There are quite a few research papers about tree defoliation because this can be caused by insects, creating a problem for the forestry industry. Defoliation is used on deciduous trees in bonsai to completely regrow a deciduous tree’s leaves, resulting in ramification and smaller leaves. This isn’t a practice for conifers, or at least, not for most of them, as many conifers simply can’t regenerate very easily and the effect will be weakening of the tree and not ramification. Although I must note here that my 2022 summer watering disaster caused a small larch forest of mine to defoliate and it looked fantastic after the foliage regrew!
Complete defoliation is a pretty drastic practice from the tree’s perspective and a double whammy – as not only does the tree have to use its stored energy reserves to regrow its leaves, it doesn’t have any energy coming in until those leaves are regrown. Defoliation significantly reduces the total stored carbon in a tree, and there is a point at which mortality occurs – one study found that once stored carbohydrates were less than 1.5% of the usual level, this will kill the tree.ref
As described in this article about the effect of grazing animals, “Plants adjust to conditions of chronic defoliation and the associated reduction in whole-plant photosynthetic rates by altering resource allocation patterns and reducing relative growth rates.”ref Although the article is focused on grasses, which are a different branch of the Plantae family to trees, it says that “root elongation essentially ceases within 24 hours after removal of approximately 50% or more of the shoot system…[and there is]…a rapid reduction in nutrient absorption”. So basically by defoliating 50% or more the roots will stop growing and nutrient absorption will reduce. Interestingly, several studies reported that photosynthetic capability of the remaining leaves on defoliated plants actually increases – perhaps a result of the resource allocation pattern change mentioned above.
The effect of defoliation is to force a deciduous tree to use the stored energy it has built up in the growing season straight away, instead of leaving it for the next season. Because of this, the tree doesn’t have the energy reserves to grow a full set of leaves at the same size it would normally, so it compensates by growing smaller leaves. Since this technique uses up stored energy, there isn’t much left for other types of growth, so it’s not a technique you would use if you were trying to thicken a trunk or grow branches.
This studyref found that a 50% defoliation of prunus saplings reduced their growth rates for the following 5 years and brought forward bud burst for a similar period, while this oneref found that larch recovered well from defoliation, but pinus did not. This oneref said that partial and complete spring defoliation reduced first-year diameter, height, and volume growth of 4-year-old loblolly and slash pines.
This article says that “scientists found that growth was reduced in both half and entirely defoliated trees in the short and long-term…both half and entirely defoliated trees had less leaf area than control plants. Defoliated trees also allocated more carbon for storage than control trees with no defoliation.”ref This suggests that defoliation in some way teaches your tree to divert resources to storage instead of foliage, not just once but into the future. Which means you really don’t want to do this while you are still establishing the branch structure and ramification because these will slow.
Interesting, Harry Harrington reports that some species don’t respond to complete defoliation by growing smaller leaves, instead they grow a small number of large leavesref. So overall a complete defoliation may be an unnecessarily unpredictable and heavy-handed way to achieve leaf reduction. One could hypothesise that defoliation of a tree which follows a fixed growth pattern (read more in Extending Shoots) might result in a greater leaf reduction effect, because buds and nascent leaves are not sitting there waiting to burst, they need to be completely regrown. But one could also hypothesise that this type of tree might struggle to regrow any leaves at all, depending on the weather conditions.
There are less drastic options than removing the entire foliage of a tree all at once – you can remove half of it for example, or do it in stages, so that new leaves can grow before the remove the next batch. It seems like you should be able to achieve a similar effect with constant low-level leaf pruning throughout the growing season, combined with bud pinching at the start of the season. A more gradual approach would allow photosynthesis and energy generation to continue, without stopping root extension and nutrient uptake, while still regrowing leaves and increasing ramification. It may be however that the shock of something more drastic is what’s needed to reduce leaf size significantly because the resources to regrow are shared more widely. An experiment for someone?
The timing of defoliation is really important. The tree needs to have had enough time with its new leaves to generate good energy stores for the next season and enough time to regrow and harden its leaves against frost. Somewhere in the middle of the growing season allows for both of these to happenref.