Abstract
The key to understanding our current and future environments is a comprehensive understanding of the processes which produced and continue to shape them, many of which are complex in nature, without a single cause, and dynamic over an extended period of time. These therefore pose challenges when it comes to our interpretation of the data available to us, if we do not utilise a combination of information sources and a broad knowledge base.
An interdisciplinary approach is essential in gaining a full appreciation of this complexity. Knowledge of physiological processes is needed to understand how the data obtained from remote sensing relate to the life processes and interactions of the vegetation. Likewise, an appreciation of plant ecophysiology and imagery is required to incorporate the multiple factors in play for comprehensive environmental modelling representing the complexities of environmental systems and to fully exploit new information sources resulting from remote sensing.
Data collection techniques presented in this book focus on the principles and assessment of plant physiological processes, and remote sensing observations (possible over the past approximately 40 years) aiming to detect related spectral or structural change. The subject is therefore on relatively near-term vegetation dynamics and could be considered somewhat marginal to the long-term change and the typical readership of the Holocene. However, it effectively discusses the combination of multiple information resources, at different (relatively recent) temporal and spatial scales, in order to provide a basic contextual knowledge required to understand means of measuring, observing and modelling the dynamic factors affecting our vegetated ecosystems over time.
I therefore read this book in anticipation of a means to provide links between the three disciplines. It falls short of achieving the subject integration throughout the text that I would have liked to have seen. This is due to the ambitious task of teaching the essential background of each discipline within a single book, without assuming substantial prior knowledge. However, these are brought together in the case studies.
As stated in the title and preface, this book aims to provide a synthesis and impart fundamental knowledge in the fields of (1) plant physiology and ecophysiology related to functional behaviour of natural systems; (2) remote sensing analysis of vegetated environments; and finally (3) the modelling of vegetation structure and function. It achieves this through considering each of these in turn within a dedicated section. The fourth section of the book combines this knowledge through the presentation of several case studies, using global examples across a range of applications.
Opening Section 1 of the book, Chapter 1 introduces the broad-scale relationships between the factors determining the Earth’s climate and weather systems, which reflect the distribution of vegetation (effects of latitude, elevation, aspect, etc.), atmospheric and oceanic circulation systems, classification of global biomes and of vegetation into plant functional types to enable modelling using common behaviours and characteristics. Their use within Dynamic Global Vegetation Models is also discussed and demonstrates the necessity of such generalisation to reduce global complexity to a manageable level for incorporation within models. The understanding of these principles is essential for the reader to acknowledge the value of the simplification required of modelling and remote sensing–derived inputs, discussed in the following sections. The chapter successfully describes the commonality between species groups and therefore the relevance of modelling steps. Chapters 2 and 3 provide a thorough and comprehensive summary of the principles of plant physiology ranging from structure, processes of photosynthesis and here also touch upon pigments and their detection using remote sensing. To cover such fundamental knowledge, which itself could warrant a dedicated book, is admirable. The information is presented thoughtfully, though will be a challenge to those entirely new to the subject.
The fundamentals of remote sensing as applied to vegetation analysis are presented in Section 2, Chapters 4–7. Chapter 4 introduces the principles of remote sensing, providing an overview which is expanded upon subsequently. Relevant subjects are presented in Chapters 5 and 6, including the electromagnetic spectrum and its relation to vegetation (a challenge for the non-expert reader); the advantages of spatial, spectral and temporal resolution of observations; and touching on time-series analysis, of great benefit for the monitoring of vegetation dynamics. An introduction is made to satellite orbits and to a range of sensor types, attempting to link these to the assessment of vegetation properties. However, the focus of the section is on optical imagery. The synthesis of other sensor types in Chapter 6, each into a short paragraph, falls short of the depth of information required to understand them, their principles and their potential applications. A reader without prior knowledge would be unable to understand LiDAR, for example, from the brief description given. In the case of airborne sensors, the implication that ‘these normally involve new, experimental sensors which are flown in research and development’ in support of satellite development, calibration and validation may be the case from the perspective of space agencies, but is not strictly true as the dominant purpose when one considers the wealth of commercial airborne data suppliers. In effect, this feels merely a cursory nod in the direction of alternatives to satellite optical imagery, as opposed to a genuine educational commitment to these.
Chapter 7 discusses the application of remote sensing for the estimation of biophysical properties. This chapter focuses not on the estimation of survey data commonly collected in the field (vegetation height, canopy cover, etc.) but rather on the generation of vegetation indices using optical imagery. However, it does a good job of linking these with the physiological principles discussed previously.
To complement this and to provide a complete picture, I would have liked to have seen conveyed an appreciation of the breadth of biophysical parameter estimates possible from other sensors, which are also useful to drive models and to monitor vegetation dynamics. Examples include vegetation height and structure, leaf area, biomass and fractional cover from LiDAR (Los et al., 2012); radar as a volumetric scatterer for vegetation biomass change (Solberg et al., 2014); and the capabilities of long-term satellite observations to detect vegetation trends and forest histories such as harvesting, decline and recovery (Kennedy et al., 2012; 2014), and observing and monitoring photosynthetic light use efficiency (Hall et al., 2012; Hilker et al., 2012).
Chapters 8–13 focus on modelling plant ecophysiology. This subject is introduced in brief in Chapter 8 which outlines the factors influencing light use efficiency and photosynthetic rates, their feedbacks and interactions. The need for upscaling from local leaf to canopy observations and beyond takes us from field or laboratory-based data and experiments to the realm of remote sensing scales. The modelling of carbon and water fluxes at these scales is the subject of Section 3; however, the focus is on the modelling of processes at these scales rather than making explicit where models can be driven by alternative sources derived from the other disciplines.
Chapter 9 presents the modelling of incident solar radiation at the top of the canopy and subsequent modelling of within-canopy radiation distribution, as essential precursors of carbon and water flux modelling presented in the chapters which follow. Leaf and canopy scale photosynthesis modelling is presented in Chapter 10, relating this to responses to incident light previously discussed, both in terms of enhancement and inhibition, and the necessity of simplification of real systems and factors to be considered to enable meaningful canopy level modelling.
The fundamentals of stomatal and canopy conductance, vegetation transpiration and landsurface–atmosphere interactions are summarised in Chapters 11 and 12, and this knowledge is brought together well in Chapter 13. The essential understanding required by the reader is thus logically stepped-through, building on preceding content.
The back cover of the book highlights the diverse and inter-disciplinary case studies, and it is this which is the highlight of the entire volume. It is in Section 4 that the three disciplines are integrated to apply the knowledge gained previously to real-world situations. To present examples across the range of global vegetation ecosystems and issues of global concern is commendable. In Chapter 14, the reader gains an appreciation (and admiration) for the struggle for survival within Boreal forests, against inhospitable freezing conditions and the short growing season, threatening their internal structure and limiting their potential to thrive … and yet they serve us as an abundant store of carbon.
The role of the globally important grassland biome is presented in Chapter 15, focusing on arid and semi-arid environments and factors influencing seasonal and intra-annual variability of carbon fluxes. This is similarly addressed, and expanded upon, for Savannas in Chapter 16, additionally using remote sensing data to illustrate phenology and trends in productivity. Chapter 17 discusses the complex seasonality of productivity of tropical forests (less distinct than temperate systems) using the globally significant Amazon Basin. Fundamental and disputed questions of which factors drive or limit productivity of this vital resource are examined (i.e. light or water limitation), including the challenges and enlightenment provided by remote sensing, modelling and the collection of field data. Although in one subsection the views summarised tend to align with the author’s own, the controversy and ongoing debate remains clearly with the reader.
The distinct issues relating to high elevation and less-studied tropical montane cloud and rainforests are explained in Chapter 18, including their response to the greater stresses associated with their environment using cross-disciplinary data. The significance of groundwater-dependent ecosystems, framed within the pressures posed by anthropogenic actions and the dependency of less developed nations on groundwater resources, is discussed in Chapter 19. Means to identify locations of these important ecosystems and their use of groundwater are presented using both field observations and remote sensing data.
The volume concludes with an examination of forest mortality and the effects of global-change type drought resulting from increased levels of atmospheric CO2. This concluding chapter aptly combines the three disciplines with observations from remote sensing vegetation indices and field sources, and the mechanisms which lead to tree mortality at times of drought.
Some basic inconsistencies are present which distract the reader from what is already a challenging task to absorb such a large volume of new information, for example, the use of both maximum and mean temperatures, and imperial and metric temperature or precipitation units within the same figure, which prevents a direct comparison. In other cases, figures could be more carefully selected (if sourced elsewhere) or preferably adapted, and captions could provide better explanation for the non-expert reader. Nevertheless, the book is written in an accessible yet knowledgeable way, providing good contextual and complementary information. While I would have liked to see a greater effort to integrate the disciplines throughout, I believe that this book will be a valuable resource for students and interested readers wishing to obtain a foundation in each of the disciplines and an appreciation of their linkages.