Taoists, Taiji players, and Qigong practitioners have been saying this for years (and we don't really consider it any more a mystery than anything else in our experience). It's why we practice with the elements, regardless the conditions:
From the article: "Spending time in nature provides protections against a startling range of diseases, including depression, diabetes, obesity, ADHD, cardiovascular disease, cancer, and many more, research shows. How this exposure to green space leads to better health has remained a mystery. After reviewing hundreds of studies examining nature's effects on health, an environment and behavior researcher believes the answer lies in nature's ability to enhance the functioning of the body's immune system."
"The realization that there are so many pathways helps explain not only how nature promotes health, but also why nature has such huge, broad effects on health,...Nature doesn't just have one or two active ingredients. It's more like a multivitamin that provides us with all sorts of the nutrients we need."
From Science Daily