'Getting a handle on the transition of the Earth to a forested planet' looks into new fossil forest finds, dating to 386mya. It is a forest but not as we know them (Jim). For a start the trees are a long way apart. and there are only really two main sorts. One is a 10m-tall leafless tree with a swollen base and the other something like a pine, but with fern-like fronds growing from the trunk.  
This seems to provide the basis for further evolution.

Roots

"The scale of the root systems were really striking" and they would have altered soil chemistry. Roots particularly of Archaeopteris (one with fern fronds from trunk) spread broadly from trunk, and could be ten metres in diameter. They are all quite near surface and are thought to have been preserved because they were covered in silt. (NB Silt not soil). Archaeopteris roots also have many of the features of tree roots today - tapering, with branching, and ending as rootlets. 
This photograph stands on what is going to go on underneath for next 40 million years - when you can dig deeper.
Credit: Charles Ver StraetenCredit: Charles Ver Straeten
Roots in Devonian times

The relative sizes, morphologies and rooting depths of rooting systems through the Early to Late Devonian.
Envisaged and drawn by Algeo and Scheckler (
1998, fig. 3, p. 117)






Vascular plants
In 'Birth of the Earth' we see how the soil was formed. 'My theory' is that this occurred at around the same time that Higher Plants evolved. They evolved together,  along with small soil animals. They all needed each other to create this new world of living soil. 

But how did these plants arrive? How did they grow up from what had gone previously - mainly ferns (inc fern trees) and mosses. These could all exist in moist places, not needing soil. You can’t have a stem without roots. nor leaves without a stem, all differentiated cells working together. It seems roots are pretty important in process - to provide water to the structure - hence 'vascular'.  To produce strong structure, lignin is important to hold the water. Then the plant can differentiate in its search for the sun. The roots also provide nutrients - provided there are nutrients in the silt/soil. 

How could this come about? Only with soil.

We can see how these elements were essential for the development of seed bearing plants. Each of the four main differentiated cells root, stem, leaf and seed, require different essential elements to function properly. All the essential elements have to be there together in order to evolve from big tree ferns into differentiated Higher Plants - those with distinct roots , stems, leaves  - and then seeds.  The mineral deemed essential to plant growth enabled plants to grow larger and live long enough to produce seeds. 

There essential elements have to be in the same place at the same time and the only place that happens is in volcanic dust - minerals coming from deep down in the Earth's crust.  The seas in late Devonian were said to be over fertilised by volcanic ash and plant debris - which shows they were together in the same place and the same time. And there were a lot of volcanoes around at the crucial period. 

Mycorrhiza
The evolution and geographical spread of trees with deep, complex rooting systems and their role in the development of soils is widely regarded as the ‘Devonian engine’ that drove major changes in global biogeochemical cycles as the planet became increasingly forested. This is known as the ‘Devonian plant hypothesis’ (Berner 1997; Retallack 1997; Strullu‐Derrien et al . 2014) and molecular clock (Simon et al . 1993) evidence indicates that the rooting systems of land plants co‐evolved and maintained symbiotic arbuscular mycorrhizal (AM) associations that have played an important role in land colonization and phosphorus uptake by most plants since at least the Early Devonian. It may have started in early Devonian, butu did not 'take off' till later when there was more soil about..

Studies indicate a key role for arbuscular mycorrhizal (those living inside roots) fungi in soil–plant processes and especially in unlocking the limiting nutrient phosphorus in soil via C a‐phosphate dissolution mineral weathering. This suggests co‐evolution of roots and symbiotic fungi since the Early Devonian (around 400mya) could well have triggered positive feedbacks on weathering rates whereby root–fungal P release supports higher biomass forested ecosystems. There is uncertainty in this paradigm around: (1) limited fossil record documenting the origin and timeline of the evolution of tree‐sized plants through the Devonian; and (2) the effects of the evolutionary advance of trees and their in situ rooting structures on palaeosol geochemistry.

Springers

Let's not forget the small soil animals. We know they were around then, but we have never plugged their role into the co-evolution of soil and trees. Yet without the small soil creatures, there would be no trees as we know them nor soil. It has been shown that Onychurid springtails accidentally pick up fungal spores (by sticking to their Hapteron), so that when the small white springtails go to the roots - to feed, they bring with them the fungal spores. This enables the spores to spread to where they need to be. The Onychurids are acting like 'the bees of the soil'. Here is another  link between plants and soil  via the small soil animals which are interacting with micro-fauna and bacteria. 

Many people think that soil creatures are there to just break down debris (like oribatid mites do), but others (springtails) play - and presumably played, a distinct role in keeping roots clean and bringing fungal spores to the roots, to establish mycorrhiza. But they are also responsible for keeping plants alive and healthy. Twas the springtails wot kept the plants fed and upright by enabling roots to absorb water and nutrients - in the form of poo from the small creatures, after they eat bacteria.

Fungal hyphae and rootlets could have started to wrap silt particles with organic matter, including Glomalin and springer poo, to form organic peds - the building blocks of soil.

Devonian Explosion 
And so this event created new conditions, into which plants evolved, consisting now of differentiated parts – particularly stems leaves.. and roots. Soil animals could evolve rapidly too. Springers may well have come from mosses and lichens (common today) to graze, but accidentally groom, the roots, eating dead fungi and encouraging fungal transmissions. Others – mainly mites, helped the fungi break the dead matter down which was then pooed back as nutrients for roots. Both these groups and the other main group of soil mesofauna – nematodes, are all pretty primitive creatures (pre-insecta). They colonised the volcanic dust, silt from flooding, and plant debris,  mixing it all up into  very many but small lumps. Lumps is a good word as it lead to peds - basis of soil. Insects evolved later - the first flyers include dragonflies, a pretty primitive insect evolved around 350mya. But that does not say they were abundant then.  There was a relatively fast evolution in the soil parallel with tree growth. By the end of the end of the Carboniferous, there were trees everywhere (hence the name!). The development and spread of soil animals would have been in parallel, and explains why soil fauna are similar the world over, where the same three groups - springtails, mites and nematodes, dominate. insects do not dominate the soil. But they do dominate IN the air and ON land. They come  into land/soil later.

The development of insects was probably a way to avoid trying to colonise an already populated ecosystem. Some soil animals moved out of the soil and developed to cope with dry conditions and took to the air . Virtually all insects are named after their wing parts, the common characteristic of insects.

If there was ever going to be a 'Punctuated' period of the equilibrium, then it could well be when two cotinents bump into each other for a 100 million years to make one continent. in the process volcanoes are common (why?) and they leave their ashes into which creatures from the water come and colonise, and evolve higher forms of plants aided by soil animals. The trigger was quite a short time - 10-15 mya that enabled/encouraged rapid development of plants and soil animals together, that goes on to this day.

Perhaps this goes somewhere else..
Punctuated Equilibrium
This is a good excuse to bring a man who I have much respect for – the late Stephen Jay Gould. His theory of evolution is that evolution doesn’t happen gradually, but consists of punctuated equilibrium. That means most of the time nothing much changes – same environment, similar occupation of niches etc. But then ‘suddenly’ something happens to change conditions and creatures evolve relatively rapidly to fill the voids. The punctuated part here is from about 380 to 350 mya.

Exploring this further, have a look at:

CLASSIFICATION to show how plat taxonomy /classification fits with the development of the Higher Plants
MANY CHANGES DUE TO GOING TO GROUND 



THAT LED  HIGHER PLANTS