From the picture it almost appears that each polygon is a specific layer (Attribute Table reads "0 out of 1 Selected", Layer name is area7_poly, all polygons are different colors), but the excel file, not to mention your post, both indicate that there are many polygons.
If they are indeed each separate layers, then this answer will not suffice, but since I can't tell, I'll carry on with what my answer would be if they aren't all separate.
To bring the tabular data (your excel file) into ArcMap and associate it with your polygons, you will need a unique identifier field that is common between the shapefile and the excel document (but obviously unique for each record). Adding this to your excel document is as easy as naming a new column something like ID and populating it with incrementally growing numbers.
You will then want to put in a unique ID field in you Attribute Table so that the ID in one file corresponds to the ID in the other (the ID for any given shape matches the ID for its information). You can add a field to the Attribute Table from the options menu. Pick a short integer and name it ID.
Right-click your new field and select Field Calculator. At the top of the window, check the box next to Python, then check Show Codeblock and paste this in the top box:
pStart = 1
pInterval = 1
if (rec == 0):
and in the lower box paste:
Click OK and let it run. Your new ID field should soon be populated. If you don't want to mess with this bit of code you can instead choose to Edit the polygon layer, then open the Attribute Window (on the Editor toolbar). This will let you select the polygons one by one and fill the field, one polygon at a time (one must be selected for the Attribute Window to show anything).
Once you have the ID fields in both files, bring in the excel file either through your catalog or through the Add Data button. Once it is there, right-click your layer, select Joins and Relates > Join. Assuming your excel file is the only one you've brought in, it will be selected by default. Pick your ID field in both the drop-down boxes and click join (Validate the join if you wish, just to test that they match). Soon enough you will be able to open the Attribute Table and see all the fields from your excel document joined.
For the second part, you will perform either a Definition Query (Properties > Definition Query) or a Selection > Select by Attributes. This is fairly straightforward in either case. You will select the field you are filtering by, say Acer campestre where it grows strongly or dominates, so you will use the basic operators and produce something like "ACERC >= 2" to select out those specific areas you wish to analyze.