By Michael Johnsen
With all the talk of being “carbon neutral,” I’m often asked, usually by friends strolling through my garden, how much carbon dioxide my plants are absorbing, storing, and generally combating global warming. But the simple question: “How much CO2 does my garden absorb?” is actually more complicated than the typical sound bite can handle.
Different plants in different soils absorb CO2 at different rates. Even the same plants in the same soils will absorb at different rates depending on every factor that affects plants, from available water to disease. The impacts are real, however, and some scientists believe the effects of climate change are offset by the re-growth of the forests in the Eastern U.S. But before I even get into all that, let me back up and look at the big picture.
The overall CO2 level in the earth’s atmosphere at the start of the Industrial Revolution was around 280 parts per million (ppm) and increased to about 379 ppm by 20051. That’s most but not all the CO2 released from fossil fuels2; some is absorbed by the oceans (making them more acidic, but that’s for the aquaculture column) and the rest is absorbed by plants and soils (yes, there’s lots of carbon in soil — well, in good, organic-rich soil).
OK. So we’ve established that plants and soils — what our gardens are all comprised of (sans gnomes) — absorb some of this fossil-fuel CO2 we’re releasing. Plants use the carbon they pull from the atmosphere to build their cells and structure. Determining what plants absorb the most CO2 is a bit trickier and really depends on where your garden is. A tree’s dry-wood is widely accepted to be around 50% carbon, but newer research (not much has been done) shows that trees can differ, from about 50 percent for maples to over 55% in the very old heartwood of Sequoias3.
Yet another factor is how many different species of plants you grow in your garden. Scientists at Brown University, in research plots on the steppes of Patagonia, found that biodiversity makes a big difference in the amount of organic matter a system produces. The scientists found that patches of diverse planting produced more organic matter than patches of single species even with equal rain, nitrogen and sunlight4.
And it’s exactly the amount of organic matter or biomass produced that determines how “productive” scientists consider whole ecosystems to be. While the amount of carbon differs between plant types, that biomass is made up of carbon. As you might guess, a tropical rain forest is gangbusters with a score of 2,200 grams per square meter per year5. A temperate evergreen forest clocks in at 1,600 and a temperate deciduous forest at 1,2006. Thus a picture starts to emerge as to the effects of different plant choices.
The next big question is: Where are you PUTTING all of that biomass in your garden? Before overrunning your garden with a bunch of kudzu, Norway Maples, and pokeweed, think about where it all ends up. Meaning, what do you do at the end of the season when you cut your garden back? Do you compost those stems, branches, and weeds? Send it to a landfill or incinerator?7. These options for disposal all release some (or most, in the case of incineration) of that carbon back into the atmosphere, so letting it grow and NOT cutting back keeps it out of the atmosphere. But compost is returned to your soil and because soils lock away carbon — until the soils are disturbed — composting stores the carbon long-term.
