Mountains and Moorlands. W. Pearsall H.

late-maturing individuals should appear at higher levels. No individuals of this type have been seen, nor has it been possible to find signs of rushes which might have been infected in this manner. It seems to be only possible to explain this apparent absence of the mature moths at higher levels by assuming also a temperature bar to their development such as we have already encountered in the flatworm Planaria alpina.

      There are many further observations that could usefully be made on this matter. It appears that Coleophora is generally confined to lower altitudes on the Eastern Highlands as compared with the Lake District, and, at first, it seemed that lower mean temperatures might explain this. However, I have not seen this moth at all in the Western Highlands or Islands—perhaps because I have generally been too early or too late for the moth and too early for the larval cases. Certainly the creature seems to be much less common in this area of high rainfall, a result that could not be easily explained on the grounds of temperature alone.

      The brief summary of upland climates and the analysis of their possible effects on animals and plants suggests that temperature has much to do with the zonation of plants and animals we observe on ascending a mountain. It controls the distribution of some organisms because they are not able to live in the higher average temperatures of the lowlands. In other cases, it seems that the low montane temperatures so lengthen the life-cycle that it cannot be completed in the short mountain summer. Perhaps more often low temperature retards some part of the developmental cycle, so that we get short-winged insects (see here), or plants unable to produce flowers and fruit. For these reasons, some zonation of organisms is inevitable as altitude rises and temperature falls.

      In practice, the most widespread influence of altitude is the change in the character of the prevailing plant communities, with all that it implies in its effect on animal habitats. Most noticeable is the disappearance of woodlands and trees with their varied faunas and ground floras. As this commonly takes place at about 2,000 ft. and as the restricted montane species appear above that level, we may take it as a convenient altitudinal separation of montane and sub-montane zones.

      Within the limits thus defined by temperature other factors must play their part. Every naturalist knows that shelter from wind is often vitally important, so that here and there among the mountains there are oases in which the frequency of plant and animal life is altogether different from that found on the exposed and wind-swept faces. Within the limits imposed by temperature, humidity also exerts its restrictions, not only by presenting a range of habitats running from pool or rivulet to desiccated rock, but by influencing the character of the soil. It is to the consideration of these soil conditions that we must now turn.

       CHAPTER 4

      SOILS

      THE second important group of factors in upland habitats is the nature of the soil covering—or perhaps more strictly, of the surfaces available for plant growth. Geologically, as we have seen, these surfaces may be classified either as stable or unstable, depending on whether they are still subject to active erosion or not. As habitats for plants there is a more profound difference between these two classes. Most of the unstable surfaces are rocky or are covered by rock fragments in various stages of disintegration, and even their physical properties differ greatly from those of fertile lowland soils. They are, in fact, soils in the making, and it is one characteristic of upland areas that they exhibit in profusion all the varied stages of soil-formation. We see the native rock breaking down under the action of frost and other weathering agents to rock fragments, which become progressively finer as the process is longer continued, ultimately to yield the small mineral particles which form the basal material of most soils. The weathered material may remain in situ, covering the original rock surface, or it may be removed by erosion and redeposited elsewhere by streams and rivers as banks of silt or alluvial plains, by solifluction or rain-wash, or formerly in Britain by widespread glacial movements.

      The raw mineral material is, however, comparatively sterile. It is converted into what we call a soil partly by chemical modifications resulting from the presence of water, often charged with carbon dioxide or humic acid, and partly resulting from the gradual accumulation of organic materials derived from plant remains. This latter material is called humus, and is particularly important because it forms a medium upon which can grow various micro-organisms, mainly bacteria, moulds and protozoa. With the accumulation of humus and the gradual colonisation of the material by these organisms comes a final stage, when it is usual to imagine that the original particles of mineral substance have become covered by a jelly-like mass of colloidal material—in part gelatinised minerals but also including humus—on and in which the population of soil micro-organisms lives.

      It will be evident from this brief summary that upland soils can usefully be considered as belonging to a developmental series. But it is true of any soil that one of its outstanding characteristics is its capacity for change. Soils are inherently dynamic systems even when they are developed in physically stable situations, and to a far greater extent is this true of mountain soils, most of which are of geologically recent origin, even if not physically unstable.

      Five types of environmental factor control the development of a soil mantle. First comes the nature of the rock or other parent material, from which soil is formed by physical and chemical weathering. Climate also exerts a marked effect on the weathering process, affecting both its physical and chemical parts, and, in particular, determining the amount of rain-water percolating through the soil in any season, a process known as leaching, which is responsible for the removal of soluble substances, bases like lime as well as plant nutrients like nitrates. Relief influences the lateral movement of percolating water down a slope, the degree of drainage and the stability, and thus affects the degree of leaching. But none of these effects is instantaneous and so there is a time-factor to be considered. Lastly, there are the obvious biological factors, of which the action of vegetation is most significant. Vegetation derives part of its sustenance from the soil and so incorporates a portion of the soil material which is returned to the soil on the decay of the plant tissues. The fertility of a soil is the result of this cyclic exchange. An efficient type of plant which draws heavily on the soil nutrients keeps them in a form of biological circulation which mitigates the losses due to leaching. Thus there is a natural mechanism for maintaining soil fertility, which, by drawing on the deep layers of the soil, is capable even of increasing the fertility of the surface layers provided the leaching factor is not too intense. Further, in any environment where the climatic factors have remained reasonably stable for a long time, it is possible for the soil-vegetation system to achieve a measure of temporary stability. In upland Britain, however, the soils are generally in dynamic states moving along definite trends of soil development. The trends due to a severe climate are particularly marked, and they operate during the different stages of soil development in the following manner.

      SKELETAL AND IMMATURE SOILS

      The initial stages of soil development in which rock fragments predominate are what we can only call skeletal soils. They are found principally where the surface is unstable or where further development is retarded by hard rocks or low temperatures. Of these factors, the low temperatures have also distinct qualitative effects, because while they greatly retard chemical modifications, the associated physical disintegrations caused by frost and solifluction are especially vigorous. There may thus be much physical commination of rock fragments with little chemical change. Thus the soils, even if finely divided, are immature in the developmental sense because of the deficiencies in their chemical and biological equipment. Soils of this general type occur on mountain-tops, where they are found under the mountain-top detritus except, perhaps, where it is especially coarse and deep. Generally, however, the detritus seems to be a superficial layer of stones extruded from below during the frost-caused or solifluction movements of the materials. Beneath the stones there is commonly a sandy loam, generally brown in colour and little leached. When vegetation