as biology OCR Help

We got some questions to do with biodiversity in class but I don't know how to answer them, any help is awesome thanks! I've looked through my 3 biology text books and revision guides and can't find the answers anywhere!

Sampling -
Why would an area of trees (in a field) affect the sample?
What variable would this effect?
Why is it better to place the 10x10 sample away from the trees?

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My thought was that the trees are only a small percentage of the field so taking a sample at this area would make the data not representative of the whole area, however I don't understand what the variable is?
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Why would a grazed field have lower species richness than a mown field?
What faults could be determined by the method used? How could they be combatted?

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Thanks for any help