I recently purchased a new, single-stem amur maple tree. It was grown with a bamboo stake alongside the central leader. What I did not realize is that the stake, while having a straight lower part, gently curves to one side once the side branching begins, which caused the tree's leader to grow curved as well. Once I removed the stake and planted the tree, the tree has a noticeable curve. What is the best way to straighten it? Should I attach a rope support two-thirds of the way up and pull the top of the tree in the opposite direction in an attempt to get it to straighten over time? Should I just replant the tree and split the curve between the top half and lower half, so that the main lower trunk is leaning slightly in one direction and the top half is leaning slightly back in the other direction, therefore giving the tree an overall straighter look? Thanks.

I would remove the stake and see what happens. unless prevailing winds or light conditions interfere, I believe it will straighten itself.

Hmm, a bit of a pickle. Here's why: woody plants need to be able to move in the wind so to strengthen their trunk and branches. Staking usually prevents this, with damage as a result once the stake is removed.
However, if the wood isn't completely lignified and can be bend, you're still able to decide the direction of growth and shape. However, this usually is done by staking, which as I said above is not advised long term.

So here's something that MIGHT work, taken from the bonsai community. If the part of the central leader that's bent is still flexible (and not too thick!), you might be able to use heavy duty wiring to straighten it out. However, this will need to be cut, loosened or removed periodically as not to inhibit secondary growth (girth) and possibly girdle it.

Or learn to live with it. I myself have planted a larger hornbeam specimen this year, and when you look at it closely, from the trunk up to the tip has a light S shape. I don't really mind at this point.

I know it's not a solution any arborist would recommend, because you want a new tree to sway in the wind, but I used the rope idea from my first post. I held the lower trunk in place with one rope and pulled the top of the tree the other way with another rope. The curved part of the trunk was just flexible enough to be forced straight. It has now been three months and the tree is mostly straight on its own. One more month should finish the job. I will then remove the ropes and let the tree go free. I'll keep an eye on it, though. On windy autumn days I could provide some loose supports so the tree doesn't whip around too much.

Just put in a taller stake. Tie it back to the stake to straighten it. Once it developes some girth and strength, you can remove the stake.
Straight line winds can snap mature trees rather easily. I would not have faith that if left unsupported, it will be fine with the wind and breezes.

BigBill said: Just put in a taller stake. Tie it back to the stake to straighten it. Once it developes some girth and strength, you can remove the stake.
Straight line winds can snap mature trees rather easily. I would not have faith that if left unsupported, it will be fine with the wind and breezes.

This tree isn't growing in a wind prone area like the Highlands or a mountain top. In those places you can indeed see trees growing at a fair angle (near horizontal actually).
In a 'normal' suburban garden setting this is rarely an issue.

I am not thinking about mountain tops or the highlands. The owner lives in East Central Iowa, I have family there.
Spring thundershowers/tornados can cause a lot of wind damage and they are a constant threat.
Why not be safe then sorry?

BigBill said: I am not thinking about mountain tops or the highlands. The owner lives in East Central Iowa, I have family there.
Spring thundershowers/tornados can cause a lot of wind damage and they are a constant threat.
Why not be safe then sorry?

So then how does your earlier advice of 'supporting' it with a stake improve the current/future situation? Seems like a lost cause. Trees must be a rare sight in Iowa because of that...

I suggested a stake to support and help strengthen a young tree. But in my mind, any support is better then no support.