What is the meaning of lines 53-54 of Yevgeny Yevtushenko's poem "Babi Yar" ("We are denied the leaves, / we are denied the sky")?

Yevgeny Yevtushenko’s famous poem “Babi Yar” is a
meditation on a particular place in the Soviet Union (a place in fact named in the
title) where tens of thousands of Jews were killed by Germans during World War II.
Yevtushenko’s poem takes this particular place as a symbol of a broader, more widespread
anti-Semitism which has plagued the Jewish people at least since the times of ancient
Egypt (see lines 6-7). Yevtushenko also alludes to French anti-Semitism during the
so-called “Dreyfus affair” (line 10) as well to anti-Semitism inflicted on the Jews
during other times and places, including anti-Semitic pogroms (or persecutions) in
Russia (see links below).

At one point, Yevtushenko alludes
to the well-known story of Anne Frank, a young Jewish woman who lived in Holland during
World War II. She and her family, along with another family, were forced to hide
themselves in a small, secret annex during the war in the hope that they would not be
sent off to concentration camps, as so many Jews in occupied Holland were at that time. 
In the following lines, Yevtushenko alludes to Frank and, apparently, to her romantic
relationship with Peter, a youth her age, who was also an occupant of the
annex:

It
seems to me that I am Anna Frank,
Transparent, as the thinnest branch in
April,
And I'm in love, and have no need of phrases,
But only that
we gaze into each other's eyes.
How little one can see, or even
sense!
Leaves are forbidden, so is sky,
But much is still allowed -
very gently
In darkened rooms each other to
embrace.

The phrase “leaves
are forbidden, so is sky” seems to allude to the cramped interior conditions in which
Frank and Peter conducted their brief romance.  They were shut off from the outside
world, prevented from enjoying the kind of natural surroundings that might have nurtured
and symbolized their love in better times. Nevertheless, they were able to embrace and
kiss despite their surroundings. In fact, Anne, in her diary, vividly describes their
kisses and their brief but poignant romance.

Later,
Yevtushenko uses natural imagery in different ways when he
writes,

Wild
grasses rustle over Babi Yar,
The trees look sternly, as if passing
judgement.

Earlier, the
reference to leaves and the sky had suggested the beautiful, open, vital aspects of
nature.  Here, however, the beauty of the grasses contrasts with the ugliness of what
Babi Yar now symbolizes. Trees, rather than being symbols of life, now almost seem to
stand like stern judges, harshly condemning the ugliness that anti-Semites have created
in a place that might otherwise have seemed a beautiful part of
nature.