Answer:
Since moose are herbivores, they could cause a plant population to decrease in size.
Since wolves are carnivores and hunt moose, there would be less moose feasting on plants, so the plant population would be allowed time to grow back larger.
Depending on how many wolves there are to moose, the plant population will tend to vary.
For example: If the moose population is greater than the wolf population, there will be a smaller population of plants. On the other hand, if the wolf population is greater than the moose population, there will be a greater population of plants.
Hope this makes sense :)