Introduction

Spatiotemporal variation in environmental conditions is ubiquitous and can help or hinder species coexistence. It is not always clear, however, when we should expect environmental variation to promote coexistence nor what forms of variation can do so most effectively. This chapter seeks to answer two questions. First, when does spatiotemporal variation most promote coexistence and why? It turns out that spatiotemporal variation promotes coexistence most effectively via spatial mechanisms and this leads us to the second question: is spatiotemporal variation more or less able to promote coexistence than purely temporal variation and why? For those who are interested, this chapter also gathers together mathematical calculations which have been scattered throughout the appendices of several papers (Snyder 2006, 2007, 2008). Here I state in one place how to get from environmental autocorrelation functions to spatiotemporal population distributions to a calculation of how much variation increases or decreases an invader’s long-run growth rate.

I explore these issues using a model of competing annual plants that is presented in the next section. The model discussion is followed by an overview of the mathematical methods, which should be accessible to everyone, and a more detailed presentation of the mathematics, for those who may wish to use these methods themselves. Next I present the results of a numerical experiment in which I systematically vary each species’ dormancy, seed dispersal distance and competition distances, and determine how much environmental variation promotes coexistence for each combination of traits. I find that regardless of the scale of spatial or temporal autocorrelation in environmental conditions, spatiotemporal variation most promotes coexistence when species have traits that encourage population aggregation and species segregation. While low dormancy and short-range between-species competition contribute to these tendencies, short-range seed dispersal is essential. I also find that spatiotemporal variation has a much higher capacity to promote coexistence than temporal variation does, and I discuss reasons for this. The chapter ends with a discussion section.

Introduction

Crops can be made resistant to pestiferous insects by genetic modification. The possibility that the modified genetic material may escape into a population of wild relatives is a threat looming over all such endeavours. Since insect infestations fluctuate from year to year, or over longer periods (locusts are an extreme example), the fitness of transgenes escaped from agriculture will be affected by this temporal dynamic and that is the subject of this chapter.

There are two ways in which genetically modified crops may pose a threat to the environment. The first is by spreading beyond arable land and simply outcompeting wild relatives by virtue of the insect resistance modification. The second is by hybridising with wild relatives so that the insect resistance transgene invades and transforms the wild population. The first is unlikely; crops are selected for yield and require tender loving care in order to thrive. The second threat is more insidious and has usually been addressed through population genetics. The role of ecological interactions has been comparatively neglected and the effect on relative fitness of temporal fluctuations in various factors, such as herbivore densities, falls into this category.

Seedlings and the temporal niche

The facilitation of plant coexistence via temporal variation in plant recruitment is increasingly studied (see this volume plus Pake and Venable 1996, Chesson and Huntly 1997, Kelly and Bowler 2002, Verhulst et al. 2008). For the most part however, corroborating studies have examined fluctuations in abiotic factors and the role of biotic agents has been largely overlooked. This omission is symptomatic of the plant coexistence literature in general; the role of predators, herbivores, pathogens and parasites in maintaining species coexistence is more often assumed than demonstrated (but see Kelly and Bowler 2009a). Nonetheless, while a number of agents, biotic and abiotic, result in the death of entire seedling cohorts, foremost among the factors limiting seedling recruitment is herbivory (Moles and Westoby 2004, Fenner and Thompson 2005). Herbivore attack has obvious effects on seedling demography (Lindquist and Carroll 2004, Maron and Crone 2006, Maron and Kauffman 2006), but even beyond population-level considerations, selective seedling removal also exerts long-lasting effects on plant community composition. We propose that temporal fluctuation in herbivore populations, and consequently variation in the intensity of herbivory experienced by plants during their regeneration phase, exerts a powerful influence over plant species contribution to the established community.

There are four necessary conditions of any temporal dynamic involving herbivory. First, seedling herbivores must be capable of moderating plant community composition in established vegetation. Second, herbivores should select preferred seedlings on the basis of readily apparent ecophysiological characteristics. Third, and related to the previous assumption, any variation in seedling susceptibility to herbivore attack (i.e. defensive traits) will most probably correlate with competitive ability. Finally, herbivore populations must show fluctuations in numbers and therefore variation in their influence on regenerating plants. Consequently, before it is possible to develop any conceptual framework to explain how temporal variation in seedling herbivory influences species coexistence, we must first evaluate the evidence for these conditions.

Physical and chemical properties of water

Water is an essential plant component, being a major constituent of plant cells, and ranges from about 10% of fresh weight in many dried seeds to more than 95% in some fruits and young leaves. Many of the morphological and physiological characteristics of land plants discussed in this book are adaptations permitting life on land by maintaining an adequate internal water status in spite of the typically rather dry aerial environment.

The unique properties of water (see Franks, 1972; Kramer & Boyer, 1995; Nobel, 2009; Slatyer, 1967 for details) form the basis of much environmental physiology. For example water is a liquid at normal temperatures and is a strong solvent, thus providing a good medium for biochemical reactions and for transport (both short-distance diffusion and long-distance movement in the xylem and phloem). Water is also involved as a reactant in processes such as photosynthesis and hydrolysis, while its thermal properties are important in temperature regulation and its incompressibility is important in support and growth.

The properties of water derive from its structure (Figure 4.1), and from the fact that it dissociates into hydrogen and hydroxyl ions that are always present in solution. The angle between the two covalent O–H bonds and the asymmetry of the charge distribution along the bonds gives rise to a marked polarity of charge so that water is a dipole.

I have been delighted, and somewhat surprised, at the continued widespread use of this text, in spite of the fact that much has changed in associated fields since the previous edition was published around 20 years ago. Perhaps the major change in plant biology over this period has been the explosion of research on the molecular and genetic basis of plant responses to the environment, though there have been important advances in other relevant fields such as in remote sensing. Although I have not attempted to cover molecular aspects in any detail, as there are many suitable alternative texts, I have tried to relate recent advances in molecular sciences to our understanding of whole-plant responses to the environment. In this context I have aimed especially to indicate the ways in which the powerful new molecular tools and other ‘omics’ technologies can contribute to advancing our understanding of the biophysical interactions between plants and the atmosphere. As in the previous editions, however, I have continued the approach of describing only briefly the biochemical and molecular mechanisms involved in plant responses to the environment, so interested readers are referred to specialist reviews and books mentioned at appropriate places in the text.

For this third edition I have chosen largely to retain the general structure and aims of the successful previous editions.

Plants can survive the whole range of atmospheric temperatures from -89°C (recorded at Vostok in Antarctica www.ncdc.noaa.gov/oa/climate/globalextremes.html#sites) to 56.7°C (recorded in Death Valley, California; El Fadli et al., 2012) that occur on the surface of the Earth, as well as the associated higher temperatures (up to about 70°C) that occur in the surface of desert soils and in the surface tissues of slowly transpiring massive desert plants such as cacti (Nobel, 1988). The even higher surface temperatures of up to 300° C that occur in bushfires can be survived by fire-tolerant plants. Seeds are particularly hardy, though other tissues of some species can also survive an extremely wide temperature range. Most plants can only grow, however, over a much more limited range of temperatures from somewhat above freezing to around 40° C, while growth approaches the maximum over an even more restricted temperature range that depends on species, growth stage and previous environment. Useful information on plants and temperature may be found in Larcher (1995) and Long and Woodward (1988).

In this chapter the physical principles underlying the control of plant temperatures are described and the physiological effects of high and low temperatures outlined. The final section considers the more ecological aspects of plant adaptation and acclimation to the thermal environment.

Physical basis of the control of tissue temperature

As outlined in Chapter 5, the temperature of plant tissue at any instant is determined by its energy balance.

The evolution of the stomatal apparatus was one of the most important steps in the early colonisation of the terrestrial environment. Even though the stomatal pores when fully open occupy between about 0.5 and 5% of the leaf surface, almost all the water transpired by plants, as well as the CO2 absorbed in photosynthesis, passes through these pores. It is only in rare cases, such as in the fern ally Stylites from the Peruvian Andes, that significant CO2 may be absorbed through the roots (Keeley et al., 1984). The central role of the stomata in regulating water vapour and CO2 exchange by plant leaves is illustrated in Figure 6.1. This figure also shows some of the complex feedback and feedforward control loops that are involved in the control of stomatal apertures and hence of diffusive conductance; these are discussed in Section 6.6.1. It is the extreme sensitivity of the stomata to both environmental and internal physiological factors that enables them to operate in a manner that optimises the balance between water loss and CO2 uptake.

This chapter outlines the fundamental aspects of stomatal physiology, their occurrence in plants, their morphology, their response to environmental factors and mechanics of operation, including a description of the various control loops illustrated in Figure 6.1. The role of the stomata in the control of photosynthesis and of water loss is discussed in more detail in Chapters 7 and 10.

This chapter considers a number of related aspects of the aerial environment that have not been adequately treated elsewhere in this book – these include wind and the effects of altitude, climate change and the ‘greenhouse effect’ and their implications for plant growth, and the effects of atmospheric pollutants. All these areas bring together principles that have been introduced earlier. Further details of these and other features of the microenvironment of plants are discussed by Geiger (1965), Grace et al. (1981), Campbell and Norman (1998), Garratt (1992), and Monteith and Unsworth (2008) while information on the scientific consensus on aspects of climate change may be found in the fourth consensus report of the Intergovernmental Panel on Climate Change (Core-Writing-Team et al., 2008; Solomon et al., 2007).

Wind

Not only is wind directly involved in heat and mass transfer by forced convection (see Chapter 3), but it is important to plants in many other ways including the dispersal of pollen and seeds and other propagules, and in shaping vegetation, either directly or, particularly at coastal sites, by means of transported sand or salt (Grace, 1977).

Measurement and variability

Wind is very variable both in direction and velocity. In general wind speeds tend to be greater during the day than at night (Figure 11.1), largely as a result of the convection processes set up by solar heating of the Earth's surface during the day.