What influences seedling establishment of encroaching species?

Luis Weber, a second year Environmental Life Sciences PhD student in the Sala lab, is in his second field season at the Jornada LTER. Luis’ research is a part of the USDA-AFRI project focusing on exotic grass and woody plant encroachment. Specifically, Luis focuses on the seedling establishment phase by asking “How do precipitation and herbivory interact to influence the probability of woody shrub and non-native grass seedling establishment in native grassland?” 

This project assesses the influence of precipitation change, grazing, and seed predation on the probability of honey mesquite(Prosopis glandulosa) and Lehmann lovegrass (Eragrostis lehmanniana) seedling establishment.

Let’s take a closer look at the treatments

(1) precipitation
Automated Rainfall Manipulation systems used for this project consist of 10  precipitation-interception plots (-80% of ambient precipitation), 10 irrigated plots (+80% of ambient precipitation), and 10 control plots (total: 30, 2.5x5m plots).

(2) grazing
There are two grazing treatments within all 30 plots. Every 2.5x5m plot is divided in half. The grazing simulation consists of clipping all the vegetation down to 15cm from the ground surface every 2 weeks on one side of the plot.

(3) seed predation
Treatments for seed predation consist of various exclosures to exclude: ants, rodents/lagomorphs, ants AND rodents/lagomorphs. There will also be treatments with no exclosure. Each seed predation treatment has 2 replicates – one is used to plant 5 scarified honey mesquite seeds and the other to plant 0.1g of Lehmann lovegrass.

To monitor the progress of the seeds under the combination of all these variables, Luis follows their germination with colored toothpicks for at least 8 weeks consecutively. Luis differentiates among weekly cohorts of seedlings using different colored toothpicks and is able to record a weekly rate of mortality and recruitment.