Author ORCID Identifier

0000-0001-9617-8011

Defense Date

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Integrative Life Sciences

First Advisor

Julie C Zinnert

Second Advisor

Rima Franklin

Third Advisor

Christopher Gough

Fourth Advisor

Spencer Hays

Fifth Advisor

Donald R Young

Abstract

Shrub encroachment is a global phenomenon driven by direct and indirect anthropogenic influence which alters plant communities and ecosystem function. Many studies have investigated drivers and consequences of woody plant establishment, but mesic landscapes are underrepresented in the literature. My objective was to assess the mechanisms of Morella cerifera encroachment into coastal mesic grassland, the potential for self-reinforcement, and consequences on community composition, nutrients, and landscape productivity. I studied temperature and water microclimate modification by Morella cerifera presence and removal to understand ecosystem engineering and community composition changes. Additionally, I examined the influence of shrubs on surrounding grassland species traits and soils. Lastly, I assessed the macroclimatic and age drivers of shrub productivity and habitat change across a chronosequence (i.e. a space for time substitution) and over time.

As shrubs establish and form thickets, species diversity is reduced while transpiration and annual productivity increase. Shrub presence modifies microclimate, reducing extreme temperature events and variance; shrubs were 18 °C cooler in summer and 2 °C warmer in winter. Woody vegetation in arid habitats does similar microclimate modification. Removal of established thickets led to lower soil moisture coupled with a wider range of temperatures. Grass plots have a lower water table than shrub plots by ~1 cm but shrub plots had higher water table temporal variability, suggesting hydraulic lift. Clear cut soil exhibited intermediate characteristics between grass and shrub, indicating that shrub removal could revert the habitat to some grassland-like characteristics. Proximity to shrubs and supplemental N fertilization impact the adjacent temperature extremes as well as plant functional traits, demonstrating edge effects due to N addition by shrubs on adjacent grassland. While temperature is the dominant driver of shrub establishment, shrub age and water dynamics (rainfall and water table depth) exert stronger controls on shrub productivity. Rainfall is typically coupled strongly with depth to water table, however, our annual rainfall and water depth did not correlate and water table depth increased more than would be due to sea level rise, supporting the possibility that M. cerifera engages in hydraulic lift. Soil carbon and N increase with shrub age and were higher in shrub soils than grassland soils but many nutrients leach into water table. Shrubs in mesic habitats establish via similar mechanisms as those in drylands - microclimate modification, increased soil moisture – but result in a monospecific thicket rather than creating refugia for other species, as is common in arid habitats. I conclude that M. cerifera establishment and persistence has ecosystem engineering capabilities that results in notable habitat feedbacks and modifications, expanding our understanding of shrub encroachment in mesic systems.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

8-2-2021

Available for download on Tuesday, August 02, 2022

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