PARENT SESSION
Contributed Oral Session 33: Grassland and Shrubland Communities
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 522 A, Level 5, Palais des congrès de Montréal
Spatial rearrangement of woody plant (Prosopis velutina) cover over 60 years in a desert grassland.
Browning, Dawn*,1, Archer, Steven1, Asner, Gregory2, Hoggatt, Elsbeth1, McClaran, Mitchell1, Wessman, Carol3, 1 University of Arizona, Tucson, AZ, USA2 Carnegie Institue of Washington-Stanford University, Stanford, CA, USA3 University of Colorado, Boulder, CO, USA
ABSTRACT- To better understand impacts of climate and land cover change on the global carbon cycle, more reliable regional estimates of terrestrial carbon (C) pools are needed. To assist in that effort, we quantified changes in the abundance and distribution of velvet mesquite (Prosopis velutina (Woot.), whose abundance has increased during the 1900s in the southwestern U.S. Abundance and spatial pattern of velvet mesquite patches were quantified on uplands of nine 1.0 to 7.2 ha landscapes in the semi-desert grasslands of southeastern Arizona using a time series (1936, 1966, and 1996) of aerial photographs. Shrub patches in 1936 and 1966 images were delineated with a supervised classification using the panchromatic band and a texture layer; 1996 patches were classified using color-infrared image bands. We tracked total woody plant (WP) cover, canopy area of 72 individual shrub patches and the fate of 127-50 m2 patches unoccupied by shrubs in 1936. WP cover in 1936 landscapes averaged (+ SE) 37 + 4.0% on coarse sandy loam soils (CSL) and 22 + 5.8% on sandy clay loam soils (SCL). Cover change from 1936 to 1966 was variable; cover on CSL soils decreased slightly with no change in mean patch size while cover on SCL soils increased 87% (from 22% to 37 + 5.4%) with an associated increase in mean patch size. On SCL soils, more individual patches (40% vs. 27% on CSL) increased canopy size and more bare patches (55% vs. 25% on CSL) were occupied by shrubs in 1966. The sharp increase in cover on SCL soils 1936 to 1966 was due to both growth of existing canopies and recruitment of new patches. Both soils experienced decreases in WP cover from 1966 to 1996 to 27 + 2.1% on CSL soils and 30 + 2.4% on SCL soils; declines in WP cover were accompanied by an increase in small patches. From 1966 to 1996, trends in shrub patch canopies were similar on both soils (30% grew, 45% lost canopy area, and 25% did not change). Shrub colonization rates increased on both soils (42% and 62% on CSL and SCL soils, respectively), indicating that shrub establishment overrides fragmentation in the appearance of numerous small patches from 1966 to 1996. Shifts in shrub size distributions under small changes in overall cover represent spatial rearrangement of woody plant cover which can profoundly impact the potential for C storage and uptake in arid rangelands.
Key words: aerial photography, Santa Rita Experimental Range, shrub encroachment, woody plant dynamics