Riparian Research and Management

Chapter 1. Understanding Gains and Losses of Riparian Habitat: Interpreting Change, Its Causes and Consequences Steven W. Carothers and Dorothy A. House Introduction One of the most wide-ranging scientific treatments of riparian vegetation of the American Southwest (exclusive of New Mexico and southwestern Colorado) can be found in the 2007 book, The Ribbon of Green by Robert Webb, Stanley Leake, and Raymond Turner (Webb et al. hereafter). In The Ribbon of Green, the authors use sets of matched photographs to evaluate and interpret historical changes in riparian vegetation along southwestern streams that have taken place since the late 19th century. As stated in the book’s preface, the authors “concentrate on the question of long-term changes, addressing whether we have endured a net loss of woody riparian vegetation” over that time (Webb et al. 2007:x). More narrowly, the authors challenge the accuracy of a widely-repeated claim that 90 percent (or a similarly large percentage) of riparian vegetation in the southwestern United States has been lost due to human activities. In The Ribbon of Green, Webb et al. either explicitly or implicitly conclude that: (1) the Southwest has not endured a net loss of woody riparian vegetation over time; (2) claims of such loss have been exaggerated and misguided; (3) the 90 percent loss figure is myth; (4) riparian vegetation has changed to some degree, but change is to be expected in a dynamic ecological system and is neither bad nor good; and (5) the role of human actions in bringing about change in riparian systems has been overblown relative to the influences of natural processes. In this chapter, we examine the validity of those conclusions and the methodology that led to them, and offer further information and interpretation. For their study, Webb et al. compared 2,724 sets of repeat photography with native and nonnative woody riparian vegetation visible in at least one of the matched photos. Of the 2,724 sets examined, 215 sets totaling 480 individual photographs are published in the book. Lengths of time spanned by the published sets vary widely, from a few recent decades (e.g., 1988 compared to 2000) to more than a century (e.g., 1863 compared to 2000). The book is more than just a comparison of photographs, however. Webb et al. also summarized the biology and distribution of riparian ecosystems in the study area, drawing upon numerous publications and research projects completed prior to 2007. They look at human influences, climatic fluctuations, hydrologic changes, and the life history strategies of dominant woody vegetation species in the selected riparian ecosystems. The Ribbon of Green is a remarkable reference book for many things riparian in the geographic areas they address. That said, we believe Webb et al. are doing a disservice to the very resource they celebrate by painting too sanguine a picture of the history and status of southwestern riparian ecosystems. The scientists who initially raised concerns about the historical trajectory, condition, and future of riparian habitats were leaders who drew attention to conservation needs for these ecosystems. In their preface, Webb et al. dispute the concerns of those scientists and generally understate the level of threat to riparian vegetation that existed in the mid-20th century. At its core, the book is revisionist, an attempt USDA Forest Service RMRS-GTR-411. 2020. 1 to correct a record that, in the authors’ view, had been distorted. But in their enthusiasm to counter often-exaggerated estimates of historical riparian losses and their interest in promoting the effects of climate and other natural processes on riparian systems as opposed to human impacts, the authors risk tipping the scales too far in the opposite direction. There is danger in understating human impacts on southwestern riparian habitats. The risk is a desensitization of policy makers, resource managers, and the public to existing and future threats. Another risk is a weakening of the political will to avoid, minimize, and mitigate such threats. Vigilance is particularly important now because southwestern riparian habitats are undergoing highly visible changes resulting from release of the tamarisk leaf beetle (Diorhabda spp.), a biocontrol effort that was in its infancy when Webb et al. wrote The Ribbon of Green. The long-term ecological consequences of the beetle release are unknown and much debated, but in the short term it is clear that the amount of woody riparian vegetation available to wildlife is shrinking as nonnative, invasive tamarisk (Tamarix spp.) defoliates and dies across the western United States (see Carothers et al., this volume; McLeod volume 1). Have We Endured a Net Loss of Woody Riparian Vegetation? Webb et al. pose the question of riparian losses in the book’s preface, and then they answer it in the book’s final two chapters with the following statements: “In general, riparian vegetation has had either an [a small] increase (49 percent [of the photographs]) or a large increase (24 percent [of the photographs]) in comparisons involving all years ....” (Webb et al. 2007:388) “The time series of change ... suggests that, at least in the specific reaches for which we have documentation, riparian vegetation began to increase after about 1940. Increases in density of plants appear to have accelerated after the 1970s, followed by increases in the size of plants. Many deviations from this general pattern have been noted in the chapters on specific reaches.” (Webb et al. 2007:388) “Our interpretations of the available imagery ... suggest more gains than losses within our photographic views.” (Webb et al. 2007:403) “Although we observed local decreases in riparian vegetation in many places, reach-scale decreases in the 20th century were present only in a few locations.” (Webb et al. 2007:405) In these statements, the authors couch their findings in terms of the specific locations photographed, or, at most, the river reach in which the photographs were taken. These geographic constraints are appropriate considering the spatial limitations of repeat photography, which we discuss at length below. However, a stated purpose of the study is to address allegations of riparian habitat loss across a very large area, the southwestern United States. Given this regional premise, readers can be forgiven if they understand the scope of authors’ ultimate conclusions to be region-wide as well. The book’s unmistakable message is we have not endured a net loss of woody riparian vegetation in the American Southwest despite more than a century of intensive agricultural, urban, and water resource development. In the book’s preface and accompanying notes (page 413), Webb et al. place the responsibility for originating the “perception of diminishing or degraded riparian areas” 2 USDA Forest Service RMRS-GTR-411. 2020. with academic and agency scientists who, for the most part, began publishing their concerns in the late 1970s. Specifically, Webb et al. report the claim of a 90 percent loss as having been traced back to a single paper (i.e., Ohmart et al. 1977). Webb et al. imply that the concerned scientists—many of whom pioneered the discipline of riparian ecology— exaggerated the extent of riparian losses right from the beginning. How does one account for the conflicting views of Webb et al. and those they disagree with? Several explanations are possible. First, Webb et al. and the early concerned scientists came to the issue from different professional perspectives, and those perspectives carried with them different priorities and biases that likely colored their conclusions. Second, Webb et al. and the early concerned scientists defined the issue somewhat differently. The scientists who first raised the alarm were concerned about the loss of native riparian vegetation, not all riparian vegetation, and they were concerned about the reduction of wildlife habitat quality, particularly that posed by the proliferation of nonnative plant species. Webb et al., although they differentiate between native and nonnative species, concentrate on changes in the total amount of riparian vegetation. They pay little attention to habitat quality. Third, Webb et al. and the concerned scientists arrived at their conclusions via different pathways, and their divergent approaches influenced their conclusions. Fourth, Webb et al. and the concerned scientists operated in different timeframes. The circumstances affecting riparian areas in the early 2000s, when The Ribbon of Green was written, differed in many ways from those in the 1970s, when the alarm was first raised. Moreover, Webb et al.’s temporal scope was necessarily limited to the periods when photographs were taken, while the concerned scientists were troubled by changes that have taken place since first European settlement. We discuss these points in the following sections and revisit the question of where that 90 percent loss figure originated. Different Perspectives, Value Judgements, and the Issue of Habitat Quality Robert Webb and Stanley Leake are hydrologists/geomorphologists, and their interest and expertise in physical processes is evident in the attention they pay to climatic variation, streamflow, flood records, and stream channel morphology. Raymond Turner, a plant biologist, has long favored climate shifts as the primary driver of vegetation changes in the Southwest (see Bahre 1991; Hastings and Turner 1965). In The Ribbon of Green, a focus on physical processes as opposed to human influences (as well as a conviction that relatively little riparian vegetation has been lost) is revealed in this statement: “That some patches of vegetation, growing as they do in a dynamic geomorphic milieu, are gone is to be expected” (Webb et al. 2007:ix). This is not to say that Webb et al. do not acknowledge human impacts—they do—but they place decidedly more emphasis on climatic fluctuations, hydrology, and geomorphological changes to explain the differences in riparian vegetation between compared photographs.

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