Climate, Geology and Soils

The Devon landscape: climate, geology and soils he landscape of Devon reflects the complexity of interactions between climate, geology, soils and land use Tthat have combined to produce a wide range of plant environments. The diverse flora of the county has adapted to these environments, and continues to change in response to land use pressures and on-going and future climate change. This introduction to the climate, geology and soils of Devon provides an overview of broad edaphic patterns and relationships, thus explaining the essential controls on the diversity of flora that can be observed throughout the county. Climate The climate of Devon varies widely from the low-lying coastal zones exposed to mild, maritime influences to the harsher upland environments of Dartmoor and Exmoor. The combination of altitude, exposure and distance from the ameliorating influences of the sea produces markedly contrasting climatic environments. This section gives a brief summary of key elements of the climate of Devon. For more detailed information, see Meteorological Office (2012). Rainfall The topography of Devon exerts a strong influence on the distribution of rainfall. Figure 1 shows the mean annual average rainfall, recorded over the period 1981– 2010. The two elevated areas of moorland, Dartmoor and Exmoor, receive the highest annual rainfall total. Parts of northern Dartmoor lie 600 m above sea level Figure 1. Annual average rainfall 1981–2010 (Crown Copyright, reproduced courtesy of Met Office, National Climate Information Centre). A New Flora of Devon 11 The Devon landscape: climate, geology and soils (asl), with the highest summits of High Willhays and Yes Tor reaching 620 m Annual rainfall on the northern slopes of Dartmoor ranges from 2,000–2,600 mm per annum (p.a.). The southern zone of Dartmoor attains somewhat lower altitudes, typically 400–500 m asl, resulting in annual rainfall totals in the range 1,800–2,200 mm. Exmoor rises to 520 m, with an annual average rainfall of just over 2,000 mm. Annual rainfall declines rapidly as altitude declines, with gradients being most marked on the more sheltered, east facing slopes which lie in the rain shadow of these two moorland massifs. In consequence, rainfall falls to a minimum in the Exeter area, with the lowest 30 year annual mean of 760 mm occurring just east of Exeter. Within a distance of 40 km the annual rainfall falls to around 30% of the total on the highest slopes of Dartmoor. This marked rainfall gradient exerts a strong influence on local soil hydrology and edaphic conditions. The pattern of rainfall varies throughout the year, with the months of May, June and July being consistently the driest, and November, December and January, when the sea is relatively warm and the Atlantic depressions most vigorous, being the wettest (Meteorological Office 2012). For example, the long term monthly average for Plymouth ranges from 45 mm in July to 125 mm in January. Temperature Air temperatures in Devon are primarily controlled by solar radiation, modified by the land reflectivity (albedo), altitude and the influence of the sea. Devon lies at a latitude of 50 –51 ºN, which in combination with the maritime influence yields a long term average annual air temperature ranging from 7–9 ºC on Dartmoor and Exmoor to 11–12 ºC on the coastal fringes of the county (see Figure 2). Due to the northerly latitude, mean air temperatures tend to reach a maximum in the months of July and August, and a minimum in the months of January and February. For example, average air temperature at Teignmouth on the English Channel coast Figure 2. Mean annual average air temperature 1981–2010 (Crown Copyright, reproduced courtesy of Met Office, National Climate Information Centre). 12 A New Flora of Devon The Devon landscape: climate, geology and soils ranges from 6.4 ºC in January to 17.1 ºC in July. Princetown, situated in central Dartmoor at an elevation of 400 m asl is much cooler, with January and July average air temperatures of 3.4 ºC and 14.3 ºC respectively (Meteorological Office 2012). Altitude and exposure also influence the number of frost-free days, another important factor that influences the distribution of more sensitive plants. In general the coastal zones are less frost prone, with the South Devon coast being particularly mild in that respect. On average, Teignmouth experiences 15 air frosts per year, while Princetown experiences 45 (Meteorological Office 2012). The north coast around Barnstaple also experiences earlier springs than other parts of Devon, due to the influence of the Gulf Stream. Local variations in the distribution of frosts will be controlled by aspect and topography, with lower lying, north facing valley bottoms being more vulnerable to damaging frosts. Exposure Devon has coastlines that face south into the English Channel and north into the Bristol Channel and the Western Approaches. The prevailing westerly winds bring not only rain but also high winds and a significant salt burden, particularly during periods of prolonged cyclonic activity. Exposure to these influences can exert a strong influence on edaphic conditions. Marked local variations in microclimate can occur, dependent on aspect and exposure. For example, the cliffs and coastal fringes of the South Hams between Plymouth and Start Point are often exposed to salt-laden desiccating winds. Plant water availability The regional climate exerts a controlling influence on plant water supply. The balance between rainfall input and evapotranspiration, driven by solar radiation and temperature, controls the amount of water available to be stored in soil for plants to transpire. Typically evapotranspiration rates exceed rainfall inputs in lowland Devon during the period April –September, which can lead to periods of water stress on the less water retentive loamy and sandy soils. Examples include the heathlands of East Devon and the Bovey Basin. This complex interplay of climate and soils further complicates the distribution of natural and semi-natural plants. Solid and Superficial Geology The landscape and diverse plant communities of Devon are underlain by a complex pattern of rocks of varying ages and characteristics. The interaction between these geological materials and the climate has produced an equally varied range of soil materials that support the diverse flora of the county. The wide age range of the rocks, and their contrasting resistance to erosion, have dominated the landforms in the county. This section will describe the characteristics and age of key geological materials that outcrop in the county, and indicate their significance in controlling soil and landscape development. By convention, the solid or bedrock geology of an area is described in age sequence with oldest first, followed by successively younger rocks, and then finally the overlying superficial deposits which while more spatially variable, can be particularly significant in explaining contrasts in local soils and habitats. This chapter draws on the geological summary given in Ivimey-Cook (1984), and both have used Durrance and Laming’s (1982) detailed description of the geology of Devon, including a description of geological timescales. For a less detailed but nonetheless informative overview of Devon’s geology the reader is referred to Hesketh (2006). Detailed geological information at a range of scales of can be obtained from the British Geological Survey publications (BGS 2012). Many of the major rock types are found in Devon, in a geological record that extends over 400 million years. While this represents less than one-tenth of the age of the Earth (4,500 my), a considerable diversity of rock types outcrop in the county. The three-fold classification of rocks is based on their genesis or mode of formation; igneous rocks such as granite and basalt are formed from molten liquids; sedimentary rocks such as sandstone and limestone are formed from accumulations of materials at the Earth’s surface and metamorphic rocks such as slate and marble are formed by changes in state of a former rock in the solid phase, due to elevated temperature or pressure, without becoming molten again. Examples of all these rocks are found in Devon. A New Flora of Devon 13 The Devon landscape: climate, geology and soils Figure 3. Simplified surface geology of Devon (redrawn from British Geological Survey data). Both solid and superficial geology are shown, to illustrate the dominant soil parent materials. 14 A New Flora of Devon The Devon landscape: climate, geology and soils Geological materials can be dated by a range of techniques some of which are very sophisticated and rely on precise chemical analysis. Absolute dating is achieved using measurements of radioactive decay rates in selected minerals. Relative dating can be carried out using fossil evidence and relative position in the geological column. This combination of techniques allows geologists to construct a detailed picture of the geological evolution of an area, and can help to explain patterns of soil development and landscape evolution. The main periods represented in Devon’s geological record are the Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary. Key features of these rocks are described here, and presented in Figure 3. Devonian The oldest rocks in the county date from the Devonian period, approximately 350–410 million years ago (mya), when much of central and northern Britain was part of a large continental land mass. A variety of rocks of Devonian age occur in Devon, and were first mapped and placed in stratigraphical context here, such that the county has given its name to this geological period. The oldest rocks in Devon are probably the green schists and mica schists (metamorphic rocks) that outcrop on the South Devon coastline between Start Point and Bolt Tail (Durrance and Laming 1982). The Devonian rocks in North Devon, including Dartmoor, are made up of mudstones, shales, sandstones and occasional limestones, all deposited in shallow water.

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