Start by creating a Nurbs Circle at the origin; don't worry about the size just yet. We will be adjusting the size later.
Select the circle you just made and using Control+D duplicate the circle 5 times in the Y direction. Adjust the Scale of each circle how you see fit, I decided to try some different shapes for my tree trunk.
Start at the bottom circle and Shift select the circles to the top.
Loft a surface between the circles.
Click image for larger version.
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