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United States Department of Agriculture

Agricultural Research Service

Title: High-Resolution Images Reveal Rate and Pattern of Shrub Encroachment over Six Decades in New Mexico, Usa

Authors
item Goslee, S - NEW MEXICO STATE UNIV
item Havstad, Kris
item Peters, Debra
item Rango, Albert
item Schlesinger, W - DUKE UNIVERSITY

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 2002
Publication Date: August 1, 2003
Citation: GOSLEE, S.C., HAVSTAD, K.M., PETERS, D.C., RANGO, A., SCHLESINGER, W. HIGH-RESOLUTION IMAGES REVEAL RATE AND PATTERN OF SHRUB ENCROACHMENT OVER SIX DECADES IN NEW MEXICO, USA. JOURNAL OF ARID ENVIRONMENTS. 2003. V. 54(4). P. 755-767.

Interpretive Summary: Encroachment of shrub honey mesquite into semiarid grasslands is a serious concern in the southwestern U.S.; yet, little is known about the long-term dynamics of the invasion process. We used 10 high-resolution aerial and satellite images taken from 1936-1996 to track population dynamics and spatial pattern of all mesquite greater than 2m in diameter on a 75-ha area in southern New Mexico. Shrub cover and numbers increased from 1936-1970s then stabilized at 43% cover and 83 patches/ha. Individual patches were extremely persistent; nearly all area occupied by shrub patches in 1936 was still occupied in 1996 and annual losses in area were very small. Recruitment was highly variable over time. Patch shape complexity (mean shape index) increased from 1936-1983 as adjacent shrubs merged and then declined as those clusters filled in and became rounder. The spatial pattern of shrubs showed a distinct trend over time: strongly clustered in 1936 at lag distances up to 250m, then random arrangement at all scales; by 1983, the pattern was regular at lag distances greater than 100m. Changes in abundance or pattern were not related to precipitation. The use of remote sensing imagery allowed us to examine one site over time and revealed patterns in population dynamics and spatial pattern that would not otherwise have been visible. Comparison of field estimates collected in 2001 with 1996 image data suggest the canopy cover estimates were accurate, but shrub densities were seriously underestimated in the satellite photographs which do not show shrubs smaller than 2m diameter. As long as limitations of the imagery are understood, these methods can be applied over a larger and more heterogeneous area to examine environmental correlates of invasion success.

Technical Abstract: Encroachment of the shrub Prosopis glandulosa (honey mesquite) into semiarid grasslands is a serious concern in the southwestern United States, yet little is known about the long-term dynamics of the invasion process. We used 10 high-resolution aerial and satellite images taken from 1936 to 1996 to track population dynamics and spatial pattern of all P. glandulosa greater than 2m in diameter on a 75-ha area in southern New Mexico. Shrub cover and patch numbers increased from 1936 to the 1970s, then stabilized at 43% cover and 83 patches/ha. Individual patches were extremely persistent: 95% of the area occupied by shrub patches in 1936 was still occupied in 1996. Recruitment into the 2m size class was more variable: 0.6 to 5.2%/yr (mean 0.8% yr). Patch shape complexity increased from 1936 to 1983 as adjacent shrubs merged and then declined as those clusters filled in and became rounder. Spatial pattern of shrubs showed a distinct trend over time: strongly clustered in 1936 at lag distances up to 250m, then random arrangement at all scales; and, by 1983 the pattern was regular at lag distances greater than 100m. There was no clear relationship with precipitation. The use of remote sensing imagery allowed us to examine one site over time and revealed patterns in population dynamics and spatial pattern that would not have been visible otherwise. Comparison of field estimates collected in 2001 with 1996 image data suggest the canopy cover estimates were accurate, but shrub densities were seriously underestimated in the satellite photographs which do not show shrubs smaller than 2m diameter. As long as limitations of the imagery are understood, these methods can be applied over a larger and more heterogeneous area to examine environmental correlates of invasion success.

Last Modified: 7/27/2014
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