In her 2008 book, Between Earth and Sky; Our Intimate Connections to Trees, Nalini M. Nadkarni wrote on page 43,
I calculated that the world supports sixty-one trees for each person on Earth [in 2005]. … When I told my husband […] he reflected for a moment and then voiced wonder that the ratio was so small. “Each person gets sixty-one trees in a lifetime? That seems hardly enough to supply just the firewood we’ll use in our woodstove for the next few winter seasons, let alone the lumber that’s in our house and the paper I put through my printer.” His reflections […] reinforced the sense that I need to think about ways to look after my sixty-one trees, wherever they might be growing in the world.
To see how she arrived at 61 trees for each of us, see the two scanned images of her text below.