A challenge to the would-be food gardener in the 'Bush Capital' is the amount of gum trees planted throughout Canberra as street and garden trees. Eucalypts are surprisingly water-hungry and pump huge amounts of water up from the soil to their leaves. Their light dappled shade allows more sunlight to reach the ground under the canopy, so that, unlike the heavy shade zones under deciduous trees in European and North American forests, where it's often quite damp, it's usually dry and crunchy under a gum tree. Combine this with the fact that gum trees produce chemicals to inhibit the growth of competing plants, you'll see that the hard, dry soil near to a gum isn't going to be a great habitat for your next planting of lettuce.
But of course, it must also be remembered that gum trees, for all their allelopathic and fire-attracting qualities are native, and provide habitat for a huge range of other species. In fact, we have the most gorgeous and stately yellow box gum tree (a local native) that stands right on the border of our garden and our neighbour's. It's beautifully symmetrical, probably about 20 metres tall, with 4 main trunks. Truly the most aesthetic specimen in the street. In flower it provides a huge amount of pollen for our neighbour's bees - which in turn pollinate our fruit trees and vegetables. Plus we get honey and wax from our neighbour. It provides good tucker for a raucous bunch of cockatoos, parrots and countless other species. It's unfortunate that it hangs over half of our roof and front garden - currently it's provocatively extending a large dead branch right over our garden and dropped a sizeable branch last Christmas day. And it's barely 2 metres from the wall of the house.
In the ACT there are rules about chopping down trees - once they get to a certain size it's hard to get permission from the local government to cut them down. So is there anything we can do to work with large gum trees and other mighty specimens that are already in our gardens when they can't be removed?
The first point is that, unless you're prepared to provide a lot of supplementary watering, it's going to be very hard to establish new edible species - fruit trees, nuts, berries and especially annual vegetables - around existing large plants. The extensive and existing root systems of gums (or any large trees for that matter) have the competitive edge when it comes to extracting nutrients and water from the soil.
You may have some luck trying local edible native species already adapted to growing in dry woodland (you could try silver wattle (acacia dealbata) for its edible seeds, kangaroo apple, and of course, lomandras and dianellas), but for growing the fruits, nuts and vegetables that are the mainstay of our modern western diet, we need a different approach.
A few years ago, in our previous house that had a tiny yard, I trialled growing annual vegetables right underneath a gum tree - in a large wicking bed. You can find more information about wicking beds elsewhere on this site, but the basic principle is that it's like a giant self-watering pot which waters the soil from underneath. Because it's a self contained system with a barrier between it and the ground, there is no interaction between the soil in the ground and the soil in the bed. That pesky gum tree wasn't able to get its roots into the bed. Our yard was small and surrounded by large gum trees, so there was nowhere else to grow vegetables, so we built a bed about 3 metres long by 2 metres wide out of old fence palings at the foot of the tree. We lined it with builders plastic (cheap alternative to pond liner) and filled it with some light soil (vegie mix) from a local supplier. It was on a very dry, north facing slope. But, planting it beneath the gum tree meant there was some dappled shade which I think was an advantage - so we turned a problem into a solution, a key permaculture approach.
So - how did it turn out? We successfully grew cape gooseberry, broad beans, pumpkins and capsicum. We tried potatoes but they were not successful (but I think this may have more to do with potatoes not liking to be sown too deeply in a wicking bed than the location under the gum tree).
I wouldn't say we got bumper yields, but a great deal more than we could have grown in the ground! I also later discovered that some of the commercially available soil mixes sold in Canberra are not pH neutral so this may have also played a role in limiting yields, but I'm not sure. (Note: I have another post on the subject of pH here - before you buy any soil mix from a supplier, read this first!)
To sum up - for space constrained situations with large established trees, I think this is a technique for growing vegetables that is definitely worth further exploration.