Forest zonation and site quality

2.4.1 Influence of climate and soil type

This section is concerned with zonation within forested regions. The ecology of forest margins and the features which mark both the beginning and the end of tree communities along climatological and altitudinal gradients are dealt with in Section 3.5. However, these timberlines, the biological boundaries marking the limit of a forest either high up on mountain sides, in frigid polar regions or adjacent to grasslands or deserts, are the subject of intense interest, particularly as there is evidence that their location is changing in response to global warming.

Soils are complex entities, shaped in response to underlying rock, climate and the influence of vegetation. In return, plants respond to the wide variations in soil that can occur in composition, structure, moisture, oxygen, pH and nutrition. All these factors together are summed up as site quality. Standing back and looking at the landscape, it is possible to see an overall zonation, a change within the vegetation as site quality alters. Sometimes, a change in the forest is abrupt, as with a striking change in the underlying rock type, but normally transitions in forest composition and structure are gradual, producing almost endless variation. Being human, however, we like to impose a pattern on this continuum, to divide up the endless variation in forests into discrete types that we can name and understand.

The woodland and scrub communities of Britain provide a large-scale example of how forest zonation and site quality are influenced by climate and soil type. Figure 2.6 shows 13 main NVC communities (National Vegetation Classification communities - see Section 1.7.1) grouped in relations to these two factors, with the six mixed deciduous and oak-birch woodlands lying centrally. It is important to realize that the name used for each group applies to the normal dominant assembly of species. A particular W17 woodland, for example, may not contain birch Betula, while oak Quercus is a common tree in many W8 communities named for the dominant ash Fraxinus, field maple Acer and dog's mercury Mercurialis. Within this central group, the soils diminish in fertility from left to right corresponding, in the warm, dry lowlands of the south-east, to a W8 Ash-field maple-dog's mercury - W10 Oak-bracken-bramble - W16 Oak-birch-wavy hair-grass woodland sequence. As Fig. 2.6 shows, the woodland sequence in the cool, wet north-western submontane zone runs W9 Ash-rowan-dog's mercury - W11 Oak-birch-wood-sorrel - W17 Oak-birch-Dicranum moss woodland. The latter includes, despite its south-western position, the vegetation on the bank of large granite boulders (clitter) of Wistman's Wood which is described in Box 1.3 and Section 9.4.1. Here pockets of acid, free-draining brown earth soils have accumulated amongst the clitter; these contrast with the wet peats and gleys of the adjacent floodplain and grass-moor plateau.

At the bottom of the figure are the W13 yew woodlands whose origins on the thin dry calcareous chalk soils of south-east England are discussed in Section 9.4.4. Above them are placed the three communities characteristic of the zone of natural beech dominance (W12, W14 and W15), again arranged in order of decreasing fertility from left to right, with dog's mercury Mercurialis perennis being

Salix-Luzula scrub

Juniperus-Oxalis woodland

Pinus-Hylocomium woodland

Fraxinus-Sorbus-Mercurialis woodland

Quercus-Betula-Oxalis woodland

Quercus-Betula-Dicranum woodland

Fraxinus-Acer-Mercurialis woodland

Quercus-Pteridium-Rubus woodland

Quercus-Betula-Deschampsia woodland

Fagus-Mercurialis woodland

Fagus-Rubus woodland

Fagus-Deschampsia woodland

Taxus woodland

NORTHERN UPLANDS AND SUBALPINE ZONE

COOL AND WET NORTHWESTERN SUBMONTANE ZONE

WARM AND DRY SOUTHEASTERN LOWLAND ZONE

ZONE OF NATURAL BEECH DOMINANCE

LOCALLY IN SOUTHERN BRITAIN

COLD

NORTHERN UPLANDS AND SUBALPINE ZONE

COOL AND WET NORTHWESTERN SUBMONTANE ZONE

WARM AND DRY SOUTHEASTERN LOWLAND ZONE

ZONE OF NATURAL BEECH DOMINANCE

LOCALLY IN SOUTHERN BRITAIN

Rendzinas and brown Brown earths of low Rankers, brown podzolic calcareous earths base status soils and podsols

Figure 2.6 The influence of climate and soil type on the distribution of three north British communities (W18-W20), the six mixed-deciduous and oak-birch woodlands (W8-W11 and W16-W17), the woodlands in the zone of natural beech dominance (W12, W14 and W15) and the southern yew woods (W13) of the National Vegetation Classification (Data from Rodwell, 1991. Woodlands and Scrub. British Plant Communities. Vol. 1. Cambridge University Press.)

associated with the most fertile, and wavy hair-grass Deschampsia flexuosa with the least. In contrast to these are W20 (willow-woodrush scrub), W19 (juniper-wood-sorrel woodland) and W18 (pine-Hylocomium moss woodland), which form the sequence of decreasing fertility shown for the cold northern uplands and subalpine zone of the north of Britain.

The 12 NVC woodland communities not shown in Fig. 2.6 are woodlands W1-7, which are characteristic of damp places and involve alder Alnus, willow Salix, downy birch Betula pubescens and ash Fraxinus excelsior, on the one hand, and the scrub and underscrub communities of W21-25 on the other.

Dog's mercury and hart's tongue fern Phyllitis scolopendrium grow well on chalk soils. Both were present in the sycamore wood on the chalk escarpment at Arundel, Sussex. Stinging nettle Urtica dioica, also found here, will flourish only if the soil has an adequate phosphorus content. While most natural soils have been subjected to extended leaching, excessive fertility is often a problem when former agricultural land is converted to forest. This is particularly the case when the intention is to create a high-quality herb layer beneath the developing tree canopy. A modern solution, widely and successfully used in Denmark and the Netherlands, is top-soil inversion. A huge plough is used to turn over a spit a metre deep, thus burying the nutrient-rich surface soil and bringing poor subsoil to the surface. This method has recently been employed at Wheeldon Copse, near Chester in Cheshire, England. This 7-ha former arable site is being developed by the Woodland Trust and Landlife, an environmental charity. Here cornfield annuals - corncockle Agrostemma githago, corn marigold Chrysanthemum segetum, cornflower Centaurea cyanus and a sprinkling of poppies Papaver rhoeas - were sown before the young trees were planted, with the intention of diminishing the invasion of rank weeds and grasses from the field edges while the trees established.

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