Bava Basra 60b: A Chasan puts ashes on his head in the place where Tefilin are worn.

Ta'anis 16a: In fasts due to lack of rain, we put ashes on the heads of the Nasi and Av Beis Din, the place where Tefilin are worn.

Radak: It is an Avel's practice to put ashes on his head.

Malbim: A mourner is lowly and disgraced in three matters. (a) His place. He sits on the earth and his horn wallows in it. In place of this, I will give to them glory. Also refer to 61:3:2:2, 61:3:3:2.

Radak: This is like "Shemen Sason me'Chaverecha" (Tehilim 45:8). A mourner does not anoint; the woman from Teku'a proves so (Shmuel II, 14:4). This reveals about Pe'er Tachas Efer (that it is a contrast to Aveilus ¹ )

Malbim: An Avel chooses to be lowly and disgraced. He does not bathe or anoint. I will give to you Shemen Sason in place of the oil that you did not anoint with before.

Radak: This is a metaphor, for Ma'ate Sehilah is not the opposite of Ru'ach Kehah. We find Ma'ate (wrapping) and covering regarding Ru'ach, e.g. "v'Ru'ach Lavshah Es Amasai" (Divrei ha'Yamim I, 12:19). Kehah is a feminine adjective; the Ru'ach is short and weak, like "v'Chihasah Chol Ru'ach" (Yechezkel 21:12). Yisrael's Ru'ach is short in Galus.

Malbim: An Avel's inner Ru'ach is lowly and disgraced. His heart is worried, and his Ru'ach's light is weak and lowly in him. In place of this, I will give to you Ma'ate Sehilah, that you will glorify yourselves in public.

Rashi: This is an expression of Ilanos (trees), like "me'Eilim Asher Chamadtem" (1:29). "Mata Hashem" affirms this. Malbim - they used to plant trees with certain associations, e.g. trees of freedom. Some trees signify Tzedek Bein Adam l'Chavero; people used to gather around them for a sign for acting righteously. Yisrael will be called trees of Tzedek that Hashem planted to be glorified. They will send their branches far. From Yisrael, Tzedek will be glorified and send out branches.