Riparian Soil Seed Banks and the Potential for Passive Restoration of Giant Reed Infested Areas in Webb County, Texas

Native landscapes like riparian areas continually undergo changes through land management practices, exotic species invasions, and/or natural processes (Poff et al. 2011, Racelis et al. 2012). Overtime, heavy disturbances and/or exotic species invasions could deplete soil seed banks (Reid et al. 2009). As a consequence, soil seed banks become an increasingly important ecosystem component that can prevent extirpation of native plant species (Moody-Weis and Alexander 2007) and facilitate survival of rare species for future generations (Coteff and Van Auken 2006).

Currently, remote sensing studies by Yang et al. (2011) have shown that Giant Reed (Arundo donax), an exotic invasive grass, has invaded the banks of a major portion of the Rio Grande River in Texas and especially in Webb County, where the study took place. Giant reed, is receiving attention in the United States and Mexico due to the deleterious effects it has on native riparian plant communities (McGaugh et. al. 2006, Goolsby et al. 2009). In optimal conditions the above ground shoots of giant reed can grow up 10 cm per day (Iverson 1994, Bell 1997) and form a dense underground network of woody rhizomes. Once established, giant reed has the ability to competitively exclude native vegetation for water, space, and nutrients, as well as rapidly diminish the presence of a persistent seed bank (Herrera and Dudley 2003, Watts and Moore 2011). Researchers have tested an array of methods to determine their effectiveness on controlling giant reed with varying success. Current leading methods for control include fire, herbicide treatments (Spencer et. al 2008), mechanical removal, and utilizing biological control agents from giant reed’s native habitat which ranges from the Mediterranean to Southern Asia (Goolsby et al. 2009).

There is little information on the potential for passive restoration in riparian areas where giant reed has been removed or controlled, based on persistent soil seed bank. In this study, we conducted a seed bank study following the selective removal of above ground biomass of giant reed along the Rio Grande in Webb County, TX (Racelis et. al 2012). The aim of this was to evaluate questions about the composition and viability of seeds at various depths and to utilize different methods to identify collected seeds. We tested two hypotheses: 1) the greatest number of seeds and highest viability is in the leaf litter-0 cm layer; and 2) seed count and viability decreases significantly at 5 cm and 10 cm.

Prior to seed bank collection, a survey of the vegetation surrounding the sampling plots was conducted, and when available, seeds were collected from each plant. The seeds collected consisted mostly from tall grasses, cool season forbs and late succession trees and shrubs characteristic of Tamaulipan scrubland and riparian types. The information gathered was used for the identification of seeds collected in the soil seed bank study.

Soil seed bank sampling occurred in May 2010, after the spring growing season and following the completion of a 27 month giant reed removal study that began in December 2007 and was completed in January 2010 (Racelis et al. 2012). We sampled a total of eleven 25 m2 plots within four sites previously used in the giant reed removal study. Latitude/longitude coordinates for the four sites are as follows: A) 27°36.200 N 099°34.874 W; B) 27°36.681 N 099°32.969 W; C) 27°34.744 N 099°31.658 W; and D) 27°29.195 N 099°28.758 W. Samples within these plots were extracted using a soil corer with dimensions of 13.5 cm (diameter) and 15 cm (chamber length). Five sampling locations were chosen randomly in each plot; soil was collected within each plot at depths of 0 cm-leaf litter, 5 cm, and 10 cm, and placed in brown paper bags. Samples were cold stratified at 4°C for approximately 60 days (Coteff and Van Auken 2006). Following cold stratification all soil samples were processed with a tower sieve (Hubbard Scientific® Chippewa Falls, WI) with mesh...