Where can I find information about indicator plants?

I'm trying to do a school project on indicator plants. (eg: clover indicates low nitrogen on the soil) (i think) maybe that was a bad example, but that's kind of what I'm looking for, in other words how the presence of certain plants tell you things about the soil or environment. I went to the library but I couldn't find much. Can anybody point me in the right direction? I'm shooting blanks here.

Where can I find information about indicator plants?
I know more about plants being wetland indicators (I used to work doing wetland delineations) so I'll post some info for you about that. I also found these websites, which seem to be more along the lines of what you're asking about: http://science.jrank.org/pages/3553/Indi...

http://www.ext.vt.edu/departments/enviro...

http://www.treesearch.fs.fed.us/pubs/291...

http://www.fs.fed.us/gpnf/forest-researc... - use the lists on the left to find species - the links will tell you what each is an indicator of



Plants also have preferences as to how "wet" of a soil they'll grow in. Some, like cattails, are always found in wetlands and are considered "obligate" wetland plants. Others are never found in wetlands, and are considered "upland" species. And a lot fall in between, and are classified as to how likely it would be to find them in a wetland. This link explains the categories: http://plants.usda.gov/wetinfo.html#cate... These plants are used to identify wetland areas prior to graning permits for new developments or expansion of existing man-made features. You can use this link: http://plants.usda.gov/wetland.html to search for plants that are wetland indicators in your area if you're in the US or one of their territories.