Unique Values

The way I'd solve for this would be two parts,

Part 1: Create a Formula Text field on Table be to combine the "Year" field on Table B, and the "Related Tree". (for now lets call this the "Tree Year Formula" field,
 - The formula will be as follows - [Related Tree] & " " & [Year]

(Note that you can get pretty creative with the concatenating formula to combine values, ie, between the quotes if you wrote " last measured " the returned value would read "Related tree last measured 2017" for example.

The second part is just a scalability safeguard since Formula fields can't currently be used as the Key field for a table. Let's say at some point you needed this value to be the "Key" field, I always suggest taking one extra step and making one final field (which will be your "key") and call it "Tree Measured Year". This field will be a plain-old text field, BUT it will be populated with a Dynamic form rule.

The form rule will need to be written as follows -  when "Tree Measured Year" is equal to Blank then change "Tree Measured Year" to the value in the field "Tree Year Formula" and "Make Read-Only"(blank just means no selection)

Once all this is done, any time a user creates a Measurement from a "Tree" Record, your key field will autopopulate with the appropriate naming convention, and the user will NOT be able to save the record if another record...with the added bonus that this field can now be used as a key field if need be.

Two quick tidbits...
1. If you do not need the tree/year to be the key field, you can skip the second part and simply make the formula field "must be unique"
2. The second part will not be an option if your tree measurements are being imported as it relies on a Dynamic Form Rule which (unlike a formula) requires that a record be manually edited/saved to "Fire".

Hope htis helps!