Over an ideal growing season a stem may grow say 1 metre and have leaves every 50mm down the stem. These 200 leaves will thus vary in age according to the length of the season. However, over this time some of these leaves will become shaded by newer growth, damaged by pest, disease or mechanical abrasion. They will become old and less efficient. They will once again become parasitic on the tree just as growing foliage and twigs are. Green leaves but unproductive – taking and not giving.
The productive life of a leaf is about three months after full maturity. The flush of growth occurs on plants, in their natural setting, in harmony with the most favourable time for their survival. Some will flush in the spring, notably deciduous trees, but others in the late summer or autumn as for many Eucalypt species.
With deciduous trees, the buds and branch tissues are filled with nutrients in the autumn and through the winter periods. During the low growth or dormant period, nutrients are stored in the buds and twigs in readiness for the next flush of growth. When this occurs the buds burst, perhaps flower, and then the massive extension of vegetative growth occurs. This is driven by the store of starch and nutrients that are mobilised for this event, until the leaves mature.
A period of consolidation occurs after the initial flush whereby the reserves of starch are rebuilt for the next season. This rebuilding of starch is normally substantially completed within the first three months after flush. While some growth increment will occur through the rest of the summer most effort is now put into ripening fruit or seeds. The older leaves become senescent and parasitic gradually returning nutrients to the plant, as indicated by the leaf colour change and finally falling.
Insect predation or fungal infection of leaves that occur during growth flush can be very detrimental to the plant. However, there is often a lag phase between the hatching of insects or the infectious period of a leaf disease, that allows the plants to do most or all of its energy rebuilding before the pest or disease builds to a significant and leaf damaging level.
As an example, Elm leaf beetle attack normally does not build up until after the leaves have had time to restore normal starch reserves. They cause nasty disfigurement of the leaves but have little effect on the health of the tree. This is obviously in the best interest of the Insect. Firstly, it wants leaves that are full of carbohydrate (Starch) in order to provide them with energy and secondly, if they killed the tree they would be limiting their food resources. So it makes sense to wake late, feast and build the population of trees that are also going to do well. Given the amount of Elm leaf beetle excrement found on the pavement under Elm trees, they provide considerable nutrient recycling.
Similarly, some diseases only attack leaves once they become senescent and these have little effect on the long-term health of the tree. There are pathogens like Phytophthora and Armillaria species that, when allowed to, build to extreme levels that will kill trees. However, generally these are in background population levels that simply maintain the species without killing the host.