Suggestions for soil improvement
Vegetation develops over time. First, pioneer species arrive, which prepare the soil. Pioneer species are a very diverse group of plants.
On a bare rock, pioneers like moss and lichen will be the first. But if you already have loose fine minerals like clay, sand, or gravel the story is a little different. Not only will mosses and lichens appear, but also other types of pioneers, from herbs to trees. Some species are very good at making nutrients available for others. Eucalyptus can exude acid from its roots and dissolve minerals and rocks. Legumes are able to fix nitrogen. Tilia can bring up calcium from deep in the soil and drop it with its leaves on the surface. Juglans is known to drop phosphorus with its leaves. Others like Celtis are experts in splitting rocks with a force of up to 50kg per cm2, or 500t per m2. That's why this tree is called rock splitter in Italy. There are even tropical species which are able to drill into solid rock via aggressive root exudates, resulting in fingers sized drill holes.
Often under appreciated is the role of pine trees on rocky mountain terrain, where there is not much soil at all. Pine trees cover the rocks with a sour (acidic) blanket of needles, which speed up the weathering process of the rock, helping form soil and prepare for other species.
The short lifetime of many pioneer species is also a key to soil formation. When the life of a tree ends, it falls over and is colonized by basidiomycetes, a class of mushrooms. These mushrooms not only grow in the wood but also in the area surrounding the wood. Similarly, logs colonized by the (oyster?) mushroom will colonize the soil in a radius 60cm around the wood. Fungi have the ability to loosen up all types of compaction and even dissolve solid rock, although this is a slow process. By doing this, fungi make the minerals available and transport it to where is needed. Fungi are also able to fix nitrogen with the help of bacteria.
Fungi are the transport system of the soil. They build a network across species. The fungal network wants to maximize photosynthesis in an area. To achieve this, it transports nutrients, but also carbohydrates, directly from one tree species to a other. Suzanne Simard describes this in her book: "Finding the Mother Tree," how mycorrhizal fungi transport carbohydrates from Douglas Fir to Paperbark Birch in spring and the opposite direction during summer.
If a forest becomes stagnant and weak, fungi like armelaria melea take more extreme measures. This mushroom is able to kill weak trees within weeks. It presses the reset button, sometimes killing whole forests and making the trees available for soil building organisms. In a healthy forest its presence is hard to detect as it does no harm, but when a soil is lacking in organic matter and woody material it can become problematic. This often happens in
city parks or private gardens.
Tree pruning is a rare event in nature, mostly due to storm damage or fire. The woody material resulting from such an event are of great benefit for the forest over the long run, as it will build up soil via fungi. When trees are topped by a storm, the fungal network will help to regrow the trees as fast as possible. But not only will large trees regrow fast after a storm event, but also the small trees in the shade will awaken from their sleep and start growing fast. A storm or topping event induces a growth impulse in the whole plant community, so that photosynthesis is maximized. Also trees start fruiting more heavily as this provides the optimal conditions to establish their seeds.
Understory plants like Yew often wait for centuries for such an event to occur. Storm damage or tree topping brings new live to a forest and lets it produce a bounty of fruits. Ernst Götsch observed this in his cacao plantation in Costa Rica. The plantation was a forest garden with many different tree species and cacao as understory plant. After a cyclone topped all tree in a 60m wide strip, the trees started regrowing quickly, but also the cacao responded very well to the storm event. The yield went up by 100% for this season.
As the storm showed Ernst Götsch the benefits of topping trees, he does it now annually. The decaying woody material helps to build up soil very quickly, and the growth impulse given by the topped trees helps all plants near by. Rich topsoil is built at a rate of 1 to 3cm a year. Usually you can find topsoil increase by only 0.5 to 1mm a year according to scientific literature.
You can use this knowledge to improve your soil and make it sutble for demanding tree species. In nature, pioneers species are followed later by other species. But it's possible to plant stages of succession at once. Even climax species can be planted right from the beginning. The species will start flourishing as soon the soil conditions are improved to match their needs. Fast growing species will produce large amount of biomass early on. The topping of this pioneer species makes the woody biomass available early on for the basidiomycets fungi. This will speed up a process which in nature takes centuries down to only a decade or two.
Fungi will build up soil rapidly. Often exotic tree species can greatly accelerate the process: Eucalyptus, Robinia (
Black Locust), or Ailanthus (Tree of Heaven) will produce lots of biomass very quickly, but also Alder, Elm, Birch, and Poplar perform this function well.
Lets do a example for Central Switzerland, with deep but biologically inactive soil and about 600mm or more rainfall. The intended production species for this scenario are Cherry, Plum, Peach,
Apple, Walnut, Oak, and Chestnut. To start with you plant the named species. But also, next to it, plant Silver Willow, Alder, Elm, Ash, Maple, Birch, and Lime (Linden). Maybe Poplar but it makes root suckers. Also exotics like Hippophae Salicifolia (Willow-leaved Sea Buckthorn), Paulownia, Gleditsia Triacanthos Inermis (Thornless Honey Locust), Toona Sinensis (Toon Tree, Garlic and Onions Tree), Xanthoceras Sorbifolium (Yellowhorn), Sophora Japonica (Styphnolobium japonicum, Japanese Pagoda Tree), or Morus (Mulberry) to name a few. They are planted at high density, about 10 plants per m2 (~10 square feet). From the first year onward, you top the pioneer species and drop the pruning material. This will maximize growth right from the beggining. With time you will take out more and more of the trees. Some you let grow tall and top them at a a height of 5m (~16.5 ft). If you do this with Maple, Fraxinus (Ash), Gleditsia (Honey Locust), Cherry, Walnut, Chestnut, or maybe Morus you can produce high quality timber along side.
Some species like Willow continually regrow from the stump. You can cover it with plastic to prevent that or plant them in a margin so that you can mow them. Also
cattle and sheep will love to eat the new shoots and after a while it will give up. Brambles also help to improve the soil quality. You can cut them at the beginning. When the soil becomes loose over time and they have fulfilled their duty, they will be easy to pull.
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