DEFOLIATION AND BELOW-GROUND HERBIVORY IN THE GRASS MUHLENBERGIA-QUADRIDENTATA - EFFECTS AN PLANT PERFORMANCE AND ON THE ROOT-FEEDER PHYLLOPHAGA SP (COLEOPTERA, MELOLONTHIDAE)

In this study we evaluated (1) the combined effects of simulated defol iation and below-ground herbivory (BGH) on the biomass and nitrogen co ntent of tillers and roots of the bunchgrass Muhlenbergia quadridentat a and (2) the effect of defoliation on the survival of third-instar ro ot-feeder larvae of Phyllophaga sp. The experiment was performed in a pine forest area at an altitude of 3200 m above sea level. The grass a nd the root-feeder species were native and dominant in the understory and in the macroarthropod root-feeder communities, respectively. Plant s were established in pots in the field and were subjected to the foll owing treatments in a factorial design: simulated defoliation (three l evels) and BGH (with or without root-feeder larvae) with ten replicate s per treatment. Plants were defoliated three times at 2-month interva ls. The interaction between defoliation and root herbivory was signifi cant for all components of plant biomass. In every case, light defolia tion with BGH decreased live aboveground, root and total plant biomass , and the number of live tillers by more than 50% with respect to the same defoliation level without root-feeders. Plants apparently did not compensate for the carbon drain by root-feeders when a high proportio n of older leaves were not removed by defoliation. Plants under heavy defoliation were not affected by the presence of root-feeders and show ed a greater live/dead above-ground biomass ratio than lightly defolia ted and control plants. Defoliation and BGH did not change tiller and root N concentrations but root herbivores did decrease live-tiller N c ontent in lightly defoliated plants. Root-feeders but not defoliation decreased the root/shoot ratio by 40% and the live/dead above-ground b iomass ratio by 45% through increased tiller mortality. Survivorship a nd final biomass of Phyllophaga sp. larvae were not affected by defoli ation treatments during the 6-month study period.