Home & Garden Columns
We get just enough sun in between the rains to keep us hoping, this time of year; just enough life showing in the trees and plants, wild and tame, to make us believe that there’s more to the world than cold and mud. The plums have blossomed and are starting to get down to summer’s business, unfurling their leaves to catch the sun of longer days. The buckeyes—just look at the bunch in the center strip on Sacramento south of University!—are spreading translucent green hands out to the plenty flowing from the sky. The sun itself, as the world turns our side to face it straight-on, begins to touch us with palpable energy.
That energy is light and heat together, and plants have the trick of using it, in particular the light. That’s the part that’s least palpable to us, only visible, and we have enough of it to see by all year in daytime and artificial means of making it by night, so we rarely appreciate its influence. Trees, though, reach for it with every fiber of their aboveground being, and use it to run the biggest manufacturing operation on the planet, photosynthesis, as they build themselves out of air and the water and minerals they stand rooted in.
At eight photons a pop, a plant sorts out a carbon dioxide molecule in the air to make a carbon compound, a sugar, for its own use, and just by the way drops a molecule of oxygen back out into the air around it. As waste goes, this is rather environmentally benign, especially to those of us who need to breathe it. It does this trick using pigments, usually green, that get excited and start tossing electrons around when they see light.
After that beginning, a chain of chemical reactions follows; some of those can happen in the dark. Plants don’t actually sleep at night; they just perform another set of metabolic chores. But some of them, the deciduous perennials including lots of our trees, seem to sleep in winter.
In some climates, this is a drought adaptation of sorts. Water that’s frozen in the soil is just as unavailable to tree roots as water on the other side of a rainshadowing mountain range. So deciduous trees in cold climates drop all their leaves at once to save water, and even some trees here where there’s more water in the soil in winter than in summer have retained that habit. Some of them, like those buckeyes, strip bare when the year’s water reserves in their particular bit of soil are tapped out, even if this is just late summer. Others like bigleaf maples and creek willows have inherited the deciduous habit, apparently, as they share it with their northern relatives.
Even in thoroughly deciduous species, there’s more than timing going on. I know a few people whose own trees—apple, ash—never did lose all their leaves this past winter, for the first time, and they’re poking new ones out now anyway. The consensus about global warming gets solidified by close-to-home instances like this as well as the news about glaciers and Arctic thawing.
What, besides the availability of water, drives deciduous trees’ “decisions” to grow or drop leaves is still less well understood than arborfolk would like. We know at the molecular level something about how trees do it—but the why, the triggers that set the process in motion, are as far as I can tell still just a bit mysterious. We know they use photopigments, chemical compounds that sense light levels and changes. Soil temperature drives the process too, especially the temperature in the top soil layer; so does air temperature. We can see a lot of raggedly-timed, out-of-step leaf-drops in introduced landscape trees here; look at the sweetgums along MLK Way. (Some of that syncopation depends on what cultivar the individual tree is.)
Whatever drives the spring budding and unfurling, you can almost feel it if you have your hands on enough trees. If you’re pruning them, they’re starting to bleed all over you; but just touching, you can feel the leaf buds firm up, swell, and loosen as cells multiply and embryonic leaves stretch themselves out and the tree stirs itself to greet the sun.
Infant leaf and leafbud of Corylus cornuta, our native hazelnut.