The woodland environment; Part 2, Soil etc.

This is about all the woodland environmental factors except for light. Light is the most important factor for woodlands and I  dealt with it  in part 1.

Other factors which affect which affect the woodland environment are to do with the soil. If you want to sound nice and scientific then these are called edaphic factors. They are probably the second most critical factor after light. Also temperature, wind speed and , humidity  soil and air temperature have their effects.

 

There are various ways to analyse a woodland soil. Firstly you might want to look at what is called a soil profile. You could do this by digging down and producing a mini cliff face so that you can view the different zones, known as horizons.  Alternatively you could use a soil auger which is like a giant corkscrew. You screw it into the ground to a depth of about 10/15 cm and then carefully take out the core of soil.  the soil is then removed from the auger and placed onto a tray as shown. Then the soil auger is placed back into the hole and screwed in another 10/15 cm. Then removed and the next layer of soil is detached and place on the tray. This process is repeated several times and thus a core of soil is removed and reconstructed onto the tray and you will be able to see its structure.

Subsequently small samples can be removed and analysed. You can test the pH using universal indicator and there are various kits to record nitrate and phosphate levels. These are the most important nutrients in the soil but it would be possible to test for other minerals like potassium or iron.  Water levels can also be ascertained but to do this you would need a slightly larger sample, enough to fill a petri dish and then it is simply a matter of weighing it fresh, then drying it out and reweighing it then the decrease in weight represents the amount of water and this can be converted into a %.

This is a sequence showing a soil profile being taken from Ninewells wood. Usually the profile shows a narrow layer at the top of leaf litter, followed by a thin layer of organic debris in which remnants of eaves and twigs can still be made out. This merges into a dark organically rich layer where the organic matter and soil blend. These three layers normally only extend down a short distance. Then what normally happens is that the soil gradually looses any dark organic colour and the normal sub soil colour is assumed.  In some cases you will get a build up of rocky particles and stones before getting down to the bed rock. This is in situations where the soil is shallow. In Ninewells wood the soil is quite deep and I did not get any indication of this.

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What this all means for the plants is that the surface is rich and well supplied with nutrients and so surface rooting is a good idea. Tree roots will penetrate further down to gain some stability but most plants restrict their root mas to a layer no more than 50cm deep.

The pH of the soil is also important and usually soil sample from different depths are tested, say 5cm, 20cm, 35cm and 50cm . This is what I did and the sample is mixed with distilled water then a few drops of universal indicator are added and the colur compared with a colour chart. Alternatively electronic pH meters are   available but in my experience they are expensive fragile and whilst more accurate they are far more fiddly to use. The soil at Ninewells is pH 5 ie quite acidic and it remains the same at all depths that were tested. Sometimes at deeper depths the pH is different from the surface especially if the underlying rock is say chalk or limestone.

The pH will affect what type of trees grow in an area, you will get Oaks on soil with a lower pH and Beech where it is higher. However I have both growing in my patch of Ninewells wood and that is quite acidic. The classification of woodlands can become quite complex as you rarely get a woodland which is just Oak or just Ash, there are normally associated species. If you do get a woodland with just one species then it is almost certainly planted. There was a woodland in Norfolk which was pure Poplar, it was next to quite a nice mixed woodland in the village of Stow Bardolph. I think it had been planted to provide wood to make matches, quite a lot of Poplar was planted in the 1950’s for this use. Of course now  the demand for matches has considerably reduced so it just sits there. If you are interested to find out the official classifications then click the link.

In terms of the wild flowers then there are some which will only grow where the pH is below 7, they are called Califuges, I like to think of it as a plant seeking refuge from Calcium. Examples are Heather (Calluna vulgaris) and  Bilberry. Others like and alkaline pH and are called Calcicoles. Examples of these are quite a few of the orchids like Early Purple Orchid. There are also quite a lot of plants which do not really mind and will grow in a range of pH’s from say 6 through to 8 , just not too happy in the extremes.

One thing to bear in mind about woodland soils is that as a result of a constant input of dead leaves, every Autumn, a layer of leaf litter builds up and gradually decomposes producing what my Father called leaf mould, but in more scientific terms is an organic rich layer. Good though this is, it will be acidic as leaves contain chemicals like tannins which are mild acids. The term humic acid is sometimes used though I think this is somewhat similar to leaf mould,  ie old fashioned and not very scientific. According to wikipedia it is ‘a complex mixture of many different acids containing carboxyi and phenolate groups so that the mixture behaves functionally as a dibasic acid  or, occasionally, as a tribasic acid.’  Thus anywhere where a woodland has been in existence for more than a few years will have a soil pH which is lower than the surrounding areas on which there is no woodland. It could lower the pH by 1 or 2 on the scale. Conifers tend to make the pH lower than deciduous trees.

The next most important thing about the soil is the water content, which will vary depending on whether the soil is clay or whether it is a more open soil (loam). If it is wet then trees like Alder and Ash will be favoured.  Dryer soils tend to favour Silver Birch. Also the layer of leaf litter will be beneficial in terms of maintaining a good water level. The leaf litter will hold water but will also have air spaces so it will be damp, not wet and waterlogged and not dry, ie just right.

Woodland soils will tend to be richer in nutrients, especially nitrates and phosphates because of the breakdown of the leaf litter by the small invertebrates, fungi and microorganisms.

Finally woodland soils will tend to be slightly warmer than other soils and again this is down to the leaf litter and humus which is decomposing and decomposition produces heat. Also warmth enters the soil via air spaces and this layer will have lots of air spaces.

Now, I have dealt with Light ( in a previous page), I have dealt with the soil, all that remains is the air. There are several aspects to woodland air which will affect the plants growing there. But they are all beneficial, ie its ‘nicer’ in the woods than it is outside.

TEMPERATURE, the woodland temperature is more even than it is outside a woodland, in the winter it is warmer in a wood than it is outside and in the summer it is cooler. This will be to the plants liking and indeed the animals.

HUMIDITY, it is  always damper in a wood compared to outside, partly due to the evaporation from the moist leaf litter layer and due to the transpiration from the leaves.

WIND SPEED, this will be less in a wood than it is outside due to the leaves and branches slowing the air flow.

These three factors combine to reduce the rate of transpiration, which is just as well because woodland plants need large thin leaves in order to make good use of the restricted light levels. Large thin leaves are not what you want if evaporation rates are high.  So that is lucky then!

There are various ways to analyse a woodland soil. Firstly you might want to look at what is called a soil profile. You could do this by digging down and producing a mini cliff face so that you can view the different zones, known as horizons.  Alternatively you could use a soil auger which is like a giant corkscrew. You screw it into the ground to a depth of about 10/15 cm and then carefully take out the core of soil.  the soil is then removed from the auger and placed onto a tray as shown. Then the soil auger is placed back into the hole and screwed in another 10/15 cm. Then removed and the next layer of soil is detached and place on the tray. This process is repeated several times and thus a core of soil is removed and reconstructed onto the tray and you will be able to see its structure.

Subsequently small samples can be removed and analysed. You can test the pH using universal indicator and there are various kits to record nitrate and phosphate levels. These are the most important nutrients in the soil but it would be possible to test for other minerals like potassium or iron.  Water levels can also be ascertained but to do this you would need a slightly larger sample, enough to fill a petri dish and then it is simply a matter of weighing it fresh, then drying it out and reweighing it then the decrease in weight represents the amount of water and this can be converted into a %.

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