Answer to question regarding benefiting from the fragrance of blossoms of an orlah tree:
The Radbaz (Shu"t 1:44) discusses this topic and rules that, if the tree was not planted for its fruits or if only the blossoms are present and the fruit has not yet grown, the fragrance is permitted (i.e. there is no problem of orlah preventing enjoyment of the ...
Another answer (suggested here in the Sefer "Kerem Efraim") is that because the tree was created through a miracle, it was not obligated in Orlah. He brings support from the Radak (Malachim Beis 100:4) who writes that the oil that was miraculously created for the wife of Ovadia did not require Maaser to be separated from it.
He explains that for the first ...
Thanks to Alex's comment elsewhere (which I only saw now), I discovered the Pische S'shuva, YD 294:13, who cites Parach Mate Aharon as saying the soil must last three years, and Shivas Tziyon as qualifying that that's only in eretz Yisrael: in chutz laaretz, he says, [where safek orla is permitted,] the soil must last "a few days".
Orlah (fruit from trees planted within 3 years) is forbidden for all benefit, see e.g., Mishne Torah Maachalot Assurot 10:9
Whenever anyone plants a fruit tree, it is forbidden to partake of
or benefit from all of the fruit the tree produces for three years
after being planted
For details see e.g., here from aish. Note this applies both in and ...
There is Orla only for a tree that was planted for eating.
See Rash Orla 1.1
פטור מן הערלה. כדדריש בירושלמי (הלכה א) (ויקרא יט) ונטעתם כל עץ מאכל את שהוא למאכל חייב
So, because roses are generally not cultivated for eating, there is no din Orla and you can smell their perfume.
Ruled in SA YD 294.1
הנוטע עץ מאכל מונה לו ג' שנים מעת נטיעתו וכל הפירות ...
The Pitchei Teshuva in Yoreh Deah 294 sk 13 quotes a number of opionions but rules that in Israel where we rule stringently about doubts relating to orla, there must be enough dirt to survive three whole years; in the Diaspora, however, where we rule leniently about such doubts, it only needs to be enough dirt to last a couple of days.
As already stated, this site does not replace a Rabbi. But here are some sources.
The Mishan in Mas. Orla (3:10) says:
ספק העורלה--בארץ ישראל אסור, ובסוריה מותר; ובחוצה לארץ יורד ולוקח, ובלבד שלא יראנו לוקט.
If in doubt, then fruit that could be Orla are forbidden in Eretz Yisrael, but permitted in the Diaspora.
That's also have the Shulchan Aruch ...
After CYLORing this, the answer I got is:
If the tree is planted exclusively for beauty - there is no problem to enjoy from the fruits during first 3 years, and even to eat them.
If the tree is planted also for eating the fruits - it is forbidden to enjoy from the fruits, even from observing (להתבונן) their beauty. However, there is no problem just to look ...
Have there been any (scientific) studies that prove the benefits of Orlah?
Well, as you will see from the answer to your second question, it's not about benefits.
You can see from articles like this one that most fruit trees don't bear (real) fruit for the first 3 - 4 years.
You then asked:
What is the reasoning behind Orlah prohibition?
Shulchan Arukh YD 294:9 (based on Kiddushin 39a) rules that doubtful (safek) Orlah in the Diaspora is permitted; only certain (vadai) Orlah is prohibited. Even if you know the fruit came from a orchard with Orlah trees, if you don't know which tree it came from then it is permitted. So any fruit you find in the grocery which was not imported from Israel and ...
Rambam Maaser Sheni 9:7 (based on Mishna Maaser Sheni 5:1):
מי שהיה לו נטע רבעי בשנת השמיטה, שיד הכול שווה--צריך לציינו בקוזזות אדמה, כדי שיכירו בו, ולא יאכלו ממנו, עד שיפדו. ואם היה בתוך שני עורלה--מציינין אותו בחרסית, כדי שיפרשו ממנו: שאם ציינו בקוזזות אדמה שמא יתפרדו--שאיסור עורלה חמור הוא, שהיא אסורה בהניה. והצנועין היו מניחין את המעות בשנת השמיטה ...
In most Halachic cases, we say that Mitzvoth are violated only if there is a Ma'aseh - an action. However, this article asks the very question you ask, with regard to aesthetic trees, but it does not give an answer (because the "tree" in his case was not a Halachic one). In fact, he implies both that the prohibition is strictly on eating the fruit, and that ...
Reminds me of this (Sanhedrin 59b -- summarized and text below):
Rabbi Shimeon ben Chalafta was walking on the road when lions met him and roared at him. Thereupon he quoted from Psalms: “The young lions roar for prey and to beg their food from HASHEM,” and two lumps of flesh descended from heaven. They ate one and left the other. This he brought to the ...
According to this answer, it would seem that this is permitted.
The Radbaz (Shu"t
discusses this topic and rules that, if the tree was not planted for
its fruits or if only the blossoms are present and the fruit has not
yet grown, the fragrance is permitted (i.e. there is no problem of
orlah preventing enjoyment of the fragrance):
Since the majority of produce on the market is not Orlah, we can assume that any given fruit etc. it is permissible, using the halachic principle of rov.
(Regarding Reva'i, I'm pretty sure it doesn't apply in the diaspora. The mishna in Kiddushin only mentions Orlah.)
(I'll try to add an explicit source later).