Growers of bonsai know: Trees have a lot of nutrients in their leaves. To get the plant to grow and make more leaves we spend a lot of time and energy providing the right fertilizer throughout the year. As soon as the tree is not getting the right blend of nutrients, we see the leaves change colour or even display patterns in colour such as yellowish leaves with dark green veigns. So what happens in fall?
In fall, deciduous trees take precautions for winter. They empty out the leaves of the more mobile nutrients. A recent study* tried to quantify to what extent this really is substantial. They found for three tree species (Oak: Quercus robur, Pine: Pinus nigra and Alder: Alnus glutinosa) there are substantial differences, but all do resorp a lot of macro nutrients and some of the trace elements.
The figure here shows the percentage of each element that remains in the leaf upon leaf-drop. So we see that for all three species, half of the Nitrogen (N) is retrieved. For Alder and eik it is clear that Calcium, Magnesium, Phosphorous, Sulfur and Potassium are resorbed in high levels. Interestingly, pines do not seem to resorp these all that much.
Now what does this mean? One thing to consider is the ever-lasting dogma that people think you should not fertilize trees in fall. However, if you look at deciduous trees, they act in a way that indicates that nutrients are indeed dear to them. This could mean that fertilizing strongly in fall will help trees prepare for the next year.
* Original article: Leaf mineral nutrient remobilization during leaf senescence and modulation by nutrient deficiency (Maillard et.al. Front. Plant Sci. 6:317.=, 2015)