dpb wrote, on Tue, 28 Oct 2014 09:21:42 -0500:

While it is true that the end that is resting on the ground does have
that support, it's not necessarily so that the remaining load on the
cable is only half; it depends on the actual geometry of the
configuration. It _might_ be half; could be more, could even be less.

Thanks for all the insight.
Below is the owner's response to your concerns.

BTW, I created an animated GIF of the entire process, as I see it,
but I can't get Flickr to show the animations since Flickr turns
an animated GIF into a static JPG.

I'll post the animation separately, if I can figure out how to
preserve the animation, but here is the starting point static JPG:
https://c2.staticflickr.com/6/5605/1...4969faac_z.jpg

Here is where we are right now:
https://c4.staticflickr.com/8/7575/1...ed414ac3_z.jpg

And here is the penultimate ending point static JPG:
https://c2.staticflickr.com/6/5600/1...66bb66b1_z.jpg

Here's the owners response to your valid concerns ...

I wonder if they realize how huge the final redwood tree is?
The tree probably weighs in excess of 2,000 tons, and has a 30 foot
circumference.
The smaller set of redwood trees I would estimate weighs 15 tons.
In a wind of 50 mph, the small tree experiences 200,000 pounds of force
due to the wind.
The idea that 28,000 pounds of tension on a cable is more than it
encounters in a light wind does not seem tenable.
The root structures of both trees routinely handle much larger forces
during a typical day.

A wind blowing at 100 miles per hour generates 25 pounds of force per
square foot.
If that wind were blowing straight down on 800 square feet of deck, we'd
have 20,000 pounds of force.
I consider that unlikely. :-)

Edge-on, we have 67 square feet, or 1,666 pounds of force. But that is
also somewhat unlikely.

Sideways forces will add a little to the cable tension, but will mostly
be taken up pushing against the trees and the support posts.

The deck will weigh in the neighborhood of 5,000 pounds, and has 800
square feet of maximum surface area. Lifting that, requires 6.25 pounds
per square foot, or a wind speed straight up of 50 miles per hour. But
the deck is held down at the ends and in the middle by either trees or
posts, which also limit the amount it can tilt or twist. The surrounding
trees limit the wind considerably.

The assumption that the engineering is "seat of the pants", or that the
mathematics have not been done is incorrect, but the ideas are all good
because I don't want to miss something, by not thinking about it at least.

Let them know that I appreciate their advice!
(Please invite them to lunch on Wednesdays in Redwood City if they're
local.)