Tipping Points

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A grassland at the tipping point near the Jornada LTER site in the mid 1930s
USDA ARS, Jornada Experimental Range archive

Tipping points resulting in state changes (or regime shifts) have been documented in many ecosystem types around the world, and are particularly important in dryland ecosystems, including those of the southwestern US. Dryland state changes include the conversion of agriculturally productive grasslands to unproductive shrublands, conversion to dominance by invasive species, or the loss of perennial vegetation altogether. In addition to the US, dryland state changes are common in throughout Asia, Africa, and Latin America, contributing to increased poverty and global political instability.

A long history of research (going back to 1915) at the Jornada LTER-USDA site (JRN) in southern New Mexico provides unprecedented perspectives on the mechanisms of tipping points. The simplistic view of dryland tipping points had been that a combination of overgrazing and drought cause them. Jornada reseach has shown that tipping points are the result of a complex interplay of management decisions and feedbacks among vegetation, soils, and climate occurring over a range of time periods and several spatial scales. In addition to providing insights to understand and mitigate desertification, this work has led to a more general perspective on the importance of cross-scale interactions in complex systems. Cross-scale interactions result when fine-scale biotic and abiotic processes, ecological patterns, and broad-scale physical drivers interact to generate nonlinear responses in state variables (such as vegetation cover or atmospheric carbon concentrations). These interactions produce surprises that challenge our ability to predict system behavior at one scale based on information at finer or broader scales.

Long-term observations at JRN coupled with modeling efforts have revealed that drivers at broad scales (climate and livestock management) often interact with properties of the soil and vegetation at multiple scales to govern shifts in plant dominance. For example, state changes can occur as a consequence of primarily local or landscape effects. Studies at the JRN using manipulations and natural variability in rainfall have shown the importance of water, nitrogen, and soil properties in grass-woody plant interactions. Additionally, the spatial distribution and density of plants and associated bare soil interspaces (gaps) are critical indicators of tipping points. JRN research has been pivotal in understanding how spatial context relative to the redistribution of limiting resources and disturbances emerging from neighboring locations can explain state change. Recent studies have also shown that the importance of processes controlling state change vary over time.

JRN researchers have been at the forefront in incorporating these insights into new tools and techniques to improve management of these systems, and to provide early warning indicators of impending tipping points. JRN findings on tipping points share many commonalities with arid and semiarid regions across the globe where we work, including Argentina, Mongolia, Mexico, eastern Europe, China, Australia, and central Africa.

Long-term data and vegetation maps from the JRN LTER show thresholds (shown by change in slope of line) in the increase in areal extent of shrubs and decrease in perennial grasses through time. Thresholds occur when the dominant process driving shrub expansion changes, from biotic (recruitment in early 1900s and clonal expansion between 1935-1985) to physical (wind erosion and deposition) leading to dune formation beginning in 1985.
Peters et al. (2004)
For further reading: 
Bestelmeyer BT. 2006. Threshold concepts and their use in rangeland management and restoration: the good, the bad, and the insidious. Restoration Ecology 14: 325-329
Bestelmeyer BT, Tugel AJ, Peacock GL, Robinett DG, Shaver PL, Brown JR, Herrick JE, Sanchez H, Havstad KM. 2009. State-and-transition models for heterogeneous landscapes: a strategy for development and application. Rangeland Ecology and Management 62: 1-15.
Peters DPC, Pielke RA Sr, Bestelmeyer BT, Allen CD, Munson-McGee S, Havstad KM. 2004. Cross scale interactions, nonlinearities, and forecasting catastrophic events. Proceedings National Academy Sciences 101:15130-15135.
For further information: 
Dr. Debra P.C. Peters
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