Bli Neder, I would like to research this further, but at the moment it seems to me that the basic explanation is this: From Devarim 8:9 alone it appears that the land of Israel will possess many species, even scarce ones.

But we do not know exactly how far that range of produce variety extends. Now, Rebbi Meir infers that the law of Orlah in Vayikra 19:23 refers not only to actual fruit trees, but it includes even less prominent species such as the peppercorn. Well, since Vayikra 19:23 refers to produce in the land of Israel (Orlah does apply to produce of Chutz la'Aretz in some circumstances (see Berachos 36a and Kidushin 38b), but the verse is clearly referring to after we enter the Land), we may conclude that the abundant and varied produce of the land of Israel -- which Devarim 8:9 is discussing -- will include even the peppercorn despite the fact that it is relatively unprominent (compare it, for example, with those species mentioned immediately prior in Devarim 8:8).

Baruch she'Kivanta! The Aruch la'Ner (Sukah 35a, DH v'Ein Eretz Yisrael Chaseirah Klum) address your question. Let me credit Rabbi Moishe Cohen who helped me to understand what the Aruch la'Ner says.

According to Rashi (Sukah ad loc., DH Chayavin b'Orlah), since peppercorn grows low to the ground, one would expect it to be Patur from Orlah. (Notably, Tosfos ad loc. DH l'Lamedcha argues.) The words "Kol Etz Ma'achal" teaches us that it is nevertheless Chayav.

But ponder this: Why, according to Rashi, doesn't the Torah include peppercorn in the obligation of Orlah by simply stating "Kol Etz O Rotem," meaning "any tree or shrub" (trees being tall, shrubs being low)?

The answer is, says the Aruch la'Ner, because that would only obligate shrubs which happen to exist in Eretz Yisrael, but it would not necessarily include peppercorn. Maybe there is no peppercorn in Eretz Yisrael!

Therefore, the Torah instead uses the phrase "Kol Etz Ma'achal" which indicates a Pri whose Etz has a taste like the fruit itself, i.e. peppercorn. Now, if peppercorn is definitively obligated in Orlah, then we know it must grow in Eretz Yisrael. Thus, the verse, "Lo Techsar Kol Bah," includes even peppercorn as species that will exist in Eretz Yisrael, which answers the original question.

Warmest regards,

Yishai Rasowsky