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Title: Spatial patterns of grassland-shrubland state transitions: a 74-year record on grazed and protected areas

item Browning, Dawn
item FRANKLIN, JANET - Arizona State University
item ARCHER, STEVE - University Of Arizona
item GILLAN, JEFFREY - New Mexico State University
item GUERTIN, D. PHILLIP - University Of Arizona

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2013
Publication Date: 9/1/2014
Citation: Browning, D.M., Franklin, J., Archer, S.R., Gillan, J., Guertin, D. 2014. Spatial patterns of grassland-shrubland state transitions: a 74-year record on grazed and protected areas. Ecological Applications. 24:1421-1433.

Interpretive Summary: Livestock grazing is a widespread practice in arid and semi-arid savannas and grasslands worldwide; however long-term perspectives capturing effects of grazing on woody plant dynamics and stand structure are rare. We made use of repeat census data for velvet mesquite in a historically grazed grassland in southern Arizona to examine the spatial ramifications of livestock removal over 74 years. The use of multiple spatial pattern metrics to quantify patterns in the arrangement of mesquite plants and associated patterns in autocorrelation on areas continuously grazed and those protected from livestock since 1932 revealed that livestock removal coincided with increased heterogeneity and patchiness of mesquite plants. The mechanism generating clustered patterns where livestock were removed was that of shrub establishment in previously unoccupied areas (i.e., infilling). These results highlight the need to integrate spatial processes in efforts to monitor and assess rangeland resources. It is only with an improved mechanistic understanding of the effects of livestock removal can land managers envision outcomes of grazing deferment on ecological potential. In this manner, our findings are of benefit and interest to state and federal land managers, private land owners, and ecologists.

Technical Abstract: Tree and shrub abundance has increased in many grasslands, causing changes in ecosystem carbon and nitrogen pools that are related to patterns of woody plant distribution. However, with regard to spatial patterns, little is known about (i) how they develop; (ii) how they are influenced by grazing; or (iii) the extent to which intraspecific interactions dictate them. We addressed these questions by quantifying changes in the spatial distribution of Prosopis velutina (mesquite) shrubs over 74 years on grazed and protected grasslands. Livestock are effective agents of mesquite dispersal and mesquite has lateral roots extending well beyond its canopy. We therefore hypothesized that mesquite distributions would be (a) random on grazed areas and clustered on protected areas; and (b) that clustered or random distributions at early stages of encroachment would give way to regular distributions as stands matured and density-dependent interactions intensified. Assessments in 1932, 1948 and 2006 supported the first hypothesis, but we found no support for the second. In fact, clustering intensified with time on the protected area and the pattern remained random on the grazed site. Although shrub density increased on both areas between 1932 and 2006, we saw no progression toward a regular distribution indicative of density-dependent interactions. We propose that processes related to seed dispersal, grass-shrub seedling interactions, and hydrological constraints on shrub size interact to determine vegetation structure in grassland-to-shrubland state changes with implications for ecosystem function and management.