Final Technical Report Award: DE-FG02-06ER64232 Recipient: The Board of Regents of the University of Wisconsin System Project title: Impact of elevated CO2 and O3 on insect-mediated ecosystem processes in a northern deciduous forest Project PI: Richard L. Lindroth Date: Nov. 20, 2011 Project dates: July 1, 2006 – June 30, 2009, with no-cost extension through December 2010 Distribution limitation notices None Executive summary Rising concentrations of atmospheric CO2 and O3 are altering the structure and function of forest ecosystems. Herbivorous insects are the major consumers in temperate deciduous forests, with the capacity to dramatically alter tree growth (via outbreaks), forest community composition and ecosystem dynamics (e.g., nutrient cycling). Until recently, however, experimental quantification of the impacts of CO2 and O3 on canopy herbivore communities and rates of defoliation and nutrient flux has not been addressed. This research, conducted at the Aspen FACE (Free Air CO2 Enrichment) facility in northern Wisconsin, U.S.A., evaluated the independent and interactive effects of CO2 and O3 on 1) the abundance and diversity of forest canopy insect communities, and 2) rates of insect herbivory and transfer of material (leaf greenfall and insect frass) from the canopy to the forest floor. Results of studies of individual insects revealed that elevated CO2 and O3 influence the performance of individual species of damaging insect pests, but the magnitude of impact is influenced by both insect species and their host tree species. Censuses of canopy insects showed that some species were positively affected, some negatively affected, and some not affected by elevated CO2 and O3. Moreover, overall species diversity was generally not strongly affected by CO2 and O3. In summary, the effects of CO2 and O3 on forest insects is highly variable among species and over time, and thus difficult to generalize across broad taxonomic groups. Estimates of foliar damage revealed that CO2 and O3 have pronounced effects on canopy damage by insect herbivores. Averaged over three years, foliar biomass lost to insect feeding increased 86% in high CO2 environments and decreased 12% in high O3 environments. The increases/decreases were greater for aspen than for birch, indicating that the selective pressure of insects will shift across tree species in forests of the future. Herbivore-mediated material (green leaf tissue, insect frass) transfer from the canopy to the forest floor increased 37% in elevated CO2 and decreased 21% in elevated O3. Nitrogen transfers paralleled those results: 39% increase in elevated CO2 and 19% decrease in elevated O3. 1 In summary, this body of work indicates that in future environments with elevated CO2 and O3, the performance of individual species of tree-feeding insects will change, but that effects will be highly variable among insect and tree species. Overall insect diversity and abundance may not be strongly affected. In contrast, the foliar damage caused by tree-feeding insects will likely increase significantly under elevated CO2 conditions, but decrease under elevated O3 conditions. Comparison of accomplishments with goals and objectives The objectives of this research were to evaluate the independent and interactive effects of CO2 and O3 on 1) the abundance and diversity of forest canopy insect communities, and 2) rates of insect herbivory and transfer of material (leaf greenfall and insect frass) from the canopy to the forest floor. The original goal was to conduct this work in three forest community types: aspen- birch, aspen-maple, and mixed aspen genotype. Due to the magnitude of work involved, and small stature of maple trees at the site, we excluded the aspen-maple community type from our research activities. In addition to the work originally proposed, we conducted targeted performance studies with several species of tree-feeding insects, and also evaluated the effects of CO2- and O3-mediated changes in leaf litter chemistry on performance of litter-feeding invertebrates. Summary of project activities and results The overall objective of this research program was to assess the independent and interactive effects of CO2 and O3 on herbivorous insect communities (abundance/ biodiversity), and feedback effects of insects on rates of herbivory and conversion of primary production to organic substrates, in forest canopies at the Aspen FACE (Free Air CO2 Enrichment) site in northern Wisconsin, U.S.A. Activities and results are summarized below, organized by original hypothesis. Hypothesis 1. Elevated CO2 and O3 will alter the abundance and biodiversity (richness and relative composition) of herbivorous canopy insects at Aspen FACE. Insect abundance, diversity and community composition were monitored via routine trapping and visual censuses in two forest types (aspen-birch and mixed aspen genotype) at the Aspen FACE facility during the summers of 2005-07. The number of insects in the visual censuses alone numbered over 36,000. Data were analyzed via standard statistical time-series analyses (abundance data) and state-of-the-art community analyses (e.g., nonmetric multidimensional scaling [NMDS] and analysis of similarity [ANOSIM]). Controlled bioassays with individual insects revealed that elevated CO2 and O3 influence the performance of individual species of damaging insect pests, but the magnitude of impact is influenced by both insect species and their host tree species. Field censuses of insect abundance revealed that elevated CO2 tended to increase the abundance of phloem-feeding insects and decrease the abundance of the chewing and galling insects. Insect abundance under elevated O3 typically had the opposite response of insect abundance under elevated CO2. However, despite some general patterns, insect responses to elevated CO2 and O3 varied considerably depending on the insect species and sample year. Analyses for idiosyncratic effects revealed multiple strong 2 two- and three-way interactions between CO2, O3, arthropod species, and year indicating that the effects of eCO2 and eO3 on arthropod abundance are species-specific and temporally variable. The total abundance of insects surveyed was rarely affected by elevated CO2 and O3 because family/species responses were offsetting and idiosyncratic. Similarly, elevated CO2 and O3 rarely altered the species composition of forest canopy insect communities. A consistent finding in this research is that the effects of elevated CO2 and O3 vary among tree species, insect species, and over time, such that population- and community-level responses appear to be idiosyncratic and species-specific. Complementary bioassays with detritus-feeding invertebrates (earthworms and springtails) showed that elevated carbon dioxide reduced nitrogen and increased condensed tannin concentrations in leaf litter. These changes were associated with decreases in earthworm individual growth, earthworm growth efficiency, and springtail population growth. Elevated ozone increased fiber and lignin concentrations of leaf litter. These changes did not influence earthworm consumption or growth, but accelerated springtail population growth. These results suggest that changes in litter chemistry caused by increased carbon dioxide concentrations will have negative impacts on the productivity of diverse detritivore taxa, whereas those caused by increased ozone concentrations will have variable, species-specific effects. Hypothesis 2. Elevated CO2 and O3 will alter herbivory rates and reapportion foliar herbivory among tree species and genotypes at Aspen FACE. To evaluate the effects of CO2 and O3 on rates of canopy herbivory, we measured foliar damage in two forest types (aspen-birch and mixed aspen genotype) at Aspen FACE during the summers of 2006-08. A sample of 10-12 randomly selected leaves was collected from each of two canopy levels, for each of four trees, for each of four genotypes/species (3 aspen genotypes & birch) for each FACE ring, three times during the growing season (June, July, August), for a total of ~14,000 leaves/year. All leaves were digitally scanned and leaf damage estimates were assessed via image analysis (WinFolia software). The sets of 10-12 leaves were then processed for subsequent chemical analysis, including carbon, nitrogen, simple sugars, starch, fiber, lignin, tannins and phenolic glycosides. Estimates of foliar damage revealed that CO2 and O3 have pronounced effects on canopy damage by insect herbivores. Averaged over three years, foliar biomass lost to insect feeding increased 86% in high CO2 environments and decreased 12% in high O3 environments. The increases/decreases were greater for aspen than for birch, indicating that the selective pressure of insects will shift across tree species in forests of the future. Changes in phytochemistry under elevated CO2 and O3 explained variation in gypsy moth and forest tent caterpillar performance in insect bioassays. Variation in phytochemistry also explained overall canopy damage patterns in aspen, but not birch. This finding suggests that combinations of factors (i.e., biotic and abiotic) will drive patterns of canopy damage in northern temperate forests in the future, and will vary among tree species. 3 Hypotheses 3. Elevated CO2 and O3 will alter the quantity and quality of organic substrates (frass and green leaf litter) produced by insects and transferred from the canopy to the forest floor. To determine the effects of CO2 and O3 on the rates of insect-mediated transfer of organic substrates (frass and