I have two favourite times of year here in Los Angeles, spring and fall. And yes. L.A. has both spring and fall. Spring is easier to see since California has more varieties of wildflowers than any other place on earth. Partly because we have more varied ecosystems and partly, well, no one really knows why and it can give a budding botanist a headache. The key guide for California plants will always be the Munz and mine is well thumbed and has way too many leaves stuck in it. If you have to have one, here it is but be warned it’s all keys and few pictures. You want pictures get a Sunset Western Garden Book or an Audubon Plant guide. http://www.amazon.com/Flora-Southern-California-Philip-Munz/dp/0520021460
But my other favourite season is now, autumn or fall. And contrary to non-native belief, it isn’t all brown. We do have colour. Some of the colour is from non-native species but others are from natives. Fall/Autumn also involves one of my favourite English words – abscission. What is Abscission? Abscission is why leaves change colour. Abscission is the process that makes leaves separate and fall from the trees. Trees form what is called the abscission layer between the leaf and the tree. Deciduous trees do this seasonally. Evergreens and conifers do it all the time and isn’t as noticeable
The layer forms at the base of the petiole. A petiole is a fancy word for stem of the leaf. Leaves turn colour as the chlorophyll recedes from the leaf back into the tree. (well, they don’t really turn colour, the green fades away.) Some plants do this chemically or functionally such as the light changing as the day gets shorter or the temperature changes or changes in salinity and some do it hormonally with hormones like ethylene and auxin. Either way, it activates the abscission layer and says “Hey! Time to give a show and drop your leaves!”
Interesting fact: Trees that turn yellow are trees that are found in open areas and trees that turn red are trees that have a longer time to send nutrients back to the tree and need more protection from the sun. The red is called anthrocyanin and it is a sunscreen to protect the leaf just long enough to send more food back to the trunk. Warm sunny days followed by cold nights bring the brightest reds out of the red turning trees, according to the US Forest Service.
The yellow is caused by carotene. The same thing that makes carrots, orange and is what’s left when the chlorophyll is gone.
In California, we have mostly trees that turn yellow that are native like alders and cottonwoods. But in the city you find whole streets of liquid amber trees that are specifically bred to change to certain colours. If you buy one in a nursery you can choose a burgundy or a scarlet “Palo Alto” or yellows and oranges. It’s a lovely sight to see whole streets lit up in fall. It’s native in the Americas and was introduced to Europe in 1681 where they call is Sweet Gum. I’ve never heard anyone call it that here.
So now you know why the leaves change and fall and you know one of my favourite words – abscission, and what it is.