I had an idea for a new world that included cactus forests, with cactuses the size of a giant sequoia. Is this realistic?
Both the Saguaro and the Giant Cardon cacti can reach 60 to 80 feet in height ... 1/4 the height of a mature sequoia. It's thus plausible to think that, in world with lower gravity than earth and fewer competitors for the cacti, they could reach over 200 feet high (sorry, don't have the math to say how low).
The lower gravity would be important; the main reason the saguaro is so tall is to hoard water while not increasing sun exposure the way a round body would (see the first link for this). This would also suggest that a longer period between rainy seasons would make saguaro-like cacti likely to grow taller.
So, if you want sequoia-tall cacti, you want a world with lower-that-earth gravity, and a large arid continent with hot sun where it rains heavily every 2-3 earth years, but very little otherwise.
What this doesn't get you is a dense forest of 200ft cacti. The water-hoarding cycle of large cacti requires them to be spaced pretty far apart, so that each plant can gather as much moisture as possible.
Tall trees have been done. But what about a cactus tree that is as wide as it is tall? A pyramid cactus?
Some dry adapted plants are nearly as wide as they are tall. The baobab tree is one.
Minimizing surface area is a fine strategy for dry-adapted plants and there are lots of cactus that are barrel shaped or nearly spherical.
Your giant cactus would take this to the extreme - a colossal wide trunk slowly tapering up to the top. Most of this internal volume is occupied by water kept in gel phase, cactus style. The cactus trees might cheat, tending to grow adjacent to rocks which capture rain and dew and let it run off to the tree. As the tree grows adjacent rocks are used for support.
In the desert, these cactus trees have no competition for sunlight*. They are so large because its water volume must sustain it through prolonged dry periods. The great pyramid trees shrink and wrinkle during these dry times.
Giant green living pyramids in the desert wastes would be pretty cool for a fiction. As opposed to living on top of these "trees" the inhabitants would live inside burrows that they excavate within the living tree. I could imagine the tree might be OK with animal life - nitrogen and phosphorus can be hard to come by as well. If you are the desert's master waterhoarders, you can trade a little of your hoard in exchange for nutrients that the animals bring you in the form of their wastes, and eventually their bodies.
* I wonder why the saguaro cacti get so tall. They have no competition for sunlight either.