Mechanisms and drivers of alternative shrubland states

Authors: ARCHER, STEVEN - University Of Arizona; Peters, Debra; BURRUS, DYLAN - New Mexico State University; YAO, JIN - New Mexico State University

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2021
Publication Date: 4/3/2022
Citation: Archer, S., Peters, D.C., Burrus, D.N., Yao, J. 2022. Mechanisms and drivers of alternative shrubland states. Ecosphere. 13(4):Article e3987. https://doi.org/10.1002/ecs2.3987.
DOI: https://doi.org/10.1002/ecs2.3987

Interpretive Summary: At the Jornada Basin USDA-LTER research site, records from 1850s show that perennial grassland-dominated landscapes had transitioned to mostly shrublands by the early 1900s. Subsequent surveys showed that by 1998, >90% of the Jornada was shrub-dominated. This broad-scale change resulted from livestock overgrazing in conjunction with periodic drought, and represents the classic “grassland to shrubland” regime shift. However, finer-scale observations suggest a more nuanced view of this state change -- one that accounts for the dynamics of shrublands dominated by woody species representing different functional groups -- is warranted. We analyzed the vegetation data using a fine-scale grid to quantify the frequency of (i) basin-scale transitions from grassland to shrubland, (ii) transitions between shrub functional groups, and (iii) transitions from shrub to grass dominance on contrasting soil-geomorphic units . Results show that: (i) shrubland communities were spatially and temporally more dynamic than has been generally presumed, (ii) in some locations, shrubland communities subsequently transitioned to communities dominated by a different shrub functional type, with these changes in shrub composition likely involving changes in soil properties, and (iii) some shrub-dominated locations have transitioned back towards grass dominance. Understanding the mechanisms and drivers of transitions between shrub states defined by woody plant functional types will be important to predicting future landscape change in the Chihuahuan Desert.

Technical Abstract: At the Jornada Basin USDA-LTER research site in the North American Chihuahuan Desert in southern New Mexico, USA, records from 1850s General Land Office township/section surveys followed by 1915 US Forest Service reconnaissance surveys show that perennial grassland-dominated landscapes had transitioned to mostly shrublands by the early 1900s. Subsequent surveys showed that by 1998, >90% of the Jornada Basin was shrub-dominated. This broad-scale change ostensibly resulted from livestock overgrazing in conjunction with periodic drought, and represents the classic “grassland to shrubland” regime shift. However, finer-scale observations suggest a more nuanced view of this state change -- one that accounts for the dynamics of shrublands dominated by woody species representing different functional groups -- is warranted. We therefore analyzed the Jornada Basin historic vegetation data using a fine-scale grid to quantify the frequency of (i) basin-scale transitions from grassland to shrubland, (ii) transitions between shrub functional groups, and (iii) transitions from shrub to grass dominance on contrasting soil-geomorphic units within the basin. Results from our spatially-explicit, decadal time-scale perspective show that: (i) shrubland communities developing on former grasslands were spatially and temporally more dynamic than has been generally presumed, (ii) in some locations, shrub communities initially developing on grasslands subsequently transitioned to communities dominated by a different shrub functional type, with these changes in shrub composition likely involving changes in soil properties, and (iii) some shrub-dominated locations have transitioned back towards grass dominance. Understanding the mechanisms, drivers, and influence of cross-scale interactions between patterns and processes on transitions between shrub states defined by woody plant functional types will be germane to predicting future landscape change in the Chihuahuan Desert.