It takes a lot of trees to have a meaningful effect on atmospheric carbon dioxide. In the United States, for example, our vast national forests store about 16 percent of the carbon we annually produce. All of the street trees in U.S. cities together store only less than 1 percent of our carbon emissions. Still, the 360 metric tons of carbon that street trees store won't warm our climate.What if we converted half of subirdia's lawns to trees?
. . .
[T]hink about the effect that we could collectively have on carbon if only half of America's turflands—twenty million acres—could do what my trees do. Turf consumes carbon just as does any plant, but unlike trees, grass doesn't put on an annual layer of wood. Its use of carbon dioxide increases the soil's carbon stores. . . . [A]n acre of U.s. grass annually adds about eighteen hundred pounds of carbon to the soil, and it continues to do so for about thirty years after it is planted. An acre of my trees did about three times that amount, in aboveground carbon storage, and they have been doing so for seventy years. Their annual storage capacity will slow as they age, probably in a century or so, but their overall capacity to buffer our climate is magnitudes beyond what grass could ever hope to do. . . . [A]n additional twenty million acres of trees could absorb another 1 percent to 2 percent of our national carbon emissions. In this accounting, every little bit helps!
John M. Marzluff, Welcome to Subirdia: Sharing Our Neighborhoods with Wrens, Robins, Woodpeckers, and Other Wildlife (New Haven: Yale Univ. Press, 2014), pp. 223, 225.
According to the Arbor Day Foundation, "National Arbor Day is always celebrated on the last Friday in April, but many states observe Arbor Day on different dates throughout the year based on best tree planting times in their area." In Washington State, it's the second Wednesday in April.