Full Idea
In the 'aggregative' understanding of a sum, it is spread out in time, so that exists whenever any of its components exists (just as it is located at any time wherever any of its components are located).
Gist of Idea
An 'aggregative' sum is spread in time, and exists whenever a component exists
Source
Kit Fine (Things and Their Parts [1999], §1)
Book Reference
-: 'Midwest Studs in Philosophy' [-], p.62
A Reaction
This works particularly well for something like an ancient forest, which steadily changes its trees. On that view, though, the ship which has had all of its planks replaced will be the identical single sum of planks all the way through. Fine agrees.