MECHANISMS AND MITIGATION OF SOIL LEGACIES OF INVASIVE GRASSES

Abstract

The grass-fire cycle can be accurately re-described as the human-grass-fire cycle, with the majority of invasive grass introductions being deliberate, and the majority of ignitions being anthropogenic in nature. With increased fire risk, and altered fire regimes favoring invasives, inhibiting natives, and depressing the resilience of native ecosystems, the stakes for restoring grass-invaded areas are higher than ever. Invasive grasses often grow in monocultures, outcompeting and suppressing other species, but even when removed, an abundance of research has demonstrated evidence of soil legacies that can hinder restoration efforts. Thus, there is a need to identify specific mechanism of soil legacies associated with invasive grasses that limit or inhibit restoration success, in order to translate that knowledge into mitigation strategies that better target these mechanisms. To begin, I gathered studies examining grasses for allelopathic abilities, to see if phylogenetically conserved allelochemicals, including benzoxazinoids, which are mostly Poaceae specific, points to allelopathy as a key soil legacy mechanism that contributes to disproportionate invasive success in the grass family. By narrowing the frame of the landmark allelopathy analysis by Zhang et al. (2020), I found support for the novel weapons hypothesis in invasive grasses; specifically, the allelopathic impact on native recipients, was more negative when the allelopathic species was a non-native grass, compared to when the allelopathic species was a native grass. Additionally, I found support for the phylogenetic distance hypothesis, supporting other research suggesting that allelochemical impacts depend on the phylogenetic distance of the target plant. I did not find support for the biotic resistance hypothesis, specifically that the allelopathic impact when the allelopathic species was a native grass was more negative on a non-native recipient, than a native recipient. Through this analysis, I showed evidence suggesting that land managers ought to consider testing for allelopathy, or considering allelopathy-informed restoration practices, when trying to restore grass-invaded areas. Next, in a field study at Camp Pālehua (Kapolei, O’ahu, Hawai’i) I compared soil characteristics and the soil microbial community between a Megathyrsus maximus invaded area, and an area that was formerly invaded by Megathyrsus maximus but had been restored by community partner Malama Learning Center 18 months prior to the comparison. The two sites shared a long history of grazing, climate and soil characteristics, and slope. While I could not rule out inherent differences between the sites that existed prior to the restoration, I sought to identify differences in soil characteristics and the soil microbial community that could be attributed, at least in part, to the restoration. While the restoration practice at the Malama Learning Center section was relatively successful, the site continued to require frequent hand-weeding, so I was particularly interested in evidence of a soil legacy in the soil microbial community, specifically any “hold-over” or “hold-out” taxa, that may be contributing to on-going re-emergence of grass species in the native ecosystem. I found a genus of fungi (Glomus) and bacteria (Candidatus Udaeobacter) were abundant at both sites and these genera have been identified in the literature as being associated with pastures, suggesting that, in the restoration site, these genera were hold-overs. This suggests that specific members of the soil microbial community could be contributing to on-going, long-lasting soil legacy effects, such that modifying the soil microbial community could mitigate some of these effects. In the third chapter, I conducted a greenhouse plant-soil feedback study, using a whole soil inoculum design, comparing germination, survival, and above/belowground growth in inoculum added soil to control soil. By using inoculum in less the 5% volume (w/w), I was able to isolate for the impact of the soil microbial community while holding soil characteristics and nutrients constant, to test for a soil microbial community mechanism for a soil legacy of an invasive grass (Megathyrsus maximus). In addition, I implemented a moderate drought treatment at 60% pot capacity to test for the impact of drought on any plant-soil feedback, since drought is expected to impact ecosystems in Hawai’i in the future. The native species used to test for feedback from the invasive grass microbial community was the endemic dry forest shrub Chenopodium oahuense. I found evidence that there was positive con-specific feedback (the grass benefited from its own soil microbial community) and negative hetero-specific feedback (negative impacts of the grass soil microbial community on the native), impacting primarily the belowground growth of both species, suggesting that the soil microbial community mediates belowground competition for space and nutrients. In addition, I found an interaction between the presence of the grass soil microbial community and drought that was associated with delayed germination of Chenopodium oahuense, suggesting that a soil legacy effect may contribute to phenological mismatch for native species as climate change progresses. In the fourth chapter, I conducted an experimental restoration at a separate Megathyrsus maximus invaded section of the Camp Pālehua property in Kapolei, O’ahu, Hawai’i using a soil amendment of activated carbon, alone and in combination with a locally sourced biowaste-based biochar fertilizer, to determine whether these amendments might mitigate the soil legacy mechanisms I had previously investigated. Two native species were used: Plumbago zeylanica and Dodonaea viscosa. I found that soil raking alone prior to planting improved the width of Plumbago zeylanica by 1 cm after 1 year. Raking had two effects that could have benefited plant growth: disruption of soil compaction, and suppression of the no-raking indicator fungi Bionectriaceae. In addition, I found that the height of Dodonaea viscosa increased 4 cm with 0.5 kg/m^2 of activated carbon added. Two potential explanations are the neutralization of any present allelochemicals, and the suppression of specific fungi which were indicators of the treatments lacking activated carbon, including Coniophora, which has been found in other studies examining the soil microbiome of restoration projects on former pastures. The hypothesis that after the initial suppression, the activated carbon treatment would create opportunity for the outplants to recruit new members to the soil microbial community, resulting in indicator species for the activated carbon level, was not supported. Overall, I was able to contribute evidence that allelopathic ability and the soil microbial community contribute to the soil legacies of invasive grasses, and in Megathyrsus maximus in Hawai’i, activated carbon can be used to suppress fungi and/or allelopathy to improve outcomes for some native plants, while raking alone can improve outcomes, potentially by disrupt soil compaction suppressing certain fungi. Activated carbon could be used in small scale-projects to create sustaining native ecosystems that can later be used for soil transplants or to provide soil inoculum in larger areas. Additionally, some of the identified fungi suppressed by activated carbon could be tested in isolation, or in different combinations for their impact on native plants.