It is understandable to use tree seedlings in various types of experiments where replicates are needed, but from the defense chemical point of view this makes no sense at all – just as a word of warning
Do control for variation, but do not change the chemistry with seedlings
Scientific experiments with plants typically require the use of multiple plant individuals so that the chemical variation between plant individuals can be controlled and its effects minimized. This requirement is easier to handle with non-woody plants than with woody plants. It may be fully fine to use 50 tiny Arabidopsis thaliana plants in the experiments, while the same with tiny oak or birch seedlings may not be that wise at all. But often these seedlings are used, since their origin can be better controlled than that of fully mature trees. Some words of advice though: full-grown, mature trees are much more relevant than seedlings in all studies dealing with plant chemistry.
Woody plant seedlings are not trees yet from the chemical point of view
It is easy to understand that annual and biannual plants do not have as much differences in the defensive chemistry of their different life forms as is found with trees. Biannual plants may have differences between e.g. the rosettes produced during the first year and the flowering parts produced during the second year, but since they flower only once, the flower chemistry of one individual can only be studied within the season. On the other hand, young woody plant seedlings can be studied year-by-year and they may be hugely different from fully mature trees e.g. in their foliar chemistry. This can be interpreted so that young seedlings must direct all the photosynthetic energy into as rapid growth as possible. This way they minimize the time spent in this most vulnerable stage.
Especially the youngest leaves of seedlings are really atypical for the species
Once trees have reached a certain critical size and have accumulated enough photosynthetic tissue, their machinery starts to direct energy to defensive chemistry in a different manner. At the same time this may be witnessed by the production of normal sized leaves, since very young seedlings maximize their photosynthetic potential by first producing quite unusually large leaves. Especially these large leaves differ a lot from the normal sized ones in their chemical composition. Do you want to use such abnormal leaves in your experiments, if their chemical composition is not at all typical to that species?
If the ecological phenomena are linked to grown trees – use grown trees then
The above-mentioned factors must be taken into account when experiments are designed to e.g. study the effects of biotic or abiotic factors on the biosynthesis of hydrolysable tannins. This should not be done with tiny seedling of hydrolysable tannin producing species, since those seedlings may produce no such tannins at all or their composition may be quite different from that found in the mature foliage. After all, most ecological phenomena are linked to full-grown trees and such should not be modeled with tiny seedlings. Yet, this is too often done.
“This is absolutely the last time I touch seedling chemistry!”