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8 Historic Changes in Vegetation Along the Colorado River in the Grand Canyon R. ROY JOHNSON, University of Arizona, Tucson, Arizona ABSTRACT: Glen Canyon Dam has irreversibly changed the Colorado River in Grand Canyon. Prior to the dam, sediments and associated nutrients were transported into the canyon from upstream, with a large percentage of those sediments and nutrients then flushed from the sys- tem as output, supporting downstream systems. These predam sedi- ments and nutrients supported aquatic and terrestrial organisms of the river corridor throughout Grand Canyon. The dam has drastically re- duced upstream input, making the Colorado River ecosystem in Grand Canyon more nearly a closed system. Downstream systems, e.g. the fishery in Lake Mead, have also been impacted by the loss of nutrients trapped in Lake Powell. Both riparian and aquatic ecosystems have been impacted by postdam flow regimes. Prior to the construction of Glen Canyon Dam the com- ponents of the natural riverine ecosystem of the Colorado River in Grand Canyon were driven by high energy, pulsing events typical of south- western rivers. Three pulsing features of the river affected by the dam included, in decreasing importance to the riparian ecosystem (1) seasonal flow patterns and maximum/minimum flows (2) nutrient and sediment transport and turbidity and (3) fluctuations in water temperature. Ripar- ian vegetation was dependent on moisture, nutrients, and sediments sup- plied by the river. Flooding is a natural phenomenon in riparian habitats, thus riparian species are generally flood adapted. However, in the Grand Canyon predam high flows and sediments transported by those flows scoured vegetation from the riparian zone on a regular basis. 178
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HISTORIC CHANGES IN VEGETATION... Riparian ecosystems are ecotonal, interfacing between the upland zone of the terrestrial environment and the adjacent aquatic environment of the river. As such, species richness and population densities both tend to be higher in riparian ecosystems than on adjacent uplands because of the edge effect. The term naturalized ecosystem has been used for the riparian ecosystem in Grand Canyon, an ecosystem in which native processes have not been appreciably interrupted and/or nonnative (ex- otic) species have become naturalized, causing no loss of native spe- cies. By contrast, the aquatic ecosystem in the Grand Canyon is an exotic ecosytem, an ecosystem that has been so extensively modified that it consists of a humanly created environment in which native pro- cesses have been interrupted, nonnative species abound, and native spe- cies have been extirpated. Early investigators in the Southwest applied the concept of desertifi- cation to "water projects," notably reclamation projects and other river management activities. Desertification, leading to increasingly xeric con- ditions, has resulted in losses of 90% to 95% of the riparian habitat throughout the Southwest lowlands. By contrast, a more constant post- dam water supply has resulted in the riparian environment in Grand Canyon becoming increasingly mesic. The Colorado River in Grand Canyon is the only major riverine ecosystem in the lowland Southwest where there has been an appreciable increase rather than a decrease in riparian vegetation and associated animal populations during the l900s. The importance of ripanan habitat to wildlife and humans has been extensively documented. The riparian zone in Grand Canyon is used extensively by whitewater recreationists, hikers, and a large number of native animals. Thus, the proper management of riparian vegetation in the canyon is of primary importance. INTRODUCTION AND OBJECTIVES 179 The Grand Canyon, one of the original seven natural wonders of the world, is located in northern Arizona, on the Colorado Plateau Province, a region called by the noted geologist Hunt (1967) "easily the most colorful part of the United States." The canyon is of great interest to resource managers, recreationists, scientists, and politicians. At least two ex-presi- dential candidates have written about the canyon Arizona Senator Barry Goldwater (1940) and former Governor Bruce Babbitt (1978~. It serves as a textbook example for stratigraphic studies in its more than a billion years of exposed geologic history. Grand Canyon National Park receives more than three million visitors annually, approximately 22,000 travel down the Colo- rado River, considered by many to be the world's premiere whitewater river. Approximately 100, 000 additional recreationists use the river be- tween Glen Canyon Dam and Lees Ferry, half as fishermen, the others on raft trips. The region's biological importance was assessed thusly by one of
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180 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT this century's greatest mammalogists (Hall, 1981), "Concerning subspeciation, which is a step toward the formation of species, the southwestern quarter of the United States...has a larger number of subspecies than any other conti- nental area of equal size in the world...The reasons seem to be high degree of relief...deeply cut, steep, rock-walled canyons (the Grand Canyon of the Colorado, for one), resultant wide range in temperature, sharply marked zonation of plants, and tremendous diversity of types of soil." It is this environmental diversity, so important to both natural and cultural systems, and the impacts of water management systems on this diversity that we address here. Glen Canyon Dam has irreversibly changed the Grand Canyon environ- ment. Interactions between humans, vegetation, and flow levels are often complicated. Some plants and animals are adapting to postdam conditions (Carothers and Dolan, 1982), others are not. Some changes are local, e.g. the gradual downhill displacement of sand into the river as a result of camper's footsteps (Valentine and Dolan, 1979~. Prior to construction of the dam, this lost sand would have been replaced almost on an annual basis by high spring flows. Other changes are large scale, impacting not only the riverine system within Grand Canyon but also affecting ecosystems hun- dreds of miles downstream. For example, the Colorado River formerly transported sediments and associated nutrients into the canyon from tens to hundreds of miles upstream. The dam has drastically reduced this input, making the Colorado River system in Grand Canyon more nearly a closed system. These predam sediments and nutrients supported aquatic and ter- restrial organisms of the river corridor. A large percentage of those sedi- ments and nutrients were flushed from the canyon as output, supporting downstream systems. Thus, the dam has also impacted systems further downstream, e.g., the fishery in Lake Mead that suffers from a loss of nutrients trapped in Lake Powell (Evans and Paulsen, 1983; and numerous other papers by Paulsen et al., in Adams and Lamarra, 1983~. The possible loss of integrity of the ecological fabric of the region, or even modification of critical features of the region, is of interest to us here as we evaluate vegetational changes, specifically changes in riparian veg- etation along the Colorado River. Johnson and Carothers (1987) called the riparian ecosystem in the Grand Canyon a naturalized ecosystem. A natu- ralized ecosystem is one in which native processes have not been apprecia- bly interrupted and/or nonnative (exotic) species have become naturalized, causing no loss of native species. Attempts to reconstruct lists of the predam biota during studies in the 1970s and 1980s have failed to demonstrate the loss of riparian species since the construction of Glen Canyon Dam in 1963. Instead, the addition of nonnative species along the river corridor has in- creased species richness within riparian biotic communites, apparently without the deleterious effects often associated with nonnative invaders. However,
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HISTORIC CHANGES IN VEGETATION. 181 thirty years is an infinitesimal length of time in the evolution of natural communities such as those in the Grand Canyon. By contrast, the aquatic ecosystem in the Grand Canyon is an exotic ecosystem, an ecosystem that has been so extensively modified that it consists of a humanly created envi- ronment in which native processes have been interrupted, nonnative species abound, and native species have been extirpated. Riparian ecosystems are ecotonal, interfacing between aquatic and terres- trial ecosystems. As such, species diversity and population densities both tend to be higher in riparian ecosystems than in adjacent upland ecosystems because of the edge effect Johnson, 1979; Odum, 1979~. The newly cre- ated rivenne environment in the Grand Canyon consists of a series of subecotones, or subzones, even further expanding the edge effect and increasing biotic diversity and population densities. Since the terms subecotone and subzone are not in common usage, the riparian zone of the Colorado River in Grand Canyon has, by convention, been further divided into even smaller "zones." The terms Old High Water Zone and New High Water Zone are so widely used that they are commonly abbreviated OHWZ and NHWZ (and will be so designated throughout this paper). This loose use of terminology, or perhaps better, nonheirarchical use of a nontechnical term, zone, leads to confusion if one does not realize that the NHWZ and OWHZ are actually subcategories of the riparian "zone." In examining the riparian ecosystem of the Colorado River in Grand Canyon, vegetational studies are critical. Plants making up riparian vegeta- tion serve as processors of environmental conditions, biological computers if you wish. Unlike animals, plants cannot move from one location to another, thereby escaping changes from day to day or year to year. There- fore, they respond to changes in temperature, soil, moisture, slope, aspect, and the multitude of other climatic, edaphic, and related conditions of their environment. In turn, vegetation serves as a substratum for animal life, including humans prehistorically serving a critical subsistence function, today a greatly sought after recreational function. Thus, vegetation is prob- ably the best component for assessing the "health" or condition of the ripar- ian segment of this riverine ecosystem. By correctly interpreting changes in vegetation over time we can thereby evaluate the past and present "health" of the ecosystem and postulate what is to come. Objectives Although our task here is the discussion of historic changes in riparian vegetation before the 1980s, projections for the future of species and pro- cesses along the river corridor necessitate some discussion of research dur- ing the 1980s, especially recent findings from the Glen Canyon Environ- mental Studies (GCES). Thus, in order to make our story complete, within
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182 COLORADO RIVER ECOLOGY AlID DAM MANAGEMENT space limits, we can stay neither entirely within that time period, the ripar- ian zone, nor vegetation. Other essential areas of discussion include floristics since riparian vegetation of the river corridor is composed of individual plants of numerous species. And finally, a basic ecological principal, sim- ply put, states that in nature everything is connected to everything else. This, then necessitates our addressing the interaction of plants with water and other components of the aquatic ecosystem as well as the interdepen- dence of terrestrial animals, and to a lesser degree aquatic animals, on riparian vegetation. HISTORY AND BACKGROUND OF BOTANICAL EXPLORATION ALONG THE COLORADO RIVER IN GRAND CANYON The first humans cognizant of riparian vegetation along the Colorado River in Grand Canyon were prehistoric groups to which certain floristic components of the vegetation were of subsistence or ceremonial impor- tance. Since these early canyon inhabitants left no written record we must rely on plant and animal lists from archeological excavations—for summa- ries see Fowler, et al. (1969), Euler (1984) and Jones (1986~. Paleontologi- cal records provide additional biological information from riverside sites (for summaries see Euler, 1984) but most palaeoecological studies deal with upland vegetation, especially those analyzing fossilized plant remains from packrat middens (Cole, 1990~. Thus, 3,000 to 4,000 year old split-twig figurines recently excavated from riverside Stanton's Cave (Euler, 1984) that had been made from obligate riparian willow and cottonwood are a good indication that those species grew along the river andlor its tributaries at that time. (Scientific names of plants in text are listed in Appendix A; animals are given parenthetically the first [and usually only] time the spe- cies is mentioned.) However, human proclivity for carrying things around and the movement of plant materials by wind and water make both paleon- tological and archeological analyses of limited value for our purposes. It is difficult to extrapolate back in time from limited information, espe- cially for determining changes in an area such as along the Colorado River in Grand Canyon, one of the most inaccessible regions of temperate North America. Unfortunately, we find extrapolation necessary not only for pale- ontological and archeological information but for much of the historic record as well, noted more for its gaps in information rather than for usable bio- logical information. Checklists for the flora and fauna of the river corridor were incomplete until intensive surveys of the 1970s. For example, Dicoria brandegei ("Unknown Compositae #1" on my early 1970s collecting expe- ditions along the river) is a common and widespread native plant about the size of a small, nonnative Russian thistle. Dicoria commonly grows as a pioneer species on sandy deposits along with Russian thistle, especially
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lIISI ORIC ClIANGES IN VEGETATION... 183 along the upper reaches of the canyon. After being left from early plant lists for Grand Canyon, it was finally included in the latest checklist (Phillips et al., 1987) as "common on sand dunes along the Colorado River from CRM [Colorado River Mile] 38-232." The species was not listed by MacDougall (1947) in the Grand Canyon Natural History Association's third edition of the park plant checklist (first edition in 1932) nor by Clover and Jotter (1944~. Since the species could hardly be overlooked it is presumably a post-dam invader, adapted to sandy deposits above the current normal high water mark, sites that were often scoured clean of fluvial sands, vegetation, and seeds by predam high flows. However, in addition to excluding dicoria, McDougall (1947) also excluded tamarisk from his checklist although Clo- ver and Jotter (1944) had previously listed tamarisk as common at several localities. Although some botanical purists do not list nonnatives, e.g. tamarisk, McDougall listed numerous other nonnative species, e.g. clovers, sow thistle, etc., with annotations that they were exotics so we are left with the uncertainty of an unsolvable mystery in relation to dicoria's estab- lishment in the Grand Canyon. PHOTOGRAPHY AND THE BOTANICAL RECORD The first systematically collected vegetational information from the Grand Canyon was by photographers. Earlier explorers, e.g., Powell, commonly took notes on the geology and even Indians but little was mentioned in their journals about vegetation. The following information about photographic expeditions were taken largely from Graf (1978), Turner and Karpiscak (1980), and Stevens and Shoemaker (1987~. The earliest scientific records for the Colorado River in Grand Canyon were accompanied by thousands of photographs that have been recently used for a series of comparisons with more recent photos. The first of these were by the Powell Expeditions of 1869 and 1871-72, especially the second expedition. Later expeditions of the late 1800s and early 1900s included the Wheeler Expedition of 1871-73, the important Brown and Stanton Rail- way Survey of 1889-1890, and miscellaneous still photography discussed by aforementioned authors. Photographic records from the early 1900s in- clude the first motion pictures by E.L. and E.C. Kolb in 1911. The U.S. Geological Survey (USGS) photographed potential damsites during the early 1920s and sponsored Stephens and Shoemaker's special expedition through the canyon in 1968 (Stephens and Shoemaker, 1987), one year before the Powell Centenniel, for comparing photos with those taken a century earlier by Powell's photographers. These photos show and occasionally mention vegetation but they were largely concerned with geology. Although these numerous earlier expeditions had photographed vegeta- tion along with the geology of the canyon the botanical value of most of
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184 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT these photos was generally limited and even the vegetation in these photos was not critically examined until the 1970s. Turner and Karpiscak's work in the 1970s, sponsored by the USGS and MNA (Museum of Northern Arizona) (Carothers and Aitchison, 1976; Turner and Karpiscak, 1980) is the most complete photo comparison of vegetation before the GCES of the 1980s. Thus, although photography was generally accomplished with little botanical intent, in retrospect much of this earlier photography has served a useful function for vegetational comparisons through time. Itecent Research and Plant Collections The first systematic plant collections and floristic notes containing veg- etational information was by Clover (1942) and Clover and Jotter (1941, 1944~. The Clover and Jotter Scientific Expedition (or Nevills' Expedition of 1938) consisted of three cataract boats that left Lee's Ferry on July 12, 1938. In addition to being the first biologists to conduct research along the river Elzada Clover, 41 year-old botany professor from the University of Michigan and her young lab assistant, Lois Jotter, were the first women to travel through the Grand Canyon. At Phantom Ranch the expedition was joined by Emery Kolb, boatman and aforementioned motion picture photog- rapher. The expedition reached Lake Mead, more than 250 miles down- stream, in late July. Clover spent part of the following year collecting at localities along the canyon that she could reach by car or foot. Botanical specimens and information collected on those two trips constituted the only critical examination of the biology of the canyon until the 1970s. Even with the impending construction of Glen Canyon Dam in the l950s and 1960s all biological "salvage" investigations were conducted upstream of the damsite in a riverine ecosystem that was to be inundated by Lake Powell (Woodbury, 1959~. The possibility of large scale changes that were to occur in downstream aquatic and riparian ecosystems were not considered until they became obvious, after the completion of the dam. In the late 1960s and early 1970s there was increasing interest in the bio- logical, physicochemical, and socioeconomic parameters of the Colorado River in the Grand Canyon. There was much concern about the"carrying capacity" for campers on the alluvial terraces, or "beaches," along the river and interre- lationships between campers, vegetation, and other components of the riverine environment (Dolan et al., 1977; Johnson et al., 1977~. In response to these concerns research planning was instituted through the Grand Canyon Research Advisory Board, formed under the auspices of the Arizona Academy of Sci- ence in cooperation with the National Park Service COPS). The board con- sisted of an interdisciplinary consortium of scientists, knowledgeable about the canyon, that advised NPS on research needs for the river. Beginning in 1970 a series of river expeditions was sponsored by this group and the Grand Canyon Natural History Association. Research expeditions varied somewhat
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HISI ORIC CHANGES IN VEGETATION... 185 in their basic missions, usually traveled by raft, and were composed of scien- tists from several different disciplines. Issues regarding recreational demands and environmental concerns, combined with a court case filed by river recreationists, eventually reached such a level that in July, 1973, the Washington, D.C. office of NPS funded a research trip through the canyon. A Colorado River craft (37 ft. pontoon raft) with two boatmen carried eight scientists and two assistants who spent 16 days on the river evaluating almost 400 campsites (Borden et al., 1975~. For the larger campsites evaluations included physical characteristics and capacity (numbers of campers), shoreline (sand vs. rock, landing condi- tions, bathing conditions, etc.), vegetation, photography available and other factors. Scientists involved in these earlier expeditions formed a core of research leaders that have contributed much of the information in this current paper, either in published papers, manuscripts, or by personal communication. By 1976, thirty projects and numerous river research expeditions had been funded by NPS under the Colorado River Research Program. The studies were coordi- nated from 1973 to 1976 by NPS Research Scientist R. Roy Johnson and the earliest riparian studies were conducted by a research team from the Museum of Northern Arizona under the leadership of Steven W. Carothers, Curator of Biology. NPS allocated more than 3/4 of a million dollars for studies and eighteen research reports published by NPS Western Region Office Johnson, 1977~. Also, during the 1970s and 1980s a great proliferation in publications for the Colorado River accompanied these research efforts. Numerous other reports and papers were published in other series and the open literature. Most publications through the 1970s were referenced in a bibliography (Spamer et al., 1981~. In addition to referencing the more important summary and synthesis papers from that bibliography, throughout this paper we also refer- ence works from throughout the 1980s that are most important to our discus- sions of vegetation (see also USDI, 1988~. DISCUSSION Prior to the construction of Glen Canyon Dam the riparian and aquatic components of the natural riverine ecosystem of the Colorado River in Grand Canyon were driven by high energy, pulsing events typical of south- western rivers. Three pulsing features of the river affected by the dam included, in decreasing importance to the riparian ecosystem (1) seasonal flow patterns and maximum/minimum flows (2) nutrient and sediment transport and turbidity and (3) fluctuations in water temperature (Carothers and John- son, 1983~. Riparian vegetation was dependent not only on moisture from the river but also on nutrients, and sediments carried by the river's water. Although fluvial sediments provided an ideal substratum for riparian veg- etation, high flows and sediments transported by those flows also scoured vegetation from the riparian zone on a regular basis.
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186 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT Increasing salinity of western rivers is one of our most serious problems (Pillsbury, 1981), not only from a standpoint of human usage but also be- cause of its impacts on natural riverine ecosystems. However, little is known of salinity and other critical riverine issues in Grand Canyon. The roles of impoundments and associated evaporation and dissolution processes in increasing salinity of water and soil further downstream are well known (Paulson and Baker, 1983~. However, total dissolved solids (TDS) have not increased appreciably in Grand Canyon and below Hoover Dam (Paulson and Baker, 1983) during recent monitoring. Presumably most of these salts have settled out in reservoirs above the dam but too little time has passed to evaluate long term effects. Paulson (1983) advocates the use of dams in controlling salinity in rivers. The interrelationships of riparian plants with soil and water salinity are being studied for our area by Stevens (ms and personal communication) who finds salt levels in soils of the newly estab- lished riparian zone along the river (NHWZ) of approximately 1/10 the level of predam fluvial terraces. In addition to interrelationships between riparian and aquatic ecosystems the riparian zone is also closely linked to adjacent upland terrestrial ecosys- tems. Riparian ecosystems throughout the Southwest present a verdant ribbon of vegetation, contrasting sharply with more arid, upland landscapes. These cooler, more mesic environments are heavily utilized by hikers, whitewater recreationists, and wildlife. Riparian ecosystems of arid regions are noted for their high recreational and wildlife values (Johnson and Carothers, 1982~. The riparian zone along the Colorado River in Grand Canyon is no excep- tion. The green of tamarisk, willows, seepwillows, mesquite and other riparian species often ends abruptly at the talus or desert uplands where it is replaced by browner and yellower hues of creosotebush, ocotillo, barrel cactus, numerous species of Opuntia, other cacti and other typical desertscrub species. PREDAM AND POSTDAM VEGETATIONAL COMMUNITIES Prior to construction of Glen Canyon Dam spring floods resulting largely from melting of headwater snow in the Rocky Mountains and Colorado Plateau attained an average high of 86,000 cubic feet per second (cfs) (2,430 cubic meters per second [ems]), but commonly exceeding 100,000 cfs (2,830 ems). At least one historic flood approximated 300,000 cfs (8,490 ems) at Lee's Ferry (Dolan et al. 1974~. These floods, and the heavy silt load that had given the river its Spanish name, Rio Colorado (Colored or Red River), scoured vegetation from the rocky canyon sides and replaced sandy terraces and their vegetation with new, unvegetated sand. Woody riparian plants were provided with adequate water but generally grew above the scour zone at the OHWZ.
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HISI ORIC CHANGES IN VEGETATION... 187 A few years after the completion of Glen Canyon Dam in 1963, a prolif- eration of woody riparian vegetation became apparent. However, this veg- etation was situated lower on the Colorado's banks in the NHWZ, or post- dam fluvial sediments of Dolan et al. (1974~. This new relationship between flow levels and vegetation regimes was first discussed by Dolan et al. (1974) and more recently by Johnson and Carothers (1982~. Prevalent species of the OHWZ, or high predam flood terraces of Dolan et al. (1974) are shown in Figure 8-1. CLASSIFICATION OF DESERTS AND PLANT COMMUTITIES The upland vegetation of the Inner Gorge of the Grand Canyon consists of Great Basin Desertscrub and Mohave Desertscrub. This was discussed early by Bennett (1969), one of the first to examine postdam riparian plants. The river corridor was mapped as Great Basin Desertscrub from Lee's Ferry downstream to approximately 10 miles below the Little Colorado River and as Mohave Desertscrub from there downstream past the Grand Wash Cliffs (Brown and Lowe, 1980~. Detailed maps of this riparian vegetation were prepared in 1977 by the Museum of Northern Arizona staff at a scale of 1:9600, with contour intervals of 200 feet (Table 8-1~. The Grand Canyon region is best known in the natural community literature as the site used by C. Hart Merriam (1890) in formulation of his Life Zone concept. Riparian ecosystems from Lee's Ferry to Lake Mead all occur in the Upper and Lower Sonoran zones. coo 80 UJ 60 11 40 20 ......... ...................... ,. . .. ........ ....... ,.. ..... . .... ..... . ........ ,. ... .. ........ ,..... ........ . .... . ,.. . . ... .... .. . .......... ...................... 1. ~ .,.... ~ TALUS _~ CANYON WALL UDSZ / IL OHWZ O- i IFDZ UDSZ Upland Desertscrub Zono - Unintiuenr7sd by river regime (stabb community) OHWS Old High Wator Zono - Woody vegetation: Prosopis. Acacia Fallugia [U,oper Canyonl. Bacchans sarathroidos Down Canyon] (Stable Community) IFOZ Intormodiato Flood Disturbance Zone (Second overbanh torraco) - Short lived invasive species: Albagi Sakoh, Doscuraini~, Blames runs (Unstable Community) NHWZ Now High Wator Zono - Tamanx, Salix. Pluchoa. Bacchans Jpp., Equisotum (Rapid Proliferation) NHWZ Zone of ~ Human Impact i FLOOD ZONE At_ RIVER POST DAM HIGH WATER RELEASES RIVER FIGURE 8-1 Profile of the vegetative zones of the Colorado River floodplain in the Grand Canyon prior to the construction of Glen Canyon Dam (after Johnson and Carothers, 1982~.
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188 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT TABLE 8-1 Vegetation types and common genera of the 1977 MNA- NPS map. VEGETATION TYPE COMMON GENERA Winter~eeiduous orthophyll forest Deeiduous selerophyll forest Deeiduous selerophyll woodland Evergreen slerophyll scrub Deeiduous orthophyll scrub Mierophyllous deciduous thom scrub Closed deciduous scrub in scattered trees Seasonal orthophyll marsh Season sclerophyll marsh Desert scrub Salt bush desert Microphyllous deciduous desert scrub Seasonal & evergreen desert herb vegetation Acer, Fraxinus, Salix Tamarind sparse Tamaruc Pluchea Baccharis, Brickellia, Salix Acacia, Prosopis Fallugia, Cercis, Celtis Epipactis, Mimulus Phragmites, Typha Agave, Larrea Atriplex Acacia Abronia, Eriogonum SOURCE: Museum of Northem Anzona, 1977. On file, CPSU/UA, NPS, Tucson. Riparian plant communities are simpler than upland communities; that is, fewer riparian plant communities occur along a given elevational or geo- graphic gradient than along the same gradient in adjacent upland communi- ties. This is due, at least in part, to the greatly ameliorating affect of water in the adjacent stream course (Johnson and Haight, in press). However, microhabitats and associated niche availability to animals are generally greater in riparian environments, especially in arid regions where more synusia (vegetational strata) are commonly present. Plants are also more closely spaced in riparian communities than in adjacent upland communities. Ad- equate moisture in riparian communities results in differing community dy- namics from adjacent uplands where underground competition for moisture serves as a critical selection factor (Johnson et al., 1989~. Miller et al. (1982) added "Colorado River Beach," in dealing with am- phibians and reptiles of the Grand Canyon. This was an additional riparian biotic community to Hoffmeister's (1971) "Riparian of Inner Gorge." Since riparian means, most simply, "streamside" (Johnson and Haight, in press), vegetation is not a requirement of a riparian environment and in the Grand Canyon much of the riparian zone consists of bare soil, rocks, or sand. THE COLORADO RIVER AS AN EXCEPTION TO RIPARIAN DESERTIFICATION The concept of desertification was originally applied to desert upland regions, usually in third world countries, notably Africa. Early investiga-
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196 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT Reptiles in general are adapted to xeric environments, and while many of them are nonriparian or facultative riparian species others occur in riparian habitats either as obligate or preferential riparian species (terms after John- son et al., 1984~. Rattlesnakes, for example, are often considered inhabit- ants of xeric, often rocky habitats. However, the best known of the canyon's reptiles, the Grand Canyon (or pink) rattlesnake (Crotalus viridis abysses) frequently occurs in riparian habitats (personal observation; Miller, 1982~. Warren and Schwalbe (1985) listed seven species of lizards for the riparian zone along the river. Stabilization of river flows has probably resulted in an increase in herp populations through greater vegetational biomass, provid- ing in turn more cover and an increase in insect availability, as discussed below for birds. Birds The importance of riparian habitat along this corridor to the avifauna of the region has been documented by a series of recent studies by Brown, Carothers, Johnson, and associates. During the Colorado River studies of the 1970s Carothers and Johnson (1975) published new distributional records for 20 birds in the Grand canyon region, seven of them additions to previ- ously compiled faunal lists. By 1978 the number of species known for the Grand Canyon region had grown to 284 (Brown et al., 1978), adding more than 100 additional species to the 180 in the first Grand Canyon checklist (Grater, 1937~. By 1987 this had increased to 303 avian species (Brown et al., 1987) of which 250 (83%) have been recorded from the Colorado River corridor (Brown et al., 1985~. The avifauna is the best studied biological component of the Colorado River. The high visibility of birds provides relative ease of scientific study, and the interest of amateurs (including river runners) results in a larger number of records than for other animals. Thus, several recent findings about the avian ecology of the river may be used to partially illustrate the role of vegetation in the riparian ecosystem. Since 1964 and publication of The Birds of Arizona (Phillips et al., 1964), nesting populations of at least six riparian species have become established in the Grand Canyon (Johnson and Carothers, 1987) and populations of several other nesting species have greatly increased. The gradual upstream movement of Bell's Vireo with increasing riparian vegetation was documented by Brown et al. (1983~. Popu- lations of nesting birds in tamarisk approaches the highest numbers for temperate North America (Brown and Johnson, 1989~. The most extensive general studies with riparian birds in the Grand Can- yon have been by Brown (1987, 1989~. His findings, broadly generalized, are that in the Grand Canyon obligate riparian nesting birds select tamarisk over native plant species for nesting habitat. Additionally, individual nests are preferentially placed in a tam arisk bushes instead of native plants, e.g.,
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HISTORIC CHANGES IN VEGETATION... 197 coyote willow, arrowweed, or seepwillow, even when other species are equally available. Proliferation of vegetation in the postdam riparian ecosystem is largely responsible for the increase in numbers and species of birds. The reasons for selection of tamarisk over native species are not so clear. However, studies by Stevens (1976,1985) have shown a disproportionately large number of individuals and species of insects supported by tamarisk. Since nearly all breeding avian species along the Colorado River are insectivores these high insect populations presumably provide an abundant avian food source. However, this still does not explain the preferential selection of tamarisk over other species for nesting. Mammals Ruffner and Carothers (1975) published new distribution records for twelve mammalian species, including two new species for the park since the publi- cation of Mammals of Grand Canyon (Hoffmeister, 1971~. Many of these additions were from the river riparian zone. Some of these species, as with herps and birds, have undoubtedly increased in numbers and/or distribution in the region because of proliferation of riparian vegetation with resultant increased cover and food availability. Of the 78 species of mammals listed by Hoffmeister (1971), Ruffner and Carothers (1975), Ruffner et al. (1978), and Suttkus et al. (1978), Jones et al. (1982) and Stevens (1989c); 40 (51%) occur in the Inner Gorge. Most occur in the riparian zone, either as obligate riparian mammals, e.g. beavers, or some lesser category of usage, e.g. desert bighorn sheep that only use it on occasions. An outstanding treatment of mammals of the area is found in Hoffmeister (1986~. The importance of vegetation to animals, as food and shelter, has been emphasized earlier. In some situations, e.g., for herbivorous mammals, animals may, in turn, have a profound influence on vegetation. Burros caused notable damage to riparian vegetation by foraging and trampling (Carothers et al., 1976) and reduced populations of some plant species by selective foraging prior to the removal of these feral mammals from the canyon. One of the major factors in the lack of establishment of Goodding willows and Fremont cottonwoods along the river is their utility to beavers (personal observation). Beavers also eat the smaller, coyote willow and even tamarisk. At a small beach at Tapeats Creek beavers totally stripped coyote willows and tamarisk, leaving nothing but seepwillows by the fall of 1989 (L. Stevens, personal communication). Such foraging could result in significant biogenic succession, producing a '`beaver disclimax" on some beaches, if beaver populations reach adequate levels. One of the more poorly understood vegetational parameters for most woody riparian species is seedling establishment. Ecesis (germination and establishment) of ripar- ian species in the canyon is being studied by Stevens (1989a). Findings
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198 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT from these studies and subsequent monitoring programs are critical to the management of the riverine ecosystem in Grand Canyon. THREATENED AND ENDANGERED (T & E), RARE, AND ENDEMIC SPECIES Riparian The Colorado River corridor, with its linear ribbon of open water and riparian habitat, comprises the largest amount of this type of habitat in the region. Moisture furnished by the river serves as an ameliorating influence, providing a series of more mesic environments in this generally arid land- scape. Still, the Grand Canyon region is notably short on endemic, rare, and T & E species for such a large, diverse area. Two previously undescribed species of flowering plants, FlaYeria mcdougallii and Euphorbia aaron- rossii were mentioned earlier in the paper (Carothers and Aitchison, 1976; Phillips et al., 1987~. This small number of special status species is prob- ably due in large part to the Colorado River's running the entire length of the region, bisecting and connecting the various sections of the Colorado Plateau and providing a relatively uniform environment that serves as a dispersal route for many species of plants and animals. Of as great interest as the presence, or lack thereof, of special emphasis species are species that are lacking. The house mouse (Mus musculus), for example, is missing as are species of old world rats (Rattus) that are found in many of the more "civilized" parts of the United States. Feral burros, on the other hand, built up to such high populations along the Colorado River during the 1970s that they unfavorably impacted both the natural riparian environment and recreational activities (Carothers et al., 1976~. Endangered Species Two endangered birds of the riparian zone are the Bald Eagle (Haliaeetus leucocephalus) and Peregrine Falcon (Falco peregrinus). Bald eagles have become regular winter visitors since the mid-l9SOs (Brown et al., 1989) in the vicinity of Nankoweap Creek and the river where they feed on fish. Numbers have increased to more than 50 individuals (B. T. Brown; L. Stevens, personal communication). Recent studies by S.W. Carothers and B.T. Brown (personal communication) have demonstrated that the Grand Canyon supports the highest concentration of breeding Peregrines in the lower 48 states. A total of 71 different Peregrine breeding areas were documented during partial investigations of the region in 1988 and 1989 (Brown, 1990~. A large percentage of the Peregrine's diet consists of birds caught over the riparian and aquatic zone of the inner canyon.
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HISTORIC CHANGES IN VEGETATION... 199 AQUATIC Aquatic ecosystems are extensively discussed elsewhere in this volume. However, this discussion of the riparian zone would be incomplete without some mention of the interactions with the adjacent river with which it so intimately communicates. For example, riparian birds are largely insectivo- rous, with different species foraging to varying degrees on insects that have aquatic stages in their life cycles. Although insects have been studied in the Grand Canyon (Stevens, 1976, 1985), more definitive information is needed to ascertain the role of vegetation in insect life histories, and the role played by insects as food for riparian as well as aquatic organisms. In addition to sedimentological studies discussed elsewhere in this volume, the contribu- tion of sediment, nutrients, and detritis from the riparian zone to the aquatic food chain needs to be further investigated. The Lee's Ferry area is one of the nation's finest rainbow trout fishery. By the early 1900s, the transplanting of nonnative species into the Colorado River had become common largely because of the advent of trains, the tank car, and other efficient means of transporting fishes. "Game fish" were common enough at Lee's Ferry by the 1950s to attract fishermen (S. Carothers, personal communication). As mentioned earlier, approximately 1/2 of the 100,000 annual visitors to the Lee's Ferry area are fisherman that use the riparian zone extensively for camping, shelter, and bank fishing. The role of these introduced fishes in reducing and/or extirpating native fishes within the Grand Canyon is not clearly known. Johnson and Carothers (1987) have proposed that predation by nonnatives has played a larger role in extirpation of these native species than some have noted. Many of these species are voracious predators on young of larger native species or adults of smaller natives. Additionally, carp had arrived in Arizona prior to 1885 and although they generally do not feed on small fishes they do eat fish eggs (Minckley, 1973~. Thus, during most of this century native fishes of the Colorado system have had to compete with these introduced species (Minckley and Deacon, 1968~. Carothers further proposes that, based on his studies of the fishes of the Colorado River in Grand Canyon (Carothers and Minckley, 1981), several native species were nearing extinction before the construction of Glen Can- yon Dam (see also Minckley, this volume). SUMMARY AND CONCLUSIONS The Colorado River in Grand Canyon, as other riverine systems through- out the arid Southwest is characterized by a close interrelationship between aquatic and riparian components of the ecosystem. Riparian ecosystems provide nearly classroom examples of ecotones, occurring at the Interlace
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200 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT of aquatic and te'Tes~ial ecosystems. As such, they demonstrate the edge effect by showing an increase in species richness and population densities. We have here discussed both riparian organisms and processes as well as aquatic-riparian ecotonal interactions along the Colorado River in Grand Canyon. The importance of this riparian habitat to the nesting avifauna of the lowland Southwest, other wildlife, and humans has been widely documented. The riparian zone in Grand Canyon is used extensively by whitewater recreationists, hikers, and a large number of native animals. Since these riparianlands are used by transients and wintering birds as well as breeding birds, the Colorado River riparian ecosystem constitutes one of the most important avian resources in the North American Southwest. Riparian desertification is a major problem throughout the arid South- west. However, the riverine environment in Grand Canyon has become increasingly mesic, contrasting with increasingly xeric conditions of most riverine ecosystems of the Southwest. Thus, riparian vegetation in the canyon is uniquely valuable since the Colorado River in Grand Canyon is the only major riverine ecosystem in the Southwest where there has been an appreciable increase rather than a decrease in riparian vegetation and asso- ciated animal populations during the 1900s. Continuing scientific investigations in the Grand Canyon need to exam- ine the interrelationships between water releases from Glen Canyon Dam and the riparian and aquatic ecosystems of the downstream Colorado River. Biological components of these ecosystems need to be closely examined in relation to the physicochemical environment. Discussion of some of the research needs in examination of the intricate interrelationships between the riparian and aquatic ecosystems of the Colorado River in Grand Canyon are mentioned in the aquatic section, above. Of equal importance is the deter- mination of socioeconomic factors and the examination of carrying capacity and visitor satisfaction as discussed by numerous investigators (Heberlein and Shelby, 1977~. Although the native riverine ecosystems are no longer extant, strategies for the proper monitoring and management of the existing naturalized riparian ecosystem and exotic aquatic ecosystem must be ad- dressed if the viability and utility of the Colorado River in the Grand Can- yon is to be maintained. ACKNOWLEDGMENTS A number of scientists contributed freely of their time and knowledge in assisting me with information for this paper. My understanding of the Canyon's ecology was enhanced by discussions with Bryan Brown and Steven Carothers for riparian systems and Steve Carothers, Gerald Cole, and Wendell Minckley for aquatic ecosystems. I attained a better depth of knowledge of
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HISTORIC CHANGES IN VEGETATION... 201 the vegetation and flora during early discussions with Paul Martin and Ray Turner and, more recently, with Peter Bennett and Laxity Stevens. Lois Haight assisted with numerous technical suggestions, and Larry Stevens and Dave Wegner critically reviewed the manuscript in its entirety. REFERENCES Adams, V. D., and V. A. Lamarra (eds.). 1983. Aquatic resources management of the Colo- rado River ecosystem. Ann Arbor Science, Ann Arbor, Mich. 697 p. Anderson, B. W., A. Higgins, and R. D. Ohmart. 1977. Avian use of saltcedar communities in the Lower Colorado River Valley. Pages 128-136 in R. R. Johnson and D. A. Jones (tech. coords.). Lrnponance, preservation and management of npanan habitat. USDA For. Ser.Gen. Tech. Rpt. RM-43. Ft. Collins, Cola. 217 p. Babbitt, B. 1978. Grand Canyon: an anthology. Northland Press, Flagstaff, Ariz. 258 p. Bennett, P. S. 1969. Some notes on the vegetation of the Inner Gorge, Grand Canyon. Pages 25-30 in Geology and natural history of the fifth field conference: Powell Centennial River Expedition. Four Corners Geol. Sac. Boldt, C. E., D. W. Uresk, and K. E. Severson. 1979. Riparian woodlands in jeopardy on Nonhem High Plains. Pages 184-189 in R. R. Johnson and J. F. McCormick (Tech. coords.). Strategies for protection and management of floodplain wetlands and other ripar- ian ecosystems. USDA For. Serv. Gen. Tech. Rpt. WO-12. Washington, D.C. 410 p. Borden, F. Y. 1975. Grand Canyon National Park campsite inventory. Prog. Rpt. No. 3, Natl. Park Serv., Washington, D.C. Bnan, N. J. 1982. A preliminary study of the riparian coyote willow communities along the Colorado River in Grand Canyon National Park, Arizona. N. Arid Univ., Flagstaff. Unpubl. Thesis. 84 p. Brown, B. T. 1987. Ecology of riparian breeding birds along the Colorado River in Grand Canyon, Arizona. PhD. Diss. Univ. Ariz., Tucson. 66 p. Brown, B. T. 1989. Breeding ecology of riparian birds along the Colorado River in Grand Canyon, Arizona. Coop. Natl. Park Resources Stud. Unit Tech. Rpt. No. 25, Natl. Park Serv., Univ. Ariz., Tucson. 42 p. Brown, B. T. 1990. Draft Grand Canyon Peregrine Falcon monitoring protocol and handbook. Grand Canyon Natl. Pk., Anz. 41 p. + maps. Brown, B. T., and R. R. Johnson. 1985. Glen Canyon Dam, fluctuating water levels and riparian breeding birds: the need for management compromise on the Colorado River in Grand Canyon. Pages 76-80 in R. R. Johnson et. al. (tech. coords.) Riparian ecosystems and their management: reconciling conflicting uses. USDA For. Serv. Gen. Tech. Rpt. RM-120. Rocky Mtn. For. and Range Exp. Sta., USDA For. Serv., Ft. Collins, Colo. 523 p. Brown, B. T., and R. R. Johnson. 1989. Ecology and management of riparian breeding birds in tamarisk habitats along the Colorado River in Grand Canyon National Park, Arizona. Pages 68-73 in M. R. Kunzmann, R. R. Johnson, and P. S. Bennett (tech. coord.) Tamarisk control in southwestern United States. Coop. Natl. Park Resources Stud. Unit Spec. Rpt. No. 9, Natl. Park Serv., Univ. Ariz., Tucson. 141 p. Brown, B. T., S. W. Carothers, and R. R.Johnson. 1983. Breeding range expansion of Bell's Vireo in Grand Canyon Arizona. Condor 85:499-500. Brown, B. T., S. W. Carothers, and R. R.Johnson. 1987. Grand Canyon Birds. Univ. Ariz. Press, Tucson. 302 p. Brown, B. T., P. S. Bennett, S. W. Carothers, L. T. [Iaight, R. R.Johnson, and M. M. Riffey. 1978. Birds of the Grand Canyon region: an annotated checklist. Grand Can. Nat. Hist. Assn. Monograph No. 1. 64 p.
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204 COLORADO RIVER ECOLOGY AND Do MANAGEMENT Lowe, C. H. 1964. Arizona's natural environment. Univ. Ariz. Press, Tucson. 136 p. McDougall, W. B. 1947. Plants of Grand Canyon National Park. Revised checklist with supplement. G. C. Nat. Hist. Assn. Bull. No. 10. Merriam, C. H; 1890. Results of a biological survey of the San Francisco Mountain region and the desert of the Little Colorado, Arizona. N. Amer. Fauna 3:1-136. Miller, D. M., R. A. Young, T. W. Gatlin, and J. A. Richardson. 1982. Amphibians and reptiles of the Grand Canyon. G. C. Nat. Hist. Assn. Monograph No. 4. 144 p. Minckley, W. L. 1973. Fishes of Arizona. Ariz. Game and Fish Opt., Phoenix, Ariz. 293 p. Minckley, W. L., and J. E. Deacon. 1968. Southwestem fishes and the enigma of "endangered species." Science 159:1424-1432. Odum, E. P. 1979. Opening Address: ecological importance of the riparian zone. Pages 2-4 in R. R. Johnson and J. F. McCormick (tech. coords.). Strategies for protection and management of floodplain wetlands and other riparian ecosystems. USDA For. Serv. Gen. Tech. Rpt. WO-12. Washington, D.C. 410 p. Paulson, L. J. 1983. Use of hydroelectric dams to control evaporation and salinity in the Colorado River system. Pages 439-456 in V. D. Adams and V. A. Lamarra (eds.). Aquatic resources management of the Colorado River ecosystem. Ann Arbor Science, Ann Arbor, Mich. 697 p. Paulson, L. J., and J. R. Baker. 1983. The effects of impoundments on salinity in the Colorado River. Pages 457-474 in V. D. Adams and V. A. Lamarra (ads.). Aquatic resources management of the Colorado River ecosystem. Ann Arbor Science, Ann Arbor, Mich. 697 p. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ. Ariz. Press, Tucson. 212 p. Phillips, B. G., A. M. Phillips, and M. A. S. Bemzott. 1987. Annotated checklist of vascular plants of Grand Canyon National Park: 1987. Grand Can. Nat. Hist. Assn. Monograph No. 7. 79p. Pillsbury, A. F. 1981. The salinity of rivers. Scientific Amer. 245(1):54-65. Pucherelli, M. J. 1988. Evaluation of riparian vegetation trends in the Grand Canyon using multitemporal remote sensing techniques. Pages 217-228 in USDI, Glen Canyon Environ- mental Studies: executive summaries of technical reports. Burl Recl., Salt Lake City, Utah. 41 1 p. Phillips, B. G., and A. M. Phillips. 1974. Spring wildflowers of the Inner Gorge, Grand Canyon, Arizona. Plateau 46: 149-1 57. Rea, A. M. 1983. Once a river: bird life and habitat changes on the middle Gila. Univ. Ariz. Press, Tucson. 285 p. Ruffner, G. A., and S. W. Carothers. 1975. Recent notes on the distribution of some mammals of the Grand Canyon region. Plateau 47:154-160. Ruffner, G. A., N. J. Czaplewski, and S. W. Carothers. 1978. Distribution and natural history of some mammals from the Inner Gorge of the Grand Canyon, Arizona. J. Ariz.-Nev. Acad. Sci. 13:85-91. Sheridan, P. 1981. Desertification of the United States. C.E.Q., U.S. Gov. Print. Off., Washington, D.C. 142 p. Spamer, E. E., G. H. Billingsley, W. J. Breed, R. C. Euler, and G. Keroher. 1981. Bibliogra- phy of the Grand Canyon and the lower Colorado River:1940-1980. G. C. Nat. Hist. Assn. Monograph No. 2. 119 p. Stephens, H. G., and E. M. Shoemaker. 1987. In the footsteps of John Wesley Powell: an album of comparative photographs of the Green and Colorado Rivers, 1871-72 and 1968. Johnson Books, Boulder, Colo. and The Powell Soc., Denver, Colo. 286 p. Stevens, L. E. 1976. An insect inventory of Grand Canyon. Pages 123-127 in S. W. Carothers and S. W. Aitchison (eds.). An ecological survey of the riparian zone of the Colorado
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206 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT APPENDIX A SCIENTIFIC NAMES OF PLANTS IN TEXT Apache Plume - Fallugia paradoxa Arrowweed - Tessaria (Pluchea) sericea Bennuda Grass - Cynodon dactylon Brome, Foxtail or Red - Bromus rubens Bulrush - Scirpus americanus and S. validus Cactus, Barrel - Echinocactus polycephalus and Ferocactus acanthodes Camelthorn - Alhagi camelorum Catclaw, Catclaw Acacia - Acacia greggii Cattail - Typha domingensis and T. Iatifolia Clover, Sweet - Melilotus spp. Cottonwood, Fremont - Populus fremontii Creosotebush - Larrea divaricata (tridentata) Desertbroom - Baccharis sarothroides Dicoria - Dicoria bra ndegei Horsetail - Equisetum spp. Mesquite, Westem Honey - Prosopis glandulosa Ocotillo - Fouquieria splendens Russian-olive - Elaeagnus angustifolia Seepweed - Suaeda torreyana Seepwillow - Baccharis salicifolia (glutinosa) Seepwillow, Emoryils - B. emoryi Tamarisk, Salicedar - Tamarix ramosissima Thistle, Russian - Salsola iberica Thistle, Sow - Sonchus asper and S. oleraccus Waterweed - Baccharis sergiloides Willow, Coyote - Sal~c exigua Willow, Goodding - S. gooddingii
Representative terms from entire chapter: