Physiological ecology of forest production: principles, processes, and models

Physiological ecology of forest production was originally planned as a second edition to the well-known book by the same name, published 25 years ago (Landsberg 1986). The original book has served as an important reference for many graduate students over the years, particularly for those who wanted to increase their quantitative understanding of ecophysiological processes that determine tree growth. This new edition is much more than an update to the classic book, as the authors became ‘painfully aware of how much progress had been made in our field’ since the first edition. The common thread of the book is the 3-PG forest production model, which is explained in detail in the penultimate chapter, and is referred to many times throughout the book. Along the way, many physiological processes are described in detail in a way that is easy to follow, yet thorough. For those aiming to sharpen their quantitative skills, a comprehensive Excel-based toolkit is also made available. The content of the book is divided into 10 chapters, each dealing with a particular aspect of the mechanistic basis of forest production. After a brief introductory chapter, Chapter 2 deals with the nuts and bolts of ‘Weather and energy balance’ and, like the original book, provides a very useful reference to many important physical processes. Many clear figures illustrate the otherwise somewhat technical topics, such as solar radiation, leaf energy balance and turbulent transfer in canopies. Chapter 3, ‘Physiological processes’, deals mostly with leaf-level processes such as photosynthesis, stomatal conductance and respiration. These processes are described in enough detail to assist analysis of leaf gas exchange data. My main criticism to this chapter relates to the coupled photosynthesis–stomatal conductance model that the authors describe. Their solution is not conventional, and is much more complex and cumbersome to use than other implementations (see, e.g., Leuning 1990). The final section in this chapter, ‘Allocation of Biomass’, is somewhat out of place here, and provides only a very brief description of allocation processes, without reference to recent important meta-analyses (e.g., Litton et al. 2007). The topic of Chapter 4, ‘Stand structure and dynamics’, has rarely been summarized, and provides a thorough treatment of processes such as mortality, height–diameter relationships, allometric scaling and leaf area index. As in most other chapters, many equations are presented, and as the authors state in the Preface, the equations ‘are simply an efficient way of describing how the particular processes under discussion work’. I found the equations to be explained remarkably well, including the ever-annoying units. Overall, this will be a very useful chapter for many readers, as it covers material that is not described in detail elsewhere. Chapter 5, ‘The carbon balance of trees and stands’, builds on previous chapters and lays out the foundations for any mechanistic model of forest carbon balance. The first section explains in detail several approaches for modelling radiation interception by forest canopies, and is clearly written, although some graphs may have been useful here to aid the reader in this technical topic. The next section explains how canopy photosynthesis can be modelled, and is basically a review of existing methods with varying complexity. The light-use efficiency model, the backbone of the authors' own 3-PG model, is described along with a summary of typical values for forest stands. I am somewhat surprised that the wealth of progress in understanding canopy light-use efficiency from eddy-flux data is not mentioned at all in this section. In the final section of this chapter, an example of a biomass allocation model (the one implemented in 3-PG) is given. As in Chapter 3, the treatment of biomass allocation is perhaps more brief than expected, given that other processes are described much more thoroughly. The important topic ‘Nutrient dynamics and tree growth’ is discussed in Chapter 6, and provides an excellent summary of nutrient cycling, nutrient uptake and the relationship between tree growth and nutrient availability. The first half of the chapter describes nutrient cycling in a way that is useful for this field, without getting lost in too many details. The focus is on nitrogen, as this is typically the most limiting nutrient, but other macronutrients are not ignored. The authors, perhaps wisely, do not discuss the recent, often heated, debate on nitrogen deposition and its purported control on forest carbon balance (Magnani et al. 2007). The final two sections of the chapter describe models of nutrient dynamics, in particular the well-known CENTURY model, and give a fairly detailed account of nitrogen uptake by plant roots. In Chapter 7, ‘Hydrology and plant water relations’, all the components of forest water balance are described, along with water transport in soils and plants. This brief overview of forest hydrology will be very useful for students. However, in contrast to some other chapters, few quantitative tools are presented. It is also strange that much space is devoted to the detailed process of stem water storage and how this affects models of water transport, but very little to the effects of water stress on stomatal closure and forest production. The latter is an important component of any forest production model, while the former, at sufficiently long time scales, is not. Before the many details of their flagship model, 3-PG, are presented, the authors quickly review other existing models of forest production in Chapter 8, and discuss some general aspects of modelling (such as model testing and sensitivity analysis). Although short, this will be a useful chapter for many readers, as it is difficult to choose a forest production model from the many existing ones in the literature. The selection of models reviewed is of course not complete, but it does form a good starting point for the first-time model user. The penultimate chapter describes the 3-PG model, and reads entirely like a manual for users of the model. Since there is a significant user base, this chapter will satisfy many readers. The 3-PG model is unique in that it sits exactly between the many detailed process-based models of canopy function and the entirely empirical forest growth models. As such, in the section ‘Possible Improvements’, the authors are clear in pointing out that adding mechanistic detail to a model like 3-PG does not necessarily improve its usefulness, or even its accuracy, in predicting forest growth. Overall, the book has summarized a wealth of information in a quantitative way that is still easy to read. The book does have some issues with organization: some topics are described more than once, fragmenting the material somewhat. I also found little justification as to why some processes are described in much more detail than others. Nonetheless, it seems inevitable that Physiological ecology of forest production will be an important textbook for graduate students in this field, and a reference for many researchers.