What do you mean by a 'tech tree'?
There are several new buildings added in this mod. Along with these buildings, there are also several new items and metals and the like. Varying buildings allow you to do varying things of increasing coolness, but in order to do those things one needs to find certain metals, acquire certain items, or otherwise unlock the next tier by random chance. It's not incredibly difficult to progress along the tech-tree, nor is it necessary, but it does take time.
I don't want to spoil anything, and narhiril can probably give a much much better explanation than I ever could, but here's an example: The dwarves of defensepicks have a robust alchemy program, it hasn't really provided them much of use until now. While mining out the hauler's quarters, they came across a vein of a strange ore which they were quick to dub "Orichalcum". After smelting this ore into bars, the metal was tossed off into a backroom stockpile for a number of years until one day a curious alchemist took one bar to his workshop in order to test its properties. While working, he discovered that the metal had a strange shape-memory property, and he was soon able to create a set of tools out of them.
A passing mechanic saw this set of tools, recognized them for what they were, and dashed off to build an Advanced Mechanics Shop. Here, he was able to create more of those tools with the right metals, as well as advanced mechanisms which were put to use in creating obstacle courses, mechanical training dummies, and various training courses for the fort's medics.
As these dwarves trained, they documented their progress in a variety of ways. Alchemists took these documents and studied them, eventually coming up with a variety of scientific advances, one of which allowed the fortress to harness the raw energy found in a stone called Lustrum, essentially removing their reliance on wood for fuel.