New Year's Day for Trees

Jacob Schor ND, FABNO

Photo by Gilabrand via Wikipedia, used
under the Creative Commons License.

The 15^th day of the Jewish month of Shevat, or Tu B’Shevat, begins at sunset on February 7^th this year and marks the Jewish New Year for trees. In Biblical times, knowing the age of a tree was important because the fruit from a tree less than three years of age was considered inedible. This date was set to approximate when the earliest blooming trees in Israel ended their winter dormancy, but during the Middle Ages, Tu B’shvat grew to be an esoteric holiday.

In the middle of the 16^th century, the renowned Kabbalist Rabbi Yitzchak Luria of Safed and his disciples began to celebrate the day with a ritual meal. Each of the fruits of Israel was assigned symbolic meanings to represent concepts from the Kabbalah (the image of a tree is often used to represent the spiritual realms of the universe) and they were eaten in a ritualized order with the appropriate blessings and with adequate quantities of wine. It was believed that this would bring human beings and the world closer to spiritual perfection.

In more recent years, the holiday’s theme has shifted to reflect conservation and ecological themes, a concept that probably traces back to 1890 when Rabbi Zeev Yavetz celebrated the day by having his students plant trees in the agricultural colony of Zichron Yaakov. In the early 1900s, the Jewish National Fund devoted the day to planting eucalyptus trees in the Hula Valley to stop malaria and in 1908 the Jewish Teachers Union (and later the Jewish National Fund) copied their activities and turned the holiday into an annual effort to reforest Israel. This has grown into an annual day of tree planting; this year on Tu’Bshvat over a million Israelis will take part. This holiday provides a suitable excuse to review some recent research on trees and how they impact our well being.

Green Space and Mental Health

The idea that “green space” is fundamental to mental well being has become a popular research subject in recent years, to the degree that a recent Dutch paper employed the term, “Vitamin G” for green space, implying that it is essential for our health and sanity. [1] What is it about green space that is so healthful? It’s not what you might think – a 2008 study proved that green space proximity does not correlate with time spent exercising. [2]

Something about green space does change the way people perceive their lives. The further people live from open green spaces, the more lonely they perceive themselves to be and the fewer social contacts they actually have. [3] The closer one lives to a green space, the less of an impact the stressful events in people’s lives seem to have on them. [4] Though not proven as well as we might hope, there is a growing hypothesis that people need a minimum daily requirement of green space exposure to maintain a state of mental health. [5]

Forest Bathing

Photo by It's Just C via Flickr, used
under the Creative Commons License.

This brings us to recent research on a traditional Japanese practice called Shinrinyoku, translated as ‘forest bathing’ or ‘taking in the forest atmosphere.’ The practice entails just what it sounds like, walking quietly through a forest. That such a practice would leave someone feeling better seems intuitively correct, but the degree to which scientists have proven this is quite surprising.

We first noticed papers on forest bathing in 2007 when three separate journals published human trials. In March 2007, Tsunetsugu et al. reported that college students who either sat for 15 minutes admiring a forest view or walked for a similar period of time both felt better and also had lower blood pressure, heart rate and cortisol levels compared to doing similar activities in an urban setting. [6] In the same month Park et al. reported data on a similar experiment, finding that the forest not only lowered cortisol but prefrontal cortex activity as well. [7]

Other studies watched participants’ responses, finding that forest exposure was associated with significantly decreased scores for hostility and depression on personality testing and increased ‘liveliness.’ The higher the initial stress level in the individual, the more pronounced the benefit of forest exposure. [8] Further studies by Park et al. tested subjects in 24 different forested areas, finding that “The results show that forest environments promote lower concentrations of cortisol, lower pulse rate, lower blood pressure, greater parasympathetic nerve activity, and lower sympathetic nerve activity than do city environments.” All the different forests appear to have a similar impact. [9]

Unfortunately none of these (or the many similar) papers have yet explained the source of the benefit. Is it the sounds of the forest, the frequency of light filtered by leaves, the various chemicals that trees emit into the air, or that lack of urban discord? One can hypothesize that it could be any one of these factors, a combination or something yet unknown. Will an indoor fountain that mimics the sound of a forest brook have the same impact as being in a forest? Will abundant houseplants act as a trigger to relaxation? Could we bottle and aerosolize the odors given off by trees to our advantage? These questions remain unanswered. Suffice to say we humans feel better in the midst of trees.

Our Duty

One is acutely aware of this here in Denver. We live in a place where trees do not grow naturally. Old photographs of homes in our neighborhood when they were first built show these structures standing alone in the middle of an empty prairie. The trees that line our streets and shade our yards are there only here because people have planted, watered and cared for them over the century.

Even with the value we might put on trees locally, 4,500 acres of forest disappear globally per hour, adding up to approximately 18 million acres of forest per year. [15] People in Sub-Saharan Africa, where trees have long been in short supply are burning wood for fuel at a rate that is 30% to 200% faster than it can grow back. Trees store carbon and destroying them hastens the carbon release that causes climate change.

Here in Denver we have a lovely organization called Denver Digs Trees that distributes free trees to residents for planting. I take pleasure in a lovely tree supplied by this group that my neighbor Bill and I planted a decade ago. The group supplies free trees for planting twice a year, Spring and Fall.

If you resonate with those bumper stickers that say “Think globally / Act locally,” then this program is for you. But if you really are the sort who wants to change the world, then you need to know about a group called Green World. As the name implies (and guessing from the context of this article), this group plants trees on a global scale. Their goals are lofty but their plan is simple. Donations are used to plant trees in parts of the world where they will do the most good.

Trees can enrich our lives. We would be far less without them. For this New Year Day I hope that you will take a moment to look about and appreciate the trees that do bring a bit of sanity to our world. I also encourage you to help plant a few more trees either in your own neighborhood or somewhere else in the world.


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1. Groenewegen PP, van den Berg AE, de Vries S, Verheij RA. Vitamin G: effects of green space on health, well-being, and social safety. BMC Public Health. 2006 Jun 7;6:149.

2. Maas J, Verheij RA, Spreeuwenberg P, Groenewegen PP. Physical activity as a possible mechanism behind the relationship between green space and health: a multilevel analysis. BMC Public Health. 2008 Jun 10;8:206.

3. Maas J, van Dillen SM, Verheij RA, Groenewegen PP. Social contacts as a possible mechanism behind the relation between green space and health. Health Place. 2009 Jun;15(2):586-95. Epub 2008 Oct 15.

4. van den Berg AE, Maas J, Verheij RA, Groenewegen PP. Green space as a buffer between stressful life events and health. Soc Sci Med. 2010 Apr;70(8):1203-10. Epub 2010 Feb 12.

5. Dean J, van Dooren K, Weinstein P. Does biodiversity improve mental health in urban settings? Med Hypotheses. 2011 Jun;76(6):877-80. Epub 2011 Mar 22.

6. Tsunetsugu Y, Park BJ, Ishii H, Hirano H, Kagawa T, Miyazaki Y. Physiological effects of Shinrin-yoku (taking in the atmosphere of the forest) in an old-growth broadleaf forest in Yamagata Prefecture, Japan. J Physiol Anthropol. 2007 Mar;26(2):135-42. Free full text http://www.ncbi.nlm.nih.gov/pubmed/17435356

7. Park BJ, Tsunetsugu Y, Kasetani T, Hirano H, Kagawa T, Sato M, Miyazaki Y. Physiological effects of Shinrin-yoku (taking in the atmosphere of the forest)--using salivary cortisol and cerebral activity as indicators. J Physiol Anthropol. 2007 Mar;26(2):123-8 http://www.ncbi.nlm.nih.gov/pubmed/17435354  Free full text

8. Morita E, Fukuda S, Nagano J, Hamajima N, Yamamoto H, Iwai Y, Nakashima T, Ohira H, Shirakawa T. Psychological effects of forest environments on healthy adults: Shinrin-yoku (forest-air bathing, walking) as a possible method of stress reduction. Public Health. 2007 Jan;121(1):54-63. Epub 2006 Oct 20.

9. Park BJ, Tsunetsugu Y, Kasetani T, Kagawa T, Miyazaki Y. The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan. Environ Health Prev Med. 2010 Jan;15(1):18-26.

10. http://www.fao.org/forestry/en/