As you can see by my profile picture, I am a fan of a flowing mane
I am not sure what the temperature gets up to in Oregon, but by the photo of your horse with a "long" mane, it seems pretty thin and not really very long (meaning not over-kill, or a Carhartt Jacket in the summertime). I have always looked at it like this (and I raise working dogs too that are exposed a lot to the outdoors), nature has a reason for what it gives animals that came from the wild.
Obviously horses in the wild don't have access to barber shops, nor do they seem to be "naturally" trying to shorten their manes. I am by far an expert on horses, but I would imagine if the horse's mane itself causes some sweat, it's a natural cooling process when they are exposed to higher temperatures. Dogs, as most know, do not sweat, and this unfortunately causes them to over-heat very quickly in higher temperatures. Human sweating is the primary means of thermoregulation (as well as waste release), so I would imagine in another animal capable of sweating, it is also a natural process with a purpose.
But again I am no expert, this is nothing more than my 2 cents on the topic