Squinting at Blind Spots
Squinting at Blind Spots
Southwestern Rangelands and the Consolidation of Successional Theory
The largest and oldest federal range research stations, the Santa Rita and Jornada experimental ranges, were founded in 1903 and 1912 in Arizona and New Mexico, respectively. Scientists at both locations struggled to make sense of the dynamics of vegetation and grazing in the face of rainfall patterns that were unpredictable and highly variable; contrary to Clements’s model, the weather, not grazing, seemed to be the dominant driver of vegetation change. By the 1930s, a further problem had emerged: even without any livestock, former grasslands were turning into shrublands dominated by mesquite and other woody plant species. Southwestern range scientists pointed out that succession was not working as predicted, but they lacked an alternative theory to replace it. But the head of Forest Service research, Earle Clapp, declared in the late 1920s that succession would serve as the basis for all range research. Clementsian theory conformed to, and ratified, the key concern of administrators and ranchers alike: stocking rates, which scientists would henceforth treat as the primary ecological variable in all rangelands. In most Western rangelands the underlying theory was flawed, however, as evidenced by its failures in drier, more variable settings such as the Southwest and Great Basin.
Keywords: Earle Clapp, equilibrium, Jornada Experimental Range, mesquite, Santa Rita Experimental Range, shrub invasion
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