UC Irvine

ABSTRACT OF THE DISSERTATION

A Novel Mechanism and Aspects of Plant Facilitationby Benjamin Marcus Schlau Doctor of Philosophy in Ecology and Evolutionary Biology University of California, Irvine, 2020 Professor Travis E. Huxman, Chair

Coexistence theory has largely focused on negative interactions (i.e. competition) between plant species despite growing evidence that positive plant-plant interactions, generally known as plant facilitation, are equally or more important to structuring biological communities. Facilitative plant species alleviate stressors for neighboring heterospecifics and can increase biodiversity. The literature on plant facilitation has largely focused on alleviation of stressors through shading, hydraulic lift, pollinator attraction, or other means that require well-developed canopies, roots, flowers, and other traits typically found only during late growth stages. Consequently, facilitation by seedlings of long-lived perennials has not been generally tested – if at all. Moreover, positive species interactions in plant facilitation systems are highly context dependent. Plant facilitation’s Stress Gradient Hypothesis (SGH) predicts that as ameliorated stressors become less severe, interactions become competitive. This includes temporal gradients in which once long-lived beneficiaries establish and are no longer vulnerable seedlings, they will kill the plant that once nursed them. However, facilitation has not been considered in combination with other mechanisms of coexistence, such as resource partitioning or complimentary competitive effects, that may make these systems more stable. Here, a series of greenhouse and field experiments test a hypothesis that the long-lived, competitive shrub Eriogonum fasciculatum acidifies soils beginning at the seedling stage and facilitates its co-dominant shrub Artemisia californica through alleviation of alkalinity stress in their native, calcareous soils. In Chapter 1, a greenhouse experiment shows E. fasciculatum seedlings increased A. californica seedling shoot growth. In concurrence with the SGH and the facilitation via soil acidification hypothesis, a field study found association rates of A. californica with E. fasciculatum had a positive linear relationship with increasing soil pH. Chapter 1 highlights the importance of investigating facilitation during early growth stages, particularly if the means of stress alleviation is physiological manipulation of soil chemistry, which may begin soon after germination. Despite plant facilitation’s ecological significance, few – if any - studies investigate how tertiary invasive species affect interactions between native species in these systems. Chapter 2 asks the question: Can A. californica protect E. fasciculatum from facilitating the harmful invasive B. nigra? As expected in an additive field experiment, adult A. californica prevented E. fasciculatum from facilitating recruitment of B. nigra. In a subsequent greenhouse experiment, A. californica seedlings outcompeted E. fasciculatum for light as E. fasciculatum continued to facilitate A. californica. Surprisingly, the presence of vertical growth dominant B. nigra elicited A. californica to grow taller, mitigating A. californica negative effects of light competition on E. fasciculatum while A. californica reduced E. fasciculatum facilitation of B. nigra. Thus, Chapter 2 demonstrates the importance of considering facilitative systems in their biological contexts of intensely invaded ecosystems. This second round of greenhouse experiments also further supported the hypothesis that E. fasciculatum facilitates via acidification of alkaline soils. Lowering soil pH mimicked E. fasciculatum’s facilitative effects on A. californica. This dissertation suggests positive facilitatory interactions between long-lived competitive natives can begin in the seedlings stage, result in stable coexistence, and potentially increase invasion resistance for the larger biological community.