Thinking about it from the nursery perspective, it's energy, the roots store energy after leaf fall and that stored energy gets pushed into the top buds first as it builds energy it pushes more bud's after they leaf out they are in the middle of their heavy energy balance cycle where the tree puts itself in a deficit to produce all the new leaves and needs to get it's energy back, we know once the leaves harden then the plant is finally putting energy in storage.
I've seen this 'stored energy' argument used plenty but never actually seen any real evidence. It's pretty obvious to anyone who has defoliated a tree or dehydrated a tree so the leaves all drop off that trees almost always have enough stores to recover. Even trees defoliated soon after leaf out will quickly send out a second burst of growth. I've had trees hit by late frost that killed all the emerging leaves but new leaves emerged soon after with no long term ill effect.
In nature, a hurricane or hailstorm can strip all the leaves and even branches off trees but they will usually recover.
Many fire resistant species can have the canopy scorched, sometimes even smaller branches burned off and can still recover. Lots of shrubs down here get burned to the ground and still grow back from buds in the base of the trunk or from the roots. Trees and shrubs get browsed by animals all the time. Sometimes completely defoliated, occasionally defoliated over and over but most still recover.
I guess its something that has developed to ensure that plants can recover from all sorts of natural events.
I'm sure that defoliation does take something from the tree. Whether that's 'energy' storage and distribution, I'm not sure. The point here being that most plants have enough stores/energy/ resilience to recover from occasional defoliation.
Back to collecting after leaf opening.
I have rarely collected deciduous trees after leaves are open but I have done some trials on root pruning trident maples later in Spring.
Trees with leaves left on soon turn brown and drop the leaves. I assume that's the tree trying to balance water availability (reduced by root removal) with transpiration (water evaporating from leaves). More than 90% of those trees produced new leaves after a few weeks.
I've seen clips of Japanese growers repotting maples with leaves. Those guys just defoliated the trees when root pruning at that stage so I tried that too. Tridents defoliated and root pruned recovered quicker than those with leaves left on. No losses, though the trial trees were all young, vigorous plants.
One of the main rules of collecting deciduous trees: Collect in late winter/early spring before trees leaf out. But, why? What mechanism, chemical process or ??? changes after the leaves come out? I've never seen an explanation for this.
I have a closely related question that's bugged me for years.
Why is it possible, even preferable, to root prune tropicals in summer, in full leaf?
We've found that Australian natives fit in this category. many of our best growers repot Aussie native plants all year round. Years ago there was a school of thought that Aussie natives have regular shorter 'dormant' periods throughout the year and we needed to coincide repotting with one of these rest times but I've now done enough to know that I can safely root prune most Aussie natives while they are actively growing.
This goes against all we've been told by our older bonsai teachers but it still works. Why?
For a while, one of the UK collectors was promoting Summer collection as better than the usually accepted Winter/Spring and apparently having good success. How does Summer root reduction of deciduous trees fit into this debate?
