Resource limitation and environmental stressors in drylands force plants to operate on their physiological margins, so abiotic variables are widely regarded to have a greater impact on ecosystem structure/function than biotic interactions like competition. An important example in arid rangelands is how scant and spatially heterogeneous soil resource availability, in combination with livestock grazing, generates positive feedbacks that lead to the desertification of formerly productive grasslands and their transformation into shrublands with low productivity.

The overall objective of Nathan A. Pierce's dissertation research is to challenge the widely held assumption that direct biotic interactions are of little or no significance in the desertification process. What happens to grass productivity when the abundance of shrub neighbors increases or decreases? Do large shrubs influence grasses more than small shrubs? Conversely, how do shrubs at different life history stages respond to the depletion of grass cover? We also assume that precipitation, rather than intraspecific (shrub-shrub) competition, sets the upper limit of woody plant cover, but direct tests are lacking. This research addresses these knowledge gaps through a series of selective removal experiments at the Jornada Experimental Range in southwestern New Mexico. The goal of these experiments is to reveal new perspectives that improve our understanding of mechanisms that regulate land cover change and degradation in dryland systems.


Nathan A.


PhD candidate
SNRE, UArizona

Dr. Steven R.


Professor of Natural Resources
SNRE, UArizona


Pierce NA, SR Archer, and BT Bestelmeyer. 2019. Competition suppresses shrubs during early, but not late, stages of arid grassland-shrubland state transition. Functional Ecology, 33: 1480-1490. DOI: 10.1111/1365-2435.13336 LINK

Pierce N, SR Archer, BT Bestelmeyer, and DK James. 2018. Grass-shrub competition in arid lands: an overlooked driver in grassland-shrubland state transitions? Ecosystems, 22: 619–628. LINK

Pierce NA, SR Archer, BT Bestelmeyer. "Grassland-shrubland state transitions in arid rangelands: competition matters." Poster presented at 2018 Society for Range Management meeting, Sparks, Nevada. LINK

Pierce NA, SR Archer, and BT Bestelmeyer. "Shrub dynamics in pre- and post-encroachment phases of grassland-to-shrubland transition." Presented at RISE 2015. LINK


The Jornada Experimental Range/Jornada Basin LTER site.

Reduced black grama cover and increased mesquite size/abundance and bare soil connectivity at the site.

System dominated by large mesquite and bare soil, with minimal grass cover at the site.

Established mesquite shrub with neighboring grasses removed.

Black grama grassland at the site, with the Organ Mountains in the background.

Sunset over the Doña Ana Mountains.