In previous studies in which features of lichen communities were used

Responding to a pm:

It all needs to make perfect logical sense.

- Previous studies examined lichen communities which were 1-2 meters above ground.
- It has been shown that lichen communities on lower branches are more sensitive. (Later studies examined lichen communities on lower branches)

First note that you need to use passive. Lichen communities didn't themselves show. They 'have been shown'. The scientists did the 'showing'. Hence A and C don't work.

Use of present perfect is preferable instead of simple present because it has already been shown that lichen communities on lower branches are more sensitive. Probably later studies were conducted to figure it out. Since we already have the conclusion, it means the 'showing' is already done. Therefore, 'have been shown' is better than 'are shown'. Also notice the use of 'since' in option (B). It makes complete logical sense once you put the word 'since'. Previous studies examined lichen communities which were 1-2 meters above ground because at that time, it was not known that communities on lower branches are more sensitive. Since then, studies (later studies) have been conducted on lichen communities on lower branches and it has been found that they are more sensitive. Now it's not odd that previous studies used lichen communities that were higher up.