Study area
The Jornada Experimental Range is an LTER site approximately 30 km
north of Las Cruces, in southern New Mexico, USA (32°33’37.44”N,
106°46’26.65”W). The area is a hot desert, with long-term mean
annual PPT of 245 mm concentrated primarily during the summer
(July-October) period. We carried out our experiment on the basin
floor sand sheet geomorphic unit where historic grasslands have
undergone extensive encroachment by honey mesquite (Prosopis
glandulosa). Our experiments were conducted in an area still
dominated by the C4 grass, Black Gramma (Bouteloua eriopoda).
Methods
We manipulated PPT using an Automatic Rainfall Manipulation System
(ARMS - Gherardi and Sala 2013), creating plots that received
ambient precipitation, 20% of ambient precipitation and 180% of
ambient precipitation. To test the effect of grass competition on
Mesquite seedlings, we split each plot within PPT manipulations
where in one half we clipped the herbaceous vegetation to 5 cm
every two weeks and left intact the remaining half. Plant material
from the clipping was collected and disposed outside the plots to
avert organic matter input to the soil (as would be the case in
livestock grazing).
To test seed/seedling predation, we installed a set of three
different exclosures and one exclosure control where we added
scarified Mesquite seeds. Each exclosure allowed a certain class
of predator to access seeds, whereas the exclosure control allowed
access by all predators. One type of exclosure consisted on a PVC
cylinder (D=15cm x H = 10cm) coated with an insect-a-slip barrier
(Fluon®) to prevent ant access while allowing access by rodents
and lagomorphs. Another type of exclosure consisted of a
cylindrical hardware cloth cage (D=15cm x H=30cm; 138 12.7 X 12.7
mm mesh) that prevented access by rodents and small mammals while
allowing ant access. The third exclosure type consisted of a
combination of the first two exclosures, wherein we placed the
cylindrical cage inside the coated PVC cylinder to exclude ants
and small mammals. Our control exclosure consisted of a metal rod
inserted into the ground to mark where we planted seeds without
exclosures, thus allowing any predator to access them. The
predation treatments were replicated in the clipped and unclipped
halves of each plot.
Germination and survival were tracked by placing different-colored
toothpicks on the north side of each germinating seed and
following the subsequent survival of the seedling.
