Rapid Changes in Landscape Dynamics: Separating Natural from Human-Induced Causes in Order to Develop Mitigation Strategies for Landscape Degradation
by
Stephen G. Wells
Desert Research Institute, Nevada System of Higher Education, 2215 Raggio Parkway, Reno, NV 89512 USA
Coauthors: Lynn F. Fenstermaker, Desert Research Institute, David A. Mouat, Desert Research Institute, Grant A. Meyer, University of New Mexico

Complex sets of geologic, hydrologic, and biologic conditions and interactions give rise to earth system services¹ that act collectively to sustain productive terrestrial landscapes and to support human livelihood. Increasing human interaction of these earth system services leads to rapid changes in the magnitude, frequency, and extent of landscape processes that may threaten environmental sustainability through degradation. The Holocene geologic record of landscapes provides critical information on rapid environmental changes that existed prior to significant human-induced degradation. A thorough assessment of the geologic history of landscape changes provides critical indicators on the range of these natural landscape changes. Such assessments yield information that allows degradation caused by human intervention to be separated from that resulting from natural causes. Distinguishing natural from human-induced changes is critical in the development of strategies for mitigating degraded landscapes. In this paper, we provide examples from western North America of the recent (i.e., late Holocene) history of landscape degradation that predates human activity and that results from arroyo cutting (entrenching of fluvial channels), wildfire activity, and climate change, with implications for environmental assessments and strategies for mitigating landscape degradation.

The geologic record of selected alluvial valley floors in the southwestern USA reveals alternations between periods of channel erosion (arroyo cutting), widespread aggradation, and stabilization with soil formation. In many valleys, these periods of erosion are regionally synchronous (ca. 400 yrs B.P., 1000 yrs B.P., 2800 B.P., and 4200 B.P.), indicating natural climatic variability as the driving force. Geomorphic comparison of Holocene-age arroyos shows that arroyos existing in the present landscape are similar in size, and thus regional conditions causing erosion have not changed significantly within these study areas. In these areas, the energy driving arroyo development (cutting and extension) originates from runoff within bedrock uplands and not from base-level changes or runoff generated on the valley floors where human activity typically has the most significant impacts. Such studies establish indicators that can be used to differentiate between human-driven arroyo incision and periods of arroyo cutting driven by Holocene climatic variability. The late Holocene geologic records of Yellowstone National Park in the western USA show that periods of intense fires and concomitant erosion were episodic and were related to millennial-scale climatic variations. Fire-related events cluster within the intervals of 3300-2900, 2600-2400, 2200-1800, and 1400-800 yr B.P., with a major pulse of fire-related debris-flow activity between 950 and 800 yr B.P.\ (i.e., Medieval Warm Period). Because land managers have been challenged with wildfire management for generations, such histories have significant implications for strategies of fire management and sustainable environments in the western North America. This geologic record demonstrates that periods of extensive wildfires have occurred episodically for thousands of years and supports the conclusion of W.A. Patterson (2000) that "we are going to have to accept what history has shown us-that there will always be some fires burning on the landscapes."

Although the geologic studies demonstrate that natural periods of widespread degradation have dominated the landscapes, human alteration of the terrestrial landscapes and their functionality threaten the sustainability of the earth systems services in rural regions of the western USA. Any assessment of arid landscape sustainability should involve long-term experiments to predict future differences in magnitude, frequency, and extent between the natural and human-induced consequences. An example of this predictive approach involves research underway at the Nevada Desert FACE Facility (a large scale, elevated CO2 experiment in the Mojave Desert). This research suggests profound ecosystem changes associated with stochastically high moisture periods will be exacerbated under elevated CO2 regimes expected during the next half century. Over time, these impacts will influence the vegetation community composition, structure, and function. These changes may have significant impacts on the socio-economics of arid rural areas, leading to desertification through decreased productivity of rangelands by invasive species and significantly increased fire cycles in landscapes previously dominated by fewer wildfires.

It is crucial in developing strategies that mitigate the often-negative impacts of rapid environmental change to distinguish among changes (a) that occur naturally, (b) that are caused by humans, and (c) where natural factors exacerbate those that are human. If the erosional, fire, and climate changes are predominately natural in specific regions, we need to consider how to live with these variations. If, however, rapid changes are human or largely human, then the choice is clear: understand how the system operates and work to change the human factor. Desertification, which results in reduced biodiversity and productivity within human systems, threatens the livelihoods of one billion people. The examples that we give-erosion, fire, and climate change-are all factors that impact human use systems. Policy decisions and individual human choices must recognize how the landscape responds to both the natural and the human system. It is because landscape processes can change so rapidly that we must understand the causes of change.

Footnotes:

¹Earth system services is defined by the National Academy of Sciences Board of Earth Sciences and Resources (2004, unpublished) as biogeochemical and hydrogeologic states and flows that give rise to and sustain the biospherethe thin layer of the earth system in which life can exist.

Date received: July 22, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-57.