Leaves of change

I think Wednesday was the most glorious day of fall foliage this year. The woodlands abutting cornfields seemed unchanged from summer, except for being painted in the golds and ruby reds of an autumnal palette. But that night the temperature dropped and wind moved in with the rain to litter our yard with leaves. We started to notice many bare trees in the morning, a reminder of the approaching winter.

The fall finery of deciduous forests in the northeastern US is as aesthetically pleasing as bird song or summer flowers. However, in those two examples, there's purpose to the beauty. Bird song must attract mates as well as fend off rivals, and showy flowers attract pollinators. Autumn leaf colors have no such purpose and if they inspire any humans to conserve temperate deciduous forests it's incidental. The trees have another form of conservation driving them: nutrient conservation.

This was also on my mind Thursday morning as I raked up leaves to store by the compost pile. The full bin represents my last big harvest of the growing season. Soon winter will shut down my gardening completely.

Our winters are cold and often gray. Us human residents suffer from the lack of sunlight, but so too does the forest around our house. The broad leaves of hardwood trees have a much greater exposure to the elements than the tight spruce needles that will stay green all winter. Conifers are also resinous, which conserves water and acts like antifreeze, not to mention creating creosote when you unwisely burn conifer logs in your woodstove. Shagbark hickory, beech, and other deciduous trees protect themselves by going dormant and losing those broad leaves, their most vulnerable parts.

But leaves are a big investment in terms of material, and trees absorb as many nutrients as they can before dropping them. Chlorophyll, the famous green pigment that facilitates most photosynthesis in plants, is broken down. This exposes yellow and orange pigments which were always present in small amounts, aiding chlorophyll by absorbing light energy in different spectrums. In contrast, the bright reds and purples of particularly good foliage years are produced in the fall when the leaves manufacture sugar during warm, sunny days, but that sugar is trapped and broken down during cold nights. Browns are another result of decomposition, not as striking by themselves though an important shade to round out the fall palette. Eventually the nutrients that are left will be mostly locked in to the leaf physically or chemically, and the tree will cut its losses. The leaf is released to the forest's version of a compost pile, the forest floor.

Oak leaves can take up to three years to decompose in the forest, but I'm hoping to speed up the process in my compost. By now, my reserve pile from last fall looks pretty much unchanged, while the compost pile, a mix of leaves, green yard clippings, and vegetable waste, is dense and dark brown. Leaves are high in carbon, but the other compost ingredients are high in nitrogen, making the mixture much more palatable to all the bacteria, fungi, mites, nematodes and insects which carry out decomposition.

In our region, glaciers dug down to the bedrock and prevented the reformation of top soil until they retreated around 11,000 years ago. One result of this scouring was the extirpation (that is, local extinction) of native earthworms. Without earthworms, decomposition of the leaf litter proceeds slowly. Eventually the leaves will return their nutrients to the soil and the trees which dropped them, but until then the leaf litter has an important job to do. The layer buffers shallow roots in cold temperatures, but also retains moisture throughout the year. It guards against erosion that threatens to drain away nutrients. It also shelters mice and other small creatures, something I will have to be careful about when I cover my garden beds with leaves to ready them for the long winter. Many native seedlings and wildflowers need this protective layer. Their populations can steeply decline when introduced worms eat through the leaf layer too quickly. Not only do natives decline, but invasive plants thrive in the exposed soil where worms have freed up nitrogen that weeds need to, well, be weedy.

The importance of leaf litter is why I only rake up leaves from our yard, not our forest. I also have to be careful not to introduce more worms, because although I know we already have at least one species, I don't want to add a new one. Last year, before I knew enough about how dangerous they were, I almost started a worm compost. I even ordered a batch of worms. We were too busy to pick them up from the post office in time, so they all perished in a traumatic incident still legendary to the small town postal workers (rotting worms smell like essence of death, in case you ever wondered). Now, though, I feel very fortunate not to have made a mistake which could have completely altered our surrounding ecosystem. No one has yet found an easy way to search out and destroy all the worms in a forest. We can't undo the introductions that have already happened here.