question about established trees outcompeting

Hello, all. I am a permaculture newbie, and have a question about what I witnessed while WWOOFing in the Canary Islands. I was working on a small plot of land on a a steep volcano slope with very sandy/gravely soil in a dry climate. There were a lot of plant causalities on the site. My hosts were under the impression that their newer tress were failing to produce fruit and dying because the established trees were sucking up all the water before they could get to it. Is this a likely cause? It's not illogical, but I was under the impression that established trees would be using water from much deeper, and the new trees would benefit more from the surface irrigation. I would appreciate learning from the wisdom of those more experienced than I. Thanks!

I think that your hosts are correct. Established trees will have feeder roots wherever they can find water or nutrients. I have seen in my nursery beds that wherever older trees send their roots into the bed, the seedlings are always much smaller than seedlings without this type of competition.

Established trees do not get nutrients from deeper roots. A tree sends down anchoring roots but the roots that gather the nutrients and water the tree needs will normally be only a foot or two deep, they will also be closer to the edge of the trees canopy than the trunk. Many people feed their trees in the wrong place because of miss-conceptions of a tree's root system. If you want to feed a tree you must place the new nutrients around the tree at the edge of the canopy and back in towards the trunk only about half the distance. I've seen people kill trees by driving those tree feeder stakes right next to the trunk, completely missing the feeder roots and damaging the anchor roots, causing the tree to stress and become infected, which kills the tree.

If you need to you can move those feeder roots back in towards the trunk by using a spade ( square point) to sever the roots and so cause the tree to grow new feeder roots. Do this over several years so you don't stress the tree or starve it to death.

I have used this method very successfully over the last 30 years: 1st year, cut the roots 2 feet inside the outer edge of the frees canopy(edge of the branches). 2nd year, cut the roots 1 foot inside the first year cut line. 3rd year, do same as 2nd year cut. It is very important that you go no further than 1/3 the distance from the drip line (outer edge of the branches) towards the trunk, if you do, you run the risk of diminishing the trees ability to withstand high winds. You also must be careful that you don't create a situation of thirst for the tree because you moved the feeder roots to far under the canopy and so created a situation where the new roots don't get enough water to suck up for the tree to thrive.

It is a good practice to also prune the branches back so the new root line is also the new drip line. This will ensure that the tree does not struggle to get enough water uptake.