Responses in trophic interactions –
Work package 2.3

Climatic and environmental changes affect plant tissue composition with likely impacts on the performance of insect herbivores (Buck and Callaghan 1999). For instance, elevated CO₂ may change the relative abundance of leaf chewing and leaf sucking insects (Bezemer and Jones 1998). Changes in moisture may stimulate or reduce herbivore activity (Oswald and Brewer 1997), and increased temperature with associated increased nitrogen availability is known to promote attack from herbivores (Coley et al 1985). Because herbivores interact strongly with other consumers of plant carbon above and below ground (Van der Putten et al. 2001) and with the plant itself, the climate change effects on herbivores is likely to translate into effects on the whole community of organisms associated with the plant. One of the most striking effects of insect herbivores on plant performance is their ability to change rates of root exudation of compounds utilized by belowground organisms. The exudation increases in response to attacks of leaf chewers, e.g. beetles, (Wamberg et al. 2003) but is reduced following attack from leaf suckers, e.g. aphids, in the plant growth phase (Vestergård et al. submitted). This effect of herbivores may affect the entire soil fauna and the decomposers with implications for the process rates of the soil organic matter.

Interactions between plants and herbivores in relation to climatic conditions and changes will be studied in an experiment with controlled herbivory in the treated plots. Aphids and leaf beetles will be placed on the plants in clip cages, and the change in abundance will be analyzed at intervals. The exact application, including selection of herbivores, will be developed during 2005-2006 and the full-scale experiment will run in 2007 when the accumulated effects of the treatments are expected to be strongest. Two applications will be done with start early in the season when the rise in temperature promotes herbivore activity and in summer when drought-effects are expected to be at a maximum.

The field studies will be combined with mesocosm (monolith) experiments in the laboratory in which the herbivores are subjected to similar treatments as in the field experiment. This will be combined with analysis of belowground respiration and the population growth of protozoa and nematodes, which has proven to correlate positively with carbon exudation from the plant roots (Wamberg et al. 2003). This investigation will also include determination of herbivory-induced changes in microbial diversity (methodology as WP3.3). In the mesocosms we also propose to measure the transfer of atmospheric C to rhizosphere soil, bulk soil, and soil fauna via the plant roots by following the ¹³C signature of these fractions. This will be possible in the elevated CO₂ treatments because of its particular ¹³C/¹²C profile. This part of the research is closely linked with WP4.2, and further described there.

For both the field and the laboratory studies, we expect that the results will show how performance of herbivores depends on climate change induced alterations in chemistry of the plants (analyzed in WP2.1). We also expect to learn how the likely treatment effects on the leaf herbivores translate into effects in the rhizosphere soil fauna and decomposers through the herbivores’ influences on root exudation rates. At elevated CO₂ it is revealed to what extent rhizosphere processes (bacterial channel) interact with decomposition of soil organic matter (fungal channel) as affected by the treatments.