Statistical Analysis Of Hedgerow Survey Data

I would like to compare data from a study I have been doing, but don't know which statistical tests to use.
I have surveyed 40 hedgerows:
20 from site 1
20 from site 2

I have attached some of the data to show you at what stage I'm at:

1). I need to find out the significance of difference between the 2 sites in terms of their 'woody species' composition.
Would something like a t-test work for this?

2). Do some stats on other attributes of the hedgerows.
e.g. I want to compare the 'gapiness' of the hedgerows from both sites, therefore I surveyed them into 4 categories (None, <10%, 10-30%, 30%+)what stat test could I use to show the differences between the 2 sites?

I would be grateful if you could look at the attatched files and suggest some suitable stat tests, and give a short explaination for why they should be used.

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...oups, the standard deviations and the calculate the statistic. Since you have a subject pool of 20, your degree of freedom would be 19. You have to go to a T table, and look up the critical point.

If your calcualted T is above the critical T, that ...