Invasive invertebrate herbivores and nitrogen addition affect arbuscular mycorrhizal colonization of Sitka Willow at Mount St. Helens
Abstract
Sitka willow is the most abundant woody plant colonist on Mount St. Helens’ Pumice Plain, where the 1980 eruption removed all biological materials. In this study we determined how N deposition and insect herbivory affect colonization by arbuscular mycorrhizal fungi (AMF) of willow roots. AMF are obligate biotrophs that supply their hosts with soil-derived nutrients in exchange for C. Insect herbivory and N deposition can impact the association between plants and their AMF, altering the degree to which C is allocated to them, leading to consequent changes in their overall abundance.
We measured AMF colonization in willows attacked by or excluded from weevils and subject to experimental N addition (0, 8, or 16 kg/ha/yr) for six years in two primary sites (i.e. no biological legacies) and two secondary successional sites (where above ground life was destroyed but soils remained). We hypothesized that AMF colonization would decrease with increasing N-addition, and colonization would be lower when compounded with weevil herbivory. We hypothesized that herbivory and N-addition would lead to more pronounced decreases in the primary compared to secondary sites.
Colonization was greater in secondary (8.75% ±2.1) compared to primary sites (4.5% ±0.97). There was a positive effect of weevil exclusion in primary sites on AMF colonization (p<0.01). There was a site x N interaction in secondary sites (p<0.04). The highest level of N increased AMF colonization at one site but not the other. Secondary sites appear to respond to N whereas weevil affects are more pronounced in primary sites.
About the Presenter
Adrian Nelson-Wheatley
Adrian is a junior in the Earth and Environmental Science program at WSUV. His interests include entomology, paleontology, and hiking. After receiving his undergrad, he hopes to study stream and wetland ecology at the graduate level.