Peter Sands

Conversion of canopy intercepted radiation to photosynthate: review of modelling approaches for regional scales

A fundamental component of most models of terrestrial carbon balance is an estimate of plant canopy photosynthetic uptake driven by radiation interception by the canopy. In this article, we review approaches used to model the conversion of radiation into photosynthate. As this process is well understood at the leaf-scale, the modelling problem is essentially one of up-scaling, to canopy, regional or global scale. Our review therefore focuses on issues of scaling, including model identification, parameterisation and validation at large scales. Four different approaches are commonly taken to modelling photosynthate production at large scales: the maximum productivity, resource-use efficiency, big-leaf, and sun-shade models. Models representing each of these approaches are discussed and model predictions compared with estimates of gross primary productivity derived from eddy covariance data measured above a Sitka spruce forest. The sun-shade model was found to perform best at all time scales considered. However, other models had significant advantages including simplicity of implementation and the ability to combine the model with remotely-sensed information on vegetation radiation interception. We conclude that all four approaches can be successfully used to model photosynthetic uptake and that the best approach in a given situation will depend on model objectives and data availability.

A simulation model of seasonal changes in the value of cattle dung as a food resource for an insect

Abstract In the Canberra region the larvae of the Australian bushfly feed in cattle dung, and the quality of this dung is an important determinant of the population dynamics of the insect. A measure of this quality is provided by the mean mass of puparia reared under standard conditions in the laboratory. This paper presents a simulation model by means of which this measure and its seasonal variation can be predicted from standard observations of rainfall, temperature and 09h00-humidity. The model is based on the growth index of Fitzpatrick and Nix, but incorporates a simple model for the phenological changes occurring in the pasture from which the dung derived. The model successfully simulates the masses of pupae reared in dung derived from a number of distinct pastures within 200 km of Canberra throughout the seasons 1970–1971, 1971–1972 and 1972–1973.

Chapter 9 - The 3-PG Process-Based Model

Publisher Summary This chapter presents and discusses in detail the model known as 3-PG. The acronym is an abbreviation for Physiological Processes Predicting Growth. Besides its simple, general structure, a significant factor in the widespread adoption of 3-PG has been that implementations of the model have been made freely available to all who wanted to use it. This chapter provides an overview of why 3-PG has the structure it does, describes that structure and summarizes the various data and species-specific parameters required to run the model. It discusses the assumptions that underlie the sub-models and functional relationships used in it, and it discusses the manner in which species-specific parameter sets can be established. Any model must consist of a set of statements that constitute hypotheses about the way the system being modeled works. Wherever possible these should be in a form that is testable, either by direct measurements designed to test particular sub-models, or indirectly by measurements that evaluate the model as a whole at the level of its outputs. Accordingly, this chapter provides a description of how a species-specific parameter set and 3-PG as a whole can be tested. Applications of 3-PG across a wide range of environments and species are summarized in this chapter. This allows assessment of the extent to which it fulfills the criteria for evaluation, whether it provides a framework within which one can set and evaluate current knowledge and information about tree physiology and the factors that affect and determine stand growth, and whether it is a useful practical tool. Finally, this chapter considers changes that could be made to various parts of the model and assess the implications of these changes in terms of the number and availability of the parameter values that would be required in relation to possible gains in the accuracy and precision of predictions.

Classification scheme and nomenclature for refractive-index distributions

A classification scheme and nomenclature for inhomogeneous refractive-index distributions is proposed which is consistent, unambiguous, and appeals to intuition. The classification is based on (1) the generic shape of the isoindicial surfaces, (2) the detailed dependence of the index on some position coordinate characterizing the isoindicial surfaces, and (3) whether the index distribution is given by N or N2 as some function of position. The scheme is illustrated by distributions which are of technological or physiological interest.

Chapter 8 - Modelling Tree Growth: Concepts and Review

Publisher Summary Models are essentially abstractions that should encapsulate the essential features of the system being modeled. They may serve any of a number of purposes. They may be designed as practical tools with which to simulate the behavior of a system in response to change or stimuli, such that managers or decision-makers can assess the probable consequences of those changes or stimuli. They may be developed primarily as research tools designed to provide a framework, within which current knowledge and information can be set and the relative importance of different parts of the system evaluated. Or models may be formulated as statements, or sets of statements, embodying current knowledge or hypotheses about the way systems work. This chapter starts with a discussion on the concepts and principles of models. There are three main types of forest growth model: empirical, process-based, or mechanistic and hybrid. Here, the characteristics of each are outlined in relation to its purpose, and it reviews a small sample of the models of each type that have been developed in forest ecophysiology in recent years, focusing on those of their properties that are of particular interest in relation to their purpose. This chapter gathers together and discusses some points arising from the overview of the various models presented here, such as: reinventing the wheel; parameterisation and calibration, and the reason to use process-based models. Models are tested in various ways during their construction, and the terms verification, validation, and testing are variously used. This chapter provides some rather cursory remarks to outline the main points that need to be considered in relation to modeling the growth of trees and forests.

Stand Structure and Dynamics

Publisher Summary Stem population dynamics are important to forest managers, to modelers, and to those concerned with carbon sequestration, either in relation to climate change or wood production. This chapter discusses both density-dependent mortality, also known as self-thinning, and density-independent mortality induced by environmental factors. The distribution of biomass between the various components of a tree— that is, foliage, stems, roots, etc., is important as this determines the potential for growth (leaves and fine roots), structural stability (stem and coarse roots) and economic products (mainly the stem). It also provides a summary of the statistical relationships between stem height and diameter, and discusses the mathematical expressions generally used to describe stem size distributions. The biomass of the component parts of trees (indeed, of any plants) tend to bear fixed, or at least stable and predictable relationships to one another. These are called allometric relationships, and are routinely used to estimate biomass partitioning within a tree from simple measures, such as its stem diameter. This chapter discusses the use of allometric relationships in growth models to constrain biomass allocation to the components of trees so that the resulting partitioning better mirrors that observed in real stands. Canopies are formed by the crowns of trees. The architecture of a forest canopy is described by the vertical and horizontal arrangement of foliage through the canopy space. This, and the leaf area in a canopy per unit ground area determine how much photosynthetically active radiation (PAR) is intercepted by the canopy, and hence the photosynthetic production by the canopy. Foliage dynamics: the emergence, growth, death and fall of leaves, determine the temporal dynamics of canopies, and are clearly a major determinant of the state of deciduous canopies, where the whole population of leaves on trees grows and is lost each growing season. Modelling this is difficult, since it must involve stored carbohydrates; another area where ones knowledge is uncertain. However, foliage dynamics are also important in evergreen trees: if some leaves did not fall each year, leaf area would rapidly reach very high values. Leaf loss is generally called litterfall, although litterfall strictly includes twigs and dead branches, therefore has to be accounted for in attempts to describe the growth patterns and carbon production of trees. Litterfall is also a factor in the overall carbon balance of trees, although the amounts of carbohydrate involved are small in relation to the amounts consumed by the growth of other organs. Finally, it discusses coarse and fine roots and their distribution, and outlines the distinction between fine roots as active uptake organs and coarse roots as passive/structural anchors.

Chapter 7 - Hydrology and Plant Water Relations

Publisher Summary Water is a controlling factor in the growth of forests, indeed, forests do not occur in low rainfall regions of the world. The water balance of stands depends on precipitation, interception, run-off, evaporation and drainage with the exception of precipitation all these processes are strongly influenced by tree populations, stand structure, and canopy architecture. The availability of water in the soil at any time, interacting with the evaporative demand of the atmosphere and the hydraulic capacity of the trees, determines canopy conductance and the ability of the trees to absorb CO 2 for photosynthesis. Tree–water relations are an excellent and well-documented example of processes at different levels with different response times. The hydrology of forest ecosystems is important not only because of the interactions between the soil water balance and tree growth but also because of the importance of catchments as water supply systems. The first part of this chapter provides an outline of the hydrological balance and its components, including consideration of water in root zones and water movement in soils. Then it considers tree–water relations and concludes with a brief discussion of the consequences of water stress.

Chapter 5 – The Carbon Balance of Trees and Stands

Publisher Summary Radiation interception and biomass production by forest stands are the fundamental plant ecosystem processes. Radiation interception depends on the leaf area index and canopy structure, while canopy photosynthesis depends on the photosynthetic characteristics of the foliage and is modified by stomatal behavior. Moreover, the photosynthetic characteristics of leaves are determined by the nitrogen distribution in the canopy, and there is strong observational and theoretical evidence that this distribution tends to optimize whole-canopy production. This chapter considers how canopy structure, leaf area, and photosynthetic properties can be coupled using commonly used and useful radiation interception models to give estimates of tree and canopy biomass production. Canopy production is a complex process because foliage is distributed throughout a canopy, and the main factors that influence the photosynthetic production–radiation, temperature, vapor pressure deficit, leaf nutrient status–vary temporally and spatially. Predicting canopy production thus requires integration of the equations describing photosynthesis over time and space, and the determination of how these environmental factors vary within the canopy in response to transpiration. Complete energy balance and photosynthesis modeling has been accomplished, and is outlined later. However, more simplified models suffice for most practical purposes, and especially when production is required for a stand over an extended period of time. These are the main subject of this chapter. Various factors impose constraints on canopy productivity, operating through both light interception and photosynthesis. This chapter considers how these processes can be coupled through commonly used and useful radiation interception models to give estimates of tree and canopy biomass production. In particular, the extent and distribution of foliage in a canopy clearly affect the amount of light intercepted by the canopy, with shading by the upper canopy and by neighboring trees playing an important role. But despite shading effects, some leaves deep in the canopy experience full sunlight some of the time, and the difference between the sun and shade leaves must be taken into account.

Chapter 2 - Weather and Energy Balance

Publisher Summary This book is concerned with analyzing the growth of forest trees in terms of the physiological processes that underlie and determine growth. The rates of these processes depend on external conditions and interactions with other processes going on in the plants. If the rates could be integrated and the interactions between them quantified, the state of the plant could be determined at any time in terms of its current state as described by its mass, the distribution of that mass among the component parts of the plant, and the condition of the plant. Therefore, it is needed to be able to analyze plant growth in terms of the rates, at which physiological processes operate. The condition of the plant may be specified in terms of the water or nutrient status of the plant. The external conditions that affect the rates of physiological processes include the weather variables: radiant energy, air temperature, humidity, and wind. To provide the background necessary for analyzing plant growth in relation to environmental variables, or for the analysis or prediction of observed responses at a particular level, this chapter presents the information about these weather variables. Also discussed here are the methods of estimating their values where measurements are not available.

Chapter 6 - Nutrient Dynamics and Tree Growth

Publisher Summary Plants require a number of mineral nutrients for their growth. Inadequate supplies of any nutrient will impose limits on the capacity of trees to utilize efficiently the radiant energy captured by their foliage, or to convert photosynthates into new plant biomass. Nitrogen is ultimately derived from atmospheric nitrogen, but all other nutrients have their origin in the parent minerals forming soil. There are cases where foliar absorption occurs, and under some circumstances this can be a significant contributor to the uptake of a particular nutrient by a plant. This chapter discusses some of the recent work on nutrient uptake and modeling. This chapter provides a general discussion of nutrient cycling. Switzer and Nelson proposed that the circulation of nutrients in forests be defined in terms of three cycles that are discussed in this chapter: (1) geochemical cycles encompass the gains and losses of nutrients to the ecosystem by processes such as weathering and leaching; (2) biogeochemical cycles encompass soil–plant relationships, including nutrient gains to the soil by symbiotic fixation, organic matter decomposition and losses by plant uptake; (3) ‘biochemical’ cycles encompass internal transfer relationships or translocation of nutrients within the vegetation. Nutrient uptake by plants is a part of the biogeochemical cycle. In the early stages of tree growth, most of the nutrients taken up from the soil will be retained in the tree biomass, but as trees grow older the contribution of re-translocation to the nutrients required for new growth increases. This chapter discusses nutrient re-translocation. It also discusses growth in relation to nutrition. The discussion in this chapter shows that the concept of site fertility is complex. It is the outcome of many factors rooted in soil chemistry; it varies both seasonally and throughout the lifetime of a forest stand as a consequence of changing climatic conditions; and it is affected by the withdrawal of nutrients as a result of stand growth, and the subsequent recycling of nutrients following litterfall. Models that could fully account for these various factors would be extremely complex. However, attempts have been made to avoid these complexities by devising pragmatic site fertility indices, and two of these are outlined in this chapter.