Green Planet: How Plants Keep the Earth Alive

Introduction: Remnants of Paradise

[3] INTRODUCTION remnants of paradise The alder, whose fat shadow nourisheth Each plant set neere to him long flourisheth. —WILLIAM BROWNE, CA. 1613 It certainly didn’t seem like paradise. I was up to my hips in the sucking slime of a swamp. Because of the water had very little oxygen, the leaf litter and the corpses of mosquitoes and snapping turtles did not completely decompose. Instead they produced a dark brown glue that stained my clothes like a stygian tea. Bacteria released a putrid scent of hydrogen sulfide. I had no idea how deep the muck was, and my left leg slipped in more deeply as I tried to lift my right. There was nothing to grab on to except poison ivy, the thorny branches of greenbrier and rose, or dead sticks. I was in Hudson Pond in central Delaware. The bridge for U.S. Highway 113, only a few yards from where I stood, rumbled with hundreds of cars and trucks. The passing motorists who looked down on me might have thought I was crazy if they knew that I was studying the small trees that grew in the swamp. If they knew that I had driven a thousand miles to see them, their suspicions would have been confirmed . But when I looked up at the trees that I had driven so great a distance to see, it all seemed worthwhile. There they were, Alnus maritima , the seaside alders. Each one consisted of a cluster of little gray trunks, with serrated leaves and puffy, conelike fruits (fig. I.1). What makes this species so special is that it is very rare, and that it has an unusual geographic distribution. All of the seaside alders in the entire world occur in just three populations. The first population lives along INTRO.qxd 11/12/08 11:08 AM Page 3 clear, rushing streams in two counties in Oklahoma.The second inhabits a single swamp in northwestern Georgia. The third is found on the Delmarva Peninsula east of Chesapeake Bay. I had visited the first two groups and was now exploring the third. Each of these populations is separated from the others by about a thousand miles. green planet [4] FIGURE I.1. Seaside alders, such as this tree in the middle of the Blue River in Oklahoma, are nearly extinct. Many plant species like this one are facing extinction before their importance in their habitats or to the human economy have been adequately studied. Photograph by the author. INTRO.qxd 11/12/08 11:08 AM Page 4 Part of the mystery of the seaside alder is how this species came to exist in only these three places. It apparently had a wider range in the past and then died out everywhere except in these locations. Another species of alder, the hazel alder (Alnus serrulata), lives right alongside the seaside alder in Georgia and Delmarva—in fact, while standing under a seaside alder, I could look a few yards to one side and see a hazel alder, in exactly the same habitat conditions. But there are millions of hazel alders throughout the eastern United States. Why was the seaside alder dying away, while the hazel alder was thriving? My colleagues and I have determined that it was because hazel alders can survive in the shade, and seaside alders cannot—and most swamps are shady.1 Another aspect of the puzzle is that the alder species that are most genetically similar to the seaside alder are not other American alders, but species of alder that live in Asia, such as Alnus nitida. The story of the seaside alder is a complex and fascinating story for me and my fellow botanists to investigate. Certainly botanists find the seaside alder interesting, but why would anyone else care about it? Why should you care about it? Why should you care about any other species of grass, wildflower, bush, or tree? These are reasonable questions.The purpose of this book is to answer them. A basic knowledge of what plants are and what they do is essential for everyone on the earth—not just for botanists, gardeners, farmers, and landscapers. Plants make life on Earth possible. All the oxygen in the air has come from plants. Plants help to prevent an excessive greenhouse effect by removing carbon dioxide gas from the air. Plants put moisture into the air and create cool shade, in...