I had posted this previously and the experts felt the question was not mentioned although it was. Please answer this question. 
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The poem does not make a clear mention of what the letters are about, though it does mention that they are not about the loss of green cover due to felling of trees. This is a clever technique by the poetess to draw attention to deforestation. This poem has underlying tones of feminism. Perhaps, the letters are about the poetess' support of feminism. Also, the trees are a symbol of human beings trying to break free of shackles. The letters could express her ideas of freedom.