Grantee Research Project Results
Final Report: Diversity and Abundance of Forest Soil Arthropods Under Elevated Carbon Dioxide
EPA Grant Number: R825861Title: Diversity and Abundance of Forest Soil Arthropods Under Elevated Carbon Dioxide
Investigators: Lincoln, David E. , Williams, Ray S.
Institution: University of South Carolina at Columbia
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1997 through September 30, 2000
Project Amount: $332,902
RFA: Terrestrial Ecology and Global Change (1997) RFA Text | Recipients Lists
Research Category: Climate Change , Ecological Indicators/Assessment/Restoration , Aquatic Ecosystems
Objective:
Large plant-mediated changes are predicted for grazing and detrital food webs under elevated carbon dioxide. One of the major responses of plants derives from the increased photosynthetic rate under elevated CO2, which leads to an elevation of leaf carbohydrate content and a depression of leaf nitrogen content, resulting in an increase in the leaf carbon:nitrogen ratio and an increased lignin:nitrogen ratio. Thus, the quality of the food supply for litter decomposing biota is likely to be changed by the projected increase in atmospheric CO2. The greatest reservoir of animal diversity in temperate systems is in the litter and soil: a single m2 may contain several hundred arthropod species. This complex fauna plays a significant role in litter decomposition and nutrient cycling and contributes to the metabolism of up to 90 percent of NPP. The present study experimentally examines the response of the litter-dwelling arthropods, their trophic structure, species diversity, and functional impact to elevated atmospheric CO2 and the concomitant changes in their resource base.
This study was conducted over a 3-year period in two plantation forests, an 8-year-old sweetgum forest, and a 14-year-old loblolly pine forest, using 25 m and 30 m diameter plots, respectively, which receive supplemental CO2 raising the concentration in the canopy by +200 ppm over ambient, or remain at ambient CO2 . The project has two primary elements: a biannual census of the microarthropod assemblages in the full litter layer (including pretreatment litter) and a litterbag experiment to examine the microarthropod response to treatment litter and their effect on its decomposition.
Summary/Accomplishments (Outputs/Outcomes):
The response of litter-dwelling microarthropod assemblages of the FACE site at Duke Forest, a loblolly pine plantation, was documented over the first 18 months of CO2 enrichment. The response variables included the abundance of the main microarthropod groups, most of which are comprised of, or dominated by, microbial feeders. While CO2-induced changes in litter quality might be expected to alter litter populations (Cotrufo, et al., 1998; Hattenschwiler, et al., 1999), we anticipated no litter quality effects in the present study. Because loblolly pine needles take 18 months to mature and senesce, the needles that fell before the spring of 1998 were unlikely to substantially reflect the CO2 treatment. Responses in the litter assemblages, therefore, may reflect belowground effects that have been transmitted to the litter.
The microarthropod abundances were measred over four sampling dates during the first 18 months of the CO2 treatment beginning in the fall of 1996, 5 weeks after fumigation began. Six 10-cm x 10-cm samples were taken of the organic horizon from each plot at each date. The samples include the litter and organic layers, but no mineral soil. This captured the majority of the microarthropod populations; sampling of the litter and first 20 cm of mineral soil outside the plots in the spring of 1998 that only 6.5 percent of the microarthropod abundance is in the top 10 cm of the mineral soil. This proportion is consistent with the results of extensive sampling in Duke Forest by Pearse (1946), whose 0-2 inches in depth category most likely included some of the organic horizon. Microarthropods were extracted from the samples using modified Tullgren funnels. We focused on the four prominent groups of microarthropods, Collembola and the suborders of mites, Prostigmata, Mesostigmata, and Oribatida. Unlike the other groups, the oribatid mites have immature stages that are easily distinguished from the adults; immature and adult oribatid mites were sorted separately.
As anticipated, there were no differences between ambient and elevated plots in percent carbon, nitrogen or lignin of fresh pine litter from the fall of 1996 and 1997. Likewise, there were no differences in carbon or nitrogen content in the organic horizons. Deciduous litter made up from 12-38 percent of the litterfall in 1997. As for the pine, Finzi, et al. (2001) found no differences in the litter chemistry of the dominant hardwood species or in total litterfall amounts in 1997 at this site.
Over the first 19 months of the elevated CO2 treatment, the total microarthropod abundance declined in elevated plots, to two-thirds of the abundance in ambient plots (P = 0.042) (see Table 1). The trend toward a relative decline in elevated CO2 plots is apparent, though not significant, in most microarthropod groups (Collembola P = 0.332, Prostigmata P = 0.156, Mesostigmata P = 0.063). The clearest response is evident in the dominant group, the adult oribatid mites, whose mean abundances in elevated plots declined to 61 percent of the abundance of control plots on the final sample date. Many oribatid mite species have development times of a year, with adult life spans of 1 to 3 years (Norton 1994). The adult oribatid mite response that appears a year and a half after the onset of treatment is foreshadowed by the decline of 50 percent in the immature oribatid mites in spring of 1997, only 8 months after the fumigation began.
Table 1. Abundances (numbers per 100 cm2) in Microarthropod Groups in Spring 1998 After 18 Months of Treatment. Data are means of the 3 plots in each treatment and standard errors.
Elevated CO2 | Ambient CO2 | |
Total microarthropods | 295±32 | 444±51 |
Adult oribatid mites | 159±33 | 261±34 |
Immature oribatid mites | 22± 2 | 27± 8 |
Collembola | 64± 7 | 82±11 |
Prostigmata | 33± 1 | 52±18 |
Mesostigmata | 16± 2 | 22± 2 |
At the Oak Ridge site, the CO2 elevation treatment and our sampling began in the spring of 1998. Our samples to date show a much lower abundance of animals compared to the Duke sites, but have already revealed a change in litter quality (i.e., increased C:N ratio), of the fall 1999 litter.
We also examined the extent to which stable isotope of carbon associated with the isotopic signature of the CO2 added in the elevated CO2 treatments could be used to trace trophic relationships. In the summer of 1999, we examined the 12/13C signature of the litter profile and of an arthropod food chain in ambient and elevated rings. The elevated CO2 12/13C signature was present in the upper litter horizons (fall 1998 litter) and in fine roots compared to the ambient rings (39.4 versus 28.1 delta ppt, respectively). There was not a significant elevated treatment carbon signature, in either the lower litter horizons originating from fall 1997 litter or in the bulk mineral soil at the surface. The 12/13C signature was readily detectable in collembola (fungal feeders) and in spiders (primary predators on microarthropods). Both collembola and spiders from the elevated rings were nearly 10 ppt greater than those from ambient rings, 25.9 versus 16.2 ppt, respectively, indicating that the food webs had been saturated with signal despite the retention of pretreatment litter in the lower litter profile layers.
The progression toward lower microarthropod numbers in elevated CO2 rings was reversed, following the arrival of litter with significant treatment influences. By the fall of 1999, abundances of most groups had become equivalent in the two treatments and the abundances of collembola, a microarthropod group codominant with the oribatid mites, were higher in the elevated rings. This may be attributable to greater litterfall in elevated rings providing a thicker litter layer that buffers the effects of drying or to other litter effects not associated with changes in quality.
Significance. The microarthropods of the litter and soil are a diverse group, encompassing many life-history strategies, feeding habit, and microhabitat preferences. The initial trend toward lower microarthropod abundance in elevated CO2 is consistent across the four major microarthropod groups and appears early after the onset of the treatment. This suggests a driving mechanism that acts broadly upon some aspect of the microbial resource base or habitat.
While we detected no changes to the litter, concurrent work on the same site shows that there were treatment effects on belowground root dynamics during the study period. The influence of such belowground changes on litter fauna is most likely indirect, through linkages between minerals and organic strata. Such linkage is strongly suggested by the finding of Finzi, et al. (2001) that the 1997 pine litter in the elevated plots immobilized significantly less nitrogen in its first year of decomposition. This was true whether the litter had grown under elevated or ambient conditions. An observed trend toward declining nitrogen in the full litter profile in elevated plots, though insignificant, is consistent with this finding.
The responses documented in the first 18 months may be transitory, since changes in litter input have since occurred. DeLucia, et al. (1999) report that litterfall mass was 21 percent higher in the elevated plots in 1998. This represents both an increase in the amount of habitat in the elevated CO2 plots and increased buffering of the habitat against drying. Both of these direct effects on the litter habitat are favorable to microarthropod populations. Hence, the populations in elevated CO2 may rebound over a longer timeframe.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 5 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Hansen RA, Williams RS, Degenhardt DC, Lincoln DE. Non-litter effects of elevated CO2 on forest floor microarthropod abundances. Plant and Soil 2001;236(2):139-144. |
R825861 (1999) R825861 (Final) |
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Supplemental Keywords:
global change, biodiversity, elevated carbon dioxide, litter quality, decomposition, soil arthropods, Collembola, Oribatida, C:N ratio, fine root turnover., RFA, Scientific Discipline, Air, Waste, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Environmental Chemistry, climate change, Forestry, Monitoring/Modeling, Atmospheric Sciences, plantation forests, environmental monitoring, biodiversity, forest soil arthopods, carbon dioxide, CO2 concentrations, elevated carbon dioxide, environmental stressors, habitat diversity, harmful environmental agents, litter bags, climate variability, arthropodsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.