Ray S. Williams

Loblolly pine grown under elevated CO2 affects early instar pine sawfly performance

Seedlings of loblolly pine Pinus taeda (L.), were grown in open-topped field chambers under three CO2 regimes: ambient, 150 μl l−1 CO2 above ambient, and 300 μl l−1 CO2 above ambient. A fourth, non-chambered ambient treatment was included to assess chamber effects. Needles were used in 96 h feeding trials to determine the performance of young, second instar larvae of loblolly pines principal leaf herbivore, red-headed pine sawfly, Neodiprion lecontei (Fitch). The relative consumption rate of larvae significantly increased on plants grown under elevated CO2, and needles grown in the highest CO2 regime were consumed 21% more rapidly than needles grown in ambient CO2. Both the significant decline in leaf nitrogen content and the substantial increase in leaf starch content contributed to a significant increase in the starch:nitrogen ratio in plants grown in elevated CO2. Insect consumption rate was negatively related to leaf nitrogen content and positively related to the starch:nitrogen ratio. Of the four volatile leaf monoterpenes measured, only β-pinene exhibited a significant CO2 effect and declined in plants grown in elevated CO2. Although consumption changed, the relative growth rates of larvae were not different among CO2 treatments. Despite lower nitrogen consumption rates by larvae feeding on the plants grown in elevated CO2, nitrogen accumulation rates were the same for all treatments due to a significant increase in nitrogen utilization efficiency. The ability of this insect to respond at an early, potentially susceptible larval stage to poorer food quality and declining levels of a leaf monoterpene suggest that changes in needle quality within pines in future elevated-CO2 atmospheres may not especially affect young insects and that tree-feeding sawflies may respond in a manner similar to herb-feeding lepidopterans.

Non-litter effects of elevated CO2 on forest floor microarthropod abundances

The arthropod assemblages of the litter and soil play significant roles in decomposition and nutrient cycling. At the FACE (Free-Air CO2 Enrichment) site at the Duke Forest, we assessed the responses of the litter microarthropod assemblage to elevated CO2 (200 ppm above ambient) in a loblolly pine plantation. Following the initiation of the elevated CO2 treatment, a trend toward lower microarthropod abundance under elevated CO2 emerged. After 18 months, the mean microarthropod abundance was 33% lower in the elevated treatment (P=0.04). The decline was evident in all microarthropod groups, but was significant only in the oribatid mites (P=0.04). Because these responses precede any changes in litter quality resulting from the CO2 treatment, they may reflect plant-derived changes in the soil that are being conveyed into the litter layer.

Effects of corridors on genetics of a butterfly in a landscape experiment.

Abstract To investigate the possible role of landscape connectivity on the genetic structure of isolated populations, we examined the effects of habitat corridors on the population genetics of a vagile butterfly species, Junonia coenia, within a large-scale, experimental system. Using allozyme electrophoresis, a total of nine loci were identified and scored, six of which exhibited polymorphism. Our data demonstrated consistently higher levels of expected (He) and observed (Ho) heterozygosity in butterflies sampled from patches connected by corridors compared to unconnected patches. A t-test comparing He and H0 in connected versus unconnected patches found a marginally significant difference in one locus, the glycolytic enzyme phosphoglucose isomerase (PGI). Connected patches exhibited overall lower F ST values compared to unconnected patches, indicating potentially increased levels of gene flow due to corridors. Our results support previous investigations on dispersal and population size for J. coenia, and show that higher dispersal through corridors promotes genetic variability at a locus (PGI) implicated in dispersal and fitness in butterflies.

Ground-dwelling Beetle Responses to Long-term Precipitation Alterations in a Hardwood Forest

Abstract It is widely predicted that regional precipitation patterns may be altered due to climate change, and these changes may affect areas with extensive forests. Therefore, studies investigating the role of this climate driver on forest floor fauna are timely. We examined the impact of precipitation alteration over 13 years on Coleoptera (specifically Family Carabidae) communities in a temperate forest by testing the effects of dry (33% precipitation interception), ambient (control), and wet (33% precipitation addition) treatments. We collected insects in pitfall traps and quantified forest-floor physical and chemical parameters. Beetle abundance and Carabidae tribe richness were significantly reduced in dry plots. Community similarity was substantially higher between wet and ambient plots compared to dry plots due to the substantial reduction of three dominant carabid tribes. Litter mass increased overall, litter nitrogen decreased, and carbon:nitrogen ratio (C:N) and total phenolics increased in the dry-plot Oi horizon. Beetle abundance and tribe richness were positively related to soil moisture, and beetle abundance was negatively related to litter mass. Microarthropod abundance was highest in the dry treatment. This study provides evidence that shifting precipitation patterns predicted with climate change could alter important ground-fauna communities in extensive ecosystems such as temperate forests.

Effects of Intraspecific Genetic Variation and Prior Herbivory in an Old-Field Plant on the Abundance of the Specialist Aphid Uroleucon nigrotuberculatum (Hemiptera: Aphididae).

Abstract Intraspecific genetic variation in plants can contribute to the diversity and abundance of associated insects, though many questions remain about why some genotypes support more insects than others. Since plant secondary metabolites, which may be induced after insect attack, may potentially vary among genotypes, these compounds provide a possible explanation for insect abundance variation in plants with substantial genetic variation. In this study, we examined four genotypes of the old-field plant species Solidago altissima (L.; Asterales: Asteraceae) and asked if the abundance of the specialist aphid Uroleucon nigrotuberculatum (Olive; Hemiptera: Aphididae) was affected by genotype and previous foliage damage by a specialist beetle. We hypothesized that different genotypes and prior herbivory would result in different quantities of terpenes produced by S. altissima, and that terpenes would affect aphid abundance. We found evidence of foliar terpene induction in a greenhouse environment, and significant differences in terpene production among genotypes in a field setting, though prior damage had little effect on aphid abundance in the field. There were significant effects of genotypes on aphid abundance, as well as genotype effects on terpenes and foliar nutrients (leaf N and C:N). Noteworthy was a change in the allocation of particular terpenes among genotypes that related to aphid abundance. Our analyses demonstrated that phytochemicals, and especially terpenes, related to aphid abundance. This study adds to a previous finding that variation in leaf terpenes in S. altissima provides a partial explanation for variable abundance among genotypes of a specialist aphid, and suggests that differences in the allocation of compounds is important.