If we are speaking the same, and that's not always reliable, I do know juniper! Where I live currently, one species of juniper is a super invasive species which groups often fight, while there is another species of the same family that is native but most people call them cedar! I'm guessing the two are almost the same as cedar when they are given a quick look and many will swear they can't be used in tanks as they do hold the sap for a long time. Cedar is famous for holding the sap and that makes it really popular for wood fences and decks, so I agree it does hold the sap and does have some potential for problems in the tank. But that is where we have to get further into the reasons for the potential trouble.
Cedar, juniper, and several of the woods called "softwoods" do hold their sap but not forever! So I suspect there are two things involved that have caused them to be "blackballed" by many. First point may be that the users don't always have hard alkaline water that has lots of GH/KH for buffering, so that leaves it very easy for anything they add to the tank to make drastic changes in the PH. Second is the users may not be into picking the really dry juniper or cedar. So if you stick a semi-green wood with lots of sap into a tank with little buffering, you do do get wild PH swings and it can be assumed to be the fault of the wood rather than the water and the user.
So what I see in the discus tank is really old, totally dry wood that likely has nearly no sap left and if there is a reasonable amount of KH in your water, you probably won't see any change in your PH!
I find very few things in the hobby that are certain so ,note how often I use probably and likely and all those words to hedge my bets?
Part of the fun to try it and see how it goes. We can always change it if it really does give major problems but I'm guessing the juniper you have will be fine. The aspen? Not sure how that may go, so get back with a report some day?