Abstract
The ecological impacts of invasive plants increase dramatically with time since invasion. Targeting young populations for treatment is therefore an economically and ecologically effective management approach, especially when linked to post-treatment monitoring to evaluate the efficacy of management. However, collecting detailed field-based post-treatment data is prohibitively expensive, typically resulting in inadequate documentation of the ecological effects of invasive plant management. Alternative approaches, such as remote detection with unmanned aerial vehicles (UAV), provide an opportunity to advance the science and practice of restoration ecology. In this study, we sought to determine the plant community response to different mechanical removal treatments to a dominant invasive wetland macrophyte (Typha spp.) along an age-gradient within a Great Lakes coastal wetland. We assessed the post-treatment responses with both intensive field vegetation and UAV data. Prior to treatment, the oldest Typha stands had the lowest plant diversity, lowest native sedge (Carex spp.) cover, and the greatest Typha cover. Following treatment, plots that were mechanically harvested below the surface of the water differed from unharvested control and above-water harvested plots for several plant community measures, including lower Typha dominance, lower native plant cover, and greater floating and submerged aquatic species cover. Repeated-measures analysis revealed that above-water cutting increased plant diversity and aquatic species cover across all ages, and maintained native Carex spp. cover in the youngest portions of Typha stands. UAV data revealed significant post-treatment differences in normalized difference vegetation index (NDVI) scores, blue band reflectance, and vegetation height, and these remotely collected measures corresponded to field observations. Our findings suggest that both mechanically harvesting the above-water biomass of young Typha stands and harvesting older stands below-water will promote overall native community resilience, and increase the abundance of the floating and submerged aquatic plant guilds, which are the most vulnerable to invasions by large macrophytes. UAV's provided fast and spatially expansive data compared to field monitoring, and effectively measured plant community structural responses to different treatments. Study results suggest pairing UAV flights with targeted field data collection to maximize the quality of post-restoration vegetation monitoring.
Highlights
The same measures of Typha dominance and native plant community composition tended to vary with pretreatment subplot location, which reflects the relative distance into the center of a Typha stand (A = outside Typha dominated stand → E = near stand center; Figure 2; Tables 2, 4)
Indicator species analysis revealed two species were correlated with individual stand-age categories, Carex lacustris correlated with the oldest age class (1998; IV, 39.3%; P < 0.05) and C. utriculata was associated with the subdominant group (IV, 34.3%; P < 0.05), where indicator values (IV) represent the percentage of perfect indication for each group
Our findings suggest that mechanically harvesting the abovewater biomass of young stands of invasive Typha and harvesting older stands below-water will promote native community resilience, and increase floating and submerged aquatic species abundance, which are some of the most vulnerable wetland plant guilds to plant invasions (Stiers et al, 2011)
Summary
The ecological impacts of invasive plants tend to compound temporally (Strayer et al, 2006; Mitchell et al, 2011; Lishawa et al, 2013; Simberloff et al, 2013), resulting in biodiversity losses, plant community restructuring, and eventually the alteration of biogeochemical cycling with time since introduction (Suding et al, 2004; Suding and Hobbs, 2009; Zedler, 2009). Theory suggests that long-invaded ecosystems can enter alternative stable states (Beisner et al, 2003) outside of their historical range of variability, and become increasingly difficult to restore (Suding et al, 2004). The shorter the duration of time between invasion and when control activities occur, the more likely the invasion will be stemmed, preventing an ecosystem from reaching an alternative stable state. Evidence from invasive plant eradication efforts support this perspective: as invasions progress in age and size, eradication becomes less likely and more expensive (Rejmánek and Pitcairn, 2002). It is not surprising that large-scale invasive plant control tends to be an expensive, labor and chemical intensive process, which often results in unsatisfactory outcomes (Martin and Blossey, 2013; Simberloff, 2014)
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