INVERTEBRATE HERBIVORY AND NITROGEN LIMITATION INTERACT TO SHAPE SOIL DEVELOPMENT DURING SUCCESSION ON MOUNT ST. HELENS

Rebecca Evans

Ecosystem development in primary succession is constrained by nutrient limitation, low biological inputs, and strong biotic filters. On the Mount St. Helens, the foundational shrub Salix sitchensis plays a central role in initiating soil formation, yet its growth and associated plant-soil feedbacks are strongly suppressed by the invasive stem-boring weevil Cryptorhynchus lapathi. Across eight years, I used a factorial field experiment manipulating weevil herbivory and nitrogen (N) addition in paired primary and secondary successional sites to investigate how top-down pressure and nutrient limitation interact to influence plant productivity, soil development, and microbial taxonomic and functional assembly. In Chapter 1, I show that weevil herbivory is a dominant constraint on early ecosystem development. In primary succession, weevil exclusion increased willow basal area growth from −1% to 20% per year, and to 53% per year when combined with N-addition, whereas non-excluded plots showed minimal growth regardless of fertilization. Exclusion increased soil carbon (C) by ~40% beneath willow canopies, and structural equation models revealed that N-addition enhanced soil C indirectly via increased plant growth. In contrast, secondary successional soils, where legacy organic matter and established root networks buffer resource limitation, showed little response to either herbivory or N-addition. In Chapter 2, I demonstrate that invertebrate herbivory and N-addition also shapes belowground microbial communities. In primary soils, weevil exclusion increased fungal diversity by ~30% and shifted both fungal and bacterial community composition (each ≈10% variance explained) Weevil exclusion increased the proportional abundance of mutualistic fungal guilds and reduced saprotrophs and pathogens, consistent with enhanced plant productivity and greater belowground C inputs. In secondary soils, exclusion reduced richness, likely reflecting non-target pesticide effects on soil arthropods, highlighting that herbivore removal has context-dependent consequences across successional stages. Mycorrhizal responses mirrored these patterns: AM fungi increased by ~40% under exclusion in primary soils but were more strongly influenced by N-addition in secondary soils. In Chapter 3, using shotgun metagenomics, I reveal that herbivory also regulates bacterial functional succession in primary successional soils. Weevil exclusion increased richness of C and N cycling genes (24–27%) while reducing evenness, indicating proliferation of a subset of resource-acquisition pathways. Resource-acquisition-associated genes showed the strongest treatment responses, particularly under moderate N-addition. Stress-tolerance genes remained conserved across treatments, reflecting persistent abiotic stress in primary successional soils. Nitrogen addition alone produced limited functional shifts, emphasizing that herbivory, not N-availability, is the primary constraint on microbial metabolic capacity during early soil development. Together, these chapters reveal that invertebrate herbivory is a fundamental bottleneck on primary succession, suppressing plant productivity, reducing belowground C inputs, and constraining both microbial diversity and functional potential. Primary soils remain far more N-limited and resource-poor than adjacent secondary soils, amplifying herbivore effects and dampening ecosystem responses to N enrichment. By integrating plant, soil, taxonomic, and metagenomic perspectives, this dissertation demonstrates that interactive top-down and bottom-up forces govern early soil development, and that even subtle changes in herbivore pressure or nutrient availability can produce nonlinear and lasting impacts on successional trajectories and long-term C accumulation on Mount St. Helens.

INVERTEBRATE HERBIVORY AND NITROGEN LIMITATION INTERACT TO SHAPE SOIL DEVELOPMENT DURING SUCCESSION ON MOUNT ST. HELENS

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