Restoring abandoned agricultural lands in arid environments: The tradeoffs between water availability and exotic species

Background/Question/Methods

On a global basis, the area of abandoned agricultural land is growing. Modification of the environment for agriculture often results in degradation of the original ecosystem processes and a loss of the biotic and abiotic legacies necessary for recovery of the ecosystem. In arid and semi-arid ecosystems, low soil water availability coupled with exotic plant dominance can make restoration of these lands problematic.  We evaluated methods for establishing diverse native plant communities on abandoned agricultural fields that were formerly characterized by Wyoming sagebrush ecological site types in a 20 to 25 cm precipitation zone in southwest Nevada. We asked if increasing soil water availability via irrigation and mulch would result in tradeoffs between an increase in exotic plant cover and higher native plant establishment. Treatments were applied in two consecutive years using a stratified random design and included all combinations of two native seed mixes (grass and diverse) and two levels (none and treated) of irrigation, mulch and herbicide. We used a factorial design in which watering was a main plot, and seeding, mulching and herbicide application were split-plots within the watering treatment. Data were analyzed using mixed effects ANOVAs.

Results/Conclusions

Establishment of forbs and shrubs in the diverse mix was low. A strong interaction existed among seeding year, irrigation and mulch. In 2009, precipitation was below-average and plots seeded in fall 2008, mulched, and watered had significantly higher seedling densities than plots not watered or mulched. In contrast, precipitation was above-average in 2010 and water and mulch had little effect on seedling densities in plots seeded in fall 2009. High cover of exotic forbs in all plots in 2010 indicated strong competition with seeded species, and likely reduced effects of water and mulch. Herbicide (weed wiping with 2, 4-D) applied in summer 2010 only decreased cover of tall exotic forbs. Despite competition with exotics, first-year seedling densities were higher during 2010 than 2009. At study’s end in 2011, seedlings that emerged in 2009 had relatively high survival over time regardless of treatment, but the highest numbers of individuals were in mulched and watered plots. Seedlings that emerged in 2010 had significantly higher survival in watered plots, and the highest numbers of individuals were in the watered plots. Over time, the positive effects of mulch and especially irrigation outweighed the negative effects of increased exotics in the wet year.