Weeds have posed significant problems for civilization since the inception of agriculture. Although rising levels of carbon dioxide and climate disruptions can affect fundamental aspects of global biology, little is known regarding weed science. Weed Biology and Climate Change provides a unique and timely synthesis of research on current and potential biological climate-induced changes with a specific focus on the impacts and management of weedy and invasive plants.

Weed Biology and Climate Change opens with separate overviews of both weed biology and climate change. Subsequent chapters look at the impacts of climatic change on basic and applied aspects of weed biology. Chapters cover key topics ranging from climate-related impacts on weed growth and reproduction to weed management, invasive species, ecosystem functioning, and food security and how these practices will need to adapt in the face of a changing climate.

Written by two experts in the field, Weed Biology and Climate Change is the first book to provide a thorough analysis of climatic change impacts specific to weed science. This book will be an essential resource for weed scientists, global change biologists, ecologists, and crop scientists in the classroom and the field.

Key features:

• Timely synthesis of climate change research and its impacts in regard to basic and applied aspects of weed biology
• Chapters look at such applied issues as weed management, ecosystem maintenance, and food security
• Provides summary of current status and looks at future implications of increased atmospheric carbon dioxide on weedy and invasive plants

“Dr. Ziska is among the world’s leading scientists regarding research on invasive weeds and climate change.”
―Hilda Diaz-Soltero, USDA Senior Invasive Species Coordinator.

Lewis H. Ziska is a Plant Physiologist in the Crop Systems and Global Change Laboratory, United States Department of Agriculture.

Jeffrey S. Dukes is an Associate Professor in the Department of Forestry and Natural Resources and in the Department of Biological Sciences, Purdue University.

Weed Biology and Climate Change

By Lewis H. Ziska Jeffrey Dukes

John Wiley & Sons

Copyright © 2011 Blackwell Publishing Ltd.
All right reserved.
ISBN: 978-0-8138-1417-9

Chapter One

A Brief History of Weeds and Their Impact

“… the sun never sets on the empire of the dandelion”

Alfred W. Crosby, Ecological Imperialism

Weed Classification

Weeds were, and are, the largest single limitation on crop yield. Before the onset of chemical control for weeds, most of the work on the farm from June through August was hoeing; a weed control method still practiced around the world. Without mechanization, the size of a farmer’s holding and yield was determined by how well (and how fast) a family could weed its land. As suggested by Zimdahl (1993), more human labor may be expended on weeding than on any other human enterprise (Figure 1.1).

Weeds affect nearly every aspect of our lives, from the appearance of our lawns to the quality of the food we eat, even the state of our health. Weed science, a subset of botany, is a multidisciplinary science with the goal of a systematic understanding of both weed biology, and more importantly, weed control. Weed scientists represent a broad spectrum of scientific disciplines including plant physiologists, botanists, agronomists, chemists, molecular biologists, biochemists, and ecologists.

But what makes a plant a weed? If we called something a weed in the northeastern United States, would it still be considered a weed in Australia? Are there common characteristics among different weed species? In what habitats are weeds found? How do weeds spread? What makes a weed harmful? How do we measure harm? How do we manage weeds currently? How will we manage them in the future? In this first chapter we will explore these basic concepts of weed biology.

A weed is a weed is a weed? One of the greatest difficulties in introducing the subject of weed biology is a clear understanding of what is meant by “weed.” The term “weed” does not exist in nature; rather, plant species assume the mantle of “weed” when classified as such by human society.

Given the diversity of societies, many weed science books will list a number of cultural and scientifically derived definitions. While varied in scope, these characterizations fall into two broad categories: “an unwanted or undesired plant species” and “early vegetation following a soil disturbance.” The first classification is used by many laypersons as well as agronomists and weed scientists, the latter is a definition widely accepted by ecologists and environmentalists. There is also a third definition—that a weed is a plant whose virtues have yet to be discovered—suggesting that if a use were found for a weed species, it would cease to be a weed. Yet, many of the most pernicious weeds were introduced specifically because of their virtues (e.g., forage crops such as Johnson grass and kudzu, as well as aesthetic species such as purple loosestrife).

Because the characterization of a weed is determined by human activity, the definition of a weed is fluid; it can vary by time and space, and certainly by culture. For example, jimsonweed, in addition to being a common weed of roadsides, is also considered an important medicinal plant for native peoples of the southwestern United States. However, certain plants are universally reviled because it is acknowledged among disparate regions and cultures that those plants can cause widespread economic or environmental damage. One such plant is Canada thistle, considered among the worst weeds in North America (Skinner et al., 2000) for its ability to invade, colonize, and out-compete native and agricultural vegetation. In these cases, the definition of a weed is less in doubt because the impact of these plants is universally recognized.

Ultimately, the definition of “weed” is anthrocentric and therefore flexible; however, “weeds” in many societies share certain biological characteristics.

The following is a synthesis of some of these characteristics, most notably those presented in Zimdahl (1993) and Radosevich et al. (1997). No single weed possesses all of these attributes, but these characteristics encompass biological traits associated with “weediness.”

1. Long-term seed survival in the soil, including resistance to microbial degradation. In addition, weed seeds can germinate over a wide range of environmental conditions. Soil disturbance may be needed for germination.

2. Rapid growth following germination.

3. Mechanical (e.g., spines) and chemical (e.g., poison) barriers to herbivory during vegetative growth.

4. Grow over a wide range of environmental conditions, including extremes of temperature, water, and nutrient availability; high environmental plasticity.

5. Flower early during vegetative growth. Time from flowering to seed production can be rapid (e.g., 2 weeks from flowering to mature seed for Canada thistle).

6. While some weeds are self-compatible, most cross-fertilize between individuals, usually using unspecialized pollen carriers or wind. As a consequence, seeds produced by any one plant can be very genetically diverse.

7. Weeds may produce more than one seed crop per year. Individuals can produce tens of thousands of seed in a given year.

8. Reproduction, particularly for perennial weeds, may include both sexual (floral) reproduction, but also asexual reproduction from roots and/or rhizomes.

9. Weeds may have highly effective seed dispersal mechanisms, from wind (dandelion) to animals (cocklebur).

Life Cycle

Weeds can also be classified on the basis of their life history. For instance, knowing whether a plant is an annual, biennial, or perennial can help weed scientists determine where a weed could become a problem.

An annual is a plant that completes its entire life cycle (from seed germination to seed development) in less than 1 year. Annuals grow quickly, and many are prolific seed producers. Summer annuals germinate in spring, grow in summer, flower, and die with the onset of frost in the fall. Examples of summer annual weeds include velvetleaf, pigweeds, and foxtail grasses. Winter annuals, in contrast, germinate in fall or early winter, flowering and maturing in spring or early summer of the following year. Examples of winter annual weeds include chickweed, shepherdspurse, and cheatgrass.

Biennials, as the name suggests, are plant species that complete their life cycle over a 2-year period. These species typically form a vegetative rosette during the first year of growth, become quiescent over winter, and then bolt during the following spring, forming a tall inflorescence with prodigious seed development. Examples of biennials include curly dock, garlic mustard, bull thistle, and musk thistle.

Among the most damaging of weeds are perennials, those that are long-lived, multi-year plant species. These can be divided into two subgroups: simple and creeping (Zimdahl, 1993). Simple perennials reproduce and spread primarily by seed, whereas creeping perennials can spread by both sexual (i.e., seeds) and asexual (i.e., rhizomes and roots) means. Examples of simple perennials would include common lawn weeds such as dandelion or plantain, or weeds of wet damp locations such as curly dock. Creeping perennials include weeds associated with contact dermatitis such as poison ivy (rhizomes and stems), and common weeds in pastures such as leafy spurge (rhizomes) and Canada thistle (roots) (Figure 1.2).

Classification by Physiology

Differences in photosynthetic pathway provide another means to classify weeds. This type of classification will also be important when we examine ongoing increases in atmospheric carbon dioxide associated with climate change in later chapters.

Initial investigations of how plants acquire carbon led to the discovery that one of the first products of photosynthesis was the production of phosphoglyceric acid, a three-carbon compound (Calvin, 1949). For the vast majority (approximately 95%) of plant species it was discovered that this type of photosynthesis (termed C₃ because the first stable product had three carbons) was the sole means by which carbohydrates were synthesized from light, energy and water. However, in the 1960s, researchers discovered other photosynthetic pathways, most notably the C₄ pathway, where the first products of photosynthesis are four-carbon sugars such as oxaloacetate, malate, and aspartate. Although these C₄ plants, principally tropical grasses, constitute only about 4% of all plant species, a higher percentage of this type of photosynthesis appears to occur among the world’s worst weeds (e.g., Holm et al., 1977). A third type of photosynthetic pathway, crassulean acid metabolism or CAM, occurs in less than 1% of plant species. While succulent species use CAM photosynthesis, major weeds do not.

Classification by Habitat

One of the most well-recognized habitats for weeds is cropland. However, because crops cover a wide range of different environments, there may not be a specific set of characteristics associated with crop weeds per se. Interestingly, the most competitive weeds in a given cropping situation are often simply “imitators” or wild relatives of the crop. They are selected for by cultural practices such as planting date, fertilizer application, selective herbicide usage, etc. which while favoring the crop, also select for those weeds that are closely related physiologically, morphologically, or phenologically. Examples of crops and their weed relatives include potatoes and nightshade (both in the genus Solanum), rice and wild or red rice, sorghum and shattercane (both Sorghum bicolor), oat and wild oat.

Rangelands constitute those land areas characterized by grassland and shrubland used (although not exclusively) for animal grazing. Because of their use, classifications in rangelands are associated with those weeds whose presence is either directly (e.g., poisonous) or indirectly (competes with desired grasses) harmful to large animal (sheep and cattle) grazing. Examples of typical rangeland species include leafy spurge and yellow starthistle.

Forests have another unique set of weed species associated with them. James Miller of the U.S. Forest Service has identified over 33 plants or groups that are spreading rapidly through southern forests (Miller, 2003). As might be expected, vines that compete directly for sunlight such as kudzu, Oriental bittersweet, or English ivy are considered among the worst nonnative vines, but other perennial weeds such as Chinese and Japanese privet, nonnative roses, and sacred bamboo are also among the worst forest weeds. In addition, certain tree species can also be considered weeds and among these, nonnative trees such as Princess tree and tree of heaven are considered common weeds in forest environments.

Riparian weeds are those species that are found in wet, poor drainage areas. They are adapted to anaerobic soils and may rely on water as a means to spread by seed or asexually. Riparian weeds include purple loosestrife, curly dock, giant reed, or salt cedar.

Aquatic weeds are those weeds adapted to living directly in the water. They can be floating, submersed, or immersed (submersed but with a root system). They can reduce water quality and water supply as well as pose problems for navigation or power generation. Some of the most prevalent aquatic weeds include water hyacinth and hydrilla (Figure 1.3).

A Few Basic Principles of Weed Ecology

Weed ecology is that aspect of weed biology that studies weed distribution, growth, development, reproduction, and population dynamics of weed species in managed (human) and unmanaged (natural) plant communities.

Seed Biology

We can begin with seed, and an appreciation of the ability of weeds to produce them. The number of weed seeds present in soil can be enormous. Koch (1969) has derived figures that indicate between 30,000 and 350,000 weed species per square meter (or 300 million to 3.5 billion seeds per hectare). An estimate of the number of seeds produced for a single weed can be in the hundreds of thousands (Table 1.1).

What is the fate of seed once it enters the soil? Harper (1977) envisioned the soil as a seed “bank” in which “deposits” or “withdrawals” are made. An example of a deposit would be seed rain, whereas withdrawals (seed removal) would occur by germination, deterioration, or predation. The concept of a seed bank is a very useful one for weed ecology since it provides key information regarding seed diversity and longevity as well as long-term information as to efficacy of weed management.

Seed longevity in turn, will depend on a number of factors and their interaction. Obviously, how much seed a given species produces is an important consideration as is soil type. However, seed decomposition, predation, and dormancy/germination are also key considerations. Overall, observations indicate that while seeds from grass and crop species are rarely long-lasting within the seed bank, weed seeds may last for hundreds of years if not millennia (Odum, 1965). Weed seeds are almost always present in the soil.

Seed germination and emergence is dependent on dormancy. Although there is a recognized physiological aspect of dormancy (seed ripening), most dormancy is imposed by the environment. Dormancy, in turn, can be broken by a given environmental factor or combination of factors, usually those that occur in seasonal cycles. For example, freeze/thaw combinations may be necessary to remove seed coat restrictions. Alternatively, many weed seeds need light (Sauer and Struik, 1964), which is usually abundant following a physical disturbance in the environment.

Seed germination is associated with rapid metabolic activity, radicle emergence, and then shoot appearance. Germination is a perilous ecological proposition, and natural selection has caused the timing of germination to coincide with favorable environmental conditions (Probert, 1992). However, even with such selection, given the large seed populations of some weed species, it is evident that many of them fail to germinate, decompose, or get eaten.

What then are “favorable environmental conditions” that promote weed seed germination within the seed bank? Whether human or natural, any disturbance exposes soil to light, alters water and temperature regimes, and, assuming temperatures above thermal limits, can allow seeds to break dormancy and germinate. As we have seen, weeds can be a primary constituent of the seed bank; such physical (abiotic) disturbances are therefore a key factor in their establishment within the plant community.

Emergence and Competition

Given physical disturbance of the soil as a signal for extensive seed germination, what factors following emergence from the soil are associated with weed species success? What, in other words, makes some weeds more competitive than others?

It is difficult to provide a complete answer to these questions, and scientists are still addressing many aspects of competition. However, in general, success at competition reflects the ability of a given species to acquire a limited set of resources at a greater rate than its neighbors, whether those neighbors are a different species (interspecific competition) or the same species (intraspecific competition). How these resources are acquired depends on both genetic hard-wiring (e.g., rate of leaf development, nitrogen use efficiency, etc.) and spatial and temporal variability of physical resources (e.g., nitrogen distribution in the soil, rainfall, temperature, etc.). In general, plant competition is for abiotic resources, principally light, water, and nutrients between or within species.

Because light varies in duration, intensity, and quality, and must be used immediately or lost permanently, those plants that intercept light and shade their neighbors are at a competitive advantage. Plants invest heavily into new leaf formation, particularly after emergence. Indeed, a simple comparison of leaf area ratio among seedlings can be used as one measure of early competitive success between weeds and crops (Kropff and Spitters, 1991). However, too much investment in leaves at the expense of stems or branches may also result in a disadvantage, since height is an important consideration in light interception. Overall, in many cropping situations where water and nutrients may be nonlimiting, light competition may be one of the principal factors influencing weedcrop interactions (Donald, 1961) and reducing crop productivity (e.g., oat and wild oat; Cudney et al., 1991).

(Continues…)

Excerpted from Weed Biology and Climate Changeby Lewis H. Ziska Jeffrey Dukes Copyright © 2011 by Blackwell Publishing Ltd.. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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