PARENT SESSION
Poster Session #3: Herbivory.
Monday, August 6, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

The effects of grazing and mycorrhizae on growth of New England Aster.

Kalberer, Scott¹, Pfitsch, William¹, ¹

ABSTRACT- Herbivores and mycorrhizal fungi both represent a sink for carbon fixed by plants although mycorrhizae enhance plant growth by providing greater access to soil resources. Many plants are able to compensate for tissue lost to herbivores with increased growth. What is the role of mycorrhizae in compensatory growth after grazing? We simulated grazing on one-month old vesicular-arbuscular mycorrhizal Aster novae-angliae plants and followed their recovery with periodic harvests during two months. Clipped plants fully compensated for lost shoot mass by the end of the sample interval. Over time, the unclipped control plants exhibited decreased leaf area and mass ratios (LAR and LMR) and specific leaf area (SLA), while allocation to stem tissue increased and root allocation (RMR) remained constant. Clipped plants overcompensated for initial losses of aboveground tissue with greater LAR, LMR and SLA and lower RMR by the end of the experiment. Interestingly, maximum relative growth rates of control and clipped plants did not differ. In a separate experiment, we measured shoot and root respiration and biomass allocation of three-month old mycorrhizal and non-mycorrhizal plants 20 days after experimental treatment plants were clipped. Mycorrhizal plants were nearly twice the size of non-mycorrhizal plants at this harvest. Clipped plants in both treatments did not differ in total biomass from control plants. As in the timecourse experiment, mycorrhizal plants responded to clipping with increased LAR and LMR and decreased RMR. Root respiration rates of clipped mycorrhizal plants were not different from controls, but shoot respiration rates were higher. This may represent an increased carbon cost of mycorrhizae following grazing, but more likely represents increased metabolic activity of leaf tissues. There were no clipping treatment effects on allocation patterns or respiration rates of the non-mycorrhizal plants. It appears that mycorrhizae do not represent a carbon drain for Aster novae-angliae inhibitory to compensatory growth following grazing.

KEY WORDS: compensatory growth, grazing, mycorrhizal fungi, biomass allocation