High Resource Conditions Research Articles

Transgenerational plasticity in morphological characteristics and biomass of the invasive plant Xanthium strumarium.

Transgenerational plasticity (TGP) allows a plant to acclimate to external variable environments and is a potential mechanism that explains the range expansion and invasion success of some exotic plants. Most studies explored the traits of TGP associated with the success of exotic plant invasions by comparison studies among exotic, native, invasive, and noninvasive species. However, studies on the TGP of invasive plants in different resource environments are scarce, and the biological mechanisms involved are not well understood. This study aimed to determine the role of TGP in the invasiveness of Xanthium strumarium in northeast China. We measured the plant morphology of aboveground parts and the growth of three generations of the invader under different environmental conditions. The results showed that the intergenerational plasticity of X. strumarium was stronger under stress conditions. We found that the X. strumarium parent generation (F0) grown under water and/or nutrient deficiency conditions transferred the environmental information to their offspring (F1 and F2). The F1 generation grown under high-resource conditions has greater height with larger crown sizes, thicker basal diameters, and higher biomass. Both water and nutrients can affect the intergenerational transmission of plant plasticity, nutrients play a more important role compared with water. The high morphological intergenerational plasticity of X. strumarium under a pressure environment can help it quickly adapt to the new environment and accelerate the rapid expansion of the population in the short term. The root:shoot ratio and reproductive and nutrient distribution of the X. strumarium F0 and F1 generations showed high stability when the growth environment of the F0 generation differed from that of the F1 generation. The stable resource allocation strategy can ensure that the obtained resources are evenly distributed to each organ to maintain the long-term existence of the community. Therefore, the study of intergenerational transmission plasticity is of great significance for understanding the invasion process, mechanism, and prevention of invasive plants.

Resource availability governs polyhydroxyalkanoate (PHA) accumulation and diversity of methanotrophic enrichments from wetlands.

Aquatic environments account for half of global CH4 emissions, with freshwater wetlands being the most significant contributors. These CH4 fluxes can be partially offset by aerobic CH4 oxidation driven by methanotrophs. Additionally, some methanotrophs can convert CH4 into polyhydroxyalkanoate (PHA), an energy storage molecule as well as a promising bioplastic polymer. In this study, we investigate how PHA-accumulating methanotrophic communities enriched from wetlands were shaped by varying resource availability (i.e., C and N concentrations) at a fixed C/N ratio. Cell yields, PHA accumulation, and community composition were evaluated in high (20% CH4 and 10mM NH4 +) and low resource (0.2% CH4 and 0.1mM NH4 +) conditions simulating engineered and environmental settings, respectively. High resource availability decreased C-based cell yields, while N-based cell yields remained stable, suggesting nutrient exchange patterns differed between methanotrophic communities at different resource concentrations. PHA accumulation was only observed in high resource enrichments, producing approximately 12.6% ± 2.4% (m/m) PHA, while PHA in low resource enrichments remained below detection. High resource enrichments were dominated by Methylocystis methanotrophs, while low resource enrichments remained significantly more diverse and contained only a minor population of methanotrophs. This study demonstrates that resource concentration shapes PHA-accumulating methanotrophic communities. Together, this provides useful information to leverage such communities in engineering settings as well as to begin understanding their role in the environment.

Arbuscular mycorrhiza changes plant facilitation patterns and increases resource use efficiency in intercropped annual plants

Arbuscular mycorrhizal fungi (AMF) play a vital role in mediating rhizosphere interactions and plant growth, especially in resource-constrained soils. However, few studies have documented the effects of AMF on plant–plant interaction patterns and resource use efficiency along stress gradients. In this study, a growth environment-controlled experiment was carried out to evaluate the effects of AMF inoculation on maize-grass pea intercropping along three phosphorus (P) and two water gradients. The data indicated that soil water and P availability determined the plant interspecific interaction pattern, and that interaction was strongly mediated by AMF inoculation. Grass pea and maize were mutually facilitated under low P and water conditions and transitioned to a new pattern, i.e., that maize was facilitated but grass pea was neutral role or facilitator with increasing resource availability. The facilitative effect was particularly significant under P- and water-deficient conditions. Moreover, the transition of the plant–plant interaction pattern was mechanically driven by improving rhizospheric phosphatase activities by 5.08–20.02 %, rhizospheric acidification and enhanced microbial activities under resource deficit conditions. Critically, AMF inoculation was observed to significantly improve rhizospheric soil P availability and microbial biomass (p < 0.05); however, it enhanced the complementary utilization of P and water and weakened interspecific competition under high-resource conditions. Therefore, AMF inoculation positively transformed the plant–plant interaction pattern along resource gradients for higher P, nitrogen and water use efficiency. This study validates the context-dependent interspecific interaction and the role of AMF in optimizing resource use in a nutrient-impoverished intercropping system.

Resource-dependent biodiversity and potential multi-trophic interactions determine belowground functional trait stability

BackgroundFor achieving long-term sustainability of intensive agricultural practices, it is pivotal to understand belowground functional stability as belowground organisms play essential roles in soil biogeochemical cycling. It is commonly believed that resource availability is critical for controlling the soil biodiversity and belowground organism interactions that ultimately lead to the stabilization or collapse of terrestrial ecosystem functions, but evidence to support this belief is still limited. Here, we leveraged field experiments from the Chinese National Ecosystem Research Network (CERN) and two microcosm experiments mimicking high and low resource conditions to explore how resource availability mediates soil biodiversity and potential multi-trophic interactions to control functional trait stability.ResultsWe found that agricultural practice-induced higher resource availability increased potential cross-trophic interactions over 316% in fields, which in turn had a greater effect on functional trait stability, while low resource availability made the stability more dependent on the potential within trophic interactions and soil biodiversity. This large-scale pattern was confirmed by fine-scale microcosm systems, showing that microcosms with sufficient nutrient supply increase the proportion of potential cross-trophic interactions, which were positively associated with functional stability. Resource-driven belowground biodiversity and multi-trophic interactions ultimately feedback to the stability of plant biomass.ConclusionsOur results indicated the importance of potential multi-trophic interactions in supporting belowground functional trait stability, especially when nutrients are sufficient, and also suggested the ecological benefits of fertilization programs in modern agricultural intensification.6BT2faHzT3jQfWsk8Bso8NVideo

Metabolic rate shapes differences in foraging efficiency among honeybee foragers

Abstract Metabolic rate is the rate at which organisms process energy and is often considered as the fundamental driver of life history processes. The link between metabolic rate and life history is critically mediated via foraging, which shapes the energy acquisition patterns of an individual. This predicts that individuals with different metabolic rates likely vary in their foraging strategies, although such a link has rarely been empirically investigated in the context of optimal foraging theory—a powerful framework for understanding how animals maximize their foraging returns. Many central place foragers such as honeybees maximize their energetic efficiency rather than the rate of energetic gain, given the critical role of energetic costs in foraging decisions. We therefore tested if individuals with low and high metabolic rates differ in efficiency maximization, using genetic lines of honeybees with different metabolic rates. Our results show that low metabolic rate foragers visit more flowers during a foraging trip and have a higher energetic efficiency than high metabolic rate foragers in both low and high resource conditions. We discuss the significance of these results in the context of division of labor and the adaptive role of phenotypic diversity in metabolic rate in a social insect colony.

Testing the association of relative growth rate and adaptation to climate across natural ecotypes of Arabidopsis.

Ecophysiologists have reported a range of relationships, including intrinsic trade-offs across and within species between plant relative growth rate in high resource conditions (RGR) vs adaptation to tolerate cold or arid climates, arising from trait-based mechanisms. Few studies have considered ecotypes within a species, in which the lack of a trade-off would contribute to a wide species range and resilience to climate change. For 15 ecotypes of Arabidopsis thaliana in a common garden we tested for associations between RGR vs adaptation to cold or dry native climates and assessed hypotheses for its mediation by 15 functional traits. Ecotypes native to warmer, drier climates had higher leaf density, leaf mass per area, root mass fraction, nitrogen per leaf area and carbon isotope ratio, and lower osmotic potential at full turgor. Relative growth rate was statistically independent of the climate of the ecotype native range and of individual functional traits. The decoupling of RGR and cold or drought adaptation in Arabidopsis is consistent with multiple stress resistance and avoidance mechanisms for ecotypic climate adaptation and would contribute to the species' wide geographic range and resilience as the climate changes.

Direct Speech-to-Text Translation Models as Students of Text-to-Text Models

Direct speech-to-text translation (ST) is an emerging approach that consists in performing the ST task with a single neural model. Although this paradigm comes with the promise to outperform the traditional pipeline systems, its rise is still limited by the paucity of speech-translation paired corpora compared to the large amount of speech-transcript and parallel bilingual corpora available to train previous solutions. As such, the research community focused on techniques to transfer knowledge from automatic speech recognition (ASR) and machine translation (MT) models trained on huge datasets. In this paper, we extend and integrate our recent work (Gaido, Gangi, et al. 2020) analysing the best performing approach to transfer learning from MT, which is represented by knowledge distillation (KD) in sequence-to-sequence models. After the comparison of the different KD methods to understand which one is the most effective, we extend our previous analysis of the effects – both in terms of benefits and drawbacks – to different language pairs in high-resource conditions, ensuring the generalisability of our findings. Altogether, these extensions complement and complete our investigation on KD for speech translation leading to the following overall findings: i) the best training recipe involves a word-level KD training followed by a fine-tuning step on the ST task, ii) word-level KD from MT can be detrimental for gender translation and can lead to output truncation (though these problems are alleviated by the fine-tuning on the ST task), and iii) the quality of the ST student model strongly depends on the quality of the MT teacher model, although the correlation is not linear.

Hurricanes increase tropical forest vulnerability to drought.

Rapid changes in climate and disturbance regimes, including droughts and hurricanes, are likely to influence tropical forests, but our understanding of the compound effects of disturbances on forest ecosystems is extremely limited. Filling this knowledge gap is necessary to elucidate the future of these ecosystems under a changing climate. We examined the relationship between hurricane response (damage, mortality, and resilience) and four hydraulic traits of 13 dominant woody species in a wet tropical forest subject to periodic hurricanes. Species with high resistance to embolisms (low P50 values) and higher safety margins ( ) were more resistant to immediate hurricane mortality and breakage, whereas species with higher hurricane resilience (rapid post-hurricane growth) had high capacitance and P50 values and low . During 26 yr of post-hurricane recovery, we found a decrease in community-weighted mean values for traits associated with greater drought resistance (leaf turgor loss point, P50 , ) and an increase in capacitance, which has been linked with lower drought resistance. Hurricane damage favors slow-growing, drought-tolerant species, whereas post-hurricane high resource conditions favor acquisitive, fast-growing but drought-vulnerable species, increasing forest productivity at the expense of drought tolerance and leading to higher overall forest vulnerability to drought.

Metabolic Rate Diversity Shapes Group Performance in Honeybees.

AbstractThe metabolic theory of ecology posits that the functional properties at any level of biological organization are a function of the metabolic rate (MR) of its constituent units, although we know little about how heterogeneity among them shapes group-level performance. Using honeybees as a model system, we leveraged the differences in MR associated with "slow" and "fast" malate dehydrogenase alleles to breed genetic lines with low MR and high MR, respectively, and created four experimental groups with different phenotypic compositions. We then measured MR, energetic intake, thermoregulation, and survival of these groups in low- and high-resource conditions. Contrary to expectations, neither the monomorphic fast groups nor the polymorphic (1∶1 slow-to-fast) groups showed a consistent advantage over other groups, but the performance of all groups was consistently better under high-resource conditions. MR had a strong influence on the other performance traits, more so under low-resource conditions. We quantified the effect of heterogeneity in the polymorphic groups as a diversity effect and found different impacts of metabolic heterogeneity, depending on the specific performance measure in question. We discuss these results in the context of how MR plays an important role in shaping division of labor and social evolution.

Interactive Effects of Mycorrhizae, Soil Phosphorus, and Light on Growth and Induction and Priming of Defense in Plantago lanceolata.

Increasing demands to reduce fertilizer and pesticide input in agriculture has triggered interest in arbuscular mycorrhizal fungi (AMF) that can enhance plant growth and confer mycorrhiza-induced resistance (MIR). MIR can be based on a variety of mechanisms, including induction of defense compounds, and sensitization of the plant’s immune system (priming) for enhanced defense against later arriving pests or pathogens signaled through jasmonic acid (JA). However, growth and resistance benefits of AMF highly depend on environmental conditions. Low soil P and non-limiting light conditions are expected to enhance MIR, as these conditions favor AMF colonization and because of observed positive cross-talk between the plant’s phosphate starvation response (PSR) and JA-dependent immunity. We therefore tested growth and resistance benefits of the AMF Funneliformis mosseae in Plantago lanceolata plants grown under different levels of soil P and light intensity. Resistance benefits were assessed in bioassays with the leaf chewing herbivore Mamestra brassicae. Half of the plants were induced by jasmonic acid prior to the bioassays to specifically test whether AMF primed plants for JA-signaled defense under different abiotic conditions. AMF reduced biomass production but contrary to prediction, this reduction was not strongest under conditions considered least optimal for carbon-for-nutrient trade (low light, high soil P). JA application induced resistance to M. brassicae, but its extent was independent of soil P and light conditions. Strikingly, in younger plants, JA-induced resistance was annulled by AMF under high resource conditions (high soil P, ample light), indicating that AMF did not prime but repressed JA-induced defense responses. In older plants, low soil P and light enhanced susceptibility to M. brassicae due to enhanced leaf nitrogen levels and reduced leaf levels of the defense metabolite catalpol. By contrast, in younger plants, low soil P enhanced resistance. Our results highlight that defense priming by AMF is not ubiquitous and calls for studies revealing the causes of the increasingly observed repression of JA-mediated defense by AMF. Our study further shows that in our system abiotic factors are significant modulators of defense responses, but more strongly so by directly modulating leaf quality than by modulating the effects of beneficial microbes on resistance.

Effects of genomic and functional diversity on stand-level productivity and performance of non-native Arabidopsis.

Biodiversity can affect the properties of groups of organisms, such as ecosystem function and the persistence of colonizing populations. Genomic data offer a newly available window to diversity, complementary to other measures like taxonomic or phenotypic diversity. We tested whether native genetic diversity in field experimental stands of Arabidopsis thaliana affected their aboveground biomass and fecundity in their colonized range. We constructed some stands of genotypes that we a priori predicted would differ in performance or show overyielding. We found no relationship between genetic diversity and stand total biomass. However, increasing stand genetic diversity increased fecundity in high-resource conditions. Polyculture (multiple genotype) stands consistently yielded less biomass than expected based on the yields of component genotypes in monoculture. This under-yielding was strongest in stands with late-flowering and high biomass genotypes, potentially due to interference competition by these genotypes. Using a new implementation of association mapping, we identified genetic loci whose diversity was associated with stand-level yield, revealing a major flowering time locus associated with under-yielding of polycultures. Our field experiment supports community ecology studies that find a range of diversity-function relationships. Nevertheless, our results suggest diversity in colonizing propagule pools can enhance population fitness. Furthermore, interference competition among genotypes differing in flowering time might limit the advantages of polyculture.

Addressing data sparsity for neural machine translation between morphologically rich languages

Translating between morphologically rich languages is still challenging for current machine translation systems. In this paper, we experiment with various neural machine translation (NMT) architectures to address the data sparsity problem caused by data availability (quantity), domain shift and the languages involved (Arabic and French). We show that the Factored NMT (FNMT) model, which uses linguistically motivated factors, is able to outperform standard NMT systems using subword units by more than 1 BLEU point even when a large quantity of data is available. Our work shows the benefits of applying linguistic factors in NMT when faced with low- and high-resource conditions.

Bacterial Tradeoffs in Growth Rate and Extracellular Enzymes.

Like larger organisms, bacteria possess traits, or phenotypic characteristics, that influence growth and impact ecosystem processes. Still, it remains unclear how these traits are organized across bacterial lineages. Using 49 bacterial strains isolated from leaf litter in Southern California, we tested the hypothesis that bacterial growth rates trade off against extracellular enzyme investment. We also tested for phylogenetic conservation of these traits under high and low resource conditions represented, respectively, by Luria broth (LB) and a monomer-dominated medium extracted from plant litter. In support of our hypotheses, we found a negative correlation between the maximum growth rate and the total activity of carbon-, nitrogen-, and phosphorus-degrading extracellular enzymes. However, this tradeoff was only observed under high resource conditions. We also found significant phylogenetic signal in maximum growth rate and extracellular enzyme investment under high and low resource conditions. Driven by our bacterial trait data, we proposed three potential life history strategies. Resource acquisition strategists invest heavily in extracellular enzyme production. Growth strategists invest in high growth rates. Bacteria in a third category showed lower potential for enzyme production and growth, so we tentatively classified them as maintenance strategists that may perform better under conditions we did not measure. These strategies were related to bacterial phylogeny, with most growth strategists belonging to the phylum Proteobacteria and most maintenance and resource acquisition strategists belonging to the phylum Actinobacteria. By accounting for extracellular enzyme investment, our proposed life history strategies complement existing frameworks, such as the copiotroph-oligotroph continuum and Grime’s competitor-stress tolerator-ruderal triangle. Our results have biogeochemical implications because allocation to extracellular enzymes versus growth or stress tolerance can determine the fate and form of organic matter cycling through surface soil.

Experimental evidence that CO2 and nutrient enrichment do not mediate interactions between a native and an exotic free-floating macrophyte

We investigated the effect of CO2 and nutrient enrichment on competition between two free-floating fern species that co-occur in south-eastern Australia—native Azolla filiculoides and invasive exotic Salvinia molesta. The species were grown in monoculture and competition in a glasshouse experiment where the nutrient medium was replaced weekly to simulate a dynamic system. We found that the relative growth rate (RGR) of both species was greater under high resource conditions, with neither showing a suppressive response due to competition. On the contrary, A. filiculoides had a facilitative effect on S. molesta. In addition, A. filiculoides gained more biomass under high resource conditions relative to S. molesta and the opposite was true under low resource conditions. We conclude that [CO2] and nutrient concentration did not mediate competition between the species but instead influenced RGR independent of competition. These findings suggest that species composition in dynamic water bodies under future conditions may be determined by the species’ responses to environmental changes rather than by changes in competitive interactions.

Invasive annuals respond more negatively to drought than native species.

In his foundational list of 'ideal weed' characteristics, Baker (1965) proposed that weedy plants maximize reproductive output under high resource availability. Since then, the idea that invasive plant species are more responsive to fluctuating resources compared with native or noninvasive species has gained considerable traction, although few studies extend this hypothesis to include reproductive output. We revisit Baker's hypothesis in the context of invasion and drought in California grasslands, exploring whether invasives show greater growth and reproductive responses to water availability compared with the native wildflowers they displace. In an outdoor potted study, we grew eight native and eight invasive species of annuals commonly found in southern California grasslands to reproductive maturity under both well-watered and drought conditions. While drought negatively impacted plant performance overall, invasives showed more negative responses for growth and reproductive traits. Invasives also grew larger than native species, especially under well-watered conditions, and produced seed with higher rates of germination. Invasives may be more negatively impacted by drought compared with natives, but they are also able to capitalize on high resource conditions and greatly increase reproductive output. Such opportunistic responses exhibited by invasives might explain previously observed fluctuations in their abundance under variable precipitation.

Eco-physiological performance may contribute to differential success of two forms of an invasive vine, Dolichandra unguis-cati, in Australia

Invasive plant species are hypothesized as being more efficient at resource acquisition and use, resulting in faster growth than co-occurring non-invasive plant species. Nonetheless, some findings suggest that trait differences between invasive and non-invasive species are context dependent. In this study, two forms of an invasive vine species,Dolichandraunguis-cati, were used to test the context-dependent hypothesis.Dolichandraunguis-catiis a weed of national significance in Australia with two different forms: the ‘long pod’ (LP) and ‘short pod’ (SP). The two forms have different levels of distribution on the eastern Seaboard of the continent, with the SP form occurring extensively in both States of Queensland and New South Wales while the LP form is found only in isolated sites in South-East Queensland. This study examines whether differences in eco-physiological performance could be responsible for differential success of the two forms. A partially factorial experiment was set up in controlled conditions where potted plants of both forms were grown under two levels of light, water and nutrient resources (high and low) for 15 months. We measured several traits that are known to correlate with plant performance and resource use efficiency (RUE). The SP form exhibited higher values of carbon assimilation, RUE, number of subterranean tubers and leaf nitrogen than the LP form. However, the LP form produced greater biomass than the SP form, with the difference driven mainly by high resource conditions. The LP form displayed significantly higher phenotypic integration (number of traits significantly correlated) than the SP form in response to all treatments while the SP form exhibited higher phenotypic integration than the LP form in response to high resource conditions only. The SP form displayed traits that are well suited for successful colonization, possibly explaining its increased success in Australia, while the LP form possessed traits of opportunistic plants. Overall, we find that the two forms of the weedy vine deploy different carbon economies in response to resource conditions, which is evidence of the context-dependent trait hypothesis.

Nematode communities differ in Hopi landrace maize and commercial maize

Selection pressures of crop breeding in varying abiotic contexts may produce cultivars differing in their relationships with belowground organisms. Nematode assemblages associated with a Hopi maize (Zea mays L.) landrace and two commercial maize varieties were examined at three traditional farms (two irrigated, one dry-farmed) near the village of Moenkopi on the Hopi Reservation in Arizona, U.S.A. At each farm, the landrace and one commercial maize variety were cultivated in close proximity. Within the two irrigated farms, densities of free-living nematodes, but not plant parasitic nematodes, were higher in landrace plots compared to commercial variety plots; however, at the dry-farmed site the nematode assemblages associated with the two maize varieties did not differ. Nonmetric multidimensional scaling (NMDS) and multiple response permutation procedure (MRPP) showed that at the irrigated farms, nematode assemblages associated with landrace maize differed significantly from those associated with commercial maize in analyses based on functional guilds, feeding groups and non-phytoparasitic c-p groups. Assemblages at the dry-farmed site were different from those at the irrigated sites in community composition based on morphotypes, functional guilds, and feeding groups, but were similar to the irrigated commercial variety plots based on c-p groups. Nematode abundances were likely constrained by water availability at the dry-farmed site, but at the irrigated sites, nematode community dissimilarities between the landrace and the commercial maize variety may have resulted from differences in plant-soil microbe interactions for the two crop varieties. Selection pressures associated with Hopi agroecosystems might have produced landraces with different belowground allocation strategies than commercial crop varieties selected under high resource conditions.

Whole-genome duplication decreases clonal stolon production and genet size in the wild strawberry Fragaria vesca.

Clonal reproduction is often associated with polyploidy and is expected to influence polyploid establishment success, but the immediate effects of whole-genome duplication (WGD) on clonal reproduction in autopolyploids are unknown. We used synthesized neopolyploids to assess the direct effects of WGD on stolon and plantlet production in the wild strawberry Fragaria vesca by (1) comparing absolute clonal investment between diploids and neotetraploids under high and low resource conditions in the greenhouse and (2) determining realized clonal plantlet establishment and genet spatial structure using artificial field populations comprising both cytotypes. Neotetraploids produced fewer stolons and plantlets than diploids at slower weekly rates in the greenhouse when resources were high, resulting in lower total investment in clonal reproduction. Low resources led to smaller reductions in clonal biomass for neotetraploids and less pronounced differences between cytotypes. Comparisons between neotetraploids representing 13 independent WGD events and close diploid relatives revealed considerable variation in the response to polyploidization for some clonal traits. Field populations corroborated greenhouse results; neotetraploid genets were smaller than diploid genets, containing 28% fewer stolons and 46% fewer rooted plantlets. WGD significantly decreases the clonal output of neotetraploid F. vesca, which is likely attributable to slower whole-plant growth of the neotetraploids than the diploids. In natural populations, smaller neotetraploid genets could decrease the probability of polyploid establishment in this species. However, variation between separate neopolyploid lines emphasizes that the response of clonal investment to WGD may not be uniform across polyploid origins.

Belowground fungal associations and water interact to influence the compensatory response of Ipomopsis aggregata.

Although the concept that some plants benefit from being eaten is counterintuitive, there is now considerable evidence demonstrating enhanced fitness following herbivory. It has been assumed that plants growing in high resource conditions are the ones best able to compensate for herbivory. However, just the opposite has been found for dicotyledonous plants exhibiting patterns of overcompensation, with most occurring in resource-poor conditions. Long-term studies of the monocarpic biennial, scarlet gilia, Ipomopsis aggregata growing in resource-poor conditions have shown that ungulate herbivory by mule deer and elk can result in a threefold increase in plant fitness over uneaten controls. These observations led us to hypothesize that fungal associations would facilitate the compensatory response most commonly observed in this Arizona population of scarlet gilia; perhaps mutualistic associations with fungi, such as arbuscular mycorrhizal fungi, would explain the phenomenon of overcompensation altogether. Fungal removal experiments, using Captan®, a commercially available fungicide, showed that a reduction in fungal abundance altered the compensatory response following ungulate herbivory, particularly in years in which water was limited, increasing fitness compensation from equal compensation to overcompensation. A multifactorial experiment revealed that the interactive effects of water and fungicide maximized fruit production following herbivory. Our results are counter to the “modification of tolerance hypothesis” in which plants associating with mycorrhizal fungi will have higher tolerance to herbivory. It is likely that arbuscular mycorrhizal fungi and dark septate endophytes compete with plants for photosynthates following herbivory, thereby limiting the magnitude of compensation. Thus, fungi appear to be parasitic on scarlet gilia following ungulate herbivory.

Root Inputs Influence Soil Water Holding Capacity and Differentially Influence the Growth of Native versus Exotic Annual Species in an Arid Ecosystem

AbstractInvasion by exotic annual species is increasingly impacting Southern California arid lands, altering ecosystem processes and plant community composition. With climate change, the Southwestern United States is expected to experience increasingly variable rainfall. Larger rainfall events could facilitate invasion by exotic species that can capitalize on high resource conditions. Exotic annual species also have dense shallow root systems that could create positive feedbacks to further invasion by increasing soil organic matter and water holding capacity. Alternatively, fine root inputs could create negative feedbacks to exotic plant growth if they stimulate microbial nutrient immobilization. The dual influences of rainfall regime and fine root inputs on species performance were evaluated in an experiment where native and exotic species were grown individually and in combination under varying watering regimes (large infrequent or small frequent pulses, holding total rainfall constant) and root additions (with or without sterilized exotic roots). Mean soil moisture increased with larger infrequent watering events, and also with root addition. Plant growth (both native and exotic) increased with larger watering events, but declined with root addition. Exotic species growth declined more than native species growth with root additions. Mechanistically, root addition lowered inorganic nitrogen (N) availability, and microbial N immobilization increased with soil moisture content. Together these results show that increased fine root production promotes negative feedbacks to growth of exotic species via microbial N immobilization, especially under conditions of high soil moisture. Further, our results suggest that organic carbon additions are a potentially effective strategy for suppressing growth of problematic desert invaders.