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Does Grazing Favor Woody Species Expansion?
Brown, J.R. and Archer, S.  1999.  Shrub invasion of grassland: Recruitment is continuous and not regulated by herbaceous biomass or density.  Ecology 80: 2385-2396.

What was done
In an effort to understand the dynamics of the most recent 200-year-long range expansion of honey mesquite in the southwestern United States, the authors conducted a field experiment to determine this woody plant's emergence and growth response to different amounts of water and grass density.  More specifically, they set out to determine if the recent expansion of honey mesquite could be attributed to livestock grazing.

What was learned
The results of the experiment indicated that the density and level of defoliation (due to grazing) of herbaceous vegetation, or grasses, affected neither the emergence nor the survival of honey mesquite seedlings within a watering regime, suggesting to the authors that the emergence and establishment of honey mesquite occurs "regardless of resource availability and livestock grazing pressure on grasses."  To answer the question as to whether or not the recent expansion of honey mesquite in the region of southern Texas might have resulted from anomalously high precipitation years, the authors analyzed the size and age-class distribution of honey mesquite in this area and found "no indication of episodic establishment or mortality."

What it means
Many authors have suggested that heavy grazing favors the emergence and establishment of woody species.  However, as noted by the authors of this study, "woody plant encroachment into grasslands can be high, regardless of grazing pressure or herbaceous composition and biomass."

What, then, is the cause of the 200-year-long range expansion of honey mesquite reported here? With each new study of the subject, the answer continues to look more and more like the rising CO2 content of earth's atmosphere.

Reviewed 1 April 2000