Marina LaForgia

Despite the importance of early life stages in plants, we know very little about controls over seed persistence and dispersal, two seed functions that are predicted to tradeoff, but with mixed empirical support. These functions are particularly important in dryland systems where many species rely on seed banks to buffer their populations during unfavorable periods, which will become more frequent under climate change. My postdoc project with Dr. Lauren Hallett at the University of Oregon and Dr. Jennifer Gremer at the University of California, Davis utilizes biological seed collections housed in botanical gardens to conduct a largescale seed trait study on dryland annual species across an aridity gradient. To then link these traits with dispersal and persistence behavior, I am collaborating with Dr. Pierre-Olivier Cheptou at the Center for Functional and Evolutionary Ecology in Montpellier, France to build models that can predict abundance in the “hidden” seedbank from aboveground plant community data. I will be applying these findings towards understanding invasibility of California rangelands and the future of seedbanks in annual desert communities.

Rhizosphere microbes in variable environments may aid in plant host stress tolerance during dry periods, however invasive competitors have the potential to recruit beneficial microbes away from natives. I am collaborating with Dr. Cassie Ettinger, a microbial ecologist at UC Riverside, to understand how precipitation alters the rhizosphere microbiome of native forbs and invasive grasses in California annual grasslands, where precipitation that can vary 5-fold from year to year. Our previous work has found that competition between these two groups shifts the rhizosphere bacterial community. With increased frequency of extreme droughts however, we are curious if any microbes aid in drought resilience of natives, and if invasive grasses may be hijacking these microbes to their advantage. In order to understand the role of microbes in the rhizosphere, we are utilizing metagenomics. We are currently processing metagenomic data from this project.

Rhizosphere microbiomes play an important role in plant health, stress tolerance, soil nutrition, and invasive species dominance. Still, relatively little is known about how these microbial communities are altered under plant competition, and even less about whether these shifts are tied to competitive outcomes between native and invasive plant species. Dr. Cassie Ettinger and I, along with undergraduate Hannah Kang, experimentally manipulated plant hosts and used high-throughput amplicon sequencing to look at rhizosphere structure and composition. We then assessed whether competitive responses of plant hosts were tied to differentially abundant bacterial and fungal families.

We found that bacterial diversity and structure differed between invasive grasses and native forbs and shifted under competition. Further, we identified four bacterial families that were differentially abundant between competition treatments and that were significantly correlated with invasive grass dominance over native forbs. This work provides evidence that plant-rhizosphere interactions may impact plant competitive outcomes and that invader dominance over natives may be mediated by specific bacterial families. This paper is now published at Microbial Ecology.

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Invasive species don’t just compete with natives, they also change the very physical structure of the community in which they invade. In annual grassland communities, invasive annual grasses produce a large amount of standing biomass leading to a thick layer that has been shown to limit germination, establishment, and growth in native annual forbs. How this litter might act as a filter on the native community however remains unknown. I used a detailed demographic field study to understand how the effects of litter and live competition shifts the functional composition of native wildflower communities by differentially affecting resource acquisitive and resource conservative natives. I found that while invasive grasses only affect seed set in resource acquisitive natives, live grass combined with litter led to declines in both acquisitives and conservatives but with stronger declines in acquisitives. This suggests that invasive grasses have stronger effects on acquisitives, but that the presence of litter might be partially responsible for invader dominance over conservative natives. This paper is now published in Biological Invasions.

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Many native annuals in California are adapted to a variable climate through short lifespans and long-lived seedbanks. Competition with invaders however may be weakening their resilience to climate change. Together with Drs. Andrew Latimer and Susan Harrison, I manipulated rainfall and grass competition in an annual grassland and followed the demographic rates of six native annual forbs with variable resource acquisition strategies. I found that the presence of grass limited the capacity of all forbs to cope with climatic variability by making drought worse for resource acquisitive forbs through increased mortality and decreased seed set and by dampening the beneficial effects of watering on mortality in resource conservative forbs. This work was published in Ecology in 2020.

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From 2012-2015, California experienced one of the most extreme droughts in the last 10,000 years. Importantly, many annuals in California grasslands maintain seedbanks. To understand whether the seed banks are buffering these species during these drought years or whether the drought is surpassing the buffering limits of the seed bank, I worked with Marko Spasojevic and Erica Case under the guidance of Drs. Andrew Latimer and Susan Harrison to sample the soil seed bank before and after two years of extreme drought and compared belowground responses to those aboveground. Together, we found that invasive annual grasses crashed both above and belowground, but native annual forbs increased in abundance in the seed bank. Further, those with low specific leaf area, a proxy for drought tolerance, increased in aboveground cover. This work was published in Ecology in 2018.

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