Graduation Date
Summer 2025
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
Committee Chair Name
Justin Luong
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Kerry Byrne
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Erika Foster
Third Committee Member Affiliation
Community Member or Outside Professional
Keywords
Coastal grasslands, Functional traits, Climate, Soil, Community, Assembly
Subject Categories
Natural Resources
Abstract
Elucidating how plant community assembly processes interact across spatial scales to shape ecosystem function remains a central challenge in ecology. These processes are often hierarchical, with broad scale environmental filters constraining the composition of local communities. I investigated these hierarchical processes across three California coastal grassland sites spanning a ~1000 km gradient. At these sites, I measured community composition, standing biomass, four functional traits (plant height, specific leaf area, leaf dry matter content, and leaf lobedness), and estimated species’ environmental niches. To identify assembly mechanisms, I used null models to assess functional convergence or divergence and scaled these patterns with community-weighted means (CWMs) of niche properties. To link community structure to ecosystem function, I then tested whether standing biomass was better predicted by the mass-ratio hypothesis using trait CWMs or the niche complementarity hypothesis using functional diversity metrics. My results revealed scale-dependent assembly patterns driven by environmental niches. Across the broad gradient, functional divergence indicated species sorting along precipitation, aridity, and soil property gradients. Within sites, functional convergence suggested environmental filtering, while divergence among dominant species pointed to niche partitioning linked to fine-scale soil heterogeneity. These patterns were largely driven by leaf economic and structural traits. Crucially, ecosystem function was best explained by the mass-ratio hypothesis, as trait CWMs were the strongest predictors of standing biomass, consistently outperforming functional diversity metrics. My findings support a hierarchical assembly model where scale-dependent mechanisms select for species with particular traits, and the aggregated traits of these dominant species ultimately drive ecosystem function.
Citation Style
APA
Recommended Citation
Chavez- Velasco, Ernesto, "Community assembly across environmental gradients: contrasting patterns of funtional convergence and divergence in California coastal grasslands" (2025). Cal Poly Humboldt theses and projects. 2319.
https://digitalcommons.humboldt.edu/etd/2319