Dryland tree planting (specific advice)

Hi Forum

Hope you can help me with this rather specific question :)

Situation:

Central Chile, zone 8, long summer draught 8-9 months, winter precipitation used to be 400-600 milimeters, now maximum 100 milimeters. Winter minimum temp. 5°, during summer we have large stretches with afternoon temp above 30°, 30% rel. humidity and 30 km wind. The soil is basically a weathered granite, so in terms of soil, mostly sand at the surface with patches that have a bit more clay and organic matter. The soil grades into solid granite at a depth of about 1 m.

So how to kick-start tree planting here? I would like to plant mediterranean trees, either fruit trees from the mediterranean itself, or local species adapted to the climate.

Self imposed limits: Watering mostly once per week in summer, no watering in winter. Water is a scarce resource around here.

Idea:

Pits around 0.5 meters deep and wide

Watered tree logs at the bottom

Fill the earth back in, but ammended with compost, maybe biochar ...

Put a pipe in so that I can water the tree logs, rather than the surface

Plant the young tree between boulders, to take advantege of their heat leveling properties and the moring dew in witer

Mulch, of course

Plant a leguminous tree (acacia) right next to it

There is some local research indicating that native trees profit from having a local acacia species right next to them.

Suggestions?
Prickly Pears as shade plants to the north-west (nothern hemisphere would be south west) of the tree?
How to avoid that the logs rot in an unhealthy way, anaerobically or something?


>Some people indicate that the tree will become instable when the logs break down. I think by that time, the tree roots will have reached outside of the planting pit so stability should not be such an issue.

I would want to add a net/pan berm on the down-hill side of the tree in order to capture run-off and direct it towards the tree roots.  

In rocky ecosystems, I really love the huge amounts of rain that can flow from a boulder. Where that runoff leaves the rock is a great place to plant a tree.

I am not a fan of burying pipes, cause a pipe holds much less water than a tree needs, and I tire of standing around long enough to pour the water into the pipe as it gets ever so slowly absorbed. I'd rather take one second to dump a bucket of water directly onto the ground.

Thanks Joseph, great advice! Will try to install this type of berms, or maybe some sort of swale ...
I built a mini-swale once on my property (10 cms deep), I didn't work really since the soil is sandy, so the accual surface runoff is rather small. Precipitation infiltrates directly.

Do you have access to Mollison's "Permaculture a Designers' Manual"?

Chapter 11 is all about Dryland Strategies, and section 11.11 on page 390 addresses "Tree establishment in deserts".  Pictures alone are worth the cost of the book.

In addition to net-and-pan and pit systems,  other items included around this section of the book are:

*Windbreak!
*Lots and lots of mulch
*Paint stems white or wrap in foil to prevent sunburn
*Gourds/legumes/ground cover crop to cool the tree roots
*Stone mulch (as you alluded to...but use plenty of smaller boulders or larger stones to capture night condensation and cool the ground during the day)
*Hardy tree legumes nearby (as you alluded to, but I'd be somewhat hesitant to put them right on top of your plantings--don't want them to compete for resources.  Plant them nearby for chop 'n drop...seems safer)
*Shade via dead brush or long palm fronds
*Drip irrigation
*Ollas

Speaking of Ollas...for your vertical pipe, remember that you want it to provide water to where the roots are.  Young tree roots are near the surface.  A metal pipe unfortunately would leak out the water too deep into the soil.  But A broken clay pipe (30cm) with pebbles filled in it would allow the shallow root zone to get wet, too. (Section 11.9, Garden Irrigation Systems, Figure 11.66)

If you are REALLY short on water, consider pit evaporator systems which funnel condensed moisture from leafy greens (Figure 11.75 in the PaDM)

Lastly, for your sandy swales or diversion ditches or berms, consider using some buried 1mil plastic to help channel that water to your trees.

Cheers!

Hi George
Many thanks for this wonderful reply! The list gives some really good ideas - looks like I have to get the book you mention. So far I have found answers to most of my questiones just via Google.

George Yacus wrote:
*Hardy tree legumes nearby (as you alluded to, but I'd be somewhat hesitant to put them right on top of your plantings--don't want them to compete for resources.  Plant them nearby for chop 'n drop...seems safer)

Resource competition seems a solid argument to plant them at some distance. But then, how is the nitrogen getting from the legumes to the tree, or at least, how long will it take to become significant? I mean, planting two trees 2 meters apart will take years for them to join roots, right?

Your question regarding beneficial N2 spacing is a very good question, indeed.

I bet someone here on Permies.com has a fantastic article or scientific paper regarding beneficial planting distances.  Sadly I don't have any good documentation.

But I think beneficial tree interactions would occur way faster than one would initially think, though.  

I can't speak specifically to acacias in arid climates, but in a temperate climate, for instance, one Black Locust (N2-fixing) tree can easily grow +2m in height in just one year.  If its roots spread out maybe half that distance to ~1m, then already in the first year there would be significant root zone overlap (and competition) if a small fruit tree was planted, say 50cm away.

But in addition to the roots competing, the branches of the beneficial tree might scratch up the young fruit tree if not pruned regularly, and the trunks could even rub if planted too close.

That said, your regular maintenance of the tree (by chopping and dropping the branches of the N2-fixer) would encourage root die-back.

For short term N2 benefit, smaller legumes or clover ground covers near the base of the tree may be safer.