Environment – Leaves for life

It’s the time of year when stately deciduous trees become beacons of brightness in diminishing daylight. The North American ‘fall’ is legendary, but even in New Zealand, autumn is a time of beauty. Ginkgo become golden, and liquidamber looks like, well, liquid amber. Oaks turn russet, orange and red, as the good weather turns to chill.

Trees show an innate intelligence, beyond human wisdom, when shorter days at the end of summer mean the photosynthesis energy return on investment (EROI) reduces and there’s less benefit and efficiency from harvesting sunlight. In response, abscission cellular activity at the leaf stalk occurs. This is where scissor-like cells form, cutting off nutrients and water, leading to leaf drop.

If humans responded to the diminishing ROI from fossil fuel energy sources in a timely manner, we wouldn’t be facing the crisis in reserves and supplies that we are now. In trees, a pre-winter leaf ‘collapse’ forestalls a waste of energy. Our dependence on limited oil and gas has made us vulnerable to the supply chain shocks arising from the US and Israel’s war on Iran. We should have seen a fossil fuel collapse coming as existing reserves take more energy to extract them, than are yielded. It looks like we will learn the hard way.

Leaves, from deciduous and evergreen trees, have three ‘lives’.

On the trees, they act as solar panels, harvesting sunlight and transferring it to feed the tree’s body.

There’s another lesson for us. It’s about the efficiency and resilience of energy transfer from renewable resources. Live leaves also provide shade, habitat and biodiversity. They capture rain and moisture and provide food to other creatures. When droplets of moisture form on the end of leaves, it’s called guttation.

The second life of leaves is when they fall to the ground. There they act as weed-suppressing mulch. They are food for small creatures, and help retain moisture. They also protect soil from heavy rain, provide insulation and help stop erosion. Fallen leaves act as a nursery for seeds and provide nesting materials for the birds in the branches above.

The third life of leaves is after they’ve been broken down by weathering processes, and insects and worms. There they are reduced to their elements, storing carbon and building life supporting soil, nurturing the tree all over again. In their death, there is life.

If you study the (misnamed) leaf litter under native bush, you’ll find a city of life – centipedes and millipedes, hoppers, springtails, beetles, bugs, spiders and mites, as well as snails, leaf-veined slugs, lizards and peripatus, the velvet worm, NZ’s own living fossil.

They can be bane to home gardeners, but using leaf blowers to collect and then dispose of leaves is the height of absurdity, an exercise in futility, and a waste of fossil fuel and foliage fuel at the same time. Leaves are life. They’re a sign of evolutionary adaptation particular to planet Earth, and make up earth itself.