Wish I knew how to get sacrifice branched growing from close to the base!!! Neil, any tips? Would it help to graft branches down low on the outside of curves and at the back of the base to increase the girth down low?
The only reliable way I know of to get plenty of low branches is to prune very early. Nip out the tips when the trunk is only a few cm tall or cut back taller ones to leave just a few lower buds. This will only work where there are buds on the trunk that can provide the new shoots. Fast grown seedlings will have longer internodes so less low branches and more spread out, even after you cut them. Seedlings that have struggled and grown slowly will usually have shorter internodes with buds closer together so you can get more shoots growing closer to the base where you want them. I remember an old Bonsai Focus magazine article where the author defoliated very small maple seedlings to limit their energy production and therefore get shorter internodes. That could be an option if you want to follow the low sacrifice branch path but all these options will slow development to some extent. I'm not sure whether there is a way to have your cake and eat it as well???
Low sacrifice branches are a two edged sword. They will increase trunk diameter low down but they will also leave scars that will take some time to heal. When growing larger trees you can afford to let a leader or branch grow freely to help scars below heal more quickly but with smaller trees that can quickly destroy the shape you have worked hard for. We have seen occasional posts about leaving a flap of bark to sort of graft over pruning scars but I have not tried it so can't say how it would work.
I prefer to let the lateral roots do the job. Every time I root prune I am still pleasantly surprised to see the buttressed base that develops after a couple of years when the tree is totally reliant on an evenly spread lateral root system that develops with this method.
Other suggestions to selectively increase trunk diameter include wounding where you want extra size. We have probably all experienced the frustration of getting inverse taper as a result of pruning and the resulting callus and scar tissue grows faster than surrounding trunk? I have seen recommendations that it is possible to increase trunk size at a given place by a. repeatedly stabbing the trunk ; b. making vertical cuts through the bark; c. lightly hammering the bark to bruise the cambium below. All these suggestions are aimed at producing scar tissue which, in theory, should increase the trunk thickness at that point. I have been trying this on a couple of maples aver the past few years but can't report outstanding success so far. Maybe someone else has had better results?
Very few techniques will be 100% successful because we are working with living things and there are so many other factors that will influence the outcomes. My approach comes down to starting lots and selecting the best as they develop.