Ah, didn't know you could use Shears for that, cool beans.
From the looks of it, just need 4 blocks to be water (I'd say standing, not flowing), in any of the 8 blocks surrounding your tree. Before someone above mentioned the O2 Collector as the reason to why they decay (which I don't see the logic behind that personally), I tried surrounding the dirt block the tree is planted on with water, and then the same but 2 blocks deep. Unfortunately, while neither decayed just on their own (even exposed directly to space), once the O2 Collector is factored in the leaves immediately start to decay
I originally encased one in glass, stopped the movement of time so it was always noon (plants need light), gave it water, Sealed the glass structure
and encased it in an O2 bubble...
Still didn't prevent decay! I think a Sealed room WITH a Bubbler can be sufficient to prevent decay, but that's just me