INTRODUCTION

So far we have dealt with how plant systems work to promote, sustain, and preserve life. But what do we know about the processes leading to decline and death in plants?

We tend to imagine death as a process that begins at birth and progresses to an end, sometime. “Lifespan” is the maximum length of time an organism could live if all the conditions of life were at their most favorable; the human lifespan, for example, is about 120 years, but most of us do not expect to be around that long. “Life expectancy” more closely describes the reality. The question is not “How long could I live?” but rather, “How long can I expect to live?” which is dependent on prevailing environmental, social, and cultural conditions. In some parts of the world, human life expectancy may be only 30 or 40 years, about the same as it was some 2500 years ago at the height of ancient Greek culture, whereas in others we know it to be over 80 years. Disease, starvation, predation, accident, and polluted environments are just some of the hazards faced by all living things which affect how long they survive.

LIFE HISTORY STRATEGIES

All species share one basic aim in life: the survival of at least some individuals to reproductive age is crucial to the passing on of genetic traits to descendants. What is important is the different strategies living things have evolved to achieve this fundamental aim.

Introduction

Increases in atmospheric greenhouse gases that produce positive radiative forcings are having large-scale impacts on the global climate system, such as increases in temperatures, changes in precipitation patterns, and changes in the frequency of severe weather events. The greatest impact is occurring at high latitudes in the northern hemisphere, at high altitudes throughout the world, and in Antarctica and the sub-Antarctic Islands. Because of their efficient long-distance dispersal mechanisms and their high fidelity to climatically sensitive habitats, bryophytes should react quickly to changes to their environment and thus have the potential of being effective predictors of current climate change.

Since the beginning of the industrial era approximately 200 years ago, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations have been increasing exponentially in the atmosphere (IPCC 2001). Concentrations of other greenhouse gases such as halocarbons and perfluorocarbons that are produced by anthropogenic activities are also increasing in the atmosphere. As a result of greenhouse gas enrichment, global mean surface air temperatures have risen by about 0.3–0.6 °C since the nineteenth century and by 0.2–0.3 °C over the past 40 years (IPCC 2001). The largest temperature increases have occurred over the northern hemisphere land mass north of 40° latitude (Serreze et al. 2000), on widespread high-altitude mountains throughout the world (Böhm et al. 2001), and in the Antarctic (Nyakatya & McGeoch 2008). Temperature increases have also been observed at low latitudes, although to a lesser extent than at high latitudes (Dash et al. 2007; Malhi et al. 2008).

Introduction

Global climate change in the Mojave Desert will likely result in a greater intensity of summer (monsoon) rain events and greater N deposition. The nitrogen cycle has already been significantly altered by human activities to the extent that anthropogenically released N now equals natural terrestrial biological fixation (Vitousek et al. 1997; Galloway 1998). Because most bryophytes receive the bulk of their nutrients from direct atmospheric deposition (Bates 2000), this influx of N can affect the productivity of individual species and thus may alter bryophyte community structure and function. In addition to N deposition, global change models for the southwestern USA predict significant increases in summer precipitation in the northern Mojave Desert (Taylor & Penner 1994; Higgins & Shi 2001). The interaction between increased N deposition and an increased monsoon effect on bryophytes in the arid southwestern USA is largely unknown. Although growth rates of desert bryophytes are relatively low compared with bryophytes in more mesic ecosystems, the contribution of biological soil crusts (a community of cyanobacteria, mosses, lichens, algae, and fungi) to the global cycling of trace gases can be significant in regard to global budgets (Zaady et al. 2000).

Most field studies have found a rapid negative effect of N fertilization on the growth and productivity of mosses, with nutrient uptake a function of desiccation regime, temperature, and light. For several bryophyte species, high experimental N deposition rates decreased biomass production except in a widely tolerant species of Sphagnum (Jauhiainen et al. 1998).

Green plants dominate our planet yet are often taken for granted. For many people, they are merely the passive aspect of a beautiful landscape, the “backdrop” against which animals exist. For others, they are essential to their lives but are there simply to be exploited for food, fodder, fuel, furniture, clothing, transport, recreation, health purposes, and protection without thought being given to their unique qualities as living things in their own right.

In the first four Parts of this edition, I have attempted to provide some insights into the very different world of green plants. Their lives are lived at a different pace from ours, which may be one reason why we so often forget that they are living organisms capable of doing so many of the things we also do. Like us, but in their own way, they can see, they can count, they can communicate with one another, they can be sensitive to the slightest touch and they can tell time with considerable precision. But they accomplish all of these things on a different timescale from most animals. Their very slowness deceives us into believing that they do not do much at all.

We should not forget, however, that green plants are unique among all organisms on Earth in that they alone have the means to use light as a source of energy. The very substance which renders them green, the pigment chlorophyll, puts them in the position of being the very foundation of our biosphere.

‘From dust you came, to dust you shall return,’ is one sober, biblical reminder that complex organisms are built from simple chemical elements to which they will revert. From the beginning to the end of their lives, living things wage a battle against natural forces which break down their highly organized structure:

• At the cell level, complex molecules such as proteins and nucleic acids, to name but two of many hundreds, are continually destroyed by hydrolysis

• Valuable molecules are lost to the environment and have to be replaced because cell membranes are leaky

• Our atmosphere is dominated by the highly reactive molecule, oxygen, as a result of which everything on Earth, organic and inorganic, is subject to corrosive oxidation.

Yet, on all sides, we observe organisms using simple materials from their surroundings to maintain, renew, and build complex structures, to achieve which they need a constant supply of energy.

Organisms have evolved two ways of satisfying their absolute need for energy. The most crucial, photosynthesis, traps light energy from an outside source, the Sun, to fuel the building of complex organic structures from simple inorganic materials. The other, respiration, requires that there be a constant source from which chemical energy can be extracted and used for maintenance and construction.

Photosynthesis and respiration together comprise bioenergetics, how living organisms gain the supply of energy they need, which is the subject of Part I.

INTRODUCTION

Plants are able to shed surplus heatloads in a number of ways, two of which are especially important but only one of which leads to the coolness we associate with places where plants are abundant, such as forests and meadows.

CONVECTIVE VERSUS EVAPORATIVE LOSS OF HEAT

If the temperature of a leaf is higher than its surroundings, air circulation will remove heat from its surface mainly by convection. As this warm air rises, it cools, becomes more dense, and sinks, creating a convection current which removes heat from plant surfaces.

Evaporation of water from leaf surfaces withdraws heat from a plant because energy is absorbed by water as it changes from liquid to vapor (the latent heat of vaporization of water is 44 kJ mol−1). Evaporative cooling can occur even if the temperature of the leaf is below that of the surrounding air.

Which of these two ways of heat loss is the most important to a plant depends on its environment. If there is an ample supply of water then loss of it from leaves can be high without causing damage to the plant; evaporation can be a major means of cooling. Plants adapted to growing in hot, dry conditions, on the other hand, have evolved ways to conserve rather than shed water; convective air currents become the main route for shedding heat.

A typical leaf at moderate temperature dissipates about half its heatload by evaporation of water, half by convection.

INTRODUCTION

Plants not only use day to night transition to time repeated functions from day to day (see Chapter 8) but also to measure the seasons of the year. Down the centuries humans have used leaf, flower, and fruit production by local plants as signals for seasonal activities such as when to begin planting crops or when to harvest. The agriculturalists Garner and Allard (more about whom shortly) expressed it this way:

PATTERNS OF GROWTH

Away from the tropics

In temperate climates there is a pattern of plant growth arranged around a yearly period of relative inactivity, winter. The pattern is seen most clearly in annuals, which grow and reproduce during favorable weather and spend winter as seeds.

Other kinds of plants also have distinct patterns of growth. Herbaceous perennials produce annual stem growth which ceases well before winter; deciduous shrubs and trees lose their leaves in the fall but begin preparing for that event well in advance of winter. In all cases of periodic growth, some signal causes the plant to go from rapid growth to near complete shutdown in days or weeks.

Introduction

The Quaternary covers the past 2.5 million years of Earth history. This unique period is well known for a record of oscillating climatic parameters. If one wishes to understand the trajectory of future climatic changes triggered by human activities, one should also have a clear picture of the climate of the past. Fluctuating climates are reflected in peat bog profiles. Paleoecological studies using plant macrofossils, like bryophyte remains, have an important role in the reconstruction of past hydrological changes in lakes and peat bogs. Plant macrofossil analysis has been used most frequently in the oceanic regions of Europe, where the moisture gradient is reflected clearly in different Sphagnum taxa. The method of bog surface wetness predictions has not been adapted to date for the characterization of continental peatbogs. Hungary is located along the southern limit of Sphagnum-dominated peat bogs, with peat bogs restricted to the moister regions of the country. Holocene climatic events, such as severe droughts, caused significant changes in mire development and as such are traceable in the paleoenvironmental record of these bogs.

Fossil mosses used as proxies for detecting past climatic changes

Detailed paleoecological investigations of fossil mosses enable us to accurately capture the prevailing conditions in some terrestrial ecosystems, mainly those in littoral parts of various catchment basins. There are two major directions for investigation and interpretation: one is restricted to the ecological needs of the individual taxa, whereas the other is based on the ecological requirements of ecological groups and communities in the reconstruction.

Bryophytes are the most successful group of plants other than angiosperms in terms of their numbers of species, geographical distribution on all continents, and their habitat diversification. There are at least 10,000 species of mosses and over 6000 liverworts. All three groups of bryophytes, also including the hornworts, were the earliest green plants to move to the land; each group has had a very long evolutionary history, probably more than 400 million years. All three groups, derived from a green algal ancestor, evolved separately from one another and from vascular plants through this long period. Although the great diversity of tropical bryophytes is often cited, Rydin (2009) pointed out their important contribution to biodiversity in northern ecosystems: 7.5% of the world's bryophyte species are found in Sweden, whereas only 0.8% of vascular plant species are found there.

Bryophytes are unique among land plants in that their dominant stage is the haploid green gametophyte rather than the much shorter-lived diploid sporophyte. They differ from vascular plants in other ways as well, in aspects that make them excellent environmental monitors. They inhabit a very wide range of ecosystems, habitats, and specific microhabitats, including substrates on which vascular plants cannot live. Many species are able to live in nutrient-poor conditions, and are adapted to respond rapidly physiologically to intermittent periods favorable for photosynthesis.

Morphology and physiology

Bryophytes lack the roots, xylem, and phloem of vascular plants. The great majority are ectohydric, that is, without internal conducting tissues.

INTRODUCTION

Apart from water, nitrogen, among all the essential mineral elements, is the key substance limiting where and how well plants grow. The distribution of animals is also linked to nitrogen since animals are dependent on plants for food, directly or indirectly. Why is nitrogen so crucial to the living world?

The machinery used to build, drive, and sustain all living systems is directed from nucleic acid blueprints, the genetic program in DNA and RNA. The machinery itself is made almost exclusively from protein, notably the enzymes which direct the thousands of chemical reactions in living things. Both nucleic acids and proteins contain nitrogen.

The quantity of nitrogen available in usable form is a major determining factor, therefore, in how much nucleic acid and protein organisms can make. Since there is much more protein in an organism than there is nucleic acid, it is the limit to protein production that is the most critical.

SOURCES OF NITROGEN

THE ATMOSPHERE

Nitrogen is enormously abundant. Close to 80% of our atmosphere is made up of nitrogen gas and even that is only about 7% of the total nitrogen on Earth. Nitrogen was given the name “azote,” a word meaning “without life,” by Antoine Lavoisier, to contrast it with the other major gas in the atmosphere, oxygen. Lavoisier found oxygen to be very active, nitrogen gas, inert. Only when combined with other elements, like hydrogen in ammonia or oxygen in nitrites and nitrates, does nitrogen become more reactive.

INTRODUCTION

Those who answer gardening questions from the general public will tell you that surprising numbers of people have a basic misconception about plants. The belief that plants build themselves from the soil is widespread even today, more than 300 years after proof showing this not to be so. Why such a belief still exists is puzzling. Consider the common practice of removing lawn clippings. If grass was simply built from soil, a lawn from which kilograms of clippings were removed during the growing season for the past dozen years would resemble a sunken garden, but it does not. Something additional to soil must go into building a plant.

PHOTOSYNTHESIS: THE KEY

We now understand that plants construct themselves from carbon dioxide (CO2), water, and minerals with the aid of light energy. What plants make by this photosynthesis (putting together by light) is an endless supply of carbohydrates: sugars, starch, and cellulose.

Other green organisms can also photosynthesize

Plants are not the only organisms able to photosynthesize. Our oceans, lakes, and rivers are populated by a wide array of green organisms such as those algae which appear as green scum on ponds and lakes; the larger green, brown, and red algae, the seaweeds, found on or near seashores; and other microscopic organisms, the phytoplankton (certain bacteria, diatoms, dinoflagellates, and the smallest algae), which are especially abundant in our oceans.