About

Evan Siemann is the Harry C. & Olga K. Wiess Professor of BioSciences at Rice University. His research group investigates how local environmental factors such as enemies, resources, disturbance regimes, and recruitment limitations interact with post-invasion adaptation to influence the likelihood and severity of Chinese Tallow Tree (Sapium sebiferum) invasions into East Texas coastal prairie, mesic forests, and floodplain forests. Additionally, his work examines the ecosystem-level impacts of exotic tree invasions into coastal prairies. A primary focus of his current research is the role of post-introduction evolutionary changes in Chinese Tallow Tree invasions. His research areas include population and community ecology, forests, grasslands, plant ecology, insect ecology, chemical ecology, plant/herbivore interactions, wetlands, freshwater ecology, invasive species, biodiversity, conservation, algal biology, biofuels, and wastewater remediation.

Research topics

• Ecology
• Biology
• Botany
• Materials science
• Environmental chemistry
• Waste management
• Pulp and paper industry
• Chemistry
• Organic chemistry
• Environmental science

Selected publications

• Reduced foliar disease promotes mycorrhizal mutualism in invasive plants and their native counterparts: The role of root exudate chemistry

  Journal of Ecology · 2026-02-01

  articleSenior author

  Abstract Interactions among foliar pathogens, host plants and soil mutualists, regulated by plant chemicals, can influence plant performance. While invasive plants in their introduced ranges are often attacked by fewer foliar pathogens and form stronger arbuscular mycorrhizal fungal (AMF) associations compared to native plants, it remains unclear whether lower foliar pathogens are related to higher AMF associations. In particular, it is uncertain what role root exudate chemistry plays in linking foliar pathogen release and AMF promotion. In field surveys, we examined the relationships between foliar fungal disease and AMF colonization in 15 invasive and 13 native Asteraceae species in central China. In complementary experiments, we manipulated foliar fungal diseases using fungicide treatments and sooty mould inoculations to assess their effects on AMF colonization and investigated whether the flavonoid quercetin (a key signalling compound in AMF symbiosis) in root exudates mediates their interactions. In field surveys, invasives had lower leaf area diseased and higher AMF colonization than natives with a negative relationship between leaf area diseased and AMF colonization. These patterns were consistent in the experiments. Foliar fungicide application increased AMF colonization while sooty mould inoculation reduced it. Across treatments, AMF colonization was negatively related to leaf area diseased. Moreover, quercetin exudate concentrations increased with fungicide application, decreased with sooty mould inoculation, were negatively related to leaf area diseased and were positively related to AMF colonization. Synthesis . Our findings suggest that decreased foliar pathogens can strengthen AMF mutualism in both invasive plants and their native counterparts by altering root exudate chemistry. This above‐ and below‐ground linkage underscores the importance of enemy release in shaping plant–microbe interactions in plant invasions and the role of chemical signals in modulating such interactions.

  PublisherDOI

• Interactions between nutrient levels and fluctuations shape soil fungal legacies and mediate non‐native plant growth

  Journal of Ecology · 2026-03-01

  article

  Abstract Resource variability has multiple dimensions, and its complexity is intensifying under global change. While elevated resource levels and temporal fluctuations often promote non‐native plant growth, less is understood about how these factors interact to influence non‐native plants via changes in soil microbial communities, which are key responders to resource change and regulators of plant performance. We conducted a two‐phase greenhouse experiment to test the relative effects of soil fungal legacies created by different historical nutrient regimes on subsequent non‐native plant growth. In phase I, we conditioned soils with six 4‐species assemblages of native plants under factorial combinations of nutrient level (low or high) and fluctuation regime (constant or pulsed). We quantified the resulting shifts in soil fungal community structure, focussing on the composition and relative abundance of arbuscular mycorrhizal fungi (AMF) and putative pathogens. In phase II, we grew 10 non‐native plant species individually with soil inocula from phase I and harvested their biomass at the end of this phase. We found that nutrient level and fluctuation interacted to generate distinct soil fungal legacies that altered non‐native plant growth at the end of phase II. Under low nutrients, pulsed additions produced fungal legacies that significantly suppressed non‐native biomass compared with constant additions, whereas under high nutrients, fungal legacy effects did not differ between fluctuation regimes. This growth response was positively correlated with AMF relative abundance and richness from phase I, which was lower in the low‐pulsed treatment than in other treatments. In contrast, non‐native biomass was unrelated to the relative abundance or richness of putative fungal pathogens. Synthesis . Our findings demonstrate that resource variability is likely to influence non‐native plants indirectly via soil fungal legacies, with effects depending on the interactions between resource level and temporal fluctuation. By showing that low‐nutrient pulses create inhibitory soil fungal legacies, this study offers a possible explanation for inconsistent effects of resource fluctuations on non‐native performance and underscores the need to incorporate historical resource conditions predictions of invasion success under global change.

  PublisherDOI

• Co‐Invasion by Distantly Related Invasive Plants Drives Invasional Meltdown via Synergistic Increases in Putative Soil Fungal Pathogens

  Global Change Biology · 2026-01-01 · 2 citations

  article

  Biological invasion is a major component of global change, and co-invasion of multiple invasive species is becoming increasingly common under accelerating globalization, climate warming, and land-use change. Such co-invasions can generate non-additive impacts on ecosystems, either exacerbating or mitigating invasion outcomes, yet their ecological consequences and underlying mechanisms remain poorly understood. We conducted a two-phase greenhouse experiment using Solidago canadensis as a focal invader and 16 co-occurring invaders to condition soils in monocultures and mixtures, and then tested effects of these soils on a native plant community. We found that non-additive effects of co-invasions on native community biomass were generally negative, with phylogenetically distant co-invaders exerting stronger negative effects. These patterns were largely driven by synergistic increases in the richness of putative soil fungal pathogens induced by distantly related co-invaders. A complementary field survey confirmed these patterns in natural communities, showing that greater phylogenetic distance among co-invaders was associated with higher richness of putative soil fungal pathogens and stronger reductions in native plant abundance. Our study provides the first empirical evidence that evolutionary relatedness among co-invaders predicts the direction and magnitude of their combined impacts via soil microbial pathways. These findings highlight the importance of incorporating evolutionary and belowground mechanisms into invasion theory, and underscore the urgent need to recognize co-invasion as a key process for predicting and managing invasion impacts under global change.

  PublisherDOI

• Drought eliminates soil microbially mediated indirect competitive advantage among exotic plants

  New Phytologist · 2026-04-12

  articleOpen access

  Ecosystems world-wide are experiencing an accelerating accumulation of exotic plant species, posing serious threats to biodiversity and ecosystem functioning. While soil microbially mediated indirect interactions play a crucial role in shaping plant community composition, their contribution to accumulation of exotics remains poorly understood, especially under changing environmental conditions. Here, we examined how native and exotic plants impact the growth and competitiveness of later-arriving native and exotic plants through soil microbial legacies generated in different environmental conditions. We found that soil microbial legacies generated by exotics under well-watered conditions conferred a competitive advantage to later-arriving exotics over natives, but legacies generated by natives had no such effect. This indirect advantage for exotics was linked to a higher relative abundance of pathogens in soils conditioned by exotics, which was negatively correlated with native growth and positively associated with the competitive effects of exotics on natives. However, drought eliminated this pathogen-mediated indirect advantage among exotics by preventing pathogen accumulation. This study highlights the critical importance of considering soil microbially mediated indirect interactions between native and exotic species for accurately predicting the accumulation of exotic plant species under climate change.

  PublisherOA PDFDOI

• Responses of flavonoids in root exudates to nitrogen and phosphorus availability influence AMF associations and invasive plant performance

  Journal of Plant Ecology · 2026-03-18

  articleOpen access

  Arbuscular mycorrhizal fungi (AMF) can facilitate plant invasions, particularly in nutrient-depleted soils. Establishment of this mutualism often depends on both soil nutrients and secondary chemicals released by plants. Yet, it remains unclear how invasive plants maintain or enhance AMF colonization in nutrient-deficient soil. We investigated the effects of different levels of nitrogen (N: 0, 2, or 10 ppm) and phosphorus (P: 0, 6, or 25 ppm) added to sand in a factorial design on three flavonoids (isoquercitrin, quercitrin, quercetin) associated with AMF colonization in root exudates of Triadica sebifera from introduced and native populations. We assessed effects of soil nutrients alone on secondary chemical metabolism with sterilized growth substrate treatments. We found that plants from introduced populations had higher root exudate concentrations of quercitrin and quercetin plus greater AMF colonization than those from native populations under every N and P treatment combination. The effects of P availability on flavonoids depended on N availability. Flavonoid concentrations and AMF colonization decreased with P addition under no N treatment but increased with P addition under low N treatment. P addition had smaller effects on AMF colonization under the high N treatment. In sterilized growth substrate, P addition had stronger effects on quercitrin than quercetin. These results suggest that soil nutrients may influence AMF colonization by modifying flavonoids in root exudates. Our findings provide evidence that the responses of flavonoid metabolism to soil nutrients are an important driver of AMF colonization and the ability of invasive plants to sustain performance under nutrient stress.

  PublisherDOI

• Restoration with non-dominant species reduces CO2 emissions and temperature sensitivity of soil respiration in degraded meadows

  Journal of Soils and Sediments · 2026-02-01

  article
  PublisherDOI

• Species-level studies do not upscale to community-wide plant-soil feedbacks

  Research Square · 2025-03-04

  preprintOpen access
  PublisherOA PDFDOI

• Plants respond to herbivory through sequential induction of cheaper defenses before more costly ones

  PLoS Biology · 2025-08-14 · 4 citations

  articleOpen access

  Plants encounter natural antagonist threats of varying intensity and respond by activating multiple defense traits. Due to the fitness costs associated with producing defense traits, plants are expected to activate less costly traits first, reserving more costly defenses for potentially more severe damage ("cheaper first hypothesis"), but evidence to date is scarce. Here, we tested this hypothesis by measuring six putative defense traits in the annual plant Ambrosia artemisiifolia. We found that all traits were effective against insect herbivores, but production of three of them more strongly reduced plant growth, suggesting higher growth costs. When plants were attacked by insect herbivores, less costly traits were induced first, even at the lowest levels of damage, while more costly traits were activated only after higher damage thresholds. This cost-dependent sequential pattern was consistently observed in plants when challenged by 12 different herbivore species from three insect orders. These findings demonstrate that plants can employ the "cheaper first" sequential induction defense strategy, potentially allowing them to reduce defense costs and maximize fitness. Our study provides new insights into how plants fine-tune their defense responses under variable antagonistic pressures.

  PublisherDOI

• Invasive plants exhibit stronger root-shoot ratio plasticity than natives under abiotic stress: Insights from meta-analysis and experiments

  Journal of Plant Ecology · 2025-12-10 · 2 citations

  articleOpen access

  Abstract Invasive plants pose a major threat to global ecosystems, exhibiting broad adaptability to multifaceted abiotic stresses. To investigate whether root-shoot ratio (RSR) plasticity represents a consistent cross-stress mechanism underpinning invasion success, we integrated a meta-analysis of 87 studies (spanning drought, heavy metals, salinity, temperature, and nutrient deficiency) with targeted experiments under controlled drought and heavy metal stress. Meta-analysis revealed that invasive plants consistently increase RSR across multiple abiotic stresses (mean effect size: +0.45; P < 0.001), whereas native species showed no significant changes (−0.03; P = 0.72). Invasive RSR increased by 0.29−0.98 (P < 0.05) under specific stresses (e.g. drought: 0.42; heavy metals: 0.77), contrasting sharply with natives (significant only under nutrient deficiency: 0.61). While, total biomass, C/N/P stoichiometry, and microbial diversity showed no consistent stress responses. In our experimental validation, under drought, invasive Avena sativa increased RSR by 65.8-305.3% (competition) and 101.1−114.1% (monoculture); under cadmium stress (9−90 mg.kg-1), invasives (Celosia argentea, Bidens pilosa, Ipomoea purpurea) increased RSR by 11.2−60.7%. These findings suggested that enhanced RSR plasticity is a key adaptive strategy in invasives, driven by greater phenotypic flexibility and reduced biotic constraints. Future research should explore molecular mechanisms (e.g. sucrose transporters) and interactive effects of multiple stressors. These findings illuminate invasion resilience and could inform strategies to bolster native plant resistance under global change.

  PublisherOA PDFDOI

• Escaping enemies enhances invader mutualisms: role of metabolites

  Trends in Ecology & Evolution · 2025-08-26 · 4 citations

  articleSenior author
  PublisherDOI

Recent grants

• Does Lack of Herbivory and Disease Explain the Success of an Alien Plant Species?: Experimental Tests

  NSF · $308k · 2000–2004

• Do Genetic Differences in Growth and Defense Contribute to the Success of an Invasive Plant Species? Experimental Tests

  NSF · $398k · 2003–2007

• DISSERTATION RESEARCH: Predicting Restoration Outcomes In Exotic-Invaded Ecosystems

  NSF · $15k · 2009–2012

• Aboveground and belowground enemies and the invasion success of plants: experimental tests in the US and China

  NSF · $476k · 2008–2014

Frequent coauthors

• Wei Huang

  Wuhan Botanical Garden

  77 shared

• Jianqing Ding

  Henan University

  73 shared

• William E. Rogers

  San Diego State University

  42 shared

• Juli Carrillo

  University of British Columbia

  37 shared

• Jianwen Zou

  The Synergetic Innovation Center for Advanced Materials

  32 shared

• Ling Zhang

  South China Agricultural University

  25 shared

• David Tilman

  University of Minnesota

  25 shared

• Mark E. Ritchie

  Syracuse University

  25 shared

Labs

• Population and Community Ecology LabPI

  Population and Community Ecology Lab Group at Rice