|Throop, Heather -|
|Reichmann, Lara -|
|Sala, Osvaldo -|
|Archer, Steve -|
Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 23, 2011
Publication Date: January 15, 2012
Citation: Throop, H.L., Reichmann, L.G., Sala, O., Archer, S. 2012. Response of dominant grass and shrub species to water manipulation: an ecophysiological basis for shrub invasion in a Chihuahuan Desert grassland. Oecologia. 169:373-383. Interpretive Summary: Shrubs and trees have increased on all continents over the last century. The reasons for these increases are numerous and debated. Reasons often described include increased efforts to control wildfires, overgrazing of non-shrub and non-free species by 3.5 billion livestock around the world, and increased atmospheric CO2 levels that may increase the growth of woody shrub and tree species. This study simply compared the rate at which one shrub species and one grass species converted sunlight, carbon dioxide and water to plant energy and oxygen, the process of photosynthesis. The shrub species was honey mesquite, a woody plant that has increased since in 1880s throughout the southwestern U.S. The grass species was black grama, a common desert grass that has declined across the southwestern US over the last century. The mesquite consistently outperformed black grama in term so photosynthesis rates whether soils were dry or wet. The ability to maintain a high photosynthesis rate may explain why mesquite has been so successful in desert environments.
Technical Abstract: Increases in woody vegetation and declines in grasses in arid and semi-arid ecosystems have occurred globally since the 1800s, but the mechanisms driving this major land-cover change remain uncertain and controversial. Working in a shrub-encroached grassland in the northern Chihuahuan Desert where grasses and shrubs typically diVer in leaf-level nitrogen allocation, photosynthetic pathway, and root distribution, we asked if diVerences in leaflevel ecophysiology could help explain shrub proliferation. We predicted that the relative performance of grasses and shrubs would vary with soil moisture due to the diVerent morphological and physiological characteristics of the two life-forms. In a 2-year experiment with ambient, reduced, and enhanced precipitation during the monsoon season, respectively, the encroaching C3 shrub (honey mesquite Prosopis glandulosa) consistently and substantially outperformed the historically dominant C4 grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates while also maintaining a more favorable leaf water status. These diVerences persisted across a wide range of soil moisture conditions, across which mesquite photosynthesis was decoupled from leaf water status and moisture in the upper 50 cm of the soil proWle. Mesquite’s ability to maintain physiologically active leaves for a greater fraction of the growing season than black grama potentially ampliWes and extends the importance of physiological diVerences. These physiological and phenological diVerences may help account for grass displacement by shrubs in drylands. Furthermore, the greater sensitivity of the grass to low soil moisture suggests that grasslands may be increasingly susceptible to shrub encroachment in the face of the predicted increases in drought intensity and frequency in the desert of the southwestern USA.