Even though I'm not a native English speaker and I actually learned the grammar I cannot tell exactly why too.
Here's what I've found:
Grammar: Definite Article - Yes, you can learn English. - Simple English News

Just that:
The Definite article can be used with countable and uncountable singular or plural nouns.
the book, the trees, the snow, the money
However, this explanation can help a little bit more:
Definite & indefinite articles (a/an, the) in English grammar - Waylink English

By using the, we are signalling to our listener that s/he is very likely to know what we are referring to and that the context of our conversation should help them to identify this. We can use the, therefore, to
  • refer backwards to something that we have already mentioned
  • refer forwards to something that we can take for granted will happen
  • refer to our common ground or shared knowledge
Here are some examples to illustrate each of those contexts:
  • I was out the other day and I found a ten-pound note on the street. I couldn't decide whether to keep the money or hand it in. (I have already talked about this money in the previous sentence.)
  • We'll need to take an axe to cut the trees . (i.e. those trees that we find in the place that we are going to.)
  • Have you put the cat out? (i.e. our cat)

It appears, that here the author describes not just trees, but some particular patch of trees he thinks readers would know about.