HI Benjamin,
Happy New Years to you as well.
I've changed your forum settings out of Cascadia (which is what we call the area near the Cascade Mountains in the N.W. USA and South Western extremity of my province in Canada. I've placed your
thread in the Africa forum, but also in the
permaculture, and greening the desert. Even though your situation meets more of the definition of Savannah as Miles points out, the people who are attracted to greening of the desert threads are more likely to view this thread and give you quality advice.
Attempting to transplant existing larger trees as you suggest is unlikely to get you very far.
I agree with the advice of Miles. You are also correct in thinking that it is not wise to disturb the deep rooted existing trees; they will likely not appreciate it, and will likely not be able to grow another tap root, and if they do, it is not likely that they will be able to do so to the point that it would be sufficient to ensure drought tolerance to an already large existing tree. If you are going to uproot or dig up any trees for transplanting, be sure to do so from very young trees, as well as from areas which have an abundance.
Another method that you might consider is pitting the
land. There are many methods that can be used to pit the soil, and the pits can be varying in size. The purpose is to provide micro-climate areas which you can place seeds. The pits also serve as a place where windblown dust and seeds can accumulate, thus increasing the fertility and depth of soil on your land. If you are able to, put the excavated material on the down slope side of the pit. This makes a mini swale out of this micro site. You can do lines of these, roughly on the contour, between existing trees.
By leaving all the trees on the land, you give micro climates for further enhancements. If it is at all possible to do your pasturage around your trees instead of removing them, then you will gain much from their presence, and you can use each existing tree as a site for further ecosystem development. If some of the trees between your swales are in the way with your main
project, you can always allow your pasture animals to over-browse them and remove them later for
firewood; but I would do whatever I could to keep the trees in your system if I was in your situation. They are your tap to water your region.
Beyond this, sometimes in your climate the area has been overgrazed, and some of the trees have been so heavily browsed that they look like they are dead stumps. But if a person relieves the area of grazing pressure, the over-browsed stumps send up new shoots. Protect these, and use a knife to select the best of the shoots from each stump. The method is best described in the idea of
Farmer Managed Natural Regeneration . This method can also be used after you establish some young deep rooted pioneer trees.
A good thing to do, would be to establish a nursery of deep rooted pioneer trees in pots in a prime location. When you get some of these potted plants ready to go, then you wait for the perfect time, immediately before, during, or immediately after your rains. And get them into pits that you mulch as heavily as you can, and make these on contour between your existing trees.
Also, if you have more information about your water flows, wind flows, and slope of your land, this will help people make better contributions/advice to your thread.
Please post more about your projects here, and also check out our Africa forum and Greening the desert forum. You will find much valuable information in these. Great to have you posting your questions. Hopefully we will get more people involved in your thread. Good luck.