A novel form and aspects of plant facilitation

ABSTRACT OF THE DISSERTATIONA 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, ChairCoexistence 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

A novel form and aspects of plant facilitation

ABSTRACT OF THE DISSERTATIONA 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, ChairCoexistence 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

Soil microbial inoculation increases corn yield and insect attack

Effective Microorganisms (EM®) is a "biofertiliser” soil inoculant, marketed as a crop yield enhancer. However, the literature has neither comprehensively reviewed its purported effects on harvests across multiple species nor investigated its effects on plant herbivore defence other than this group's previous research on tomatoes. Here a meta-analysis of 39 journal articles and a greenhouse experiment with nine crop species afforded a nuanced assessment of Effective Microorganisms' effects on plant growth and yield. Overall, in line with predictions, Effective Microorganisms showed significant positive effect on yield and growth (0.03 effect sizes increase) in the meta-analysis, and increased growth 16% in the greenhouse, but with strong, and at times negative, species-specific responses. An additional potential benefit of Effective Microorganisms includes increased defence against herbivore attack, but inoculated corn (Zea mays) in a field and a greenhouse experiment exhibited decreased defences. Specifically, the field experiment demonstrated that Effective Microorganisms treatment corresponded to a 26% reduction in predatory insect diversity on corn plants, while not improving growth or yield but did increase water uptake. A subsequent greenhouse experiment suggested likely physiological mechanisms behind the loss of predator diversity. When non-inoculated control corn plants were set upon by caterpillars of the herbivorous insect Spodoptera littoralis, the plants increased production of defensive volatile organic compounds (VOCs) by 272%. Surprisingly, inoculation with Effective Microorganisms rendered greenhouse corn plants 51% more palatable to S. littoralis. Further localised studies are, therefore, needed to efficiently incorporate Effective Microorganisms with either conventional or sustainable agricultural management systems