To amplify on the spreadsheet suggestions. I would use a custom report with the fields you use in your normally searches. Export each tree's custom report to a CSV file.
(In Quattro Pro each page of the spreadsheet is called a Sheet, Excel has the same thing I don't remember what they are called.)
Open the spreadsheet and create a Sheet called Index with the following column headers.
First name, Last Name, XXXX, and Tree Name
the XXXX, would be whatever fields you placed in the custom report. The order of the columns would be the same as they were in the custom report.
For each of the your trees, Import the CVS file for each tree into the spread sheet. Copy all of the new rows to the Sheet called Index, Then enter the name of the tree in the column Tree Name. and copy it down for all of the new entries.
When you are done, you should have all of the people in all of the Trees in one Sheet in the spreadsheet with the name of the tree where they are found.
You can then sort the spreadsheet on last name and then first name. This will give you an alphabetical list of all of the people and the tree where you will find each person.
Since this will take some time, you could possible create a macro to import each of the CSV files.
To maintain the index, you would have to added each new person to the Index when you add the person to their Tree. Or you could periodically repeat the above process with the maros
I would further recommend that you use a good spreadsheet like Quattro Pro or Excel as there will be a lot of data to sort.