It may be annoying to know that leaf chemistry vary within the leaf crown, especially if you are conducting a feeding assay and would like to correlate herbivore performance with the chemistry of one or two leaves only
Do you know your chemistry-related restrictions in the feeding assays?
It is quite frustrating for any chemical ecologist, if there are large chemical differences within a single tree, especially if one wants to correlate tree chemistry with the performance of herbivores chewing the foliage. In perfect feeding assays it is simply impossible to use the same leaves both in feeding assays and chemical assays. For that reason the samples used for chemistry should be well replicated within the branch or canopy, and feeding assays should take the same approach, i.e. use multiple leaves of the branch or canopy.
Some feeding approaches use one half of the leaf blade in feeding assays and the other half for chemistry, but this approach has at least a couple of problems. Cutting the leaf in half may initiate induced responses in leaf chemistry and one half of a leaf is seldom sufficient for proper feeding study. In addition, it tends to dry easier than an intact leaf and that may determine insect performance more effectively than leaf chemistry. Insects do get all the water from their diet and dry leaves hardly are a feat.
How uniform is the leaf crown – short shoot vs. long shoot leaves
But what is enough so that we can consider the set of leaves to represent the whole leaf crown of the tree? Or does it need to represent the whole crown? Again, this must depend on the tree species and age, and especially on its growing habit. We have lots of data supporting the fact that in birch trees where both older long shoot leaves and younger short shoot leaves are found in the same branch at the same time, both their chemistry and effects on insect performance are different. Do you know if your species produces such short shoot leaves as well? What about waxy sun leaves such as found in the sunny side of oaks, are they different from the more shaded leaves?
Differences in growing seasons matter as well
The above situation must be complicated when we move away from Scandinavia where we have very well shaped growing seasons between the spring and autumn. In the tropics or other less cold climates there are not necessarily any such clear periods when all leaf growth would end and then again new growth begin after the winter months. Instead, when trees are evergreen, also individual leaves within the foliage may be quite different in their levels of maturity and this may have direct effects on their defensive chemistry. This aspect is highlighted in the next section that describes the seasonal variation of plant defense compounds.
“Look at that huge oak – both very nice and very frustrating. So, stop looking.”