Lumber species for outdoor beam

Not exactly framing, but I figured you folks would know a bit about lumber.

I’m looking to make a swing set by hanging a beam between two, large trees. I’ve already been brought up-to-speed on how to secure the beam to the trees without restricting sway or having it break the beam.

The trees are about 18′ apart. I was really hoping for something like 14′, as 16′ beams are easy to find. I’ve got four types of wood available nearby that should do OK outside:

  1. Pressure treated pine. For this one it’s no big deal that I can’t find it longer than 16" because I already planned on joining 2-3 layers of 2×6 or 2×8 to control the warping. since I’ll already be gluing and screwing, I could just splice together the layers with shorter boards at staggered spots.
  2. Cypress. I found a mill nearby that I can get 2x6s, 2x8s, 4x6s or 4x8s from and they may be able to give me a 20′ piece depending on what they’ve got. Otherwise, I can do the same as the PT pine and splice together a couple layers of 2x?s.
  3. Kiln dried cypress. Longest length is 16′ so I’d have to do the spliced 2x? layers here to make a longer beam.
  4. White oak. Another mill, same situation as the cypress – availability of one, solid beam would depend on what they have.

So what’s the best material for this application? Best way to build the beam? Consider that the hardwoods won’t likely be old growth so I’m probably not getting good heartwood. Should I worry about them warping or rotting? For that span, considering the light, intermittent load, would a 6" beam work or should I go to 8"? Should I add additional support with cables and turnbuckles holding up closer to the center of the beam?

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