Robert Wadlow, 8' 11" might fit your bill. The picture on wikipedia (below) makes his nearly 6 foot father look diminutive.
Of course, his untimely death points out some of the potential problems with such a large body. While the organs scaled up successfully enough, there were other health effects, such as needing leg braces to walk; and I believe I read that the difficulty of pumping blood through his feet helped to hasten the infection that killed him. So maybe not a good characteristic for a successful race, and the cause was probably not a heritable trait (gland/hormone malfunction). But it is a demonstration that such people can (and do, from time to time) exist.
Anecdotes describe him being as stronger than most adults at 9 years old and, with no end of his growth expected, he probably would have continued to gain strength and stature until some side effect became fatal. Another extremely tall man was able to lift nearly half a ton, so strength seems proportionate to stature (more on that later).
But as for a mechanism for an extra tall race, a change in pituitary gland size seems to be enough to dramatically impact the body's growth. Unfortunately such abnormally tall people often die young, as the wikipedia chart on the subject shows. A short list of health conditions:
- Anklyosis - Rigid joints, possibly resulting in the inability to move the joint
- Increased infections, possibly due to circulation problems
- Spinal curvature
- Tooth problems
- Brain hemorrhage
and you can find more on Acromegaly. But some people seem to lead fairly normal lives, but with the added benefit of not needed a chair to reach the lightbulb.
Now, as far as uprooting trees go, I was surprised to find a study discussing the exact force needed to uproot certain trees. You can find it over at the Finnish forest institute. http://www.metla.fi/silvafennica/full/sf44/sf444681.pdf
Suffice it to say that uprooting trees is extremely complex, but in general the formula for uprooting Scots Pine stumps in sandy soil is:
F = 6.542 × (D^0.6369 + e^0.041189×D – 1) (if someone wants to format that better, have at it). F is the forces in thousands of Newtons and D is stump diameter in centimeters. Maybe the formula would be different for trees that have fibrous root systems, though I assume that those would be harder to uproot.
So let's take a 5" stump. Plugging the formula into google says that would require about 7000lbs.
Given that Edouard Beaupré could lift about 900lbs, 7000 seems to be a bit much. But, for comparison, the Journal of Applied Physiology has an article discussing weight lifting. They use bench press figures I believe, but they give a formula for computing weight lifted given height:
weight in kilos = 120 * h^2.16 (where h is height in meters). Given this figure, a 6 foot man should be able to lift 975lbs, which is apparently accurate, as a youtube search will show. Comparing records of bench presses to clean and jerk lifts (unscientifically), it seems that lifting limits useful for pulling trees out of the ground would be about 25% lower than the figures given by the formula from the JAP, giving us
So to lift 7000 lbs we'd need someone about 5.2 meters tall (17 feet). A 9' person could, if they were a professional olympic lifter, lift 3/4 ton, but not anywhere near close to our 5" diameter tree.
Upshot 1: Trees are wicked hard to uproot.
Upshot 2: Formulas are weird. I'm under 6' tall and I know I've uprooted 1cm diameter trees. It was a lot of work, but it's possible. And I can't lift anywhere near 1500 lbs.
Upshot 3: if you're only 5 feet tall and you can press 650lbs, you might as well stop trying. Applied physiology says you've arrived.