Leaves

Leaves

Maple (Acer sp.) outside my window - Eva Buxton

Maple (Acer sp.) outside my window – Eva Buxton

For many years I was lucky to have a spectacular view of San Francisco Bay and the East Bay Hills while living in Tiburon. Now I have a view of trees – two horticultural maples (Acer sp.), a crabapple (Malus sp.), and a native coast live oak (Quercus agrifolia). I have learnt to really like the trees and the birds that occasionally take a rest on their branches. When I first moved in, the leafless crabapple was covered in pink blossoms and the maples were just leafing out in sheer greenery. The crabapple got its bronze-colored leaves, and the maples grew a dense crown of deep green leaves. The evergreen coast live oak remains green all year. Now the maple leaves are turning colors and dropping from the branches, reminding me that autumn is here.
We know that without leaves or other green plant parts and algae (plus Bryophytes and some bacteria), there would be no life as we know it on Earth. I was a Bay Shore Study Guide at the National Audubon Sanctuary in Tiburon a long time ago. The volunteer “teachers” took 4 to 6 graders in schools from all over the Bay Area and talked about the ecology of the Bay. While looking at all the critters attached to the rocks, hiding under the rocks, buried in the sand, or floating in the water, I would tell the students that they all need oxygen and food, and green plants and algae, some microscopic, are the only organisms that can produce oxygen and make their own food and then feed the rest of the world. Invariably, some kid would say: “But my mom can make food!” I always hoped that my answer would make a lasting impression: “Your mom can prepare food, but she cannot make food, only plants and algae can make food.”

Some Leaf Morphology and Anatomy

European silver fir (Abies alba) - Zoya Akulova

European silver fir (Abies alba) – Zoya Akulova

Leaves come in many shapes and arrangements on plants. Most leaves of higher plants are thin and blade-like and most often attached to a stem or twig with a small stalk called petiole. The outermost layer of cells of a leaf – the epidermis – contains 1000’s of microscopic pores called stomata, which make possible the passage of gases in and out of the leaf, as well as the regulation of transpiration (water loss). Coniferous leaves such as those on pines and firs, for example, are needle-shaped, an adaptation to growing in cold, snowy climates. The thin needle has a reduced surface area, which reduces transpiration; the dark color of the needle absorbs heat from the sun; and needles do not accumulate much snow, thus reducing weight on a tree branch.

Photosynthesis and Respiration

Courtesy of https://www.science-sparks.com/what-is-photosynthesis/

Courtesy of https://www.science-sparks.com/what-is-photosynthesis/

Leaves conjure up photosynthesis and respiration for me! Plants manufacture their own food through photosynthesis mainly carried out in the leaves. Plants are green because that color is the part of the light spectrum that is reflected by a pigment in the leaves called chlorophyll. Photosynthesis is the process by which plants in the presence of chlorophyll a use sunlight, water (H2O), and carbon dioxide (CO2) to create oxygen (O2) and glucose (C6H12O2). Intricate processes within the cells of the leaf (mesophyll) transform the raw products into oxygen that is released into the air and chemical energy that is stored within the glucose molecules. (In case you care to remember, the chemical equation for photosynthesis is 6CO2 + 6H2O → C6H12O2 + 6CO2.) The process of respiration in plants is the opposite of photosynthesis; it involves using the glucose (sugar) produced during photosynthesis plus oxygen to produce energy used by the plants to carry out various life processes including growth. Plants produce their own “food” to grow and survive! Herbivores then obtain this energy by eating plants, carnivores by eating herbivores, and omnivores, like humans, by eating both in addition to plants!

Deciduous Trees

Deciduous trees, like the maples outside my window, drop their leaves in the fall and remain leafless until new leaves form in the spring. The leaves drop in response to decreasing temperatures and day length. When those changing conditions happen in fall in the Northern Hemisphere, they trigger a hormone that sends a chemical message to the leaf, which in turn produces a layer of cells at the base of the petiole. This layer is called an abscission layer, and it will eventually cut the petiole from the twig and make the leaf fall. The reason leaves turn yellow, orange or red before falling is that they contain other pigments in addition to chlorophyll such as carotenes and xanthophylls and also produce anthocyanins in the fall to slow down photosynthesis. These pigments are masked by chlorophyll during the growing season, but when chlorophyll starts breaking down in response to colder temperatures and shorter day lengths, the other pigments become visible.

Forests, Groves, and Stands

Quaking aspen (Populus tremuloides) photo from Wikipedia

Quaking aspen (Populus tremuloides) photo from Wikipedia

Walking through a deciduous forest somewhere in the north-eastern part of the U.S in the fall was at one time on my bucket list, but I gave up viewing the colors from the ground once I had seen the unforgettable color display from an airplane on my way to New York, NY. Common native species turning vibrant colors in the eastern deciduous forests are oak (Quercus), maple (Acer), beech (Fagus), birch (Betula), and hickory (Carya). California does not have deciduous forests like those found on the east coast. Providing the most fall color in California – a bright golden color – is quaking aspen (Populus tremuloides). This tree, sometimes growing on several acres but more commonly in smaller groves, is found in canyons mostly on the eastern side of Sierra Nevada. My favorite area is Hope Valley just south of Lake Tahoe, where I first saw a breathtaking display many years ago. I have, however, never seen a more magical stand of aspen than the one I saw near the northern rim of Grand Canyon, AZ.

Big leaf maple (Acer macrophyllum) - Peter Stevens

Big leaf maple (Acer macrophyllum) – Peter Stevens

Marin County woodlands display no spectacular fall colors! However, the leaves of our native bigleaf maple (Acer macrophyllum) and black oak (Quercus kelloggii), both species occurring mostly singly or in small stands, turn golden or orange in the fall. Black oak is also unusually beautiful in early spring, when the new foliage is purple due to an anthocyanin pigment protecting the young leaves from sun damage.

If you want to see trees in spectacular fall foliage, walk down some streets in our Marin towns! Some of the most colorful horticultural trees in developed areas are American sweetgum (Liquidambar styraciflua) and maidenhair tree (Gingko biloba).

 

Black oak (Quercus kelloggii) - Neal Kramer

Black oak (Quercus kelloggii) – Neal Kramer

 

American sweetgum (Liquidambar styraciflua) - Panter Nursery

American sweetgum (Liquidambar styraciflua) – Panter Nursery

 

Maidenhair tree (Gingko biloba) - Doreen Smith

Maidenhair tree (Gingko biloba) – Doreen Smith