I'm going to guess that "essentially," the 90 lights are akin to a single string of lights just spliced with connectors. Are they are numbered, too so that the connections always go 1->2, 2->3, 3->4, etc.? If that's the case, I'll bet that hidden somewhere on a couple of the trees are some rectifier/current control pods spliced into the wire.
I'd measure the voltage at the outputs of each of the trees to see whether it's constant throughout or whether it decreases with each tree. If it decreases it's more indicative of a series setup while if it stays constant, it's likely more of a parallel setup.
How many wires are in the cable that goes from tree to tree and between the bulbs? If it's 3 wires, one of them is likely a "through" wire and connects elsewhere to a bulb to re-energize a new set of bulbs.
But.... I'm just guessin' here....
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