Viable seeds that occur in the uppermost layers of soils in many arid and semi-arid lands are vulnerable to surface disturbances. Trampling and vehicle activity compact the soil surface, and trenching mixes seed-rich topsoil with subsurface soils. Seed abundance is influenced by soil surface properties and seed harvesting animals, yet how these properties are altered by different disturbances is poorly understood. Evaluating the relationships between surface properties and seed abundance can help develop indicators for determining the severity of site degradation, the level of management action required, and the potential for vegetation recovery. In a recent study in the Journal of Arid Environments, USGS scientists Dr. Lesley DeFalco, Dr. Todd Esque, Jeffrey Kane and Melissa Nicklas assessed the soil properties associated with common human-caused disturbances to better understand the processes driving seed retention and movement in disturbed arid landscapes.

The study was conducted at the U.S. Army National Training Center, Fort Irwin, California, in the Mojave Desert of southern California. The scientists determined whether the abundance of viable seeds of annual and perennial species differed among compacted sites, backfilled trenches and nearby undisturbed areas. Secondly, they evaluated whether patterns in seed abundance were related to soil properties such as soil particle size, soil compaction, shrub canopy cover, ground cover of plant litter, surface slope, surface relief, and ant seed-harvesting activity.

The authors found that the abundance of viable seeds of perennials were typically rare in undisturbed areas (3-4 seeds/m²) and nearly absent within compacted and trenched sites (<1 seed/m²). Seeds of annual species in undisturbed areas were more numerous than perennial species. Annual seeds in compacted sites were depleted by one-third those of undisturbed areas but doubled within trenched sites. Greater ground cover of litter comprising the fruits and seeds of the dominant spring annuals, and low gravel content, increased seed abundances of both annuals and perennials on trenched sites. Litter cover and surface slope and relief explained perennial seed abundance on compacted sites, but litter cover and the presence of a common harvester ant influenced annual seed abundance better than any other surface properties that were examined.

This study demonstrates that soil seed abundance is influenced by changes in soil conditions due to surface disturbance as well as by the direct losses of seed-bearing plants. Even though disturbance changed surface conditions – such as the shift toward coarse-textured soils on compacted sites and the increase in surface topography on trench sites – litter had a dominant influence on viable seed abundance. Litter is also known to promote nutrient cycling, lower sediment yield and run-off, and reduce wind erosion.

Although the authors examined only disturbed sites associated with military training activities in the Mojave Desert, results may be applicable to other compacted sites including abandoned town sites, or to excavated trench sites such as aqueduct, power line and pipeline developments. Rehabilitation of these degraded sites may benefit from litter additions to facilitate the development of seed banks and accelerate the recovery of degraded lands, but this approach warrants further investigation. Whereas the authors cannot directly ascribe cause and effect to the relationships between seed abundance and site conditions in this study, these results emphasize the need to more thoroughly understand the influence of altered surface characteristics on seed banks in arid lands. Previously thought to have been detrimental to seed banks in arid systems, harvester ants and other granivores can enhance seed movements and must be considered in relation to the goals of passive or active rehabilitation.

Ground litter comprised of seeds augments soil seed banks in desert shrublands and may be a good indicator of disturbance severity and recovery potential. Photo by: L. DeFalco, USGS.

Harvester ants not only consume seeds but also distribute seeds across disturbed areas to facilitate plant establishment. Photo by: L. DeFalco, USGS.

Management Implications

• Surface disturbances deplete seed banks of perennial species, thus broadcast seeding may be required to re-establish shrubs in disturbed desert shrublands.
• Disturbances can alter the conditions of the soil surface and the resulting seed bank in a variety of ways, but ground cover of litter is consistently related to seed abundance.
• Plant litter can be rapidly and easily estimated, which makes it a prospective indicator of site condition in degraded arid lands.

DeFalco, L.A., T.C. Esque, J.M. Kane, and M.B. Nicklas. 2009. Seed banks in a degraded desert shrubland: Influence of soil surface condition and harvester ant activity on seed abundance. Journal of Arid Environments 73:885–893.

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