One of the questions that Greenback students often pose about carbon dioxide emissions runs along the lines of, "well, if there's more CO2, isn't that good for plants? And aren't plants good for the environment?"

Sometimes the student is merely curious, sometimes (s)he is being intentionally challenging. But the response I like to give is the same, either way. Yes, more CO2 generally tends to stimulate more plant growth among other effects. But not all plant growth is good for the environment. Not all plants are good, not all plants are good in all places, and more growth isn't necessarily better growth.

That we're getting more growth is the subject of a report just issued by the Smithsonian Environmental Research Center. Ecologist Geoffrey Parker led a team of scientists who measured the growth of 55 different plots of mixed hardwoods for 22 years, and their data corroborates increases in CO2 levels, increases in soil and air temperatures, and a longer annual growing season.

That faster growth isn't always a good thing is borne out by differences in maple trees over distance and time. I've always been fond of red maples and sugar maples -- as trees, as syrup sources, and as dimensional lumber. But when I was living in the southern USA, the maples that grew nearby were junk trees -- the wood grew too fast and was, as a result, insufficiently dense. OK to burn, perhaps, but no good as lumber.

Near Greenback, maple sugaring is a traditional pursuit as winter gives way to spring. It's not a major industry, but a lot of farmers supplement their incomes by boiling a lot of sap. It's been that way for generations. It breaks my heart to know that it's on its way out.

A couple of years ago, I saw some signs along the back roads: "Stop global warming. Save our syrup". I haven't seen them recently. Maybe the supply ran out. Or maybe the people who set them out have noticed -- as I have -- that their yields are trending down already. Look -- maple syrup yields vary according to the kind of spring it is; some years the sap seems to run for almost a month, sometimes it's pretty much done in a week. But regardless of season length, yields have been decreasing for probably a decade or more.

Saving our syrup is, in all likelihood, no longer possible. Some day in the future, oil imports may be down but maple syrup imports will be up. That may be a victory for corn syrup producers, perhaps (shudder!), but it might just be enough to get me to stop making pancakes on Sunday mornings. Or to start bringing back gallons of duty-free syrup from Canada on a regular basis. Maybe I'll just cut Sunday breakfasts short, take my coffee out to the shop, and work on that trestle table I'm building. At my age I may see the end of maple sugaring, but at least the lumber will still be dense and strong with a nice open grain.