Modifying Drivers of Competition to Restore Palmer's Agave in Lehmann Lovegrass Dominated Grasslands

Abstract

Dryland restoration is becoming increasingly challenging in arid and semi-arid regions, such as the southwestern U.S., due to rapid land degradation, an increase of non-native species, and climate change. Encroachment of a non-native perennial grass species commonly known as Lehmann lovegrass (Eragrostis lehmanniana), is a particularly critical challenge for land managers of the region. Lehmann lovegrass threatens and outcompetes one of the Sonoran Desert’s ecologically and socio-culturally significant plant species, Palmer’s agave (Agave palmeri). Stakeholders in Southeastern arid lands are particularly challenged by scarce rainfall, limited land resources, invasive species dominance, and frequent predation posing unique challenges to land management. Unfortunately, mechanisms and drivers of Palmer’s agave survival and growth patterns are not well understood, limiting our ability to design effective management strategies. In this study, we had a greenhouse experiment and a field study to explore survival and growth responses of transplanted Palmer’s agave. For the greenhouse study, we measured agave growth and survival under a variety of manipulated conditions, both biotic (agave age and size class at planting, and simulated mowing of Lehmann lovegrass) and abiotic (simulated precipitation and surface mulch). In the field experiment in southeastern Arizona, U.S., we assessed the utility of several management techniques to promote restoration outcomes for Palmer’s agave survival and growth, including protection from solar insulation and herbivory, and reduction in the competitiveness of Lehmann lovegrass. We found that the presence of thatch when combined with high and medium simulated precipitation levels significantly enhances agave biomass. However, we observed that Lehmann lovegrass biomass was indifferent to Palmer’s agave presence, precipitation regimes, and thatch. We found that the combination of herbivory protection and shade resulted in the highest survival of planted agaves, while the shade treatment alone resulted in the largest agaves. We conclude that stakeholders should use an integrated approach by using an additional artificial simulation of high and medium watering —especially in hot desert climates— with thatch as the surface mulch and shade provider to enhance agave growth and survival. For a long-term, large-scale restoration efforts, land managers can forgo weed management and by planting agaves under nurse plants or existing vegetation canopy could save costs by providing the same shade benefits as thatch or surface mulch. KEYWORDS: Agave palmeri, Restoration strategies, Abiotic interactions, Weed management, Straw mulch, Javelina, Eragrostis lehmanniana