Mega-Monsoon Experiment (MegaME) Vegetation Sampling Data from the Sevilleta National Wildlife Refuge, New Mexico

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.