Joe Landsberg

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.

Estimation of potential forest productivity across the Oregon transect using satellite data and monthly weather records

Detailed physiological and micrometeorological studies have provided new insights that greatly simplify the prediction of gross photosynthesis ( P G ) and the fraction of production that goes into above-ground net primary production (NPPA). These simplifications have been incorporated into a process-based forest growth model called 3-PGS (Physiological Principles Predicting Growth with Satellite Data). Running the model requires only monthly weather data, an estimate of soil texture and rooting depth, quantum efficiency ( f ), and a satellitederived Normalized Difference Vegetation Index (NDVI) correlated with the fraction of visible light intercepted by foliage. The model was originally tested in Australia where seasonal variation in NDVI is extreme. In Oregon, NDVI varies much less seasonally and fully stocked coniferous stands maintain nearly constant canopy greenness throughout the year. We compared 3-PGS estimates of P G and NPPA across a steep environmental gradient in western Oregon where groundbased measurements at six sites were available from previous studies. We first tested the simplification in data acquisition of assigning the same quantum efficiency ( f =0.04 mol C/MJ APAR) and available soil water storage capacity ( θ =226 mm) to all sites. With these two variables fixed, the linear relation between predicted and measured P G was y =1.45 x +2.4 with an r 2 =0.85. When values of θ were adjusted to match seasonal measurements of predawn water potentials more closely, and the quantum efficiency was increased to 0.05 mol C/MJ absorbed photosynthetically active radiation (APAR) on the most productive site, predicted and observed values of P G and NPP A were in near 1:1 agreement with r 2 =0.92. Because maximum greenness (NDVI) reflects the seasonal availability of water, limits on soil water storage capacity can be inferred from calculated water balances derived following the onset of summer drought. The simplifications embedded in the 3-PGS model, along with the need to acquire only one midsummer estimate of maximum greenness, make the approach well suited for assessing the productive capacity of forest lands throughout the Pacific Northwest, USA.

Water relations in tree physiology: where to from here?

We look back over 50 years of research into the water relations of trees, with the objective of assessing the maturity of the topic in terms of the idea of a paradigm, put forward by Kuhn in 1962. Our brief review indicates that the physical processes underlying the calculation of transpiration are well understood and accepted, and knowledge of those processes can be applied if information about the leaf area of trees, and stomatal conductance, is available. Considerable progress has been made in understanding the factors governing stomatal responses to environment, with insights into how the hydraulic conducting system of trees determines the maximum aperture of stomata. Knowledge about the maximum stomatal conductance values likely to be reached by different species, and recognition that stomatal responses to increasing atmospheric vapor pressure deficits are in fact responses to water loss from leaves, provides the basis for linking these responses to information about hydraulic conductance through soil–root–stem–branch systems. Improved understanding in these areas is being incorporated into modern models of stomatal conductance and responses to environmental conditions. There have been significant advances in understanding hydraulic pathways, including cavitation and its implications. A few studies suggest that the major resistances to water flux within trees are not in the stem but in the branches. This insight may have implications for productivity: it may be advantageous to select trees with the genetic propensity to produce short branches in stands with open canopies. Studies on the storage of water in stems have provided improved understanding of fluxes from sapwood at different levels. Water stored in the stems of large trees may provide up to 20–30% daily sap flow, but this water is likely to be replaced by inflows at night. In dry conditions transpiration by large trees may be maintained from stored water for up to a week, but flows from storage may be more important in refilling cavitated xylem elements and hence ensuring that the overall hydraulic conductivity of stems is not reduced. Hydraulic redistribution of water in the soil may make a contribution to facilitating root growth in dry soils and modifying resource availability. We conclude that the field of tree water relations is mature, in the sense that the concepts underlying models describing processes and system responses to change are well-tested and accepted and there are few, if any, serious anomalies emerging. Models are essentially formal statements about the way we think systems work. They are always subject to further testing, refinement and improvements. Gaps in knowledge appear within the framework of accepted concepts and mechanisms research is needed to fill those gaps. The models currently available can be used to scale estimates of transpiration from leaf to landscape levels and predict species responses to drought. The focus in tree water relations has shifted to examine the climatic thresholds at which drought, high temperatures and vapor pressure deficits cause mortality. Tree death may be caused by hydraulic collapse following irreversible cavitation or extremely low water potentials, but recent research indicates that the relative sensitivity of stomatal conductance and whole-plant hydraulic conductance plays a major role in determining plant responses to drought.

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.

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.

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.

Introduction: Looking Back and Into the Future

Our objective in this book is to describe and discuss forests and their significance in our world. Human societies need the products of forests—not just wood and wood products but all the ecological goods and services that forests provide: biodiversity and its essential benefits, carbon sequestration and storage, stable water supplies, land protection, recreation. But relatively few people are aware of these services and benefits, so we hope to contribute to raising awareness of these values and the importance of forests, and to providing the science-based information needed to guide political action and decisions about them. Toward achieving these objectives we consider how forests grow and why different types occur in different parts of the earth; what constrains their growth, why they are important to us and how they should be managed.

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.