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Spacing shallow rooted fruit trees with nitrogen fixer trees / tap root trees in forest garden
We are converting an orchard to a forest garden in Mediterranean climate. I've been searching for days the right spacing between shallow rooted fruit trees and tap rooted fruit trees or nitrogen fixer trees, a topic essential in forest garden design. I hope permies.com community can enlighten me on that.
What is the right spacing ? Recommended distance between shallow rooted fruit trees seems to be 50% more than crown size (according to Martin Crawford). Do you think that between shallow rooted and tap rooted fruit trees this distance should be smaller, as soil is better partitioned ?
Should this distance be smaller also when spacing fruit trees and nitrogen fixer trees (often tap rooted though), so they intermingle their roots and the fruit tree access the fixed nitrogen ? If it is the case how much smaller should it be ?

All trees, whether 'tap rooted', or not, will send out lateral roots...especially those native to Mediterranean climates.
The lateral roots collect the light rains typical to Med climates in the summer (growing season), while the deeper tap roots collect moisture left deeper in the soil from the rainier seasons.

In a true Mediterranean climate, I believe you need to adjust the spacing to allow the lateral (surface) roots to maintain each tree, and just consider the tap rooted ones 'lucky', in their ability to thrive while the others are struggling. Alternating tap-rooted trees with non-tap rooted ones is an excellent choice (wish I had thought of that when I was 'there'). They can be spaced a little closer, just because the tap rooted ones can source water deeper than their neighbors, but, they will all compete for that precious little water that falls in their growing season.

I built a small food forest in the eastern Med, and it seemed that whatever I did around the (pre-established) fig trees, nothing could hurt them, but the citrus trees always seemed thirsty...my very young nut (pistachio) trees didn't ever seem to falter, nor did the olives. Grapes never had a problem. In all seasons, my citrus trees always seemed to be lacking water, while everything else thrived. I planned to put in avacados, but politics forced me out of the region before I could try.

Consider your climate/soil, and plant accordingly. Trees need a lot of water...throughout their lives...they will continue to exist with a lack of water, but they will not produce fruit/nuts without enough water. Space them a little farther apart, and plant some annuals under them...if water is short, fuck the annuals, and let the trees survive.
Thanks for the reply John. Very interesting, I never thought that shallow root get the light rain of growing season and tap root from rainier season.

Our water table is not very deep (5m), so deep rooted figs thrive for years with no irrigation, and also shallow rooted mulberries, almonds and grapes. But many trees (apricot, peach, olive) make no fruits, like you said. We try to make hugelkultur buried wood around citrus and other trees to keep water at the surface for dry season. We try to find pistachio and germinate avocado to add to the garden. Seems nice the forest garden you did in there.

We'll try like you said shorter of recommended distance, like 80% of this distance, and keep 20% more than canopy width around trees between tap rooted and shallow rooted. As you mention only water as being the problem, I guess that being a nitrogen fixer tree make no difference and will suck up too much water if closer to fruit trees.
Is it possible that your peaches and apricots just aren't getting their needed chill hours to fruit?
Yes Dana, it is possible that they dont get enough chilling hours, I will look into that. But one of them is starting to flower, so it got enough chilling, I will see if it gets fruits.
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