The nature offorest soils and their influence on the ground flora

Traditionally, soils of high quality have been used for agriculture, so those beneath long-established woodlands were usually not only of lower original quality, but have not been limed, fertilized and drained. Such human manipulations have sometimes influenced the soils of modern plantations to such a degree that this causes difficulties when attempts are made to introduce characteristic woodland floras (Section 11.6.2). Conversely, in many parts of the world abandoned agricultural land has reverted to forest and after a century or so it can superficially look like original untouched forest. However, the effects of agricultural practices such as ploughing on the soil can be long-lasting. For example, a study at Harvard Forest in central Massachusetts, USA (Motzkin et al, 1999) found that farm land abandoned a century or more before still had clear evidence of a ploughed surface (Ap) horizon. Moreover, composition of the forest (from trees to bryophytes) was significantly affected by this historical land use, despite the length of time since it was last ploughed and intervening disturbances such as extensive damage by a hurricane in 1938, fire, the ravages of chestnut blight (Section 5.4.6) and widespread tree felling and planting. Former ploughed land is associated with such plants as white pine Pinus strobus, red oak Quercus rubra and red maple Acer rubrum, and undisturbed sites with eastern hemlock Tsuga cana-densis and American witch hazel Hamamelis virginiana. Although agriculture intensely affects the surface of the soil, woodland soils have been affected to a far greater depth by plant growth; though most tree roots are less than 0.3 m deep, they can penetrate to 3 m (see Sections 1.3.2 and 2.3).

The impression given so far in this chapter may be that forest soils are fairly uniform over large areas, but this is not always so. In eastern England, drift deposits of glacial till are a mosaic of boulder clay, sands and gravels, from 1-70 m thick, laid down over a range of bedrocks. The drift is very variable; boulder clay is usually heavy clay with lumps ofchalk, yet woodland soils on it include some of the most acid soils in Britain and are often sandy or silty. This is because much of the till or solid geology is overlain by wind-blown deposits, up to a metre thick, of silt and fine sand, which are almost certainly loess deposits blown in from eastern and central Europe during great dust storms in late glacial times. Again, ploughing has played a role with extensive mixing of such material into agricultural soils, so that its influence is diluted. Besides this homogenization, a combination of ploughing and sheet erosion tends to leave a low cliff at the downhill edge of ancient woodland in the UK.

Soils of old forests are more layered (have clearer horizons) than those modified by agriculture, but they may be disturbed when old trees blow over, pulling their root plates out of the ground often with large amounts of soil attached. In other cases, trunks fall after the major roots have decayed and the soil is not greatly disturbed, or unfelled trees merely rot while still standing, each simply leaving a stump and fragmented debris. But even these have their effect on shaping the soil: the vertical and horizontal channels created by rotting roots act as easy conduits for animals and water. Rooting animals, particularly wild pigs, often cause superficial disturbance, but when grazing is heavy or many walkers use an area (see Section 11.5) the soil becomes compacted. This reduces pore space in the topsoil and increases surface water runoff. The opposite effect is caused by earthworms, moles and other animals which live in the soil and help integrate organic matter and to mix the soil horizons.

Humus type is a cause of diversity both below and above ground (Ponge, 2003), besides being most important in determining plant growth. As mentioned above, acidic soils (such as podzols supporting conifers, or even acid brown earths in temperate forests beneath oak and beech) have few earthworms or other large animals and so litter accumulates as a thick organic deposit, in which L (litter), F (comminuted i.e. fragmented litter) and H (well decomposed amorphous humus with little mineral matter) layers can often be distinguished (Fig. 2.2a), sitting discretely on top of the mineral soil. The L layer may be a few centimetres thick but it is the F layer rich in fine roots and fungal growth and the H layer that make up the bulk of the organic deposit. As discussed in Chapter 7, the hard nutrient-poor litter typical of plants growing on these soils also contributes to organic build-up. The raw humus or mor of such systems is of low fertility and possesses little soil nitrogen; its C/N ratio is often above 20 and even 30-40. Endomycorrhizas are common (see Section 5.4.1), probably because the fungi involved are known for their efficient use of organic nitrogen. Mull humus, in contrast, has passed at least once through the gut of one of the larger soil animals, usually an earthworm's, to form stable mineral-organic soil crumbs with virtually no litter (L) layer. It develops under deciduous or mixed forests on moderately well-drained soils with adequate calcium and a high pH, and also in forests of cedars and of those spruces whose litter has a high calcium content. The C/N ratio of mull is usually about 10. Moder humus is intermediate or transitional between mor and mull, characteristic of fairly acidic soils often under trees with hard litter such as beech or oak. Here, earthworms are usually absent and the litter is mainly decomposed by fungi and arthropods (ants, mites, springtails, millipedes, woodlice/sowbugs, etc.), leaving a mix of plant fragments and mineral particles held together by arthropod faecal material. This gives a loose, crumbly texture to the humus.

Trees themselves have a direct influence on the nature of soil humus. In some places in the UK the soils of birch plantations have mull humus, while soils beneath adjacent Scots pine on the same parent materials undergo podzolization and mor humus is formed. Even trees in the same genus vary in their influence on soil pH. Topsoil beneath eastern red cedar Juniperus virginiana has a raised pH, while that beneath common juniper J. communis in the same Connecticut old fields is reduced by the low base status and acid nature of its leaf litter (Spurr and Barnes, 1980). Soil pH of the root zone of the eastern red cedar is lowered, but considerable amounts of calcium and other bases absorbed by its roots are returned to the soil in the leaf litter. A similar effect of leaf litter has been seen in woodlands over the acidic Coal Measures near Sheffield, UK. Mean pH of the surface soil (0-3 cm) in 80 samples taken was higher than that at 9-12 cm (Packham and Willis, 1976). Bases contributed by leaf litter are again likely to have caused this effect.

Specific examples of how the underlying geology and soil type influences forest development and the ground flora present can be found around the world. In some cases the link is very visible. For example, in Swaziland, southern Africa, sandstones with sandy topsoils and tough clay pans are widespread, but areas of igneous rock occur which carry a brown, clay-rich soil. The dominant clay pan soils support a savanna of thorny Acacia spp. which are largely absent from the clay soils, being replaced by a woodland savanna containing the marula plum Sclerocarya birrea, which is noted for the intoxicating effect of its fermenting fruits on the local elephants!

Usually, however, the picture is more complex. The Ercall is a prominent hill in Shropshire, UK made up of a variety of rocks which, with variations in slope and water regime, result in a complex set of woodlands within an area of about 50 ha. The woodlands are largely of oaks but with differences across the site (Fig. 2.4). The north-west side of the hill is composed of acidic igneous rocks with a podzol supporting oak woodland (pedunculate oak Quercus robur and sessile oak Q. petraea) with an acidic field layer including heather, bilberry and wavy hair grass (A in Fig. 2.4). On the south-east slope, sedimentary quartzite again supports a podzol but better drainage and a warmer (and hence drier) slope results in an open woodland dominated by birch with a dense covering of bracken beneath (B1 and B2). The eastern flatter slopes are on siltstones and shales and are damper and richer in nutrients giving a good brown earth soil (C). These support a rich oak woodland with a variety of other trees and a diverse ground flora. In some places boulder clay creates an argillic (clay-enriched) brown earth with a more neutral pH and greater cation exchange capacity (see above) holding an abundance of plant nutrients (D). Not surprisingly, here is an even richer mixed woodland, with diverse

WEST

EAST

WEST

EAST

Soil Catena Podzol Brown Earth

168 m

Granophyre

168 m

Granophyre

Cambrian quartzite

Cambrian Boulder siltstones and clay shales

Soils

Vegetation

pH

Topsoil

Subsoil

A

Podzols and brown podzolic soils

Oak-birch coppice; heather, bilberry, bracken or wavy hair grass locally dominant. The mosses Leucobryum glaucum and Plagiothecium undulatum are conspicuous

3.9

4.5

B1

Humo-ferric podzols and podzolic rankers

Bracken, some birch scrub and heather

3.7

4.4

B2

Gley-podzols

Bracken, some birch and bluebell

3.8

4.6

C

Typical brown earths

Oak-birch coppice, with rowan and holly, very variable field layer including male fern, ivy, honeysuckle, bluebell, bramble, creeping soft-grass, wood sorrel, yellow archangel

4.1

4.3

D

Gleyic argillic brown earths

Mixed deciduous woodland with ash, birch, wych elm and alder, with hazel, hawthorn, oak and ash regenerating; field layer includes dog's mercury, enchanter's nightshade, male fern, tufted hair grass, wood sanicle

5.2

5.7

Figure 2.4 Relationships between underlying rock, soils and vegetation on The Ercall, Shropshire, UK. Leaching has decreased the pH of the topsoils. (Redrawn from Burnham and Mackney, 1964. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

tree, field and ground layers, including the calcium-loving wood sanicle Sanicula europaea. Where a stream passes through the site, the extra moisture and alluvial deposits result in a wet woodland with alder, birch and hazel; a 5 x 5 m area looked at contained 27 species of vascular plants and 9 species of mosses and liverworts (bryophytes) with many more species within the immediate area (Burnham and Mackney, 1964).

Sometimes the effects of soils on vegetation may not be visually obvious but are nevertheless no less important. The mixed dipterocarp forests in Sarawak, Malaysian Borneo, are among the most species-rich tropical forests. A longstanding problem is to try to understand how so many different species can co-exist without one or two eventually coming to dominate. Part of the answer, it seems, is to do with soils. Potts et al. (2002) looked at 105 different 0.6 ha plots in an area approximately 500 x 150 km and found 1762 different tree species. Sixty species were judged common as they were found amongst the ten most abundant species in at least five of the plots. Of these, 23 species were confined to sandstone soils with a surface layer of humus and another 20 were restricted to clay-rich soils over shale with no humus layer. The other 17 were not exacting as to which soil they grew on. These patterns are not easy to see on the ground because of the bewildering variety of trees but differences in soils do seem to partly explain tree distribution.

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