Multiple IF statements with INDEX and MATCH to populate cells from other sheets

I am trying to build a vegetation database at work that self populates a main table from other sheets that are identical that I can import into ArcGIS and can be used for reporting. However, I am having trouble stacking muliple "IF" statements that also use "INDEX" and "MATCH" to populate cells from the other sheets to populate certain cells in the the main database. Below is what the database looks like.

I am asking for help on how to stack the "IF" statements from "Study Type" (Column D) in the Photo Point Study (the "IF" statement works and matches "Study Number" so order does not matter), Upland Trend Study, Aspen Trend Study, Riparian Trend Study, TES Trend Study, and Recreation Trend Study sheets to populate D2 in the main database sheet MLSNF Veg Studies. Right now I can get the study type value from the "Photo Point Study" sheet to populate the main database sheet ("MLSNF Veg Studies") using: =IF('Photo Point Study'!$A2="",0,(INDEX('Photo Point Study'!$D$2:$D$4720,MATCH('Photo Point Study'!$A2,'MLSNF Veg Studies'!$A$2:$A$4720,0))))

I was forced to use a ".xls" file because I use the main database sheet "MLSNF Veg Studies" to populate a access database ".mdb" to import into ArcGIS to spatially view the information. I know that only limited number of "IF" statements can be stacked in a ".xls".

Can someone help me with the "IF" statements to populate the "Study Type" in the main database sheet ("MLSNF Veg Studies") based on the six sheets ("Photo Point Study", "Upland Trend Study", "Aspen Trend Study", "Riparian Trend Study", "TES Trend Study", and "Recreation Trend Study"). I have given the example of the "IF" statement with "INDEX" and "MATCH" functions that works for one of the sheets... I just need help stacking them... I get errors when I stack them...

Thank you in advance for being awesome!