Archive | September 3, 2013

Why do leaves change colour and fall?

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.

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.

Trees and how they grow

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I’m in the mood to be instructive and because some idiot keeps peeling the bark off our sycamore trees.

Did you know that the parts of the tree are very different and complex?

Let’s talk about the trunk. Only two parts of the trunk of a tree are alive. The rest is structure or not alive. It’s not dead but like your fingernails and hair it is no longer alive. In some ways a tree grows from the outside in and not from the inside out.

The two parts of the tree that are alive are the Bast. (love that name.) which is the inner bark and carries the enriched sap from the leaves to the cells that are growing.

The other living part of the trunk is the cambium layer. This is the layer of cells that is actively growing. It will eventually be the next ring on the inside of the tree. This layer is between the sapwood and the Bast. This layer produces the bark and the new wood for the tree. The cambium layer is the most vulnerable part of the tree. Damage to this layer can kill the tree. This is why you shouldn’t tie anything tightly around a tree. This is called girdling the tree and if the cambium layer is hurt all the way around the tree it dies. If you nail something to the tree you can also introduce bacteria or other harmful things and kill the tree but more slowly. The cambium layer can be only a few microns thick to much thicker.

The next layer in is the Sapwood. This layer is the pipeline that carries water up to the leaves. This is the part that will be heartwood next.

Heartwood is the structure that holds the tree up. The heartwood can completely burn out or die and as long as the cambium layer is alive the tree will be alive. You see this with incense cedars that have been struck by lightning. They appear green and growing but the insides will be blackened and hollowed out from burning.

Leaves make the food for the tree and send it back down in the Bast or phoelm to help the tree grow. The Bast will eventually become part of the bark as the tree grows.

Today’s reading – twice in 2 days?

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Ten of Stones

The inner and outer community that sustains and supports the individual, not necessarily blood family but the trusted friends, comrades and lovers who offer security and affection. …the community is in harmony.

Coming home to oneself, an abundant home life, feeling spiritually and financially happy, loving one’s immediate environment.