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This chapter describes work on a stochastic soil moisture model that has been used to investigate the relationship between the hydrologic and vegetation dynamics ecohydrology in water-controlled ecosystems. Such systems are complex evolving structures whose characteristics and dynamic properties depend on many links between climate, soil, and vegetation. After a discussion of the soil water balance and a brief account of rainfall modeling, infiltration, and runoff, evapotranspiration and drainage are sketched. The probabilistic modeling of the soil moisture process and of long-term water balance are discussed, followed by minimalist modeling of soil moisture dynamics. The chapter concludes with a brief account of plant water stress, with an application to the Kalahari precipitation gradient.

This chapter identifies the ecological factors that influence the availability of water to California grasslands, including abiotic and biotic influences, and describes the extent to which soil water availability varies temporally, spatially, and as the species traits of the vegetation community change. The influence of climatic conditions, including precipitation, on the productivity and species composition of grasslands are also discussed. Finally, the chapter examines how the shift in community composition due to invasion of non-native species into inland grasslands has influenced soil moisture dynamics in California.

The upper few centimeters of the soil are extremely important because they are the interface between soil science and land-atmosphere research and are also the region of the greatest amount of organic material and biological activity (Wei, 1995). Passive microwave remote sensing can provide a measurement of the surface soil moisture for a range of cover conditions within reasonable error bounds (Jackson and Schmugge, 1989). Since spatially distributed and multitemporal observations of surface soil moisture are rare, the use of these data in hydrology and other disciplines has not been fully explored or developed. The ability to observe soil moisture frequently over large regions could significantly improve our ability to predict runoff and to partition incoming radiant energy into latent and sensible heat fluxes at a variety of scales up to those used in global circulation models. Temporal observation of surface soil moisture may also provide the information needed to determine key soil parameters, such as saturated conductivity (Ahuja et al., 1993). These sensors provide a spatially integrated measurement that may aid in understanding the upscaling of essential soil parameters from point observations. Some specific issues in soil hydrology that could be addressed with remotely sensed observations as described above include (Wei, 1995): (1) criteria for soil mapping based on spatial and temporal variance structures of state variables, (2) identifying scales of observation, (3) determining soil physical properties within profiles based on surface observations, (4) quantifying correlation lengths of soil moisture in time and space relative to precipitation and evaporation, (5) examining the covariance structure between soil water properties and those associated with water and heat fluxes at the land-atmosphere boundary at various scales, and (6) determining if vertical and horizontal fluxes of energy and matter below the surface can be ascertained from surface soil moisture distributions. In this chapter, the basis of microwave remote sensing of soil moisture will be presented along with the advantages and disadvantages of different techniques. Currently available sensor systems will be described.

The micro-environmental conditions of different soil habitats are influenced by prevailing vegetation, aspect, soil texture, soil colour and other variables that influence the incoming and outgoing solar energy. The variability is especially pronounced in sand dunes because of shifting substrate, burial by sand, bare areas among plants, porous nature of sand and little or no organic matter, especially during the early stages of dune development. Even within a dune system there is disparity in radiative heating of different habitats that is manifested as variation in micro-environmental factors such as relative humidity, temperature, light, moisture content and wind turbulence. The major factor affecting these changes is the establishment of vegetation that stabilizes the surface, adds humus, develops shade, aids in the development of soil structure and reduces the severity of drought on the soil surface. The system changes from an open desert-like sandy substrate on the beach to a mature, well-developed soil system with luxuriant plant communities. The principal topics discussed in this chapter include accounts of micro-environmental factors of coastal sand dunes that influence the growth and reproduction of colonizing species. The water content of the substratum in sandy soils is one of the most important limiting factors in plant growth. Sandy soils have high porosity and after a rain most of the water is drained away from the habitat because of the large interstitial spaces between soil particles and the low capacity of sand to retain water. Evaporation in open dune systems also removes substantial quantities of water. Lichter (1998) showed that evaporation was greater on non-forested dune ridges than on forested areas and the rate of soil drying was influenced by soil depth and dune location. After 3 days of a heavy rainfall there was a drastic decrease in the percentage of moisture (67–80%) at 0–5 cm levels in open habitats compared to only 30–36% in the forested dune ridges. The same measurements at 10–15 cm depths showed much lower reduction in the percentage of moisture. In the swale (slack) even though the evaporative demand was the same, there was actually an increase in moisture because of seepage from the dune ridges.

The alpine, while not extensive in global area, has several advantages for trace gas research, particularly the spatial landscape heterogeneity in soil types and plant communities. This variation can be viewed as a “natural experiment,” allowing field measurements under extremes of moisture and temperature. While the atmospheric carbon dioxide (CO2) record at Niwot Ridge extends back to 1968 (chapter 3), and NOAA has done extensive measurements on atmospheric chemistry at the subalpine climate station (e.g., Conway et al. 1994), work on tundra soil-atmosphere interactions were not initiated until recently. In 1992, studies were begun on Niwot Ridge to gain a comprehensive understanding of trace gas fluxes from alpine soils. Our sampling regime was designed to capture the spatial and temporal patterns of trace gas fluxes in the alpine. In addition, we coupled our studies of trace gas fluxes with ongoing studies of nitrogen cycling on Niwot Ridge (Fisk and Schmidt 1995,1996; Fisk et al. 1998; chapter 12). Methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) were studied because of their role in global environmental change and because they could be easily monitored at our remote sites. On a per-molecule basis, CH4 and N2O are much more potent as greenhouse gases than CO2 is (Lashof and Ahuja 1990; Rodhe 1990). In addition, N2O plays a role in ozone depletion in the stratosphere. The global CH4 and N2O budgets are still poorly understood and the relative importance of soils in these budgets is even less clear. For example, estimates of the global soil sink for CH4 range from 9.0 to 55.9 Tg per year (Dörr et al. 1993). This range is large compared with the approximately 30 Tg of excess CH4 that is accumulating in the atmosphere every year. To better assess the role of soil in trace gas budgets, our work focused on investigating landscape patterns of gas fluxes (CH4, N2O, and CO2) and environmental controls on these fluxes.

Since evaporation represents some 60% of precipitation over land surfaces, it is crucial for hydrologic purposes to know with some degree of certainty the magnitude of the water vapor flux into the atmosphere. Actual evaporation (E) from drying land surfaces is often formulated, in hydrology, as a fraction of some measure of potential evaporation (Ep), which can be written as a bulk transfer relationship: . . . Ep =CE up(qs* -q) (10.1) . . . where CE is the bulk mass transfer coefficient for water vapor, u is the mean wind speed at reference height z above the surface, r is the density of the air, q is the mean specific humidity at z, and q*s is the saturation specific humidity at the temperature of the surface (Ts) (Brutsaert, 1982, 1986).

This chapter discusses gas exchange and other environmental responses of cacti. It focuses on net CO₂ uptake and examines the influence of three key environmental factors—temperature, soil moisture, and solar irradiation absorbed by photosynthetic pigments, i.e., the photosynthetic photon flux (PPF)—on CO₂ uptake by Opuntia ficus-indica. The response of net CO₂ uptake by Opuntia ficus-indica to these three variables is important for predicting its productivity under any environmental condition and serves as a model for assessing the net CO₂ uptake, and hence the potential biomass productivity, of other cacti.

Freshwater ecosystems are naturally dynamic. The source of water, discharge, turnover, and residence times all affect which organisms can live in different freshwater habitats and are key determinants of freshwater ecosystem structure and function. Human-induced changes to the volume and timing of both surface and ground water flows are a leading driver of global declines in freshwater biodiversity and are likely to be exacerbated by climate change. The conservation of many wetlands around the world, including in some cases the preservation of unique flora and fauna, is now entirely dependent on continued human intervention and water management. Such management can only be successful if based on sound understanding of water budgets and hydrological processes informed by accurate hydrological monitoring. This chapter provides a brief introduction to hydrological monitoring—what needs to be measured and how—for freshwater ecology and conservation.

This chapter discusses the ecology, diversity, and climbing ability of rattan. It begins with an overview of the biodiversity of tropical forests as well as rattan genera, number of species, and their distribution. It then considers the range of ecological conditions under which rattans grow and the importance of water, especially soil moisture, in the diversity, abundance, growth, and development of rattans. It also examines the relationship between rattans and ants, along with the evolution of rattan's climbing organs. Finally, it describes the role of rattan in the formation of canopy gaps and in subsequent plant succession and species composition.

Biodiversity is regarded as a scientific concept, a measurable entity, as well as a social–political construct (Gaston 1996, Wilson 1993). The aim of this volume is to develop the scientific basis for biodiversity studies, and for the integration of the concept into management practice. We emphasize biodiversity as a powerful, integrative concept—one that requires careful articulation and further conceptualization before application. Diversity is a concept that refers to the range of variation or differences among a set of entities; biological diversity then refers to variety within the living world. An example of biological diversity is “species diversity,” which is commonly used to describe the number, variety, and variability of the assemblage of living organisms in a defined area or space. However, biodiversity as a concept has evolved. Current definitions expand the biological diversity concept to emphasize the multiple dimensions and ecological realms in which biodiversity can be observed. These definitions stress that biodiversity encompasses at least four kinds of diversities: genetic diversity, species or taxonomic diversity, ecosystem diversity, and landscape diversity (McAllister 1991; Solbrig 1993, Stuart and Adams 1991; Groombridge 1992; Heywood 1994, Wilson 1993). Two main problems emerge as a consequence of the broad scope that the biodiversity concept has taken at present. Cast as questions, the problems are: (1) How do we incorporate processes (e.g., foraging, energy and nutrient flows, patch dynamics) into a concept that is based on seemingly static entities (i.e., individual organisms, species, habitat types, patch types)? (2) How do we integrate across ecological subdisciplines (e.g., ecosystem, population, landscape ecology) and across scales that are involved in biodiversity studies? The two problems are not mutually exclusive. Indeed, they are inseparable and complementary. For example, to determine how species diversity and ecosystem processes interact requires incorporation of entities and processes, as well as integration of community and ecosystem ecology. The focus on both entities and processes reflects the long-recognized dichotomy of structure and function in biology and ecology. Clearly, both structure and function must be integrated in order to successfully solve ecological questions. Dealing with biodiversity brings this needed integration into focus.