Shrub encroachment is a global phenomenon. Both the causes and consequences of shrub encroachment vary regionally and globally. In the southwestern US a common native C3 shrub species, creosotebush, has invaded millions of hectares of arid and semi-arid C4-dominated grassland. At the Sevilleta LTER site, it appears that the grassland-shrubland ecotone is relatively stable, but infill by creosotebush continues to occur. The consequences of shrub encroachment have been and continue to be carefully documented, but the ecological drivers of shrub encroachment in the southwestern US are not well known.One key factor that may promote shrub encroachment is grazing by domestic livestock. However, multiple environmental drivers have changed over the 150 years during which shrub expansion has occurred through the southwestern US. Temperatures are warmer, atmospheric CO2 has increased, drought and rainy cycles have occurred, and grazing pressure has decreased. From our prior research we know that prolonged drought greatly reduces the abundance of native grasses while having limited impact on the abundance of creosotebush in the grass-shrub ecotone. So once established, creosotebush populations are persistent and resistant to climate cycles. We also know that creosotebush seedlings tend to appear primarily when rainfall during the summer monsoon is well above average. However, high rainfall years also stimulate the growth of the dominant grasses creating a competitive environment that may not favor seedling establishment and survival. The purpose of the Mega-Monsoon Experiment (MegaME) is twofold. First, this experiment will determine if high rainfall years coupled with (simulated) grazing promote the establishment and growth of creosotebush seedlings in the grassland-shrubland ecotone at Sevilleta, thus promoting infill and expansion of creosotebush into native grassland. Second, MegaME will determine if a sequence of wet summer monsoons will promote the establishment and growth of native C4 grasses in areas where creosotebush is now dominant, thus demonstrating that high rainfall and dispersal limitation prevent grassland expansion into creosotebush shrubland.
Knowledge Network for Biocomplexity (KNB) Identifier
Data Policies: This dataset is released to the public and may be freely downloaded. Please keep the designated Contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement of the Sevilleta LTER. Datasets must be cited as in the example provided. A copy of any publications using these data must be supplied to the Sevilleta LTER Information Manager. By downloading any data you implicitly acknowledge the LTER Data Policy (http://www.lternet.edu/data/netpolicy.html).
2014-05-29 - 2015-10-27
Location: The Mega-Monsoon Ecotone site is located just west of the Five Points Black Grama site.Vegetation: This site is dominated by black grama (Bouteloua eriopoda) and creosote. Other prevalent grasses include Sporobolus contractus, S. cryptandrus, S. flexusous, and Muhelnbergia arenicola. , Location: The Mega-Monsoon Cresoote Site is included within the Five Points Creosote Site. Vegetation: The Five Points Creosote site is characterized as Chihuahuan Desert Scrub, dominated by a creosote overstory, with broom snakeweed, purple pricklypear (Opuntia macrocentra), and soapweed yucca as co-occurring shrubs. The site is also characterized by numerous, dense, grass-dominated patches, reflecting proximity to the Five Points Black Grama Site. Dominant grasses are black grama, fluffgrass (Dasyochloa pulchellum), burrograss (Scleropogon brevifolia), bushmuhly (Muhlenbergia porteri), and galleta (Pleuraphis jamesii). Notable forbs include field bahia (Bahia absinthifolia), baby aster (Chaetopappa ericoides), plains hiddenflower (Cryptantha crassisepala), Indian rushpea (Hoffmannseggia glauca), Fendlers bladderpod (Lesquerella fendleri), and globemallows (Sphaeralcea spp.).
Collins, Scott (2015): Mega-Monsoon Experiment (MegaME) Vegetation Sampling Data from the Sevilleta National Wildlife Refuge, New Mexico (2014 - present). Long Term Ecological Research Network. http://dx.doi.org/10.6073/pasta/c23c1de597eff16eac1e737019017615