I visited some famous tourist hotspots in a country where I enjoyed some beautiful sceneries. However, once we were told some trees planted there were actually fake (the trees were carefully sculpted to appear natural), we felt betrayed by the __?
Fraud sounds like a good fit for your example of the fake trees. It matches one of the definitions from Merriam-Webster very well:
a copy of something that is meant to look like the real thing in order to trick people
"fake" could also be used as a noun and thus "fakes".
Ersatz is useful in this context. From en.wiktionary, ersatz means “Made in imitation; artificial, especially of an inferior quality”. Eg, “We felt betrayed because the palm trees were ersatz”.
Synonyms include artificial, faux, imitation, knock off.
Edit: Ersatz is of course an adjective rather than a noun. In terms of nouns, previously-suggested deception perhaps has the best fit in the example sentence, “We felt betrayed by the ____”. Terms like sham and bogosity have more-negative connotations than are warranted, because the question clearly says “the trees were carefully sculpted to appear natural”. The problem is not the appearance of the fake trees, but rather the engendered feeling that a confidence trick has been played. Knowing a deception's been made makes the scenery less believable. One might then fill in the blank with unbelievability, or perhaps with a longer phrase like “the incongruity or shabbiness or unreality of trying to improve on nature”.
I would suggest going with something more than a single word. Perhaps something more along the lines of "We felt betrayed by the subterfuge of using artificial trees instead of real and natural surroundings." Alternatively instead of subterfuge: fraud, deceit, dishonesty or deception.
Also, in regards to Reto Koradi's comment above in reply to jwpat's suggestion of 'ersatz'. When used in English, is it very common for the speaker/writer to intend to impress a sense of a lower quality substitute. And has long been used in this manner.
The specific quality by which you feel betrayed here, I think, is lack of authenticity. If you want a single word, then the slightly-clunky inauthenticity would work.
Especially when traveling, it seems like we're all in search of a truly "authentic" local experience, and this is exactly what's lacking at overdone tourist hotspots.
A scam is a devious scheme set up in order to cheat people, usually out of money. In this context it fits because the OP was part of a group of paying tourists. As a tourist one hopes to witness first hand the natural (and possibly unspoilt) splendours of a country. The tourist is but a client who pays a fee to the tourist agency, tourist guide/tour guide etc. and in exchange expects not to be lied to. This fee includes visiting the area's natural and authentic scenery.
Once established that some of the trees are fake, one could also call the setup, a hoax, which Merriam_Webster defines as, an act that is meant to trick or deceive people.
Simulacrum? Might not carry the 'disappointment' that you were looking for though....
Faux palm trees can be especially disturbing when you tap them and hear a metallic ringing rather than a woody thud.
Made in imitation; artificial:
Not genuine; fake or false