added or subtracted. There were long, Talmudic discussions, conducted
partly in English and partly in Spanish, about how, exactly, the recensus
should be conducted. One of the few that I could follow centered on
asymmetry. A tree trunk is not perfectly circular, so depending on how
you orient the calipers when you’re measuring, you’ll get a different
diameter. Eventually, it was decided that the calipers should be oriented
with their fixed jaw on a dot spray-painted on every tree in red.
In the plots, each tree over four inches in diameter has been tagged.
Owing to the differences in elevation, each of Silman’s plots has a
different average annual temperature. For example, in Plot 4 the average
is fifty-three degrees. In Plot 3, which is about eight hundred feet higher,
it’s fifty-one degrees, and in Plot 5, which is about eight hundred feet
lower, it’s fifty-six degrees. Because tropical species tend to have narrow
thermal ranges, these temperature differences translate into a high rate
of turnover; trees that are abundant in one plot may be missing entirely
from the next one down or up.
“Some of the dominants have the narrowest altitudinal range,” Silman
told me. “This suggests that what makes them such good competitors in
this range makes them not so good outside of it.” In Plot 4, for example,
ninety percent of the tree species are different from those species found