Passive Restoration Potential of Riparian Areas Invaded by Giant Reed (Arundo donax) in Texas

As complex environments that include terrestrial and aquatic ecosystems, riparian areas are the most threatened habitats in North America, suffering impacts from anthropogenic activities, such as extraction and regulation of water and pollution, as well as ecologically-driven changes due to climate change and invasion by non-native species (Poff et al. 2011). The current condition of the riparian corridor along the bi-national Rio Grande/Rio Bravo River, which forms much of the United States-Mexico border, is considered one of the world's prime examples of damage caused by overextraction and other human activities to riparian ecosystems in arid regions (Wong et al. 2007). A shared resource, the Rio Grande has been heavily managed to provide water to a robust agricultural economy and burgeoning urban areas on both sides of the border, and it is acutely contaminated by agricultural and industrial runoff. The Rio Grande basin has also been heavily impacted by non-native invasive plant species, including giant reed (Arundo donax), a rhizomatous woody non-native grass that has invaded at least 40,000 ha of the narrow riparian corridor along the river and its tributaries (Yang et al. 2011). Invasive non-native weeds often drive or accelerate riparian ecosystem decline by outcompeting native plant species (Mason et al. 2007). Giant reed has significantly reduced native biodiversity in riparian ecosystems across arid regions of North America, transforming riparian landscapes into thick, impenetrable monotypic stands (McGaugh et al. 2006; Yang et al. 2011). In addition, giant reed has negatively impacted water supply due to its great evapotranspirative capacity (Watts and Moore 2011), and it has decreased border security along the Rio Grande (Yang et al. 2011). These impacts have justified and informed a long-term integrated management plan for control of this weed. Current management of giant reed include a combination of mechanical control and chemical herbicides (Spencer et al. 2008), and more recently, classical biological control using natural enemies (Goolsby et al. 2009). Despite these control efforts, little is known about the successional response or potential for restoration after the removal or control of giant reed from riparian habitat. A succession of diverse native vegetation after a given treatment may signify a high potential for passive restoration (Prach et al. 2001), whereas re-invasion by the same or another non-native species may suggest a need for active restoration efforts (Zavaleta et al. 2001), such as intentional plantings or planned revegetation.

To explore the restoration potential of giant reed infested areas, we conducted a 27-month observational study of the diversity and abundance of emergent plant species after repeated above-ground removal of giant reed across 16-25 m² infested riparian plots near Laredo, Texas (27°31'28"N, 99°29'26"W). Careful attention for plot placement was given to slope, soil type, relative distance to the river, and proximity to patches of native vegetation so that plots could be considered environmentally similar across field sites. We placed each plot at least 5 m into a wide (30 m) stand of giant reed to avoid edge effects. Our first visit was in December 2007, when we removed all existing above ground biomass within each plot and cleared a 1-m buffer around the periphery of the plot to allow for minimal trampling within the plot. We cut all giant reed stems at the base using a set of loppers (Fiskars™, Madison, WI), and we measured the basal diameter of each removed stem (Fractional+©, General Tools International, New York, NY). At each subsequent visit, every 2-3 months, we removed any re-emegent giant reed stems > 1 m in length, simulating sustained biological control or repeated, selective mechanical control. We counted and identified all other plant species that were present in each plot at the time of sampling. We present information on post-treatment succession to reveal the potential for restoration of giant-reed infested areas and discuss these results in relation to giant reed management in the context of the available control options.

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