Effects of elevated CO2 and temperature on plant growth and herbivore defensive chemistry

Concentration of atmospheric CO2 and temperature have both been rising for the last three decades. In this century, the temperature has been predicted to rise by 2-5degreesC and the CO2 concentration to double. These changes may affect the primary and secondary metabolism of plants and thus have implications for other trophic levels. However, the biotic interactions in changing climate conditions are poorly known. In this study, two questions were addressed: W How will climate change affect growth and the amounts of secondary compounds in flexible plant species? and 60 How will this affect herbivores living on this species. Four clones of the dark-leaved willow (Salix myrsinifolia (Salisb.)) seedlings were grown in closed-top chambers with two controlled factors: concentration of atmospheric CO2 and temperature (T). There were four combinations of these factors, each combination replicated four times (total of 16 chambers): (i) Control CO2 (350 ppm) and control T, (ii) Elevated CO2 (700 ppm) and control T, (iii) Control CO2 and elevated T (2degreesC), and (iv) Elevated CO2 and elevated T. Stem growth and aerial biomass of the plants were determined; and the leaf phenolics, nitrogen and water concentrations were analysed. In addition the growth rate of larvae and feeding preference of adults of a specialist herbivore, the chrysomelid beetle Phratora vitellinae (L.), on the treated willow leaves were measured. Elevated temperature and CO2 concentration increased the stem biomass and elevated CO2 increased leaf biomass and total aerial biomass of the willows. Patterns of biomass allocation were different in different temperature treatments. At elevated temperature there was less branch and leaf material in relation to stems than at the control temperature. Moreover, patterns of biomass allocation differed among clones. CO2 enhancement increased the specific leaf weight (SLW) and reduced both water and nitrogen content of the leaves, however, leaf area was unaffected by the treatments. Carbon dioxide (CO2) and T enhancement reduced the concentrations of several phenolic compounds in the leaves. Phenolic compounds, nutrients, and water in the leaves might be diluted partly due to increased carbon allocation to different structures (e.g. thickening of cell wall and increase of trichomes, etc.). In some cases plant clones showed specific responses to treatments. The CO2 enhancement reduced the relative growth rate (RGR) of the beetle larvae, and in contrast, temperature elevation increased it. Adult beetles did not clearly discriminate between willow leaves grown in different T and CO2 environments, but tended to eat more leaf material from chambers with doubled CO2 concentration. At elevated CO2 adult beetles may need to eat more leaf material in order to reproduce, which may in turn prolong the life cycles, increasing the risk of being eaten and possibly affecting ability to overwinter successfully. Overall, climate change may significantly modify the dynamic interaction between willow and beetle populations.