Which statistical method to use?

#1
Background:
We have recently done an experiment on the affect of grazing pressure on plant height and coverage, but now I'm going round in circles trying to work out which statistical method I should be using to analyse the data.

We had four groups, each of which had 9 trays: 3 trays with no caterpillers, 3 trays containing 2 caterpillars, and 3 trays containing 5 caterpillers. Each tray contained 4 plant species and the height and coverage for each species was measured 3 times at one-week intervals.

Therefore for the trays with 0 caterpillars i have 12 measurements of height and coverage for each type of plant for each of the three week's sampling. Likewise for the trays with 2 and 5 caterpillars.

Questions:
1. Is it reasonable to assume that I should use the mean of the data for each type of tray before doing any statistical analysis?
2. Am i right in thinking that the data is non-parametric?
3. What would be the best statistical analysis to perform and, most importantly, why?
4. As we only have three measurements over time, how reliable can i expect any statistical analysis to be?

Many thanks,
Ucuuba