May 2022, ScientificAmerican.com 15Source: “The Number of Tree Species on Earth,” by Roberto Cazzolla Gatti et al., in Proceedings of the National Academy of Sciences USA, Vol. 119; January 31, 2022E C O L O G YHidden
Forests
Thousands of tree species
remain unknown to scienceThe world’s forests may hold more
secrets than previously thought: a new
global estimate of tree biodiversity
suggests that there are about 9,200 tree
species yet to be documented. Most are
likely in the tropics, according to the new
research published in the Proceedings of
the National Academy of Sciences USA.
Although trees are hard to miss, they
are also hard to quantify—and sometimes
even to identify. “Their crowns are hun-
dreds of feet up; they’re in between other
things; they look like similar [species],” says
Wake Forest University conservation biol-
ogist Miles Silman, who was not involved
in the new study. “It’s a rare breed of per-
son who sits out in the wild for months on
end and looks at every single tree.”
The new research drew on the efforts
of hundreds among that rare breed from
around the world. These contributors have
catalogued trees in two huge data sets:
One, the Global Forest Biodiversity Initia-
tive, records every species found in exten-
sively documented forest plots worldwide.
The other, TREECHANGE, compiles sight-
ings of individual species. Together they
suggest there are approximately 64,
recorded tree species on the planet—up
from previous estimates of around 60,000.
To arrive at their estimate of an addi-
tional 9,200 yet undocumented species, the
researchers extrapolated from the number
of rare ones already in the databases. Most
unknown species are likely to be defined as
rare, found in limited numbers in small geo-graphical areas, says study co-author and
Purdue University quantitative forest ecol-
ogist Jingjing Liang. The team’s result is
“a rather conservative estimate,” Liang
says, because scientists know less about
the preponderance of uncommon trees
in places such as the Amazon, where out-
of-the-way spots could host pockets of
unusual species found nowhere else. “If
we can focus the resources, the forest
inventory expertise and money, on those
rain forests in the Amazon and Borneo,”
Liang adds, “then we would be able to esti-
mate it with higher confidence.”
Silman says the study result is likely an
underestimate. His and his colleagues’ local
surveys suggest there are at least 3,
and possibly more than 6,000 unknown
tree species in the Amazon basin alone.
Tree species often get lumped together
based on appearance, he notes, so new
genetic analysis techniques will likely lead
to the discovery of even more biodiversity.
Kenyon College biologist Drew Kerk-
hoff, who was also not involved in the
study, wonders how many species will go
extinct before scientists describe them.
“Conversely,” he says, “how many are
already known to Indigenous peoples in
the Amazon or Congo basins—or were
known to peoples or cultures who have
themselves been rendered extinct through
colonization, disease, genocide or assimila-
tion? How many [species] already have
dried samples sitting in an herbar-
ium cabinet?”
Searching for the new species will
inform not only conservation but the basic
evolutionary science of how and why spe-
cies diversify and die out, Kerkhoff says.
“Just the fact that there are thousands of
species of something as common as trees
out there that are still left to be discov-
ered,” he adds, “I find pretty inspirational.”
— Stephanie Pappas0 4,000 8,000 12,000 16,000 20,000 24,000 28,000 32,AfricaEurasiaNorth
AmericaSouth
AmericaOceaniaObserved
tree speciesSpecies yet to
be documentedEstimated Tree Species per ContinentGraphic by Amanda MontañezSony Interactive EntertainmentSource: “The Number of Tree Species on Earth,” by Roberto Cazzolla Gatti et al., in Proceedings of the National Academy of Sciences USA
, Vol. 119; January 31, 2022E C O L O G YHidden
Forests
Thousands of tree species
remain unknown to scienceThe world’s forests may hold more
secrets than previously thought: a new
global estimate of tree biodiversity
suggests that there are about 9,200 tree
species yet to be documented. Most are
likely in the tropics, according to the new
research published in the Proceedings of
the National Academy of Sciences USA.
Although trees are hard to miss, they
are also hard to quantify—and sometimes
even to identify. “Their crowns are hun-
dreds of feet up; they’re in between other
things; they look like similar [species],” says
Wake Forest University conservation biol-
ogist Miles Silman, who was not involved
in the new study. “It’s a rare breed of per-
son who sits out in the wild for months on
end and looks at every single tree.”
The new research drew on the eff orts
of hundreds among that rare breed from
around the world. These contributors have
catalogued trees in two huge data sets:
One, the Global Forest Biodiversity Initia-
tive, records every species found in exten-
sively documented forest plots worldwide.
The other, TREECHANGE, compiles sight-
ings of individual species. Together they
suggest there are approximately 64,
recorded tree species on the planet—up
from previous estimates of around 60,000.
To arrive at their estimate of an addi-
tional 9,200 yet undocumented species, the
researchers extrapolated from the number
of rare ones already in the databases. Most
unknown species are likely to be defi ned as
rare, found in limited numbers in small geo-graphical areas, says study co-author and
Purdue University quantitative forest ecol-
ogist Jingjing Liang. The team’s result is
“a rather conservative estimate,” Liang
says, because scientists know less about
the preponderance of uncommon trees
in places such as the Amazon, where out-
of-the-way spots could host pockets of
unusual species found nowhere else. “If
we can focus the resources, the forest
inventory expertise and money, on those
rain forests in the Amazon and Borneo,”
Liang adds, “then we would be able to esti-
mate it with higher confi dence.”
Silman says the study result is likely an
underestimate. His and his colleagues’ local
surveys suggest there are at least 3,
and possibly more than 6,000 unknown
tree species in the Amazon basin alone.
Tree species often get lumped together
based on appearance, he notes, so new
genetic analysis techniques will likely lead
to the discovery of even more biodiversity.
Kenyon College biologist Drew Kerk-
hoff , who was also not involved in the
study, wonders how many species will go
extinct before scientists describe them.
“Conversely,” he says, “how many are
already known to Indigenous peoples in
the Amazon or Congo basins—or were
known to peoples or cultures who have
themselves been rendered extinct through
colonization, disease, genocide or assimila-
tion? How many [species] already have
dried samples sitting in an herbar-
ium cabinet?”
Searching for the new species will
inform not only conservation but the basic
evolutionary science of how and why spe-
cies diversify and die out, Kerkhoff says.
“Just the fact that there are thousands of
species of something as common as trees
out there that are still left to be discov-
ered,” he adds, “I fi nd pretty inspirational.”
— Stephanie Pappas0 4,000 8,000 12,000 16,000 20,000 24,000 28,000 32,AfricaEurasiaNorth
AmericaSouth
AmericaOceaniaObserved
tree speciesSpecies yet to
be documentedEstimated Tree Species per ContinentGraphic by Amanda Montañezsad0522Adva3p.indd 15sad0522Adva3p.indd 15 3/23/22 5:26 PM3/23/22 5:26 PMIn
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