Yellow period

Leaves change their color in the autumn, not all trees though and some get red leaves, some get yellow ones. There are big differences and I hadn't thought much about it. 

I searched the web and found an interesting article, in swedish, that claims we know very little about what really happens in the trees. One scientist at Umeå University in the north of Sweden, Stefan Jansson, explains that one reason is that research on trees is very frustrating and  time consuming. You can more or less only make one experiment a year and if it fails you have to wait another year to do it again. He says that very few scientists are stupid enough to start researching trees.

The most important factor is daylight, when the days get shorter the tree sucks the nourishment in the leaves back into the tree and start to break down the chlorophyl that makes the leaves go green. In this process the leaves might get damaged, that's why some leaves go red in the process, to protect the leaves from being damaged by the sunlight. Red works as a sun protection factor and the tree wants the process to be controlled in order to get as much nitrogen  back as possible. 

Trees are constantly short of nitrogen, so they try and use as little as they can and make good use of what is available. When the leaves fall off, they get converted into nitrogen by insects and worms etc. so the roots can suck it up to get strength for growing new leaves next year. Only trees in the northern hemisphere do this, and only some trees like Aspen, Birch and Maple. Apple trees, for instance, have other methods and every tree species do it their own way. Stefan Janssons research is only on Aspen trees and he says that if he was to do research on another type of tree he would more or less have to start from scratch again. 

Fascinating, I didn't know about any of this complexity before. I'm sure there are articles in english about the subject if you are interested, otherwise you have to learn swedish and read the one I linked to.