The informal consensus among the plant folks I know is that we’re having an unusually early autumn. That might be true; that poor plum tree that hangs over our fence is bald already, and started dropping leaves in early August. It’s been badly stressed though, since it was butchered so ineptly last year, so I’d thought it was an exception.
But the native-plant cabal’s e-mail correspondents in the Sierra and the foothills said the aspens and maples were turning early, and some local street trees seem to be jumping the gun a bit too. People are tossing assorted theories around—warm wet spring, hot fog-poor summer, bug populations, water table … I’ll stay agnostic for a while, myself. I’m pretty sure that there are lots of reasons.
The poor street sycamores, the London planes and their kin, are losing leaves fast and unglamorously and I suppose that’s because it’s been such a prosperous year for anthracnose, a leaf fungus they’re susceptible to. They’ve looked gray in the leaf all summer.
But what goes on in a leaf in autumn is a complicated chemical dance, and it’s ruled mostly by day length. Its intensity, in some places and species, is influenced by sunny or cloudy days, cool nights, the arrival of frost and freezing weather. But the great change itself is kicked off by shorter days, longer nights, a critical point in the amount of light a tree gets.
We have only a few native deciduous trees here, species like California buckeye (Aesculus californica, which normally starts undressing in late summer) and bigleaf maple (Acer macrophyllum) and the alders and willows that grow along wild creeks. But some of our street trees, the various ashes, mulberries, and especially the sweetgums (Liquidambar styraciflua) have been flashing gorgeous colors at us for the past few weeks. These species are giving us an exotic treat, a précis of what goes on in the deciduous forests back east. When I visited Pennsylvania last in autumn, I realized I’d forgotten how saturated those colors are, how intensely sweet to the eye.
What’s going on inside those leaves is as amazing as the beauty on the surface. Some of the new colors have been present in the leaves all along: yellow xanthophylls and orange carotenoids. They’ve been overshadowed by the chlorophyll that the tree uses to make food, in a chemical process fueled by sunlight’s energy. Chlorophyll is delicate; it breaks down fast in sunlight, and the leaf is replacing it constantly.
The waning days signal less usable light, and in many deciduous trees’ homelands, the coming of freezing weather. Among other effects, freezing makes groundwater unavailable to be absorbed by the trees’ roots—an effective seasonal drought. Deciduous leaves typically transport a lot of water out of the tree through their stomata, and get expensive to keep in a drought.
The tree starts making its abscission layer, a corky band of cells between the leaf’s petiole and the twig. It slows and stops traffic between the leaf and the rest of the tree: as less food (carbohydrates) flows out of the leaf, less mineral and water support flows in. Chlorophyll stops being made, and the sturdier xanthophylls and carotenoids are unmasked.
Another pigment set, the red and purple anthocyanins, forms in autumn in some trees, made of the sugars left over in the leaf. These show as reds where the leaf’s cell sap is very acidic, purples where it’s closer to neutral. (Anthocyanins are responsible for apples’ red skin, the purple of grapes, and some flower colors too. The result from a reaction between plant sugars and light, which is why an apple is redder on the sunny side.)
Anthocyanins are a bit more responsive to the tree’s prosperity than other pigments: if it’s been a good summer, with enough water and sun to make lots of sugars, there will be more leftovers to make more red or purple colors.
Domestic sweetgums are good showcases for all these pigments right now. Their varieties and cultivars show assorted color habits: ‘Burgundy’ is almost purple; ‘Festival’ is gaily multicolored. Some hold their bright leaves all winter in our climate. You can pick up a brilliant boutonniere from the sidewalks anytime from now to March, free and freely given.w