Symbolic: The trees rejoicing should be read on two levels. The first parallels the rest of the earth in verse 7, and is the first contextual reading. As features of the earth, as representatives of nature the trees rejoice in their rest also. Thus this verse becomes a literary parallel emphasizing the rest of all nature begun in verse 7.
On a second level, however, one cannot miss the associations of the trees with political entities. These trees are representatives of the nation of Israel and surrounding states. On this associative level, not simply the trees, but the nations for which they stand, will have rest. It is in this last context that the phrase "no feller is come up against us" has its most poignant meaning. No longer will these states fear the more powerful nations that in the past have come to cut them down.