Plants: Anatomy, Growth, and Function PDF

U N I T Plants: E Anatomy, Growth, and Function 354 Unit E 13-BIOLOGY-11SE-Ch13.indd 354 7/19/10 3:43:02 PM  Unit C...

U N I T Plants: E Anatomy, Growth, and Function 354 Unit E 13-BIOLOGY-11SE-Ch13.indd 354 7/19/10 3:43:02 PM  Unit Contents Plants play key roles in nature and society. 13 13.1 Plants and People 13.2 Disturbance and Recovery Specialized plant structures support plant functions. 14 14.1 Plant Organs, Tissues, and Cells 14.2 Primary and Secondary Growth in Plants 14.3 Plant Vascular Tissue Plant growth is regulated by internal and 15 external factors. 15.1 Plant Hormones 15.2 Plant Responses 15.3 Soil Resources Unit Task In this unit, you will learn why plants are important in nature and to human society. You will also learn about the relationship between a plant’s structures and their function and what factors affect plant growth. In the Unit Task, you will select a plant in your neighbourhood, research its importance, and discuss how its structures and growth support this role. DISCOVERING BIOLOGY Rafflesia is a flowering plant from the jungles of South East Asia. This strange plant is a parasite because it grows inside vine stems, absorbing nutrients from them. Eventually it produces a huge flower that may be up to a metre in diameter. Recent genetic analysis has shown that Rafflesia is related to the familiar poinsettia. The common name of Rafflesia is “corpse flower” because the flower smells like rotting meat. What is the function of the flower’s rotting-flesh odour? 13-BIOLOGY-11SE-Ch13.indd 355 7/19/10 3:43:06 PM  CHAPTER Plants play key roles in nature 13 and society. M any foods were in short supply in Canada during World War II, Learning especially foods with military uses. Oil was needed to lubricate Expectations machinery, and there was little oil available for cooking. Phyllis Turner, commissioner of the Wartime Prices and Trade Board, decided By the end of this chapter, that Canada should develop its own oilseed crop. Sunflowers, cherries, you will: peaches, apricots, and linseed were all considered and rejected as Relating Science to unsuitable. In 1942, Canada purchased 20 kg of rapeseed from Argentina Technology, Society, and by 1948, 32 000 ha of rapeseed plants were growing in the Prairies. and the Environment But there was a problem: rapeseed oil did not taste very good. It contained evaluate, on the basis of two compounds that give mustard and radishes their sharp flavour. Canadian research, the importance of scientists used traditional plant-breeding techniques to improve the taste plants to the growth and of rapeseed oil by reducing levels of both compounds. The new form of development of Canadian society rapeseed was renamed “canola.” Canola comes from Canada plus oleum, evaluate, on the basis of which is Latin for oil. research, ways in which Canola, the world’s only “made in Canada” plant, is now grown on five different societies or cultures million hectares across Ontario and the Prairies (Figure 13.1). It competes have used plants to sustain with wheat as Canada’s top farm export. Canola is one of the healthiest human populations while supporting environmental edible oils, with about half the level of saturated fats found in olive or soy sustainability oil. Once the oil is extracted, the remaining meal is a high-protein feed for farm animals and aquaculture. Canola oil can also be made into biodiesel, a Developing Skills of renewable fuel that replaces diesel made from fossil fuels. Investigation and In the 1990s, canola was genetically modified (GM) with the insertion Communication of foreign genes that are associated with herbicide resistance. GM canola use appropriate terminology can be treated with the herbicide glyphosate, which kills a great variety related to plants of weeds, while leaving GM canola unharmed. About 80 percent of the canola grown in Canada is GM. Most of the remaining 20 percent is Understanding Basic Concepts conventionally grown, with the application of herbicides that kill fewer explain the process of ecological weed species. Only 0.07 percent of Canada’s canola crop is organically succession, including the grown, without any application of herbicides and pesticides. role of plants in maintaining biodiversity and the survival of organisms after a disturbance to an ecosystem Figure 13.1 Canola is a “made in Canada” plant. 356 Unit E ©P 13-BIOLOGY-11SE-Ch13.indd 356 7/19/10 3:43:07 PM  13.1 Plants and People Section Summary Plants have many key roles in human society, providing food, medicines, building materials, and energy. The human food supply is based on the cultivation of relatively few species of plants. The use of biofuels has environmental, economic, and social costs and benefits. Plants provide a number of ecosystem services. Sustainable development meets the needs of the present without compromising the ability of future generations to meet their own needs. Imagine a world without plants. You may immediately think about BIOLOGY SOURCE edible plants: the fruits, vegetables, and grains you eat every day. But you wouldn’t have meat, if farm animals didn’t have plants to eat. In addition, Suggested Activity many of the building materials in the room around you were made from E1 Quick Lab Overview on plant products. Some of them are obvious: wooden benches and cedar page 365 shingles, for example. Other plant products, such as glues, resins, and paints, are less easy to spot. Textiles, packaging, and medicines also come from plants. Even the oxygen you are breathing is produced by plants as a waste product of photosynthesis. Canada’s Food Crops Early humans collected wild seeds and fruits for food. Agriculture developed as humans began sowing seeds and cultivating plants in order to have a more dependable food supply. Later, humans began to breed plants to improve food quantity and quality. Over human history, people became more and more dependent on domesticated crops as a source of food. Today, almost all the human food supply is based on the cultivation of only 14 species of plants. Canada’s main crops, in order of income generated, are wheat, canola, barley, corn, soybeans, potatoes, flax, and oats (Figure 13.2). Figure 13.2 Wheat is one of All fruit and almost all vegetable crops come from flowering plants, Canada’s main crops. which are the angiosperms. Corn, rice, wheat, and other cereal grains are the fruits of grass species. In addition to feeding humans, cereal grains are the main food source for domesticated animals such as cows and chickens. Other foods, such as avocados, lentils, and okra, come from broad-leaf plants. The most important crops in the world are cereal grains and legumes, members of the pea and bean family. Half of the protein consumed by people worldwide comes from cereal grains. Rice is the most important cereal grain throughout Asia. Wheat is the most common grain in North American and European diets. Another 15 percent of human dietary protein comes from the dried fruits of legumes. ©P Chapter 13 Plants play key roles in nature and society. 357 13-BIOLOGY-11SE-Ch13.indd 357 7/19/10 3:43:09 PM  Food comes from all plant parts. Lettuce, spinach, and tea are leaves; celery, potatoes, sugar cane, and bamboo shoots are stems; carrots, sweet potatoes, and beets are roots; coffee and cocoa are seeds (Figure 13.3). Edible fruits include bananas, apples, strawberries, and oranges. Spices are strongly flavoured plant parts. Cinnamon is from bark; vanilla and black pepper are fruits; cardamom is a seed; and cloves are a flower. Tasty herbs such as basil, thyme, and mint are the leaves of aromatic plants. One of the world’s most expensive herbs, saffron, is the male floral parts of a crocus flower, collected laboriously by hand. Many of the foods we eat contain hidden processed plant products, such as sugars from corn or beets and oil from canola or sunflowers. We use thickeners, such as guar gum and corn starch, and soy protein is often added to increase protein and improve texture. Although Canada is a major food-producing country, many edible plants such as coffee, tea, cocoa, and bananas cannot be grown in our temperate climate and must be imported. Ontario alone spends almost $16 billion on imported food and food products each year. Figure 13.3 Cocoa pods are 15–25 cm long and often grow directly from the tree trunk. Plants as a Source of Pharmaceuticals Cocoa is made from the seeds inside the cocoa pod. Plants have long been used as a source of traditional medicine. In 400 B.C.E., Hippocrates, the Greek doctor known as the father of modern medicine, recommended that the bark and leaves of the willow tree could be used to treat headaches, fevers, and pain. The active ingredient in willow is acetylsalicylic acid, better known as Aspirin™. We now know how Aspirin™ works. It inhibits the production of prostaglandins, chemicals that increase the pain sensitivity of the body’s nerve endings. More than 120 prescription and non-prescription drugs, ranging from stimulants to cancer treatments to laxatives, are extracted from plants. In fact, 25 percent of drug prescriptions contain a plant product (Table 13.1). Table 13.1 Drugs Produced from Plant Products Plant Species Drug Uses Autumn crocus Colchicine Anti-inflammatory, (bulb and seeds) anti-tumour Cowage (seeds) L-Dopa Parkinson’s disease Foxglove (leaves) Digitalis, digitoxin, digoxin Heart rhythm regulator Jimsonweed (seeds) Scopolamine Sedative Pacific yew (bark) Taxol Ovarian cancer Poppy (fruit) Morphine, codeine Pain killer Rosy periwinkle (whole plant) Vinblastine, vincristine Hodgkin’s disease, leukemia Wild yam (roots) Cortisone, diosgenin Anti-inflammatory, (estrogen) birth control Willow trees (bark) Acetylsalicylic acid Pain killer, anti-inflammatory (Aspirin™) Yellowbark cinchona (bark) Quinine Malaria 358 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 358 7/19/10 3:43:11 PM  Canada’s Aboriginal peoples used more than 500 plants for medicinal purposes. Members of Jacques Cartier’s 1535 expedition suffered from scurvy, a disease that results from a lack of vitamin C. They were treated by Iroquois healers with a tea made from eastern white cedar and other conifers that are high in vitamin C (Figure 13.4). North American plants continue to be a source of new drugs. Taxol, a potent drug used to treat ovarian and breast cancer, was originally derived from the bark of the Pacific yew tree. The bark of a 15-m tall, 200-year-old tree yielded less than a gram of taxol. Yield is the amount of an agricultural product that is collected and used by humans. Demand for taxol threatened the Pacific yew before methods were developed for making the drug. Figure 13.4 Tea made from There may be many more plant-based drugs that await discovery. eastern white cedar is high in Researchers have investigated fewer than 5000 of the 280 000 known vitamin C. plant species as potential sources of new medicines. Human activities threaten many plants before their potential uses can be assessed. The tropical rainforest is losing plant species faster than any of Earth’s ecosystems. It could be a “medicine chest” of healing plants that will become extinct before they are even discovered. Plants as a Source of Fuel Plants are the main source of fuel for human activities. Humans have used BIOLOGY SOURCE wood and peat fires to cook food and provide heat for over 50 000 years. Fossil fuels such as coal, oil, and natural gas come from the fossilized remains Suggested Activity of plants that died 350 million years ago during the Carboniferous period. E2 Decision-Making Analysis Overview on page 365 The Promise and Peril of Biofuels The search for alternative energy sources has become increasingly urgent as oil prices rise and fossil fuels run out. There is also concern about greenhouse gases that are produced when fossil fuels are burned. Biofuels are energy sources produced from renewable organic materials, including plants and organic waste. Ethanol is the most familiar biofuel. It is a form of alcohol, mostly produced from corn. All cars can run on gas/ethanol blends, while modified engines can run on pure ethanol (Figure 13.5). Biodiesel is produced from plant or animal fats or used cooking oil. The Canadian government funds programs that encourage the development and production of biofuels. Biofuels are also taxed at a lower rate than fossil fuels. Canadian production of ethanol is expected to reach two billion litres during 2010, while the target for biodiesel production from canola is 500 million litres. Biofuels offer the hope of renewable fuels with lower greenhouse gas emissions. However, using crops for fuel may reduce the availability of agricultural land for food crops and Figure 13.5 More and more increase the cost of food. vehicles are powered by biofuels. Plants as a Source of Industrial and Building Materials BIOLOGY SOURCE Plants and plant products play many key roles in building, industry, and the Explore More home. Temperate hardwoods, such as beech, oak, and maple, and tropical What is straw-bale housing and how hardwoods, such as mahogany and teak, are dense woods of angiosperms can it support Canadian agriculture and increase the sustainability of used to make high quality furniture and flooring. Softwoods come from other natural resources? gymnosperms, which include conifers, such as pine and spruce trees. ©P Chapter 13 Plants play key roles in nature and society. 359 13-BIOLOGY-11SE-Ch13.indd 359 7/19/10 3:43:16 PM  Softwoods are used for furniture and buildings as well, but their long fibres mean that they are also suitable for producing pulp, used for paper and cardboard production. Canada is one of the world’s largest producers of softwood lumber and its products. We supply about one-third of the softwood lumber used in the United States. This trade earns Canada $8.5 billion a year. About 280 000 Canadian jobs and 300 communities depend on Canada’s softwood lumber industry. Plants have many industrial uses. Linseed oil is used as a furniture finish. Castor oil is used as motor oil and as a lubricant for hydraulic systems in heavy machinery. A century ago, most industrial products were made from plant products. In fact, plastic was initially made from corn. Although oil-based products currently dominate industry, there is a new interest in industrial products based on plants. New technologies have led to the production of inexpensive plant-based industrial products. Plant-based inks, paints, plastics, and dyes are becoming less expensive and more popular. Other Uses of Plants Plants have many other uses. When you use a pencil or pen to jot down a note on a piece of paper, the pencil, the ink, and the paper all come from plants. The money you spend, the violin you play, makeup, a baseball bat: all of these products are made from plants. Many of our clothes are made from cotton, flax, hemp, or sisal fibres (Figure 13.6). Linen is made from flax. Rayon is made from chemically processed cellulose derived from Figure 13.6 These items are all wood, cotton, or bamboo. made from plants. The blouse is Plants also perform important ecosystem services. These services are linen, which is made from flax. The the beneficial processes carried out by living things that are necessary scarf is rayon, which is made from wood pulp. The socks are bamboo to sustain life on Earth. For example, plants play a key role in nutrient rayon, and the T-shirt is cotton. cycles. They are a food source for consumers and they enrich the soil by adding usable nutrients to it. Plant roots bind soil, reducing erosion in terrestrial ecosystems and coastal areas. Plants are also capable of storing or detoxifying toxins and pollutants. Some house plants, such as the spider plant and philodendron, reduce indoor air pollutants. Plants provide many ecosystem services in urban centres. Trees and other plants in parks and along city streets improve air quality, cool the air, reduce storm water runoff, reduce noise levels, and provide habitat for animals (Figure 13.7). Some studies show a reduction in crime and increase in personal relaxation in urban areas that have more vegetation. Plants also have economic benefits. They increase property values and reduce energy use through shading and wind reduction. Concept Check 1. List three different ways you used plants today. 2. Distinguish between biofuels and fossil fuels. 3. Provide an example of an ecosystem service performed by plants. Figure 13.7 Trees and other plants provide many ecological, economic, and social benefits to cities. 360 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 360 7/19/10 3:43:20 PM  Using Plants Sustainably It is important to adopt a goal of sustainable development in using plants for agriculture and forestry. Sustainability is the capacity to maintain a certain process or state indefinitely. The United Nations has defined sustainable development as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” People practising sustainable development can continue to farm and harvest timber, while still protecting biodiversity. For example, a forest corridor through farmland may be left to connect two areas of parkland, allowing wildlife to move from one area to the other. Trees may be harvested selectively, with only mature individuals taken, instead of cutting down all the vegetation in an area. Plant Harvest and Horticulture by Aboriginal Peoples The earliest evidence of crop cultivation in Ontario is found in areas near the Great Lakes around 1250 to 1500 years ago. When Jacques Cartier visited Hochelaga (now Montréal) in 1535, he said, “[The Iroquois] have good and large fields of corn.” The Aboriginal peoples of Ontario were skilled farmers who grew several varieties of maize (corn) as well as beans and squash. Maize, beans, and squash were grown in small fields beside the villages (Figure 13.8). Each field was used for a time and then allowed to lie uncultivated, or fallow, for a number of years. Kernels of maize and dried beans could be stored for several years, providing a stable food supply when game, fish, or wild rice were unavailable. The cultivation of maize, beans, and squash in Ontario probably led to significant increases in population size. These three crops all benefit from the presence of the others. The Figure 13.8 Growing maize, beans, broad leaves of the squash plants shade the soil, maintaining soil moisture and squash together is a form of and preventing weeds from establishing and competing with the crops. sustainable agriculture. The corn stalks support the climbing stems of the bean plants, while the beans house nitrogen-fixing bacteria in root nodules. In a process called nitrogen fixation, these nodules “fix” the nitrogen that is in the air pockets in the soil, changing it into a form that plants can use (Figure 13.9). The wisdom of this form of cultivation has been rediscovered in the modern world. On some Mexican farms, fields planted only with maize have been replaced with fields planted with maize, squash, and beans. The result has been a substantial increase in yield. Worldwide Sustainable Agricultural Initiatives Many farmers around the world are adopting innovative agricultural techniques — or returning to traditional methods — to increase yield. A 2001 study described over 200 sustainable-agriculture projects carried out in 52 developing countries. These sustainable initiatives increased crop yields by an average of 73 percent. Figure 13.9 Bacteria living in the There are many examples of sustainable initiatives. In Kenya, farmers nodules of many legumes are able are planting weeds in their maize fields to act as “trap plants” for stem to transform atmospheric nitrogen borer larvae. Previously, stem borer larvae ate about a third of the maize into a form that is usable by plants. produced in the region. However, the larvae prefer to attack napier grass, a local plant often considered a weed. Napier grass secretes a sticky material that glues down and kills the larvae. By planting napier grass among their maize plants, farmers increased maize yield by nearly 70 percent. ©P Chapter 13 Plants play key roles in nature and society. 361 13-BIOLOGY-11SE-Ch13.indd 361 7/19/10 3:43:26 PM  In Bangladesh, farmers are raising fish in the flooded rice paddies, turning one harvest into two (Figure 13.10). In Madagascar, a local researcher developed a method to quadruple rice yield. He transplanted the rice seedlings at an earlier stage, delayed flooding of the rice fields, and used locally available compost instead of costly chemical fertilizers. This method has been so successful that it has now been adopted in China, Indonesia, and Cambodia. One-third of Argentina’s farmers no longer till, or plough, the soil before planting their crops. Tilling puts air into the soil. This increases bacterial decomposition of organic material in soil and releases carbon dioxide from bacterial respiration. Instead of tilling, the farmers plant winter crops or spray biodegradable herbicides to prepare their fields. Untilled fields serve as major Figure 13.10 In Bangladesh, farmers can raise rice and fish carbon sinks because organic material in the soil does not degrade in the same paddies. and release atmospheric carbon dioxide, a major greenhouse gas. A one-hectare field that is not tilled before planting can absorb one tonne of carbon each year. In Thailand, small fields are cultivated for several years and then left fallow for a decade. This method is similar to that used by Ontario’s Aboriginal people. The result is productive land with high biodiversity. When fallow, the agricultural fields contain 223 plant species, nearly as many as the 319 plant species found in virgin forests in the same area. In Cameroon, farmers plant cacao, orange, mango, avocado, and cherry trees among the natural trees of the rainforest. The result is high agriculture yield and high biodiversity. The farmed forest contains more than half the number of species found in natural forests. In South America, farmers harvest Brazil nuts, cashews, and palm hearts from undamaged rainforests. Some crops, such as Brazil nut trees, will grow only in a rainforest where the overhead tree canopy is not disturbed. Some plants, such as the cashew tree, have many uses (Figure 13.11). The fleshy part is used for juice and in jams and chutney. The cashew nut is Figure 13.11 Cashew trees have roasted and eaten. The liquid in the nut shell has industrial uses. The many uses. bark, leaves, and juice from the fruit have medicinal properties. Resins and gums can be extracted from the stems and bark. BIOLOGY SOURCE Concept Check Explore More 1. Define sustainable development. Are there ways that we can eat 2. What plants were cultivated by Ontario’s Aboriginal people? to support sustainable agriculture and economic self-sufficiency in 3. Provide two examples of sustainable agricultural practices. developing countries? Sustainable Agriculture in Canada The two main agricultural practices used by Canadian farmers to increase crop yields are the use of chemical fertilizers and pesticides. The use of chemical fertilizers, however, releases greenhouse gases and may cause excessive growth of algae if fertilizers run off into lakes or oceans. Many farmers also use manure as an inexpensive source of nutrients for crops, but they must also take care to prevent run-off into streams or standing water. 362 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 362 7/19/10 3:43:28 PM  The two main types of pesticides are herbicides, which control unwanted plants, and insecticides, which control unwanted insects. About 80 percent of Ontario farms use at least one pesticide annually. There are about 5700 chemical pesticides registered for use in Canada, and Canadian farmers spend over a billion dollars on them each year. The risks associated with these chemicals vary with the amount used, how long they last before breaking down, where they are carried by wind and water, and whether they kill beneficial organisms, such as earthworms and insect pollinators. The more pesticides are applied, the higher the environmental risks. Insects and weeds reproduce quickly, evolving to become resistant to pesticides. Tilling land is also done to prepare the soil for crops and to control weeds, but it increases soil erosion and the loss of soil nutrients. In addition, tilling disrupts wild bees. Bees are important pollinators that nest in holes in soft dirt, tangled grasses, and abandoned rodent burrows. If bees did not pollinate flowers on fruit trees, we would not have fruits, such as cherries (Figure 13.12). Integrated Pest Management Many Canadian farmers have adopted a variety of pest-management Figure 13.12 Without bees, we strategies designed to increase crop yields while reducing harmful health would not have cherries or many other fruits. and environmental effects. This is called integrated pest management. Table 13.2 outlines several strategies used in integrated pest management. Of course, each of these strategies has its own risks. For example, selecting insects for biological control must be done carefully so that populations of these introduced insects do not expand uncontrollably. Table 13.2 Strategies for Integrated Pest Management Strategy Explanation Crop rotation Changing the crop grown on a plot of land each year means that each crop is less vulnerable to specialized pests and diseases. Green manure Growing plants and then tilling them back into the soil increases soil nutrients and improves crop health so they are more resistant to pests. Planting nitrogen-fixing crops Growing nitrogen-fixing crops, such as soybean, increases the nitrogen content in the soil, improving crop health. Figure 13.13 Natural predators, such as this ladybird beetle, eat Biological control Natural predators, such as birds, ladybird beetles, and pest insects, such as these aphids. spiders, can control some insect pests (Figure 13.13). Genetically modified crops Crops can be genetically engineered so they possess desirable traits. For example, Bt corn produces a bacterial toxin that kills the larvae of the corn borer when it feeds on corn. Pheromones Pheromones are chemicals used by insects to communicate with one another. Releasing synthetic pheromones may prevent insects from mating or lure them into traps, while not affecting other species. Water Management Canada has an abundance of fresh water. However, this precious resource is not unlimited. Canadian farmers use about 9 percent of available Figure 13.14 Irrigation of farm water for irrigating crops (Figure 13.14). Irrigation reduces soil erosion fields uses about 9 percent of because it prevents rich topsoil from drying out and blowing away. Canada’s water. ©P Chapter 13 Plants play key roles in nature and society. 363 13-BIOLOGY-11SE-Ch13.indd 363 7/19/10 3:43:30 PM  BIOLOGY SOURCE Irrigating farmland also carries costs. It may increase the salt content of soil, harming plants. The added water may carry pesticides and fertilizers Explore More into ground water, streams, and lakes. If water for irrigation is pumped What happens when plants are from underground sources, these sources may not refill. Farmers must removed from coastal areas and manage water use very carefully, using as little water as possible, at the riverbanks? How can reintroducing times when their crops need it the most. native plants to these areas help Most Ontario farmers maintain vegetation on the edges of streams or to prevent soil erosion? ponds. This vegetation is important because it prevents soil erosion and reduces water run-off from the farm. Therefore, it prevents valuable soil, polluting nutrients, and toxic pesticides from being washed into streams and ponds. Most farmers prevent livestock from grazing near water bodies to reduce damage to vegetation and keep manure out of streams and ponds. Sustainable Forestry in Canada Canada has over 400 million hectares of forest, representing 10 percent of the world’s forests. Large expanses of boreal forest stretch from the Atlantic to Pacific Oceans and make up 75 percent of Canada’s forested land (Figure 13.15). Boreal forest is composed mostly of coniferous trees, such as spruce, balsam fir, tamarack, and jack pine. Most of our forests are still intact. Only 6 percent of Canada’s forests have been removed and the land converted to cities, farms, mines, or roads. Canadians value their forests: 13 million person-visits are recorded annually in our national parks. Canada is the world’s largest exporter of timber and other forest Figure 13.15 Canada has about one-third of the world’s products. Canada’s National Forest Strategy pledges a commitment to boreal forests. sustainable forest management. Under this policy, forest products are harvested sustainably, preserving the forest’s health and biodiversity. Forest managers plan forest use in consultation with Aboriginal people, forest owners, environmental groups, academics, recreational forest users, and community groups. At present, 146 million hectares of Canada’s forests are classified as sustainably managed. Each year, less than 1 percent of Canada’s forests are harvested. Half of the harvested area is replanted with seeds or nursery-raised seedlings, while the other half is left to reseed naturally. However, the growth of much of this regenerating forest is limited due to climate, topography, and other factors. In addition, extensive areas of forests are lost due to fires and other natural disturbances. Therefore, we must be careful to manage Canada’s forests wisely. Figure 13.16 Canada’s National Tree Seed Centre collects seeds Preserving Genetic Variation of Canada’s Forests from all Canadian tree species. Trees are an important natural resource in Canada. Canada’s National Tree Seed Centre (NTSC) in Fredericton, New Brunswick, conserves the genetic diversity of our forests. The Centre collects and stores seeds BIOLOGY SOURCE from across the natural ranges of all Canadian tree species. The goal of the Centre is to obtain a diverse collection of genetic material. Therefore, Take It Further seeds are collected from many individual trees in each population, not The National Tree Seed Centre just from the “best” or tallest trees (Figure 13.16). Some tree populations (NTSC) collects seed from all of are adapted to the unique environmental conditions in which they Canada’s tree species. Prepare a live. Other populations may be endangered or threatened. When trees pamphlet directed to the general are harvested, the cleared areas are planted with genetically improved public explaining what the NTSC does and why it is important. seedlings grown in seed orchards. The NTSC collection is an increasingly important storehouse of genetic information. 364 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 364 7/19/10 3:43:35 PM  E1 Quick Lab BIOLOGY SOURCE The Plants Around Us Purpose To appreciate the many functions of plants in your surroundings Activity Overview In this activity, you will work in a group to identify the many plants and plant products that you use in everyday life. Your teacher will give you a copy of the full activity. Prelab Questions Consider the questions below before beginning this activity. 1. How many different kinds of plants and plant products do you eat each day? 2. How many items in Figure 13.17 can you identify that are made of plants? 3. Which textiles are made from plant products? Figure 13.17 Many of the products in this photo are made from plants. REQUIRED SKILLS DI Key Activity Gathering information E2 Decision-Making Analysis BIOLOGY SOURCE Stating a conclusion Assessing the Impact of Biofuels Issue Prelab Questions The use of biofuels rather than fossil fuels is becoming Consider the questions below before beginning increasingly popular. Many believe that switching this activity. to biofuels will reduce greenhouse gas emissions. 1. What is a biofuel? However, there are possible costs associated with biofuel use. Some believe that their use will cause food 2. Name two Canadian plants that are used to produce prices to rise as land is diverted from food production to biofuel. fuel production. 3. Why do governments encourage the development and production of biofuels? Activity Overview In this activity, you will select a biofuel and assess its total environmental impact, including the greenhouse gas output that results from its production and use and its effect on land use. You will also consider how growing crops for biofuels will affect food availability and the price of food (Figure 13.18). Students will then debate whether or not the government should require all fuel used in Canada to include a substantial percentage of biofuel. Your teacher will give you a copy of the full activity. Figure 13.18 Corn can be grown for food or for biofuel. ©P Chapter 13 Plants play key roles in nature and society. 365 13-BIOLOGY-11SE-Ch13.indd 365 7/19/10 3:43:37 PM  13.1 Check and Reflect Key Concept Review 16. Ontario farmers try to maintain vegetation on the banks of ponds and streams on their farms. 1. Why is canola described as a “made in Canada” (a) Explain the benefits of this practice. plant? (b) Describe two costs associated with this 2. List the top two economically important crops practice. in Canada and describe their uses. 17. Name three services performed by the ecosystem 3. Name one crop harvested from each plant part: in the following photo. seeds, fruits, leaves, stems, flowers, roots. 4. List three drugs that are produced from plant products. For each drug, state its use(s) and the plant it is obtained from. 5. Why is the destruction of tropical rainforests a threat to human health? 6. Explain why fossil fuels, such as coal, oil, and natural gas, are plant products. 7. What are biofuels and how do they differ from fossil fuels? 8. What is softwood lumber? Name two products Question 17 made from softwood lumber. 9. Define the term “ecosystem services.” 18. Why does the National Tree Seed Centre collect seeds from a variety of trees in each population? 10. Explain what is meant by the term “sustainability.” 19. Decisions about the use of biofuels touch on many different issues: food availability, energy 11. What are the benefits and risks of the use independence, use of renewable resources, of chemical fertilizers? reduction of greenhouse gas emissions, and the 12. Explain the term “trap plant” and describe total environmental impact of resource use. an example of how such a plant can increase How would you weigh all these different issues agricultural yield. in deciding whether to use biofuels or another source of energy? 13. Describe three sustainable strategies that a farmer might adopt for combating pest weeds 20. Should the Government of Canada encourage and insects. the growing of corn and canola for production of biofuels? Justify your answer. Connect Your Understanding Reflection 14. Some Aboriginal people of Ontario grew three crops in small fields, which were used for a 21. Have your perceptions about where our food time and then left fallow. comes from changed as a result of reading (a) Use a concept map or another graphic this section? Explain your answer. organizer to explain how these crops benefit 22. When you buy plant products, do you consider from being grown together. whether they were grown sustainably? Why or (b) Explain the significance of these agricultural why not? practices to soil fertility and local biodiversity. 15. Use a table to compare the costs and benefits of For more questions, go to BIOLOGY SOURCE tilling an agricultural field. 366 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 366 7/19/10 3:43:42 PM  13.2 Disturbance and Recovery Section Summary Healthy communities experience periodic disturbances. Following disturbance, plant communities change over time. This is called ecological succession. Primary succession occurs in disturbed areas lacking soil. Secondary succession follows a disturbance that damages a community but leaves the soil intact. Succession promotes biodiversity because it allows different types of plant communities to exist in an area. In 1980, 400 m of the cone blew off Mount St. Helens in Washington (Figure 13.19). In the blast zone, burnt trees lay “toppled like toothpicks.” Mud flowed and pumice stone and volcanic ash were flung from the volcano, destroying plants and animals up to 30 km from the new crater. Thirty years later, plants now cover about 75 percent of the ground around the crater. A stable plant community has not yet established, and it may not develop. Mount St. Helens is an active volcano that will likely erupt sometime in the next 200 years. When Mount St. Helens erupted in 1980, plant communities on the mountain slopes had not recovered from a previous eruption in the 1800s. Figure 13.19 Mount St. Helens erupted on May 18, 1980. Disturbances Influence Community Structure Although they may seem stable and unchanging, most healthy plant communities are subject to periodic disturbance. Natural disturbances include fire, flood, frost, drought, or the fall of a single tree in the forest (Figure 13.20). Less frequent natural disturbances include dramatic natural disasters such as hurricanes, volcanic eruptions, and earthquakes. Disturbances can also be caused by human activities, such as mining, deforestation, agriculture, or urbanization. Disturbances affect communities directly, by killing organisms. They also alter communities indirectly, by changing the availability of abiotic resources such as shelter, nutrients, light, or water. Changes in the abiotic environment mean that the habitat may become suited to a different set of Figure 13.20 Fire is typically a plant and animal species. large-scale natural disturbance. ©P Chapter 13 Plants play key roles in nature and society. 367 13-BIOLOGY-11SE-Ch13.indd 367 7/19/10 3:43:44 PM  The Benefits of Disturbance The effects of a disturbance are not always negative. Despite the plant BIOLOGY SOURCE and animal deaths caused by the disturbance, some organisms thrive in Explore More the wake of the destruction. Moose and deer benefit from fire because it How are plants and animals in the creates habitats where there is lots of new vegetation to browse. Seeds of boreal forest adapted to survive or some species require exposure to the extremely high temperature of a forest even benefit from forest fires? fire before they can germinate, or begin to grow. For example, cones of jack pines are sealed shut by a gummy substance called resin (Figure 13.21). The cones cannot open to release seeds until the resin is scorched by fire. Figure 13.21 Most jack pine cones open only after they are heated to about 50°C. The Impact of Disturbance The type, severity, and frequency of a disturbance will determine its impact. The type of disturbance is important because disturbances change ecosystems in unique ways. Fires, floods, and windstorms alter communities quite differently. The severity of a disturbance determines the extent and intensity of destruction. A hurricane will have a greater effect on a community than a windstorm. The frequency of disturbance determines how much recovery time there is for the ecosystem. A community that has already been altered by a disturbance may recover more slowly from a second event. Some disturbances happen with predictable frequency, allowing communities to adapt to and even benefit from disturbance. For example, the Canadian Prairies are maintained by fire because trees are destroyed by fire (Figure 13.22). Smaller-scale disturbances tend to occur more frequently. Strong winds may bring down a few trees in a forest each year, while a hurricane will affect forests much less frequently. Figure 13.22 Fires maintain the Prairie habitat. Concept Check 1. Provide an example of an ecological disturbance and describe its effects on a plant community. 2. Explain why disturbances can have positive effects on a community. 3. List three factors that determine the impact of a disturbance on a community. 368 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 368 7/19/10 3:43:47 PM  Ecological Succession Major disturbances such as clearcut logging, volcanic eruptions, or major storms may remove or greatly alter the plant community in an area. Some plant species are well adapted to colonize disturbed areas. Early establishing species are called pioneer species. Pioneer species are often adapted to high light or low nutrient environments. Alder trees, for example, are often pioneer species. Alders do well in disturbed areas that have low-nutrient soils because nodules on their roots fix nitrogen, converting it into a form usable by plants. As the pioneer plant species grow, they change the biotic and abiotic conditions in the disturbed area. The roots of these early plant colonizers bind soil, allowing it to retain more moisture. Many pioneer species fix nitrogen, adding nitrogen to the soil that other species can use. As the leaves shed by pioneer plants decompose, more nutrients accumulate in the soil. Above ground, shoots of pioneer species create shade, moderating temperatures. Over time, pioneer species are replaced or out-competed by other plants that need richer, moister soils, but are more tolerant of low light levels. As the plant species change, so do the animal species that inhabit the community. This process of replacement of the plants and other organisms that make up a community is called ecological succession. Primary Succession When a community arises in a lifeless area that has no soil, the change is called primary succession. Examples of such areas are new islands and landscapes created by erupting volcanoes or the bare rock left behind by a retreating glacier. Micro-organisms are generally the first organisms to appear during primary succession. Then lichens and mosses, which grow from windblown spores, colonize the barren ground. Soil gradually develops from the decomposed remains of the early colonizers. Once soil is (a) present, the lichens and mosses may be overgrown by grasses and other herbaceous plants. The seeds of these plants may have blown in from other areas or been carried in by animals. Eventually, shrubs and trees become established. Primary succession from barren ground to a forest community can take hundreds or even thousands of years. Another type of primary succession, dune succession, occurs along the shores of the Great Lakes. Here, grasses with long horizontal roots, called rhizomes, are usually the first species to colonize the unstable sand (Figure 13.23). These grasses stabilize the sand dune, eventually (b) allowing other species, such as sand cherry and dune willows, to colonize. A mature forest Figure 13.23 (a) These dunes along Lake Huron are undergoing community may eventually develop but this primary succession. (b) Grasses with rhizomes are often the will take thousands of years. pioneer species. ©P Chapter 13 Plants play key roles in nature and society. 369 13-BIOLOGY-11SE-Ch13.indd 369 7/19/10 3:43:54 PM  Primary Succession on Mount St. Helens When the cone blew off Mount St. Helens in 1980, the eruption affected a large area. Volcanic gases, ash, and pumice covered a fan-shaped area to the north of the crater. Mud flowed in other directions, forming a 30-km desolate landscape covered with ash and strewn with rocks the size of large buildings (Figure 13.24(a)). The new landscape was dry and low in nutrients, without any soil. (a) (b) Figure 13.24 (a) The eruption of Mount St. Helens provided The Mount St. Helens eruption allowed biologists to study the an excellent opportunity for sequence of events during primary succession. They expected that the researchers to study primary first organisms to arrive would be photosynthetic bacteria, followed by succession. (b) Lupines were often the first colonizers on mudflows at mosses and lichens. These organisms did move in as early colonizers on Mount St. Helens. the edges of the mudflow, but chance survivors also played an important role in primary succession. A few young saplings buried in snow managed to survive. Some lupines and fireweeds survived as their roots and bulbs tumbled on top of the mudflows, instead of being buried underneath them. The roots of lupines house bacteria that fix nitrogen (Figure 13.24(b)). This helped to enrich the nutrient-poor mudflows formed by the eruption, allowing other seeds to germinate and grow. Over the course of succession, species diversity generally increases. Biologists monitoring a small mudflow near the crater noted that there were no plants after the 1980 eruption, but by 1984, one species of lupine had started to grow, and by 1990, eight species were present. After 30 years, 20 plant species had established and these covered about 75 percent of the ground. It will take over 100 years for plant communities on Mount St. Helens to stabilize. That may not happen, as this active volcano may erupt again before long. Often, studying natural processes such as succession can help ecologists manage natural landscapes. Because of the new understanding gained from the study of Mount St. Helens and other ecosystems, foresters now leave 15 percent of the trees and other plants behind when they clear-cut forest. This biological legacy accelerates the process of ecological succession, increasing the rate at which a new forest grows. Secondary Succession When a disturbance damages an existing community but leaves soil and BIOLOGY SOURCE plants behind, the change in plant communities that follows is called Suggested Activity secondary succession. The remaining soil is important, because it contains E3 Inquiry Activity Overview on nutrients that support plant growth as well as a large number of living page 376 organisms. Soil is not just dirt: it is a living bank of plant seeds, fungal spores, and insect larvae that can speed the growth of a new community. 370 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 370 7/19/10 3:43:56 PM  Forest areas that have been clear-cut, burned by a forest fire, or cleared for farming and then abandoned recover by secondary succession. Light-loving and fast-growing annual species, which grow, reproduce and die within a year, are usually the pioneer species in the newly cleared area (Figure 13.25). Within a few years, grasses and non-woody perennial plants, which live for more than one year, become established. Shrubs that are shade-tolerant and require richer, moister soil may replace these first plants. Eventually, trees may replace the shrubs. Annual Perennial species Shrubs Coniferous forest Mixed forest Plants and grasses TIME Figure 13.25 Secondary succession occurs following a disturbance that leaves some soil and plant fragments behind. Secondary Succession in Algonquin Park Parts of Algonquin Park, in northern Ontario, were logged around 1900. After the forest was cut down, piles of woody debris called slash were left behind. The slash caught fire, and the resulting fires spread through areas of the park. A century later, the herbaceous plants and shrubs that established in burned areas have been replaced by a forest of poplar and white birch (Figure 13.26). Poplar and birch seedlings grow best in bright light and are tolerant of low-nutrient soils. Birch trees shed leaves that decompose rapidly, releasing nutrients. Young saplings of oak and white pine trees, both shade-tolerant species, are now growing up under the poplar and white birch canopy. Eventually, sugar maple and beech forests will replace the oak and white pine, and finally hemlock, the most shade-tolerant species, will replace the maple and beech trees. Each successional stage is defined by the dominant tree species, but it also has its own species of birds, insects, fungi, ferns, and shrubs. As the forest fills in during succession and forms an overhead canopy, the interior of the forest changes. Soil moisture increases, light intensity on the forest floor decreases, and air and soil temperatures stabilize. With these changes, the animals in the forest also change. Birds, such as white-throated sparrow and ruffed grouse, that prefer open forest are replaced by birds of the intact forest, such as brown creepers and ovenbirds. On the forest floor, seed abundance and diversity increase leading to higher Figure 13.26 Areas of Algonquin Park alternate numbers and diversity of small mammals and soil insects. between forests of poplar and white birch, as shown here. ©P Chapter 13 Plants play key roles in nature and society. 371 13-BIOLOGY-11SE-Ch13.indd 371 7/19/10 3:43:59 PM  Immediately following a disturbance, successional changes are rapid as fast-growing annual species move in and then are replaced by perennials and shrubs. Over time, however, the rate of change slows as slower-growing trees come to dominate the canopy. Secondary Succession After the Yellowstone National Park Fires In 1988, huge fires scorched 400 000 ha of Yellowstone National Park in the United States, burning over a third of the park. In the years after the fires, spectacular wildflower displays filled the burned areas and young conifers sprouted among the wildflowers to begin a new cycle of forest growth (Figure 13.27). Many of the plant species in Yellowstone National Park are adapted Figure 13.27 Spectacular to periodic fires. Before the area was designated as a park in 1872, wildflower displays followed the Yellowstone National Park Yellowstone’s grasslands burned every 20 to 25 years, while major forest fires of 1988. fires occurred every 300 years or so. After Yellowstone was designated as a park, forest fires were controlled. As a result, the 1988 fire was quite extensive because dead trees and other woody material had accumulated. Despite the severity and extent of the 1988 fires, few animals died. About 200 of the 50 000 elk in the park, 36 deer, 6 black bears, and 9 bison were the only mammalian casualties. Less than 1 percent of the park’s soils heated up enough to kill underground roots and seeds. Fire tends to burn the above-ground parts of plants, leaving the below-ground root systems unharmed. Spectacular regeneration of the grasslands and forests followed the fires. Ash from the fires brought new nutrients to the park’s soil, and wildflowers bloomed throughout the park. Lodgepole pine, the dominant tree species in the park, produces cones that are sealed with gummy resin. Fire is needed to burn the resin away before the cones can open and release their seeds. Scorched lodgepole pine cones released their seeds, producing seed densities of 400 000 seeds per hectare. Healthy lodgepole pine seedlings grew under the blackened canopy of burned trees, forming a new forest (Figure 13.28). One surprise from the regrowth of Yellowstone National Park was the patchy distribution of the new pine forests. Lodgepole pine trees produce two types of cones. One type of cone opens as soon as it matures. Figure 13.28 Lodgepole pine seedlings sprouted following The other type must be burned before it can open and shed its seeds. The the 1988 fires in Yellowstone distribution of the two types of cones was uneven in the park, leading National Park to a patchy distribution of new pine trees. Secondary Succession After Human-Caused Disturbances Of all species, humans have had the greatest impact on the natural environment (Figure 13.29). Currently, humans use 60 percent of Earth’s land, mostly for cities, farms, or rangeland. Unfortunately, human disturbances usually have a negative effect on species diversity. When a forest or grassland is converted to farmland, a diverse community is replaced with crops of a single plant species, such as canola, wheat, or corn. Figure 13.29 A clearcut hillside is an example of human disturbance to a community. 372 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 372 7/19/10 3:44:02 PM  A large-scale example of human disturbance occurred in the Greater Sudbury area. Since the late 1800s, the centre of Greater Sudbury has been degraded by many human activities, including forestry, mining, and nickel smelting. The original vegetation was killed off by a combination of logging, acid deposition, and acidic soils with high levels of certain metals. The resulting Barrens around Sudbury were bleak, desolate, and lifeless areas totalling 18 500 ha around the three smelters. Only stunted birches and red maples survived in the surrounding areas (Figure 13.30(a)). (a) (b) Figure 13.30 (a) Prior to 1972, the Sudbury Barrens were lifeless areas surrounding Sudbury’s three primary nickel smelters. (b) Young stands of deciduous and coniferous trees now grow on many areas of the former Barrens. Since 1972, air emission improvements of Sudbury’s smelting complexes have reduced sulphur dioxide and metal particle emissions by 90 percent. Reduced sulphur dioxide levels in the air allowed plants to grow, but the acidic soils with high metal concentrations were still a problem. The first step in restoration was to spread lime on the affected area. BIOLOGY SOURCE Spreading lime reduced the soil acidity, which in turn reduced the toxic effects of the soil metals and permitted plants to grow normally. Grass and Explore More legume seeds were added next. The successful grass species were tolerant How are scientists using plants to of heavy metal pollution. The roots of legumes fix nitrogen. These early clean contaminated soil? steps mimicked the natural events of secondary succession, increasing the nutrient availability and moisture of the soil. These changes allowed the survival of trees. Some tree seedlings were planted, while the seeds of other trees blew into the area on their own. Today, Sudbury is a city of parks and trees. The former Barrens are filled with deciduous woods of white birch, trembling aspen, and willow (Figure 13.30(b)). Not all human-disturbed habitats can be remediated using secondary succession. Many human-caused disturbances are too vast or repeated too frequently to allow succession to proceed. For example, land in the tropical rainforest that is cleared for forestry or agriculture will often not return to its natural state because the nutrients have been lost from the ecosystem. ©P Chapter 13 Plants play key roles in nature and society. 373 13-BIOLOGY-11SE-Ch13.indd 373 7/19/10 3:44:11 PM  Climax Communities Ecologists used to think that ecological succession in a region would always proceed through a set of predictable stages to form a climax community. A climax community was considered to be a stable, self-perpetuating community that existed in equilibrium with the area’s biotic and abiotic environment. This view held that climate was the only factor determining the nature of the climax community, and that the same stable plant community would form in an area regardless of the disturbances that began the process of succession. We now understand that many factors, not just climate, determine which community will be stable in a particular region and whether a climax community will form at all. Local differences in topography, soil composition, rainfall, and temperature influence plant community development. The other living things found in a region, including animals, fungi, and bacteria, may also alter plant succession and influence the nature of the plant community. Succession is not as predictable as we once thought. In fact, ecologists now believe that the process of succession is often directed by random events, such as the season of the disturbance or local rainfall patterns. Concept Check 1. What is the main difference between primary and secondary succession? 2. Describe the sequence of successional plant communities that are expected to form in Algonquin Park after it was logged in 1900. 3. Define the term “climax community” and explain how our understanding of this concept has changed. Succession Promotes Biodiversity As a plant community changes following a disturbance, each successional stage is defined by the dominant plants found in the ecosystem. The animals change, too, as the tree canopy fills in and as light and soil conditions change. Within a given successional stage, small-scale interactions and disturbances produce patches of different habitat types. This patchiness is important to maintaining biodiversity. Different plants and animals will be adapted to the different patch types. Thus, an ecosystem is often made up of habitat patches, each with a different set of species. Even the fall of a single tree in a windstorm or ice storm opens up a gap in the canopy and produces a patchy habitat. This was dramatically illustrated when a severe ice storm struck eastern Ontario and western Quebec in January 1998 (Figure 13.31, next page). Some trees had accumulations of up to 10 cm of ice, causing branches and even trunks to break. Many trees were killed outright, and others died over the next few years due to the damage they sustained during the storm. The damage to the tree canopy allowed light to reach the forest floor. As a result, light- loving pioneer species established in a late-successional stage forest. 374 Unit E Plants: Anatomy, Growth, and Function ©P 13-BIOLOGY-11SE-Ch13.indd 374 7/19/10 3:44:12 PM  Figure 13.31 The 1998 ice storm in Ontario and Quebec was a disturbance that initiated secondary succession. Even within old-growth forest, changing habitat patches help maintain biodiversity. Hooded warblers and Acadian flycatchers are Ontario birds at risk of extinction (Figure 13.32). Acadian flycatchers do best in old-growth forests, under a permanent canopy cover. However, the gap created by the fall of a single large tree allows hooded warblers to persist because light-loving understory plants, such as raspberries, provide nesting habitat. Both birds can survive in an old-growth forest with scattered gaps in the canopy. Figure 13.32 (a) The Acadian flycatcher and (a) (b) (b) hooded warbler are both at risk of extinction. In some of Ontario’s boreal forests, biodiversity is maintained because spruce and birch replace each other in a 300-year cycle (Figure 13.33, next page). This cycle occurs because each species creates conditions that favour the other. Some forest animals, such as pine marten and woodland caribou, do well under a spruce forest canopy. On the forest floor, spruce needles decompose slowly, releasing very few nutrients to the soil. The soil becomes nutrient-poor and spruce seedlings cannot grow. Instead, birch seedlings invade spruce stands, changing the forests and the animals that live there. Many birch trees have root nodules full of bacteria that fix soil nitrogen. In addition, birch leaves decompose rapidly, releasing nutrients to the forest soils. Spruce seedlings grow well in the nutrient-rich soils created by the birch. As a result, the spruce reinvades among the birch trees. These patches of forest types increase the diversity of the forest as a whole. ©P Chapter 13 Plants play key roles in nature and society. 375 13-BIOLOGY-11SE-Ch13.indd 375 7/19/10 3:44:12 PM  BIOLOGY SOURCE Tree Density vs. Time 500 Take It Further Ecological restoration is often based birch 400 on the changes that take place during spruce Tree Density (number per hectare) secondary succession. What ecological restoration activities are planned or ongoing in your community? How do 300 these activities accelerate the processes of secondary succession? 200 100 0 0 100 200 300 400 500 600 Time (years) Figure 13.33 Ontario’s spruce and birch forests alternate on the basis of nitrogen availability in decomposing leaves and soil. REQUIRED SKILLS Recording and organizing data E3 Inquiry Activity BIOLOGY SOURCE Analyzing patterns Seeing Succession Question Activity Overview What evidence of succession do you see in th