Some of the walks took place in June, whereas others took place in January; most people didn’t particularly enjoy trudging through the harsh Michigan winter, but their scores jumped just as much as in the summer trials.
I found this the most interesting point in the article. I would have assumed that any psychological effect of viewing trees would be largely due to their greenness, since that is their dominant visual aspect. But, assuming a largely deciduous environment, naked trees in winter would seem to have the same effect. So the effect must be stimulated by something deeper than just raw color.