Plants in the Bible generally represent facts, knowledge that can be gleaned from the world. Plants that can serve animals as food represent facts that have the potential to be put to use, ones that can lead to something good. A pasture, then, is an area that offers a great deal of this sort of useful, life-building knowledge. "Pasture" is often used as a verb in the Bible as well; to pasture animals means teaching people facts that can help them do what is good in life.
The Inner Meaning of the Prophets and Psalms #198
198. Internal Meaning of Joel, Chapter 1
1-3 To all who are of the church. (2)
4 Falsity from the sensual man and afterwards evil therefrom has consumed all things of the church. (2, 15)
5-7 Let them repent, for evil from the sensual man has destroyed the different things of the church. (2)
8-13 Mourning over the destruction of the goods and truths of the church. (2)
14 Exhortation to be converted, (2)
15 and to reflect that thus will be the last time, when the Lord will come, (1)
16-17 and that everything of the church has been devastated; (3)