Tree planting this year is three times that of last year, which means that tree planting last year was 1/3 this year.

If the number of trees planted this year is regarded as "1", then the number of trees planted last year is "1/3".

The number of trees planted this year is: 1- 1/3=2/3 (copies).

Because 1 is 840 trees.

So 2/3 is: 840×2/3=560 (tree).

So 560 more trees were planted this year than last year.