ABSTRACT

1. Effects of Eastern Redcedar and its removal on invertebrate abundance and richness in two relict prairies Erin Brogan, Jacob Gaster, Jenna Noble, Heidi Doty, Erica Boyd, Todd Tracy, and Matt Vander Molen ABSTRACT Eastern Redcedar is a conifer native to the eastern U.S. that has become an insidious invader of prairie communities. To assess the magnitude of its effect on invertebrate abundance and diversity, in September 2007 and 2008 we used pitfall traps to sample invertebrates in cedar-invaded, cedar-removed (cut and burned), and cedar-free areas in a fragment of mixed-grass prairie at the Nature Conservancy's Niobrara Valley Preserve (NVP), and in cedar-invaded and cedar-free areas of a degraded relict upland prairie in Oak Grove Park (OGP). Results from 2007 showed significantly greater ant abundance in the cedar-removed area of NVP, results consistent with other studies that show increases in ant abundance in recently burned prairie. However, in 2007 traps in cedar-invaded areas were placed only under the cover of the cedar trees. We repeated our study in 2008, but with traps in cedar-invaded areas placed both beneath the cedars and in open areas ~5m away. Our results from NVP show that that the presence of cedar cover drastically reduces invertebrate abundance and richness compared to non-cedar, burned, and open cedar areas, a trend that may be attributable to alterations in the soil and microclimate beneath the cedars. Conversely, at OGP we found higher invertebrate richness beneath cedars in 2007 and higher invertebrate abundance beneath cedars in 2008, which may be attributable to the prevalence of invasive downy brome and leafy spurge in the OGP study area. Oak Grove Niobrara X X Fig. 1 - Invertebrate abundance at Niobrara in 2007 and 2008. “Cedar open” samples were not collected in 2007. Letters indicate significantly different means. Fig. 4 - Invertebrate abundance at Oak Grove in 2007 and 2008. “Cedar open” samples were not collected in 2007. Letters indicate significantly different means. Oak Grove Park Niobrara Valley Preserve INTRODUCTION Eastern redcedar Juniperus virginiana is generally classified as an invasive species in the central U.S., despite the fact that it is a native tree.  This species is spreading (mainly via birds passing seeds through their digestive tracts) across the grasslands and rangelands of the central U.S., areas that historically have burned frequently enough and/or been sufficiently trampled by buffalo to prevent woody species from taking hold.  Until recently, its range was limited to river bottoms and cliff faces where trampling and fires were less likely to have occurred.  Its range expansion has been greatly aided by farmers, ranchers, and state Departments of Transportation planting the species as a windbreak.  Although the species is useful for windbreaks and building material, economic losses from lost rangeland and lost hunting leases, decreased stream water quality, and lowered water tables appear to outweigh the benefits of the species in many areas (Drake & Todd 2002).  Loss of plant diversity and abundance is readily apparent, but little has been noted about invertebrates directly impacted by the invasion of redcedar. In our study, we hypothesized that the presence of redcedar influences the overall species richness and abundance of invertebrates, and we predicted that invertebrate abundance and species richness would be lower in cedar-infested areas than cedar-free areas, with areas recently cleared of cedars showing intermediate values. X X Fig. 2 - Morphospecies richness at Niobrara in 2007 and 2008. “Cedar open” samples were not collected in 2007. Letters indicate significantly different means. Fig. 5 - Morphospecies richness at Oak Grove in 2007 and 2008. “Cedar open” samples were not collected in 2007. Letters indicate significantly different means. MATERIALS AND METHODS Our first round of pitfall-trapping in Nebraska took place on September 14 -15, 2007, at the Nature Conservancy’s Niobrara Valley Preserve (NVP) in north-central Nebraska, in a riparian zone along the Niobrara River. The research area was relatively flat with sandy soil and typical mixed-grass native prairie grasses and forbs. Our research areas included areas with cedar present and cedar removed in a controlled burn the previous winter, and a third, non-cedar area that had never been invaded. We flagged 10 separate sample points a minimum of 15m apart within each of the areas. Solo cups were then placed at ground level beside each flag, and cups were filled with ~2cm of a combination of 70% ethanol and dish soap, used to trap and kill the insects that fell into the cup. We harvested samples 24 hours later and poured specimens into individually labeled bags, which were then taken back to Northwestern College for identification to the morphospecies level. Our 2008 Nebraska samples were collected on Sept. 15-16 in the same area using the same methods, except that we added a “cedar open” set of cups that were placed 5m away from each cup placed beneath a cedar tree (“cedar cover”). Our pitfall-trapping in Iowa took place on September 19-20, 2007, in a degraded upland oak savanna at Oak Grove Park (OGP), where downy brome Bromus tectorum and leafy spurge Euphorbia esula are prevalent, along with a few native tallgrass prairie species interspersed with woody plants and shrubs, including eastern redcedar. The same trapping method was used, but with only cedar-infested and non-cedar areas being tested, as no cedar-removed areas were available. We paired cedar and non-cedar traps by placing one pitfall cup under the outer branches of a cedar tree and the corresponding non-cedar cup anywhere from 5 to 10m away from the tree in a clear, grassy area. Ten pairs of cups were placed and harvested 24 hours later for analysis. Our 2008 OGP samples were collected on Sept. 25 about 200m north of the 2007 site. While 2007 samples were collected on a north-facing slope, 2008 samples was taken from a south facing slope. Methods of collecting were the same as those employed in 2008 at NVP. Invertebrate abundance and richness values were determined for each sample point at both sites. For both sites in both years, we performed ANOVAs and independent-sample t-tests comparing both overall invertebrate abundance, ant abundance, and morphospecies richness among treatment areas, with paired t-tests employed in 2008 to compare "cedar-cover" samples to "cedar-open" samples. X X Fig. 3 - Ant abundance at Niobrara in 2007 and 2008. “Cedar open” samples were not collected in 2007. Letters indicate significantly different means. Fig. 6 - Ant abundance at Oak Grove in 2007 and 2008. “Cedar open” samples were not collected in 2007. Letters indicate significantly different means. DISCUSSION In this study, we examined the effects of invasive eastern redcedar on invertebrate communities by pitfall-trapping in a riparian area at the Nature Conservancy’s Niobrara Valley Preserve in north-central Nebraska and in an upland savanna at Oak Grove Park near Hawarden, Iowa. Our Niobrara data showed that redcedar adversely affected ant and overall invertebrate abundance in both 2007 and 2008 and invertebrate richness in 2007, while at Oak Grove we found significantly greater invertebrate richness beneath cedars in 2007, and higher overall abundance and ant abundance beneath cedars in 2008. Results from our Niobrara site support our hypothesis that invertebrate abundance is adversely affected by the presence of redcedar, while results from Oak Grove suggest otherwise. Differences between the two sites can be accounted for perhaps by the different terrain, substrate, and slope exposure, as our Nebraska site was sandy and flat, while our Iowa site was grassy and sloped. However, a more plausible explanation for the difference is that Oak Grove Park is heavily invaded by downy brome and leafy spurge, two invasives not directly studied in this experiment. Little is known about the effects of these invasives on invertebrate diversity, but all of our non-cedar pitfall traps at Oak Grove were placed in the ground where one or both of these species were prevalent, so an apparent preference among invertebrates for cedar-infested areas over non-cedar areas at Oak Grove may simply be an avoidance of these two invasives rather than an actual preference for cedar. Neither of these invasives were found at the Niobrara site, and no other invasives were apparent in any of the Niobrara plots. More sampling dates and sampling over multiple years are necessary to fully understand the effects of redcedar and its eradication on invertebrate communities. For example, long-term monitoring will elucidate the effects of the restoration process. Repeated sampling of the area over a period of several years will reveal whether the invertebrate community in the cedar-removed area gradually becomes more like the non-cedar area in invertebrate composition. Moreover, we know from our research of the European buckthorn at Northwestern College’s Alton forest and at Oak Grove that patterns in invertebrate abundance and richness between areas of high and low buckthorn density vary from one season to the next, and without repeated sampling we can only draw tentative conclusions about the effects of cedar in the fall. The results of this study can help us more fully understand the impacts of redcedar on its surroundings. Not only can redcedars displace native plants, alter soil properties, lower water tables, and decrease the quality of pastureland, they also appear to displace many of the invertebrates living in invaded areas. Conversely, redcedars may act somewhat as a refuge if the surrounding areas are heavily degraded (e.g., by other invasives), and this potential benefit of redcedars should be considered before they are removed in an attempt to restore a grassland ecosystem. One encouraging finding in our Niobrara study is that within two years of the removal of cedars in a heavily-infested area, the invertebrate populations have already shown signs of recovery in that invertebrate richness and abundance are not significantly different from the nearby area where redcedars have never invaded. RESULTS NIOBRARA: In 2007 we found significant differences in overall abundance across treatments, with the lowest abundance beneath cedar trees, intermediate abundance in the cedar-removed area, and highest abundance in the non-cedar area (Fig. 1). In 2008, we found significantly lower overall abundance beneath cedar trees and no differences among the other three treatments. We also found higher ant abundance in cedar-free areas than beneath cedar trees, with cedar removed areas having greater ant abundance than cedar infested areas in 2007 (Fig. 3). Samples in 2007 revealed no differences in morphospecies richness among sites, while 2008 samples showed the cedar-cover samples to have the lowest richness and non-cedar samples to have the highest richness, with cedar-open and cedar-removed samples having intermediate values (Fig. 2). OAK GROVE: In 2008 we found significantly HIGHER invertebrate abundance in cedar cover samples than the other treatments (Fig. 4). In 2007 we found a similar trends in morphospecies richness, with greater species diversity in cedar areas than in non-cedar areas (Fig. 5). 2007 showed no significant relationship, however, greater numbers of species were recorded. Though no significance was reported for ants in 2007, we found higher ant abundance in cedar cover samples then in cedar-open samples (Fig. 6) in 2008. REFERENCE Drake, B., and Todd, P.  2002.  A strategy for control and utilization of invasive juniper species in Oklahoma: Final report of the redcedar taskforce.  Oklahoma Department of Agriculture, Food, and Forestry.