RESEARCH INTERESTS

David Hooper

My research interests follow two related themes: 1) biotic and abiotic controls on biogeochemical cycles; and 2) effects of community composition on ecosystem processes. I have worked in a variety of ecosystems, most notably California grasslands and Arctic tundra. My greatest strength lies in experimental ecosystem ecology involving measurements of carbon and nitrogen dynamics. However, my interests span several levels of organization, from soil microbial ecology to global element fluxes. Processes of interest include microbially-mediated carbon and nutrient dynamics in the soil, such as nutrient availability, nutrient leaching, and soil organic matter turnover, as well as whole system carbon exchange with the atmosphere. The unifying theme in these diverse interests is how the functional traits of organisms, particularly plants, influence ecosystem fluxes of energy and nutrients. I feel that it is important to link plant functional traits at the ecophysiological level (e.g., litter quality, nutrient uptake and relative growth rate) with community dynamics (e.g., competition, facilitation) to better understand vegetation controls on ecosystem processes. I am also increasingly interested in using ecosystem restoration to establish experimental treatments to investigate these types of questions. To this end, I am beginning to interact with the Nooksack Salmon Enhancement Association, which is actively involved with riparian restoration in Whatcom County, Washington.

Effects of plant diversity on ecosystem processes

Serpentine species photos

Productivity and nutrient cycling

My Ph.D. thesis investigated the effects of plant functional group diversity on ecosystem productivity and nutrient cycling using a California serpentine grassland as a model system. The goal of this work was to examine how diversity of groups of species which differ dramatically in phenology and other properties (e.g., root:shoot ratio, litter carbon:nitrogen ratio, rooting depth and size per individual) affects ecosystem carbon, nitrogen, and phosphorus dynamics. I revegetated sites near the Kirby Canyon Landfill in southern San Jose, CA, planting experimental treatments of different plant composition and diversity. I measured integrative variables of whole system response to plant diversity (e.g. productivity and leaching losses), as well as a variety of nutrient pools and fluxes. My purpose was to understand the patterns and the mechanisms by which plant composition and diversity influence ecosystem processes. I tested the hypothesis that complementary nutrient use among different plant functional groups would lead to increased nutrient retention and greater net primary productivity in more diverse communities. While plant diversity did influence process rates, the results highlight the point that plant composition (i.e., the identity of the species present) can have a much larger effect on nutrient dynamics and productivity than plant richness (the number of species present). Furthermore, microbial dynamics, as influenced by differences in plant functional traits, had as large an effect on ecosystem nitrogen retention as did direct plant uptake. Related papers have been published in Science, Ecology, and Ecological Monographs, and BioScience. See Selected Abstracts.

Invasibility of ecosystems

Adding another dimension to this work, I have recently received a grant from the National Science Foundation to investigate how plant diversity and composition affect the invasibility of ecosystems by exotic species. Ecologists have hypothesized for decades that more diverse communities are more difficult for species to invade because greater species richness should leave less available resources for a potential invader to exploit. The proposed research seeks to address three fundamental questions: 1) Does diversity influence invasibility of communities? 2) Do functional attributes of invaders related to resource acquisition influence the success of their invasion? And, 3) Is there a greater probability of invasion success when the invader differs from community members in functional attributes related to resource capture?

Unlike previous studies that have inferred answers to these questions by surveying communities which have already been invaded, this project is making a direct experimental test of the patterns of invasion into communities that differ in both composition and richness of plant functional groups. Using the previously established plots in California serpentine grassland, described above, we will study the role of competition as a mechanism involved in invasion outcome by seeding in different species of both native and exotic plants to see if more diverse assemblages are more resistant to invasion. These experimental communities allow control of several factors, including disturbance regime, soil fertility, and community history, which confound attempts to study these questions a posteriori in already-invaded ecosystems. To complement our analysis of patterns of invasion, we are also measuring resource parameters potentially relevant to invasion success (nitrogen, phosphorus, water and light availability) in the field plots.

Controls on tundra C balance: interactions between vegetation and temperature

My post-doctoral work was also in the theme of how vegetation and abiotic controls interact to modulate ecosystem processes. Working with Terry Chapin (then at the University of California, Berkeley, now at the University of Alaska, Fairbanks), we investigated how temperature and vegetation type control carbon balance in Alaskan tundra and boreal forest soils. Our goal was to better understand responses of northern ecosystems to potential climate change. Because tundra soils contain large amounts of carbon in undecomposed organic matter, changes in net ecosystem carbon storage could be a significant feedback to rising atmospheric CO₂ levels. We used field measurements of net CO₂ flux from a series of soil cores transplanted across a natural temperature gradient running from boreal forest in the south through tussock tundra in the north. Our reciprocal transplant design across this latitudinal temperature gradient and across vegetation types within sites allowed us to compare effects of differences in environmental versus vegetation controls (e.g., differences in organic matter quality due to differences in vegetation type) on ecosystem carbon dynamics. This information will be useful for identifying the extent to which temperature changes might affect tundra carbon balance, how vegetation type might alter these responses, and what feedbacks (e.g. increased plant nutrient availability) might be important in counterbalancing short-term decomposition responses.

Selected abstracts

Curriculum vita

Hooper homepage

Biology Department Faculty