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ANCIENT PIÑON-JUNIPER WOODLANDSA NATURAL HISTORY OF MESA VERDE COUNTRY
UNIVERSITY PRESS OF COLORADOCopyright © 2003 University Press of Colorado
All right reserved.
Chapter OneINTRODUCTION AND OVERVIEW
M. Lisa Floyd, David D. Hanna, and William H. Romme
Mesa Verde Country is home to an ancient woodland of broken, twisted junipers and stout old piñon pines (Figures 1.1 and 1.2). The old-growth woodland, now rare in much of the southwestern United States, is relatively undisturbed in the deep canyons and high ridgetops of Mesa Verde. As each tree ages over the centuries, it balances the need for new leaves, cones, or pollen with the wood fiber required to resist the stress and strain of wind and snow loads. Yet a critical component of the woodland structure results when years of accumulated lignin finally give way to these physical burdens. The old trees and their detached limbs become home for scores of birds, mammals, reptiles, and even some amphibians. Germinating on the woodland floor in the shade of downed limbs sprout the newest generation of bitterbrush and sagebrush, lupines and buckwheats, muttongrass and ricegrass. Within the geographic landscape of Mesa Verde Country-given enough time free from widespread fire and intense human activity-the woodland trees grow more ragged, more twisted, and more dispersed, and the woodland's biological inhabitants become more andmore diverse.
It is unusual that Mesa Verde has remained undisturbed long enough for the ancient woodland to develop. Although the area supported a large Ancestral Puebloan population a thousand years ago, which probably altered the vegetation substantially, human influences were greatly diminished following abandonment of the region around A.D. 1300. Utes occupied the region for several centuries thereafter, but their ecological impact appears to have been localized and relatively unimportant in the rugged, inaccessible terrain of Mesa Verde. Even with the livestock and lumbering that Euro-American settlers brought to southwestern Colorado in the late 1800s, and despite recent droughts and extensive wildfires, a substantial portion of today's dense, ancient woodland has remained relatively unaffected by human-induced changes. Thus the southern portions of Mesa Verde and pockets of protected habitats in the region today support ancient piñon-juniper woodlands unknown in most other areas on the Colorado Plateau.
The following chapters summarize our current scientific understanding of the biology, ecology, geology, and climatology of this interesting portion of southwestern Colorado; but the book is also a celebration of the beautiful and remarkable natural history of the Mesa Verde region. The chapters have been contributed by a diverse set of authors, many of whom have spent nearly their entire lives in view of Mesa Verde. Their love for the land comes through as strongly as their technical expertise. As editors, we have smoothed the text, removed redundancies, and rectified technical inconsistencies among chapters. All of the chapters have been reviewed by experts to guard against errors of fact or interpretation. We have not attempted to change the basic flavor of each chapter, however. As you read the book, you will see that some chapters have a rigorous analytic underpinning and present concepts and interpretations of broad application. Others are reflections on natural history, honed by decades of close and patient observation and rich in local detail. Some chapters focus almost exclusively on Mesa Verde itself, while others range widely over the surrounding area depicted in Figure 1.3.
We wrote this book to elucidate and to celebrate the fascinating but often underappreciated natural history of Mesa Verde and the surrounding countryside. Almost everyone in Colorado and surrounding states is aware of Mesa Verde's famous archaeological sites; but few realize the exceptional nature of the ancient woodlands that, along with striking topography, form a backdrop for the abandoned cities of an ancient culture. Few recognize the myriad ways in which seemingly insignificant things-broken branches on centuries-old trees or microscopic organisms in the dry soil-are essential components of an ecological system that has supported the human and nonhuman inhabitants of the area from Ancestral Puebloan times to the present. For reasons explained in the following discussion, piñon-juniper woodlands are especially well developed in Mesa Verde, so this unique region illuminates the ecological importance of piñon-juniper woodlands and reveals some of their fascinating secrets of natural history.
We have carved out a naturally defined landscape as the focus of this book, and it is important to understand the boundaries of this region because they define a portion of piñon-juniper woodland unlike others in the Southwest. Figure 1.3 depicts this 13,000-square-km area along the eastern boundary of the Colorado Plateau geologic province, where the plateau meets the southern Rocky Mountains. It lies within the Four Corners region of the United States, an area described by the intersection of the political boundaries of Colorado, New Mexico, Arizona, and Utah. The greatest part of Mesa Verde Country lies in Colorado, with small incursions into New Mexico, Arizona, and Utah. The region is defined by a natural landform drained by major and minor rivers that originate in the San Juan Range of the southern Rocky Mountains. It is bounded by the Pine River to the east, the San Juan River to the south, Montezuma Creek to the west, and the salt basin structures of Paradox Basin, Disappointment Valley, and the 2,425 m contour to the north. Our region includes the north side of the San Juan Basin and the southern edge of the Paradox Basin; it extends up onto the slopes of the San Juan Mountains and abuts the Uncompaghre Plateau.
Extensive topographic relief characterizes this area. The landscape's surface is dissected into mesas, cuestas (similar to mesas but with one side decidedly higher than the other), buttes, canyons, and valleys. The bulk of the region sits atop layer after layer of sandstone, shale, coal, and other sedimentary rocks, in some places more than 9,000 m in aggregate thickness. Over time these strata have been lifted, tilted, folded, faulted, and eroded by water and wind to form a maze of canyons and a puzzle of disjointed surfaces (Chapters 11 and 12). The centerpiece is Mesa Verde (meaning "green table" in Spanish), a prominent cuesta that is visible throughout most of the area. Mesa Verde National Park encompasses the northeastern portion of the cuesta; most of the remainder lies within the Ute Mountain Ute Reservation. The country surrounding Mesa Verde is more or less similar in terms of geology, climate, and vegetation, so we refer to the whole area as "Mesa Verde Country." Here the high peaks of the southern Rocky Mountains, which lie to the east and north, promote an uplift of airmasses (an orographic effect) that shower plateaus and mesas with up to 46 cm of annual precipitation-a lot of moisture for the generally arid Four Corners region. The relative abundance of water (Chapter 14); unique geology (Chapters 11 and 12); diverse soils that vary with slope angle, the dip of the beds, and the nature of the parent strata (Chapter 13); and a long growing season (Chapter 15) provide the physical foundation for Mesa Verde Country's distinctive piñon-juniper woodland.
REGIONAL VEGETATION PATTERNS
To get oriented, let's look in a general way at the vegetation patterns in Mesa Verde Country. Between 1,666 and 2,275 m elevations in Arizona, New Mexico, Colorado, and Utah, woodlands of Pinus edulis (Colorado piñon pine) and Juniperus osteosperma (Utah juniper) form the dominant vegetative community. At the lowest and driest elevations, where annual precipitation may be less than 30 cm and few shrubs or forbs persist, the woodland is represented by only scattered junipers. Junipers become more abundant as mean annual precipitation increases with elevation. Piñons appear and become more common than junipers as the average annual precipitation approaches 35 cm and increases to 45 cm. Piñon-juniper stands in this upper portion of the zone grow taller and closer together than lower stands. The piñon-juniper stands continue to occupy sites on steep slopes with south exposures to about 2,600 m, especially those protected against fire by cliffs or barren areas (Chapter 16). The Utah juniper of lower elevations is replaced by Juniperus scopulorum (Rocky Mountain juniper) as moisture increases and mean annual temperature declines. Piñon pine and Rocky Mountain juniper continue as subordinate elements of the Gambel oak-ponderosa pine community up to about 2,900 m.
As a rule, the distribution of the Pinus edulis-Juniperus osteosperma (piñon-juniper) woodland is restricted in elevation and latitude to landscapes that receive between 30 cm and 50 cm of precipitation. The upper elevation boundary, however, is not a physiological limit for either species; the moister climate at higher elevations permits such mesophytic species as Pinus ponderosa (ponderosa pine), Quercus gambelii (Gambel oak), and Pseudotsuga menziesii (Douglas fir) to out-compete piñon and juniper.
Precipitation patterns alone do not drive vegetation patterns. A heterogeneous vegetational mosaic also arises out of the complex dissection of the region into canyons and mesas, hills and valleys, and south-facing and north-facing slopes. Even if the land were completely flat, recurring natural disturbances such as fire and disease would create a mosaic of habitats. Differences in soils arising out of various parent rocks, depositional processes, and weathering also create diverse microhabitats. Long-term climatic variability and local anomalies such as the rain shadow in the lee of Sleeping Ute Mountain prevent the establishment of a single uniform type of vegetation over any large portion of the area.
As a result of this physical heterogeneity, our area of interest supports a variety of plant communities in addition to the woodlands that are the focus of this book. At low elevations near the San Juan River on the southern border of Mesa Verde Country is a high desert scrubland dominated by Atriplex canescens, A. gardneri, and A. obovata (shadscale), as well as numerous forbs (herbaceous plants other than grasses, sedges, and rushes), including Eriogonum corymbosum (buckwheat), Sphaeralcea coccinea (globe mallow), and grasses such as Hilaria jamesii (galleta grass) and Sporobolus aeroides (alkali sacaton). Bare, rocky soils are common, as vegetation cover is low; microorganisms form well-developed soil crusts (Chapter 5). Scattered junipers also may dot the landscape.
At slightly higher elevations, Artemisia tridentata (big sagebrush), Gutierrezea sarothrae (snakeweed), and Chrysothamnus nauseosus (rabbitbrush) sometimes form dense stands on deeper soils. Junipers also may intermingle with these tall shrub species.
From elevations greater than 1,700 m to the upper reaches of the Mesa Verde cuesta at 2,500 m, piñon and Utah juniper share dominance, and the density of the woodland increases. Patches of young, open woodland with a dense shrub understory are interspersed with old-growth stands. Mountain shrublands abut the woodlands at the highest elevations on the cuesta. These shrublands share many floristic affinities with the piñon-juniper woodlands but have very different physiognomy. Gambel oak, Amelanchier utahensis (serviceberry), and Fendlera rupicola (fendlerbush) form dense stands that resprout vigorously after fire and dominate where fire frequency is relatively high.
Along streams and rivers and around ponds and wetlands is distinctive riparian vegetation composed of plant species that require a permanent water source: Populus fremontii (cottonwood), Salix species (willows), Typha latifolia (cattails), and a diversity of sedges, rushes, and other herbs (Chapter 14).
This simple introduction to the landscape and vegetation patterns sets the stage for the chapters to come, which elucidate special characteristics of piñon-juniper woodlands and their nearby riparian or shrubland neighbor communities of Mesa Verde Country.
SUMMARY AND ROADMAP
We begin by developing a general concept of old-growth conditions for the piñon-juniper vegetation that is so well developed in Mesa Verde Country (Part 1). Chapter 2 focuses on the trees' age and structural complexity, which supports a dense biological network. In Chapters 3 to 10, various experts describe the multitude of plants, microorganisms, mammals, birds, reptiles, amphibians, and insects that depend on the old-growth woodland in Mesa Verde Country.
Focus then turns to the physical parameters that make this biological reality possible (Part 2). Underlying the piñon-juniper woodland is a fascinating substrate: Chapters 11 to 13 examine the geologic and soil-forming processes that sustain old-growth piñon-juniper woodlands. Specific climatic conditions, especially those involving available water (Chapter 14), are needed to support the old woodland and its biological inhabitants. Chapter 15 describes the climate of the last hundred years.
We recognize that change is part of all landscapes, and the old woodland at Mesa Verde is no exception. Part 3 examines factors that have helped shape today's ancient woodland, including fire (Chapters 16 and 17) and the role of humans, both prehistoric and historic (Chapters 18 to 21).
Finally, we venture into the future, identifying potential threats to the ecological integrity of Mesa Verde Country and discussing what can be done to manage this landscape for a healthy future (Part 4).
Each part opens with prefatory remarks in which we outline the structure of the following chapters. Remember that these chapters are the product of years of study and reflection by different experts, and each tends to have its own flavor and viewpoint. All of the chapters are united, however, by their focus on this unique and complex landscape. The diversity of perspectives and writing styles in this book reflects the ecological diversity of Mesa Verde Country itself.
Chapter TwoGNARLY OLD TREES: CANOPY CHARACTERISTICS OF OLD-GROWTH PIÑON-JUNIPER WOODLANDS
M. Lisa Floyd, Marilyn Colyer, David D. Hanna, and William H. Romme
Although over twenty-four million hectares of piñon-juniper woodlands are scattered across the southwestern United States and northern Mexico, old-growth piñon-juniper woodlands are rare in today's landscape. Later chapters discuss characteristics of natural and human disturbances that have reduced the chances that woodlands would attain old age and describe the biological wealth supported by the few remaining old-growth woodlands such as those of Mesa Verde Country. But before we do that, we must develop a definition of old-growth that suits piñon-juniper woodlands. The common vision of old-growth that includes towering trees and lush moss-covered logs (such as the old-growth forests of the Pacific Northwest or montane regions of the Rocky Mountains) just does not fit the lower elevations of the Southwest. Rather, piñon pines and several species of junipers-trees that rarely attain 10 m in height-are the dominant species, and low precipitation precludes a lush understory. Nonetheless, given enough time, old-growth develops in a way characteristic of this arid ecosystem. In protected pockets of Mesa Verde Country stand very old, complex, piñon-juniper woodlands. Twisted, gnarled junipers (replete with rot pockets and strips of shaggy bark) are accompanied by dark-barked and often stately piñon pines, some of which were established soon after the Ancestral Puebloans left the region in the late thirteenth century. Our research on the structure of undisturbed woodlands at Mesa Verde provides an opportunity to define old-growth. This concept of old-growth creates the basis for all subsequent chapters; for without the complex, old, woody structure provided by the trees themselves, the biological wealth of mammals, birds, amphibians, and insects described in Chapters 3 to 10 could not be sustained. Conversely, without the physical environments and ecosystem processes described in later chapters, the old-growth trees-the very structure of the woodland-could not persist.
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