Abrupt transitions in a southwest USA desert grassland related to the Pacific Decadal Oscillation

Authors: Christensen, Erica; James, Darren; RANDALL, ROBB - Us Army Engineer Research And Dvelopment Center; Bestelmeyer, Brandon

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2023
Publication Date: 7/3/2023
Citation: Christensen, E.M., James, D.K., Randall, R.M., Bestelmeyer, B.T. 2023. Abrupt transitions in a southwest USA desert grassland related to the Pacific Decadal Oscillation. Ecology. 104(7). Article e4065. https://doi.org/10.1002/ecy.4065.
DOI: https://doi.org/10.1002/ecy.4065

Interpretive Summary: Ecosystem transitions resulting from climatic drivers are observed at multiple time scales. In the arid southwest United States, the role of climate variation in the collapse and recovery of grassland communities is poorly understood. Here we use a quality-controlled vegetation monitoring dataset initiated in 1915 to show that grass cover dynamics during the 20th century were most closely correlated to the Pacific Decadal Oscillation (PDO) index. The relationship out-performed models correlating grasses to yearly precipitation and drought indices, suggesting that ecosystem transitions attributed only to local disturbances were instead influenced by climate teleconnections. However, the relationship between PDO and vegetation broke down after 1995, indicating that warming or land degradation overwhelmed multi-decadal variation driven by PDO.

Technical Abstract: Prediction of abrupt ecosystem transitions resulting from climatic change will be an essential element of adaptation strategies in the coming decades. In the arid southwest USA, the collapse and recovery of long-lived perennial grasses have important effects on ecosystem services, but the causes of these variations have been poorly understood. Here we use a quality-controlled vegetation monitoring dataset initiated in 1915 to show that grass cover dynamics during the 20th century were closely correlated to the Pacific decadal oscillation (PDO) index. The relationship out-performed models correlating grasses to yearly precipitation and drought indices, suggesting that ecosystem transitions attributed only to local disturbances were instead influenced by climate teleconnections. Shifts in PDO phase over time were associated with the persistent loss of core grass species and recovery of transient species, so recovery of grasses in aggregate concealed significant changes in species composition. However, the relationship between PDO and grass cover broke down after 1995; grass cover is consistently lower than PDO would predict. The decoupling of grass cover from the PDO suggests that a threshold had been crossed in which warming or land degradation overwhelmed the ability of any grass species to recover during favorable periods.