The Scope of Environmental Physics

I Chapter 1 The Scope of Environmental Physics Physics has always been concerned with understanding the natural environment, and, in its early days was often referred to as “Natural Philoso- phy.” Environmental Physics, as we choose to dene it, is the measurement and analysis of interactions between organisms and their environments. To grow and reproduce successfully, organisms must come to terms with the state of their environment. Some microorganisms can grow at temperatures between 6 and 100 C and, when they are desiccated, can survive down to 272C. Higher forms of life, on the other hand have adapted to a relatively narrow range of environments by evolving sensitive physiological responses to external physical stimuli. When environments change—for example, because of natural variation or because of human activity, organisms may, or may not, have sufciently exible responses to survive. The physical environment of plants and animals has ve main components that determine the survival of the species: (i) The environment is a source of radiant energy that is trapped by the process of photosynthesis in green cells and stored in the form of car- bohydrates, proteins, and fats. These materials are the primary source of metabolic energy for all forms of life on land and in the oceans. (ii) The environment is a source of the water, carbon, nitrogen, other min- erals, and trace elements needed to form the components of living cells. 1 2 Chapter 1 The Scope of Environmental Physics (iii) Factors such as temperature and daylength determine the rates at which plants grow and develop, the demand of animals for food, and the on- set of reproductive cycles in both plants and animals. (iv) The environment provides stimuli, notably in the form of light or grav- ity, which are perceived by plants and animals and provide frames of reference both in time and in space. These stimuli are essential for resetting biological clocks, providing a sense of balance, etc. (v) The environment determines the distribution and viability of pathogens and parasites that attack living organisms, and the susceptibility of organisms to attack. To understand and explore relationships between organisms and their environment, the biologist should be familiar with the main concepts of the environmental sciences. He or she must search for links between physi- ology, biochemistry, and molecular biology on the one hand, and atmospheric science, soil science, and oceanography on the other. One of these links is environmental physics. The presence of an organism modies the environment to which it is exposed, so that the physical stimulus received from the environment is partly determined by the physiological response to the environment. When an organism interacts with its environment, the physical pro- cesses involved are rarely simple and the physiological mechanisms are often imperfectly understood. Fortunately, physicists are trained to use Occam's Razor when they interpret natural phenomena in terms of cause and effect; i.e., they observe the behavior of a system and then seek the simplest way to describe it in terms of governing variables. Boyle's Law and Newton's Laws of Motion are classic examples of this attitude. More complex relations are avoided until the weight of experimental evidence shows they are essential. Many of the equations discussed in this book are approximations to reality that have been found useful to establish and explore ideas. The art of environmental physics lies in choosing robust approximations that maintain the principles of conservation for mass, momentum, and energy. Such approximations are often described as models. These models may be either theoretical or experimental, and both types are found in this book. We have not considered models of plant or animal systems based on com- puter simulations. They can rarely be tested in the sense that physicists use the word because so many variables and assumptions are deployed in their derivation. Consequently, although they can be useful for identifying the sensitivity of systems to environmental variables, they seldom seem The Scope of Environmental Physics 3 to us to contribute to an understanding of the principles of environmental physics. Several volumes would be needed to cover all relevant principles of environmental physics, and the denite article was deliberately omitted from the title of this book because it makes no claim to be comprehen- sive. However, the topics that it covers are central to the subject: the exchange of radiation, heat, mass, and momentum between organisms and their environment. Within these topics, similar analysis can be applied to a number of closely related problems in plant, animal, and human ecol- ogy. The short bibliography at the end of the book should be consulted for more specialized treatments; for example, of subjects such as the physics of water, heat, and solute transfer in soils. The lack of a common language is often a barrier to progress in in- terdisciplinary subjects, and it is not easy for a physicist or atmospheric scientist with no biological training to communicate with a physiologist or ecologist who is fearful of formulae. Throughout the book, therefore, simple electrical analogues are used to describe rates of transfer and exchange between organisms and their environment, and calculus has been kept to a minimum. The concept of resistance (and its reciprocal, con- ductance) has been familiar to plant physiologists for many years, mainly as a way of expressing the physical factors that control rates of transpira- tion and photosynthesis, and animal physiologists have used the term to describe the insulation provided by clothing, coats, or by a layer of air. In micrometeorology, aerodynamic resistances derived from turbulent transfer coefcients can be used to calculate uxes from a knowledge of the appropriate gradients, and resistances that govern the loss of water from vegetation are now incorporated in models of the atmosphere that include the behavior of the earth's surface. Ohm's Law has therefore become an important unifying principle of environmental physics; the basis of a common language for biologists and physicists. The choice of units was dictated by the structure of the Systeme In- ternational, modied by retaining the centimeter. For example, the dimen- sions of leaves are quoted in mm and cm. To adhere strictly to the metre or the millimeter as units of length often needs powers of 10 to avoid super- uous zeros and sometimes gives a false impression of precision. As most measurements in environmental physics have an accuracy between 1 and 10%, they should be quoted to 2 or at most 3 signicant gures, prefer- 1 3 ably in a unit chosen to give quantities between 10 and 10 . The area of 2 3 2 a leaf would therefore be quoted as 23.5 cm rather than 2:35 10 m or 2350 mm2. Conversions from SI to c.g.s. are given in Appendix A.1..

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