Part 2: Soil and deadwood

INTRODUCTION

Having taken a closer look at the woodlands I most often visit in the first part of this series and getting to appreciate how the geographical location and habitat define the vegetation that can grow there, the next logical step in achieving a better understanding of the ecology is to look at substrates. The primary component in respect of cryptozoic and saproxylic species, and indeed many of the fungi featured in following articles, is deadwood, but before we start exploring that important material it makes sense to begin at ground level with the soil, leaf-litter and other forest floor detritus.

2.1: Geology and soil groups > Soil attributes - structure and water retention, soil pH, horizons and soil profiles.

Woodland soils > Humus and the importance of leaf-litter

2.2 Deadwood

Associated information:

Appendix 2A:

Appendix 2B:

Geology and soil groups

2.1 (v.1)

Despite being taught basic geology at school, I have to admit that it's one of those academic subjects that's pretty much passed me by. However, I do remember that soil is derived from the underlying bedrock, so that seems like the best place to start.

A bit of research shows that the whole of my local area of East Dorset and South Hampshire is in a geological zone broadly known as the 'Hampshire Basin' with various rock types from the Jurassic, Cretaceous and Palaeogene periods - the latter including the Eocene and Oligocene epochs. The variation in the bedrock across Dorset gives the landscape plenty of character with distinct and interesting habitats including chalk downlands to the north, sandy heaths and moors to the south, and the world-famous Jurassic coastline. There are also plenty of wooded areas, although mostly coniferous forest close to home rather than broadleaved woodland, which is why I feel blessed that as well as having the other places to visit the New Forest is just across the county border.

The subject matter here is to have a better understanding about what's under our feet when we're in the forest and how the soil varies from one location to another. In that respect it's useful to see where these rock formations lay in relation to the sites I visit, so this is my rather crude, but hopefully reasonably accurate, hand-drawn attempt at representing the important features on a map where I've marked the location of my local woodlands (as referenced in articles 1.1a & 1.1b) and the approximate boundary of the New Forest NP.

Simplified geological map of East Dorset and the New Forest area (click to enlarge)

The northernmost woodlands in Cranborne Chase are located close together on the chalk downland plateau that is part of the wide Cretaceous white chalk band that runs from the south west to the north east of the county. These chalk beds were formed during the latter part of the Cretaceous period some 100 million years ago when much of the south was submerged 200m or more below the sea. The underlying clay and sands were slowly displaced and absorbed as the chalk layers built up and, as the volume of clay decreased, the rock became purer and harder. The geology defines three age groups, the oldest and least pure being the Lower Chalk, which comprises beds as little as 30m thick, whereas the Upper Chalk, which is the youngest and purest, and the predominant type across these downlands, ranges from 250m to 400m thick. On the higher ground at Stonedown Wood and also towards the northern end of Garston Wood the chalk is capped with drift clay and flint. The soils here are mainly Rendzinas - shallow, humous-rich, calcareous and well-drained - where they are directly over the chalk bedrock, or Paleo-argyllic brown earths - well-drained, clayey and silty - where there's a Clay-with-Flint underlayer.

Running along the southern boundary of the chalk there's a narrow band of Lower Eocene ('Reading & London') clay that roughly follows a path through Wimborne up and across the Hampshire border and through the Fordingbridge area in the north of the New Forest. It encapsulates Horton Wood, but skirts around the Whitesheet and Cannon Hill Plantations that are located south east of Wimborne.

The majority of the region though, including the Verwood area of Ringwood Forest, plus the west, north and central areas of the New Forest, lies on an extensive Eocene clay area known as the 'Bagshot, Bracklesham and Barton Beds', which gently slopes down towards the coast to merge with the base-rich clays of the Oligocene 'Headon Beds' in the south of the Forest from Brockenhurst to Lymington.

The 'Bagshot Beds' are course and porous, which leach nutrients more freely than fine clay sediments. The soils in the areas where these beds are present, are acidic and nutrient poor, whereas soils derived from the clays of the 'Bracklesham and Barton Beds' are relatively base-rich by comparison. The 'Heaton Beds' in the south are even more fertile, so in broad term there are three soil types - acid, nutrient-poor sands and gravels in the north, merging into less acid, but otherwise not dissimilar soils grading into more nutrient-rich, less base-poor soils in the south. It's not that straightforward as there are actually eight or nine defined geographical soil associations in the New Forest in accordance with the formal soil classification system that identifies Soil Series as distinctive assemblages in specific landscape types.

Soil attributes

2.1a (v.1)

Structure and water retention

Soil is a complex and highly dynamic mixture of various forms and sizes of mineral particles, such as sand, silt and clay, that have originated from the bedrock, plus small amounts of organic matter, and a variable, but typically high, proportion of water and air that can jointly account for nearly half the total volume. The water content is dependent on various factors including geographical location and exposure, but also the consistency of the mass, and the type and size of the pores. For example, sandy and chalky soils are light and free-draining, whereas clay tends to be sticky with small particles that contain very few air pockets such that any excess water becomes trapped. When the ground becomes saturated the pores naturally collect water, with any overfill draining away until the soil is back to field capacity. If it carries on raining, the heavier soils will soon become waterlogged and, even if there's a drier period, it's the larger pours that will be the first to start drying out as the force necessary to draw water from soil gets progressively harder as the pores get smaller.

Soil pH

The 'potential of Hydrogen' in soils, commonly known as the pH level, is a logarithmic scale that measures the acidity or alkalinity of soil within a range of 0 to 14 where the mid-point 7 is regarded as neutral, although in practice that point is widened as neutral is usually within a range of 6.6 to 7.3. Higher pH levels of 7.4 to around 7.9 are slightly alkaline, whilst a

pH of 8 or above is definitely considered as alkaline. In the other direction, pH levels below 6.6 become progressively more acidic. Forest soils fall somewhere between pH 4.5 and 7, with anything below 4.5 regarded as strongly acidic, although the exact definition of acidic at that level is somewhat variable.

The soil maps of the area show that the calcareous downland woodlands in Cranborne Chase are towards the higher end of the pH scale from around 6.8, but right up to pH 8 in the vicinity of Garston Wood. As we move south away from the chalk bedrock to the Horton Wood area, the level drops to around pH 6, and then in the coniferous forests near Verwood it's in the region of 4.5. The New Forest covers a wide range from pH 5.5 down to pH 4.5 in most woodlands according to the topsoil values indicated on the map, but in the previous set of articles relating to the vegetation communities there was a clear reference to the strongly acidic areas having a pH below 4.

Soil horizons

Although all of the preceding information is good to know as it helps to explain what's going on deeper down in the ground, I'm more interested in what's happening on or certainly near the surface. In this respect it's worth having a basic knowledge of soil horizons, which is the term used to describe the different layers that would be visible if you were to dig down to the bedrock. They have been produced over a long period of time through interactions and processes within the soil system. The different horizons can be identified by content, colour and texture and have been widely studied, which has resulted in a universally recognised system that defines each layer or type by a specific letter. Some horizons are split at times, which can become a little confusing, but the system is quite straightforward once the basics are understood.

Without going into unnecessary detail at this point, woodland soil comprises an organic surface layer known as the 'O' horizon, which is primarily made up of fallen leaves or pine needles, twigs and other fragments of wooded debris, plus an assortment of animal and plant matter (fully described below as humus). Immediately below this loose material that can be easily cleared to one side, you have the topsoil horizon, denoted by the letter 'A', which contains most of the organic decaying matter typically giving it

a dark colour.

Most of the soil organisms that I'm able to feature in part 4 of this series either live in the organic surface layer or the first few centimetres of the top soil. The anecic deep-burrowing earthworms certainly go much deeper and some of the endogeic species could also be found further down, so although I have no intention of digging down to find them it's still interesting to know how the remaining horizons are identified.

Over time, and dependent on habitat and various other factors, clays and minerals can leach out of the topsoil (covered in more detail below under Mor humus) creating a leached sublayer, also known as the eluvial horizon, which is why it breaks the 'A','B','C' sequence and is given the letter 'E'. When present it is a well-defined zone typically lighter in colour than the 'A' horizon above, or the 'B' horizon below.

The 'B' horizon, commonly referred to as the subsoil or mineral soil, is an accumulation layer where leached minerals and nutrients accumulate, which usually gives it a distinct colour and certainly in the case of Mor humus produces a clearly defined transition between the 'A' ('E') and 'B' horizons. Deeper still we have the 'C' horizon which constitutes the weathered parent material from which the soil has formed. It contains little organic matter, but does affect the whole soil profile and the characteristics of all the upper horizons. That completes the 'A','B','C' sequence, but there is one further layer below, which is the 'R' horizon that represents the underlaying bedrock that was discussed in the first article.

Soil profiles

Soil can be classified in many different ways, so to avoid another long explanation this is just a brief summary in respect of the woodland soils in my area. Further clarification and information are given in Appendix 2A.

The three main profiles are Brown earth, specifically the moderately acid Argillic Brown Soils, which leach to some degree forming a merging boundary between the dark brown top soil and the normally paler brown subsoil; Podsols, which are characterised by their distinct 'A', 'E', 'B' horizons as covered below under Mor humus; and Gley, particularly Stagnogley, which is the typical heavy clay with poor drainage found extensively in lowland Britain.

In the New Forest, seasonally waterlogged Stagnogleys dominate the pasture woodlands with Stagnogleyic Argillic Brown Earths occurring in better drained areas, and Argillic Gley Soils in locations affected by groundwater. The most notable exception are the pasture woodlands and associated Inclosures around Mark Ash Wood in the centre of the forest where the soils are classified as loamy and clayey with impeded drainage.

Apart from White Sheet Plantation that borders Holt Heath, which has free-draining sandy and loamy soils, all of the coniferous woodlands close to home are naturally wet sites and, particularly during the winter months or indeed any time when there have been prolonged periods of rain, there are many boggy areas that can become waterlogged to varying degrees. One such area is in Ringwood Forest, so I thought I finish this long article with a photo that looks more like a shot

of the Everglades.

Admittedly this particular spot in Ringwood Forest is close to a stream, but there are many similar areas of naturally

wet woodlands that remain relatively dry during much of the year, but become totally waterlogged in the winter

Woodland soils

2.1b (v.1)

Although I've made a number of comments about soil associations relating to specific sites or within certain geographical areas, it's useful to have a general overview and comparison regarding the two principle types of woodland habitat.

Broadleaved woodland

In broadleaved forests and woodlands where the soil is neutral to slightly acid (typically with a pH of 5.5 or higher) you'd expect to find brown forest earth where the topmost layer is made up of a mixture of organic material. The main component part will be partially decomposed leaves from the previous autumn fall mixed in with twigs, plus an assortment of plant and animal matter including old faecal remains.

Technically it's called humus and is further defined as 'mull humus' as described below, but it's more commonly known as leaf-litter, or simply litter.

The depth of the litter and the rate of decomposition of the leaves and other organic matter is affected by certain environmental conditions relating to exposure, soil type, and moisture retention, but also in relation to fungi and the presence of litter and soil organisms, particularly earthworms. Another factor is that the leaves from different trees break down at different rates. Beech and Oak leaves that usually lay deep on the ground are quite tough and can take two years or longer to rot, whereas Birch leaves for example which are softer may end up fully decomposed within 6-9 months.

The dark brown colour of the humus extends down into the topsoil to a depth rarely less than 20cm.

Coniferous woodland

In coniferous plantation woodlands with pines, firs and other conifers growing on nutrient-poor, acidic soil (typically with a pH under 5.5) the situation is very different as the humus layer here mainly consists of tough evergreen pine needles, conifer cones, twigs and other woody debris that takes much longer to rot down. Earthworms are rarely found in this type of habitat and although there are probably plenty of other creatures, such as mite and springtails, living amongst the pine needles they have minimal impact on the decomposition process, such that it's not unusual to find that this form of humus is 15-20cm deep in places. It is typically quite loose, non-compacted and often noticeably springy underfoot.

Another factor in pine forests is that conifers shed their old needles at different rates and in different cycles, which means that the soil is being starved of fresh organic matter. Scots Pine for example can take 2-3 years to shed their needles with the older growth turning yellow and dropping in the autumn, but the ripening needles remain green until the following year or the year after until it is time for them to be replaced with new growth. Spruce on the other hand retains its needles for around 5 years, which makes the process almost inconspicuous. The high volume of decomposed matter in these woodlands results in a slow release of nutrients back into the soil, which restricts plant development.

The primary decomposer in coniferous woodlands is fungi.

Humus - getting to the bottom of the matter, and the importance of leaf-litter

2.2 (v.1)

The rotting process and breakdown of the materials that form the all-important organic 'O' horizon or surface layer of the forest floor that we typically refer to as leaf-litter, has been carefully studied and separated into three defined forms based on the rate of decomposition and the humus that is produced. The three forms are known as Mull, Moder and Mor, with each form representing a decreasing level of nutrient recycling.

Mull humus is a rich, dark layer of decomposed organic and mineral matter that forms a crumbly and nutrient-rich horizon due to the presence and activity of soil organisms such as earthworms which are the primary decomposers and mixers of organic material on deciduous forest floors. The feeding and burrowing activity of earthworms and other soil-dwelling invertebrates facilitate rapid recycling of nutrients into the topsoil, thereby reducing the accumulation of organic matter within the 'O' horizon as typically found in coniferous forests. Conversely, the 'A' horizon is well developed, creating optimal conditions for new plant growth and the creatures that live there. The 'B' horizon is effectively combined with the 'A' horizon as a result of the activity within the top soil.

Moder humus is characterised by a less rapid transformation of litter, resulting in a greater accumulation of organic matter at or near the soil surface. This form often occurs in mixed woodlands and broadleaved forests typically with nutrient-poor, acidic soils where there are fewer earthworms or detritivorous invertebrate species to speed up the decomposition process. In this type of environment fungi generally play a more important role, which is a subject that I intend to research in greater detail for an associated article in part 3. Due to the increased thickness of the 'O' horizon, three sub-layers are described: 'OL' being the exposed top layer where the fallen leaves or needles and other surface debris is largely intact; 'OF' which is the hidden layer below consisting of partially decomposed matter that is mostly identifiable; and then the 'OH' horizon, which is an accumulation of fully disintegrated organic matter that merges into the topsoil.

Mor humous is found in coniferous forests and in locations where woodland merges with ericaceous heathland. In many respects it is similar to Moder, but is characterised by an even slower rate of decomposition that results in deep accumulations of old pine needles, partially decomposed conifer cones, twigs and other organic matter. Below the Mor layer is a typically ash-grey coloured 'A' horizon, which has lost its minerals, organic matter and iron/aluminium compounds through a process called leaching being the percolation of acidic water through the soil. The subsoil 'B' horizon is usually a reddish-brown or yellowish-brown colour because of the metal oxides and organic matter that were leached from the topsoil. If you were to dig down to that level there would be a clearly defined transition between the lower layer of humus and topsoil with the 'B' horizon mineral subsoil.

This photo provides a pretty good view of the various soil horizons you'd expect to find in coniferous forests.

It was taken during late autumn when there were a lot of wind-blown twigs and plenty of dropped pine cones and needles laying on the ground. I carefully dug down so that the majority of that material stayed in place, and took a wide-angle shot to ensure all horizons were visible.

The surface-laying debris represents the 'OL' horizon as described above. Immediately below which there's the 'OF' horizon with fragmented and partially decomposed matter near the surface merging into the paler-coloured 'OH' horizon, which is mostly made up of crumbly fully decomposed material. The total depth through all of this loosely compacted organic matter to the point where it starts to intermingle with the 'A' horizon was around 25cm.

I then had to dig down a further 12cm or so through the dark-coloured humus and organic mix, as well as though some tree roots, before reaching the firm top layer of the leached ash-grey soil that sits at the bottom of the hole.

If I'd dug further down, I would have eventually come to the mineral deposits and the 'B' horizon subsoil.

Mor humus in coniferous woodland

Leaf-litter

The above descriptions regarding the different forms of humus are certainly useful, but leaf-litter in the context of the cryptozoa, and particularly part 5 of this series entitled 'life on the forest floor', refers to the leaves and associated organic material that are free-laying on the surface rather than the decomposing remains that are becoming absorbed into the soil.

That's not to say that the fragmented and decayed material in the ground is not important, because it certainly is as it releases essential nutrients back into the soil enriching its fertility, which is the foundation of new growth. And, of course, there are many soil organisms and other ground-dwelling species that live in that environment, but for the purpose of this series of articles they're covered in part 4.

But before we even start looking at the ecological role of leaf-litter and the various species that live or can be found there as I intend to cover in the Cryptozoa section, it's worth remembering that leaf-litter is also important for fungi. In that respect, the layer of dropped pine needles in coniferous forests fulfils a not dissimilar role to fallen leaves in deciduous woodland. The primary difference is that while coniferous forests have a much deeper layer of humus overall, broadleaved woodlands typically have a much greater volume of material laying on the surface, especially during the autumn months and particularly in Beech and Oak woodlands.

The following photos were both taken in November. The first shot taken in Shave Wood in the northwest of the New Forest is virtually all Beech leaves with just a few Holly leaves and twigs, and bottom left there's the top of a buried mushroom, which gives an idea how deep these leaves lay. The ground covering across most of this woodland and numerous similar locations all look very similar during the autumn months. The second photo taken at much the same time was from one of the more densely planted areas of pine in Ringwood Forest and, consequently, the mass of pine needles seen here is not found everywhere. It appears as though there's quite a thick covering, but when you look more closely and notice the bryophytes poking through you can appreciate that it's quite a thin layer. And, the other point to note is, that in the Beech woodland there were many leaves still on the trees that hadn't dropped, whereas most of the pine needles that were going to be shed were probably down on the ground.

Autumn leaves in broadleaved woodland

Pine needles and cones in coniferous woodland

A thick and dense covering of leaves during the winter months helps to stabilise and insulate the soil as it provides a layer of protection from the impact of frost and rain, and conversely during drier, sunnier times it helps retain moisture by reducing evaporation.