Answer
Plant diversity is a nonrenewable resource because it cannot readily be refreshed or renewed— it was produced by hundreds of millions of years of evolution which cannot be readily replicated. It is a resource since we make so much practical use of plants.
Work Step by Step
Think about renewable resources like water. If you are on the space shuttle, you can take waste water and purify it to reuse it.There is not analogous way to handle plant diversity, on which so much of our lives depend.