
- Wood of the species Pinus sylvestris.