We are besieged by leaves.

Everywhere — leaves float or cascade, bash, deluge, and slowly drift down from above. They carpet lawns and sidewalks, clog roads, curbs, boulevards, sidewalks, and accumulate on shrubs like snow. They create deep snowdrifts or autumnal quilts and change the world of green to brown.

It seems less a time of fall clean up than a time of self expression of trees, reminding us that they are living things with life-cycles of their own — and we just happen to live among them.

Some leaves are not spectacular like the silver maple, with leaves that turn a nondescript scraggly yellow, and dribble from the tree little by little. Others are a study in gorgeousness like the October Glory maple, a celebration of clear orange and red brilliance.

The tree seems lit from within in the thin fall light. Liquid amber leaves begin with smudges of red in the canopy and gradually color to a deep burgundy as leaf sugars are converted to anthocyanins.

A ginkgo’s matte green, fan-shaped leaves color to a clear, bright yellow and carpet the ground with a surreal earth-anchored painting of vibrant color.

Sycamore leaves are large, coarse and blow around like dry paper bags. Some oaks retain dead leaves through the winter and only lose them when they are pushed out by newly emerging leaves.

Many conifers are evergreen. However, fir, pines, spruce, juniper, and arborvitae drop the majority of their old needles in the interior of the tree or shrub the same time as deciduous trees do — from August through October.

For all deciduous trees and conifers, leaf drop (or senescence) is a physiological response to the progressively shorter day lengths and cooler temperatures that happen in fall.

Tree leaves are a necessary “gift” to the trees that drop them as they serve to naturally fertilize trees; each tree’s leaves contain nutrients the tree needs. In autumn, as the leaves begin to senesce (prepare to fall), many nutrients in the plant go to the leaves.

Coarse tree leaves like magnolia, sycamore and oaks are low in nutrients and high in lignin, a complex organic material (polymer) that is resistant to decomposition. Bacteria predominately break down low lignin, higher nutrient leaf fragments higher in nitrogen, calcium, phosphorus and potassium and other nutrients. Leaf skeletons have much higher rates of lignin, and fungi finally enable decomposition.

Additionally, leaves act as a mulch to suppress weeds, raise organic matter levels in soil, retain soil moisture and help develop good soil structure. In many cases, earthworms love them and readily incorporate them into the soil.

Yet we assiduously rake leaves away from trees and dispose of them elsewhere, leaving the ground underneath them bare. Over time, bare soil can lead to nutritionally deprived and stressed trees more prone to pest insects. It can also contribute to a decline in soil structure leading to poor soil porosity and thus declining water and air percolation. Porous soils are important to us as they aid in capturing and cleaning rainwater.

Laying down plastic landscape fabric is one of the worst things you can do under any tree. This physically prevents most nutrient cycling into soil from leaves, wood chips or other organic matter sources. Bark mulch has almost no nutrients and will not aid in developing soil health.

Some trees leaves are thin and break down quickly with consistent moisture. A snowdrift of this type of leaves can soon break down to a thin mulch with many happy earthworms underneath. Some examples of the type of tree leaves suitable to use for mulching are: smaller-leaf maples, ash, locust, honeylocust, sophora, elm, willow, catalpa, chitalpa, tilia, redbuds, and hackberry. These leaves can create a beneficial and nutritious mulch around shrubs or larger perennials.

Trees with large or coarse leaves that take a long time to break down and readily blow around in the wind like magnolias, sycamores, Norway maples, oaks and London plane trees, should be shredded before returning them under the tree, or have the city pick them up for composting. If possible, where space, circumstance and situation agree, leaves should be just left under the tree.

Many soil dwelling organisms live or overwinter in leaf litter. From salamanders, beetles, ladybugs, spiders beneficial to our gardens, butterfly caterpillars, eggs and larvae, and overwintering queen bumblebees- a variety of organisms use and need these dark, protected or food-filled places.

Obviously, for safety purposes and function, always remove leaves from paved surfaces like streets, gutters, sidewalks, pathways, patios and decks. Tree leaves readily smother lawns or small plants and should be removed promptly.