The world has more kinds of trees than we thought: scientists estimate that about 12 to 14 percent of the Earth’s tree species have yet to be discovered.

On the last Friday in April, we will mark the annual National Arbor Day. This year, that date is April 28. Much like Earth Day, Arbor Day celebrates nature. Its main purpose is to encourage people to plant trees. Some communities also take this opportunity to organize litter-collecting events.

I’d like to seize the occasion this year to tell you about three pieces of good news regarding trees:

1) A new study involving more than 100 scientists from around the world and the largest forest database yet assembled estimates that there are about 73,000 tree species on Earth, including about 9,200 species yet to be discovered.


Roughly half to two-thirds of all known tree species occur in subtropical and tropical forests, which are both species-rich and poorly studied by scientists.

2) As temperatures have steadily risen, the growing period for hardwood forests in eastern North America has increased by an average of one month over the past century.

3) A 30-year tree planting campaign in Portland, Oregon, allowed researchers to show that the number of trees planted on city streets is associated with reductions in mortality and that the relationship grows stronger as the trees age and grow.

More trees

Forests provide many ecosystem services to humanity for free. In addition to supplying fiber, fuelwood, timber and other products, forests clean the air, filter the water, and help control erosion and flooding. They also help preserve biodiversity, store climate-warming carbon, and promote soil formation and nutrient cycling. That’s why extensive knowledge of tree diversity and richness is key to preserving the functioning and stability of ecosystems.

This map shows the number of trees and the number of tree species per continent in the Global Forest Biodiversity Initiative (GFBI) database. The GFBI blue points were merged with the TREECHANGE occurrence-based data (purple points) to provide the estimates in the study. Green areas represent global tree cover. ©“Proceedings of the National Academy of Sciences”

So, to learn more about our planet’s trees, researchers from the Global Forest Biodiversity Initiative, Indiana’s Purdue University and the University of Michigan combined tree abundance and occurrence data from two global datasets: one from the Global Forest Biodiversity Initiative and the other from TREECHANGE. The combined databases yielded a total of 64,100 documented tree species worldwide, a total similar to a previous study that found about 60,000 tree species on the planet.

After combining the datasets, the researchers used advanced statistical methods to estimate the total number of unique tree species at biome (such as a boreal forest, a tropical rain forest or a savanna), continental and global levels—including species yet to be discovered and described by scientists.

In a paper published on January 31, 2022, in the science journal Proceedings of the National Academy of Sciences, the researchers state that the conservative estimate of the total number of tree species on Earth is 73,274, which means there are about 9,174 tree species yet to be discovered. Most of the undiscovered tree species (about 8,200) are likely to be rare, they say, with very low populations and limited spatial distribution; and roughly 40 percent of them are probably in South America—more than on any other continent.


South America has special significance for global tree diversity. The continent has the highest estimated number of rare tree species (about 8,200) and the highest estimated percentage (49 percent) of continentally endemic tree species.

Hot spots for undiscovered South American tree species presumably include the subtropical and tropical, moist forests of the Amazon Basin, as well as Andean forests at elevations between 3,300 and 11,480 feet. There might be as many as 4,000 unknown tree species in South America, and most of them could be endemic.

And that makes forest conservation a paramount priority in South America, especially considering the current tropical forest crisis from human-caused disruptions, such as climate change, deforestation and fires. Undiscovered species are especially vulnerable to such anthropogenic impacts. The study’s authors hope their new findings will help establish a quantitative benchmark that will help prioritize forest and tree conservation efforts and contribute to the future discovery of new trees in certain parts of the world.

A longer growing season

From 1883 to 1912, a Wauseon, Ohio, farmer named Thomas Mikesell recorded the seasonal changes in precipitation, temperatures and his observations of the trees and other plants near his home. In the process, he created what may be the only comprehensive, prewarming dataset of tree growing patterns in North America.


Forest conservation must become a priority in South America, especially since tropical forests are now suffering from anthropogenic disruptions, such as climate change, deforestation and habitat loss.

To see how those statistics compare with the growing patterns of today’s trees, researchers from The Ohio State University conducted a study that included present-day observations of the time span from bud burst to peak leaf coloration in seven tree species and similar documentation that was collected by Thomas Mikesell at the turn of the 20th century.

Between 2010 and 2014, the researchers traveled to Wauseon multiple times per week in the spring and the fall to make their own observations of American elm, black oak, black walnut, eastern cottonwood, staghorn sumac, sassafras and white oak trees, all of which grow well across most of the United States. They also used monthly temperature and precipitation data from the U.S. Historical Climatology Network’s Wauseon station to calculate long-term trends.

Results showed a clear connection between increased warming during winter and spring and an extended period of tree growth by about a month. The fact that leaves stayed on trees about 15 percent longer than they did 100 years ago was an obvious indicator that temperatures are profoundly different.


Warming due to climate change can affect oaks by modifying the length (delaying leaf fall) and the start (earlier bud bursting) of the growing season. But climate change can also amplify water stress on oaks by raising evapotranspiration rates.

Though all tree species did not respond to changing temperatures in exactly the same way—some budded early, and most kept their leaf color longer into the fall—two patterns stood out: 1) the average midwinter and spring temperatures in the region have increased by up to 5 degrees Fahrenheit since 1892, with six of the 10 warmest years in November and December occurring since 1990; and 2) leaves’ longer life spans into autumn indicated when most of the growing season extension took place because foliage coloration was delayed.

The dates of peak coloration were used (rather than when leaves fell to the ground) to determine the end point of the growing season to tie in with each tree’s peak period for photosynthesis. As leaf colors fade, trees become much less efficient at taking in carbon dioxide and water to obtain the sugars that sustain them.

While extended growing likely increases trees’ absorption of carbon dioxide from the atmosphere, the mix of extreme temperature fluctuations and general warming can stress trees in ways this research couldn’t detect. Overall, though, there was quite a bit of variety in the species’ responses to changing temperatures, which is a red flag for biologists.


One recent study shows that leaves are staying on trees about 15 percent longer than they did 100 years ago.

In their results, the researchers state that if we expose organisms to the exact same environmental pressures and we then see radically different responses, chances are that one of those responses is going to be better in the long run than the others. Time will tell who the ultimate winners and losers will be, and what that means for how different forests will end up looking if some species are consistently underperforming because they can’t handle the extremes we’ve introduced to the system.

These findings, reported in the journal PLOS ONE on March 3, 2023, point to the need for even more species-specific research to improve our predictions about how forests and their valuable carbon-absorption services will respond as the climate continues to change. An entire month of growing season extension is huge when talking about a pretty short period of time for those changes to be expressed.

As we work to mitigate the effects of climate change, knowing how much benefit we can get from the trees we already have and from planting more becomes important.


Between 1990 and 2019, Friends of Trees planted almost 50,000 trees on the streets of Portland, Oregon.

Healthier lives

The association between our exposure to nature and lower mortality has been accumulating for years. However, say some researchers, most studies use satellite imaging to estimate the amount of vegetation in an area, which does not distinguish the different types and cannot be directly translated into tangible interventions. So, the authors of a study that was published in the journal Environment International in December 2022 took advantage of a natural experiment that occurred in the city of Portland, Oregon, between 1990 and 2019. The nonprofit organization Friends of Trees planted 49,246 trees on the streets—and kept records of when and where the trees were planted.

The study’s authors looked at the number of trees planted in a given area (specifically, a census tract, where approximately 4,000 people live) in the preceding five, 10 and 15 years. They associated this information with mortality due to cardiovascular, nonaccidental and respiratory causes in that same area, using data from the Oregon Health Authority. What they found was that in neighborhoods in which more trees had been planted, mortality rates (deaths per 100,000 people) were lower. This negative association was significant for cardiovascular (a 6 percent reduction) and nonaccidental mortality (a 20 percent reduction), particularly for males and people over the age of 65.

Furthermore, the link got stronger as trees aged and grew: the reduction in mortality rates associated with trees planted 11 to 15 years before was double that observed with trees planted in the preceding one to five years (30 percent and 15 percent, respectively). This means that preserving existing mature trees may be particularly important for public health.


Residents of Portland, Oregon, neighborhoods where more trees had been planted had lower mortality rates. The reduction was particularly significant for males and people over age 65.

The study did take into account other factors that may influence mortality, such as education, income and racial composition of the neighborhoods. While it didn’t provide a direct insight into how trees improve health, the finding that large trees have a greater health impact than smaller ones is telling, because larger trees are better at absorbing air pollution, moderating temperatures and reducing noise (three factors linked to increased mortality).

And, finally, according to the scientists’ estimates, the benefits of tree planting greatly outweigh the costs: the annual cost of planting and maintaining one urban tree in each of Portland’s 140 census tract areas ranged between $3,000 and $13,000, while it generated about $14.2 million annually in lives saved.

A stronger connection

Since 1972, the Arbor Day Foundation, an American nonprofit membership organization, has planted more than 350 million trees in cities, communities, forests and neighborhoods throughout the world, helping us all to live healthier and longer lives.


The benefits of tree planting greatly outweigh the costs. This Arbor Day, I hope you will plant a tree—or promise yourself that you’ll do your best to take care of the ones already in your world.

Although it would be pretty amazing, I’m guessing that most of us won’t go out and plant a tree on Arbor Day. But since trees give us so much, what we can do is pledge to take care of the ones in our neighborhoods, yards and favorite parks—and in that way, we can appreciate and speak for the trees we already have, and advocate for growing more in the future.

Here’s to finding your true places and natural habitats,