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The Green Life: Hey Mr. Green, What's Better: A Vegetable Garden or Trees?

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December 15, 2011

Hey Mr. Green, What's Better: A Vegetable Garden or Trees?

Bob Schildgen is Mr GreenHey Mr. Green,

I live on a very wooded 12 acres in northern Minnesota and want to create a vegetable garden. However, I have about 20 seedlings that I'd need to remove to be able to prepare the ground. Is a vegetable garden worth the impact of removing a few trees?

—Shannon in Laporte, Minnesota

You probably wonder if surrendering these baby trees to a garden would reduce your acreage's ability to absorb carbon dioxide. If you’re a diligent gardener, I seriously doubt that uprooting the trees will do any harm. Besides, even lousy gardeners are infinitely less dangerous than the real culprits: fossil-fuel corporations, industrial plutocrats, and their political accomplices who chill in their office towers while the planet roasts. And of course, any home garden will drastically reduce the "food miles" and fuel burned to transport food.

Even something as modest as a 50-by-50-foot tomato patch might offset the trees' loss, though I hasten to add this is by no means an endorsement of wanton logging. To figure out why, I created a model assuming just a crop of tomatoes. Next, using your state tree, the red pine, as our standard, by applying Department of Energy calculations for arborial ability to absorb CO2, I found that 20 red pines typically sequester about 3,000 pounds of CO2 in 10 years. (If you’re less Minnesota-centric and plant a slower-growing conifer instead, you might only sequester two-thirds as much.)

Next, supposing that you and yours consume a lot of tomatoes, but they come from 2,000 miles away, I calculated the amount of CO2 that'd be emitted hauling tomatoes that far each year. A semi truck getting its typical 6 mpg and pulling a 60,000-pound load 2,000 miles would use 333 gallons of diesel fuel. Burning diesel emits about 22.4 pounds of CO2 per gallon, so each pound of tomatoes creates around .124 pounds of CO2 for shipping.

Which means you’d have to grow 2,400 pounds of tomatoes per year to equal the trees’ absorption of CO2 over a 10-year period. Agronomists say that in the Upper Midwest, you can reasonably expect a tomato yield of one pound per square foot (though three times that much may be possible, depending on the variety of tomato, cultivation techniques, and so on), so you could achieve parity with the trees in the aforementioned 50-by-50-foot patch. 

Also, crops themselves can sequester carbon, which favors the garden. On the other hand, if the trees have a lower mortality rate than anticipated, or they shade your house in summer and block wind in winter, or you raise more or fewer tomatoes, or entirely different crops, say, zucchinis, cabbage, or corn, the equation changes. If you get tomatoes from closer to home, the numbers could differ substantially, provided the transportation is as efficient as a semi (which it often is not, which is why I’m not a knee-jerk locavore). Other variables include agricultural inputs like fuel, fertilizer, pesticides, and irrigation. We could even factor in the calories you consume while weeding, or the emissions from your compost heap.

I could’ve calculated beyond the 10-year range, but given the baffling outcome of the recent climate conference in Durban, South Africa, longer-term projections depend so much on such a huge variety of future policies and actions that I can't project all that far into the future. Moreover, the  precise local results of climate change aren’t always predictable. You might be able to grow five times as many tomatoes if it heats up real fast; on the other hand, increasingly turbulent weather could blow you and your tomatoes into the nearest lake, provided it isn’t either dried up or flooding your land.

The overarching moral is that carbon sequestration in your garden, or anywhere, with all the tradeoffs involved, is a topic fraught with gazillions of variables. We’re talking biology here in all its multifaceted mystery, not some carbon-gobbling machine fabricated at a Boeing or GE facility. Yet another reason why environmentalists should continue to demand drastic action to slow climate disruption. In other words, heed Voltaire's famous words: "Tend your garden." But also tend what politicians say and do.

Got a question for Mr. Green? Submit it here.

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