Rambam on Planting non-fruit trees on Shemita

Question

The Yerushalmi in Sheviit 4:4 says:

וכן היה רשב״ג אומר נוטעין אילן סרק בשביעית
So too Rabban Shimon ben Gamliel would say - one may plant non-fruit trees on the seventh

then RaMBa"M goes and says in hilkhot shemita ve'yovel 1:5:

אין נוטעין בשביעית אפילו אילן סרק, ‏

One may not plant on the seventh year even non-fruit trees [are included in the prohibition]

Rambam seems to contradict an explicit Yerushalmi and then this source in the Bavli Sukkah 40a seems to strengthen Raban Shimon ben Gamliel in the Yerushalmi:

עצים בעלמא הוא, ועצים אין בהן משום קדושת שביעית, (דתנן) +מסורת הש"ס: [דתניא]+ עלי קנים ועלי גפנים שגבבן לחובה על פני השדה, לקטן לאכילה - יש בהן משום קדושת שביעית, לקטן לעצים - אין בהן משום קדושת שביעית! ‏

For this is merely wood and wood does not possess the sanctity of the Sabbatical Year, as it has been taught, Leaves of reeds and leaves of the vine which have been heaped up as a hiding-place upon a field, if they were gathered for [animal] food, they possess the sanctity of the Sabbatical Year, but if they were gathered for firewood, they have not the sanctity of the Sabbatical Year

1. Who is Rambam holding like?

2. Are there sources that I am missing in this conversation?

3. How would the Rambam explain these gemarot?

4. Do you think that this is a conversation about the prohibitions of shemita being on the land vs the person?

Answer 1

The Bavli in Sukka talking about collecting wood, not treeing.

According to the Kesef-Mishne, Rashbag says it is allowed implies that Rabanan (the other) says it is forbidden. Rambam holding like Rabanan.

Answer 2

R Yosef Tzvi Rimon adresses exactly your question in his reference book on shmita. He writes to your questions 1 & 3 (p. 90)

The Rambam's ruling is problematic because it contradicts an explicit statement in the Yerushalmi. Apparently because the Yerushalmi recorded the opinion that allows planting trees that bear no fruit only in the name of Rabban Shimon ben Gamliel, the Rambam understood the Yerushalmi to imply that the Sages disagree. According to the Sages, even the planting of trees that bear no fruit is forbidden, and the Rambam ruled in accordance with this view.

So paradoxically it is because of that Yerushalmi that Rambam rules the way he does.

To your question 2, he refers to the Kesef Mishne cited in the comments above, Mahari Kurkus ad loc. and adresses the Tosefta cited by @Fred in the comments to say (p. 91, fn 4) that Responsa Maharal Diskin allows such planting as a fence BUT Rav Kook and the Chazon Ish forbit it.

To your question 4, this is a mahloket poskim but R Rimon says that in practice we rule stringently.