Connectivity and Seed Availability in Drylands: Interactions of Vegetation and Seed Movement at Multiple Scales

Tyler Turk

Shrub encroachment into arid grasslands is a widespread phenomenon in drylands globally and the Chihuahuan desert specifically. Encroachment leads to structural and functional changes in the system which facilitate positive feedback loops that further reinforce the shrubland state. Large bare interspaces serve as areas in which wind and water erosion can act, redistributing resources including sediment, seeds and litter, to the shrub canopy. This results in greater nutrient availability and infiltration under shrubs as well as deficient interspaces where the ability of herbaceous plants to establish is greatly restricted. As encroachment worsens and bare ground increases, the interspaces become long connected pathways through which erosion can act. Sediment deposition, in turn, occurs at increasingly larger scales, further separating deposited resources from the source. An emerging framework to understand dryland dynamics describes dry-landscapes with many large, bare plant-interspaces as having a high degree of connectivity. Recently, restoration in drylands has incorporated methods to reduce connectivity to mitigate erosion and retain resources that promote herbaceous plant growth. This can be achieved using structures known as connectivity modifiers (ConMods), which have been found to increase recruitment of desirable species. Seeds are an important resource captured by ConMods: quantifying the seeds captured in ConMods will allow us to gain insight into seed movement on the landscape. We quantified the sediment and germinable seeds captured in ConMods at two long term research sites at the Jornada Experimental Range by performing a seedling emergence study on the materials collected over one growing season. We found that there were many seeds in transit in our study sites, and that perennial grasses made up a large portion of the seeds collected. Sediment and seed movement did not have similar relationships to large-scale connectivity. Our research suggests that use of ConMods as restoration tools necessitates consideration of current and past vegetation and seed predation.