Shifts in flowering phenology in response to spring temperatures in eastern Tennessee.

PREMISE: Plant phenological shifts are among the clearest indicators of the effects of climate change. In North America, numerous studies in the northeastern United States have demonstrated earlier spring flowering compared to historical records. However, few studies have examined phenological shifts in the southeastern United States, a highly biodiverse region of North America characterized by dramatic variations in abiotic conditions over small geographic areas. METHODS: We examined 1000+ digitized herbarium records along with location-specific temperature data to analyze phenological shifts of 14 spring-flowering species in two adjacent ecoregions in eastern Tennessee. RESULTS: Spring-flowering plant communities in the Blue Ridge and the Ridge and Valley ecoregions differed in their sensitivity to temperature; plants in the Ridge and Valley flower 0.73 days earlier/°C on average compared to 1.09 days/°C for plants in the Blue Ridge. Additionally, for the majority of species in both ecoregions, flowering is sensitive to spring temperature; i.e., in warmer years, most species flowered earlier. Despite this sensitivity, we did not find support for community-level shifts in flowering within eastern Tennessee in recent decades, likely because increases in annual temperature in the southeast are driven primarily by warming summer (rather than spring) temperatures. CONCLUSIONS: These results highlight the importance of including ecoregion as a predictor in phenological models for capturing variation in sensitivity among populations and suggest that even small shifts in temperature can have dramatic effects on phenology in response to climate in the southeastern United States.

How early does the selfing syndrome arise? Associations between selfing ability and flower size within populations of the mixed-mater Collinsia verna.

PREMISE: Widespread associations between selfing rate and floral size within and among taxa suggest that these traits may evolve in concert. Does this association develop immediately because of shared genetic and/or developmental control, or stepwise with selection shaping the evolution of one trait following the other? If the former, then association ought to appear within and across selfing populations. We explore this fundamental question in three populations of the mixed-mater Collinsia verna where autonomous selfing (AS) ability has been shown to be under selection by the pollination environment. METHODS: We grew clonal replicates of C. verna in a controlled environment to characterize broad-sense genetic correlations among traits within populations and to assess whether divergence in mating system and floral traits among these populations is consistent with their previously observed selection pressures. RESULTS: As predicted by their respective pollination environments, we demonstrate significant genetic divergence among populations in AS ability. However, patterns of divergence in floral traits (petal, stamen, and style size, stigmatic receptivity, and stigma-anther distance) were not as expected. Within populations, genetic variation in AS appeared largely independent from floral traits, except for a single weak negative association in one population between flower size and AS rate. CONCLUSIONS: Together, these results suggest that associations between selfing rate and floral traits across Collinsia species are not reflected at microevolutionary scales. If C. verna were to continue evolving toward the selfing syndrome, floral trait evolution would likely follow stepwise from mating system evolution.

Novel chemicals engender myriad invasion mechanisms.

Non-native invasive species (NIS) release chemicals into the environment that are unique to the invaded communities, defined as novel chemicals. Novel chemicals impact competitors, soil microbial communities, mutualists, plant enemies, and soil nutrients differently than in the species' native range. Ecological functions of novel chemicals and differences in functions between the native and non-native ranges of NIS are of immense interest to ecologists. Novel chemicals can mediate different ecological, physiological, and evolutionary mechanisms underlying invasion hypotheses. Interactions amongst the NIS and resident species including competitors, soil microbes, and plant enemies, as well as abiotic factors in the invaded community are linked to novel chemicals. However, we poorly understand how these interactions might enhance NIS performance. New empirical data and analyses of how novel chemicals act in the invaded community will fill major gaps in our understanding of the chemistry of biological invasions. A novel chemical-invasion mechanism framework shows how novel chemicals engender invasion mechanisms beyond plant-plant or plant-microorganism interactions.

New genomic resources and comparative analyses reveal differences in floral gene expression in selfing and outcrossing Collinsia sister species.

The evolutionary transition from outcross- to self-fertilization is one of the most common in angiosperms and is often associated with a parallel shift in floral morphological and developmental traits, such as reduced flower size and pollen to ovule ratios, known as the "selfing syndrome." How these convergent phenotypes arise, the extent to which they are shaped by selection, and the nature of their underlying genetic basis are unsettled questions in evolutionary biology. The genus Collinsia (Plantaginaceae) includes seven independent transitions from outcrossing or mixed mating to high selfing rates accompanied by selfing syndrome traits. Accordingly, Collinsia represents an ideal system for investigating this parallelism, but requires genomic resource development. We present a high quality de novo genome assembly for the highly selfing species Collinsia rattanii. To begin addressing the basis of selfing syndrome developmental shifts, we evaluate and contrast patterns of gene expression from floral transcriptomes across three stages of bud development for C. rattanii and its outcrossing sister species Collinsia linearis. Relative to C. linearis, total gene expression is less variable among individuals and bud stages in C. rattanii. In addition, there is a common pattern among differentially expressed genes: lower expression levels that are more constant across bud development in C. rattanii relative to C. linearis. Transcriptional regulation of enzymes involved in pollen formation specifically in early bud development may influence floral traits that distinguish selfing and outcrossing Collinsia species through pleiotropic functions. Future work will include additional Collinsia outcrossing-selfing species pairs to identify genomic signatures of parallel evolution.

Invasion-induced root-fungal disruptions alter plant water and nitrogen economies.

Despite widespread evidence that biological invasion influences both the biotic and abiotic soil environments, the extent to which these two pathways underpin the effects of invasion on plant traits and performance remains unknown. Leveraging a long-term (14-year) field experiment, we show that an allelochemical-producing invader affects plants through biotic mechanisms, altering the soil fungal community composition, with no apparent shifts in soil nutrient availability. Changes in belowground fungal communities resulted in high costs of nutrient uptake for native perennials and a shift in plant traits linked to their water and nutrient use efficiencies. Some plants in the invaded community compensate for the disruption of nutritional symbionts and reduced nutrient provisioning by sanctioning more nitrogen to photosynthesis and expending more water, which demonstrates a trade-off in trait investment. For the first time, we show that the disruption of belowground nutritional symbionts can drive plants towards alternative regions of their trait space in order to maintain water and nutrient economics.

Negative effects of an allelopathic invader on AM fungal plant species drive community-level responses.

The mechanisms causing invasive species impact are rarely empirically tested, limiting our ability to understand and predict subsequent changes in invaded plant communities. Invader disruption of native mutualistic interactions is a mechanism expected to have negative effects on native plant species. Specifically, disruption of native plant-fungal mutualisms may provide non-mycorrhizal plant invaders an advantage over mycorrhizal native plants. Invasive Alliaria petiolata (garlic mustard) produces secondary chemicals toxic to soil microorganisms including mycorrhizal fungi, and is known to induce physiological stress and reduce population growth rates of native forest understory plant species. Here, we report on a 11-yr manipulative field experiment in replicated forest plots testing if the effects of removal of garlic mustard on the plant community support the mutualism disruption hypothesis within the entire understory herbaceous community. We compare community responses for two functional groups: the mycorrhizal vs. the non-mycorrhizal plant communities. Our results show that garlic mustard weeding alters the community composition, decreases community evenness, and increases the abundance of understory herbs that associate with mycorrhizal fungi. Conversely, garlic mustard has no significant effects on the non-mycorrhizal plant community. Consistent with the mutualism disruption hypothesis, our results demonstrate that allelochemical producing invaders modify the plant community by disproportionately impacting mycorrhizal plant species. We also demonstrate the importance of incorporating causal mechanisms of biological invasion to elucidate patterns and predict community-level responses.

Inbreeding shapes the evolution of marine invertebrates.

Inbreeding is a potent evolutionary force shaping the distribution of genetic variation within and among populations of plants and animals. Yet, our understanding of the forces shaping the expression and evolution of nonrandom mating in general, and inbreeding in particular, remains remarkably incomplete. Most research on plant mating systems focuses on self-fertilization and its consequences for automatic selection, inbreeding depression, purging, and reproductive assurance, whereas studies of animal mating systems have often assumed that inbreeding is rare, and that natural selection favors traits that promote outbreeding. Given that many sessile and sedentary marine invertebrates and marine macroalgae share key life history features with seed plants (e.g., low mobility, modular construction, and the release of gametes into the environment), their mating systems may be similar. Here, we show that published estimates of inbreeding coefficients (FIS ) for sessile and sedentary marine organisms are similar and at least as high as noted in terrestrial seed plants. We also found that variation in FIS within invertebrates is related to the potential to self-fertilize, disperse, and choose mates. The similarity of FIS for these organismal groups suggests that inbreeding could play a larger role in the evolution of sessile and sedentary marine organisms than is currently recognized. Specifically, associations between traits of marine invertebrates and FIS suggest that inbreeding could drive evolutionary transitions between hermaphroditism and separate sexes, direct development and multiphasic life cycles, and external and internal fertilization.

Deer Indirectly Alter the Reproductive Strategy and Operational Sex Ratio of an Unpalatable Forest Perennial.

Environmental conditions impose restrictions and costs on reproduction. Multiple reproductive options exist when increased reproductive costs drive plant populations toward alternative reproductive strategies. Using 4 years of demographic data across a deer impact gradient, where deer alter the abiotic environment, we parameterize a size-dependent integral projection model for a sexually labile and unpalatable forest perennial to investigate the demographic processes driving differentiation in the operational sex ratio (OSR) of local populations. In addition to a relative increase in asexual reproduction, our results illustrate that nontrophic indirect effects by overabundant deer on this perennial result in delayed female sex expression to unsustainably large plant sizes and lead to more pronounced plant shrinkage following female sex expression, effectively increasing the cost of reproduction. Among plants of reproductive age, increased deer impact decreases the size-dependent probability of flowering and reduces reproductive consistency over time. This pattern in sex expression skews populations toward female-biased OSRs at low deer impact sites and male-biased OSRs at intermediate and high deer impact sites. While this shift toward a male-biased OSR may ameliorate pollen limitation, it also decreases the effective population size when coupled with increased asexual reproduction. The divergence of reproductive strategies and reduced lifetime fitness in response to indirect deer impacts illustrate the persistent long-term effects of overabundant herbivores on unpalatable understory perennials.

Do selfing species have greater niche breadth? Support from ecological niche modeling.

We explore the relationship between plant mating system (selfing or outcrossing) and niche breadth to gain new insights into processes that drive species distributions. Using a comparative approach with highly selfing versus highly outcrossing sister species, we test the extent to which: (1) species pairs have evolved significant niche divergence and less niche overlap, (2) selfers have wider niche breadths than outcrossers or vice versa, and (3) niches of selfers and outcrossers are defined by significant differences in environmental variables. We applied predictive ecological niche modeling approaches to estimate and contrast niche divergence, overlap and breadth, and to identify key environmental variables associated with each species' niche for seven sister species with divergent mating systems. Data from 4862 geo-referenced herbarium occurrence records were compiled for 14 species in Collinsia and Tonella (Plantaginaceae) and 19 environmental variables associated with each record. We found sister species display significant niche divergence, though not as a function of divergence time, and overall, selfers have significantly wider niche breadths compared to their outcrossing sisters. Our results suggest that a selfing mating system likely contributes to the greater capacity to reach, reproduce, establish, and adapt to new habitats, which increases niche breadth of selfers.

Pollen performance decreases with plant age for outcrosser but not selfer: evidence for cost of male performance.

PREMISE: Declines in reproductive capabilities with increasing age are common across the tree of life. However, in plants, mating system traits have rarely been tested for signs of senescence. Since reproduction is often resource limited, we might expect outcrossing and selfing taxa to allocate these resources differently, especially as a plant ages. Compared with selfers, outcrossers are expected to produce showy, rewarding flowers that attract pollinators and high-quality pollen that can successfully compete for ovules. Yet, this resource-intensive strategy of outcrossers may result in declines in floral allocation and pollen performance metrics, relative to selfers. METHODS: To explore age-related changes in reproduction, we measured flower size and pollen germinability over the flowering period for multiple populations of an annual sister species pair, Collinsia linearis (outcrosser) and C. rattanii (selfer), in a growth chamber experiment. RESULTS: We found that flower size decreased significantly with age in both species. The outcrosser expressed a significant and dramatic (88%) decline in pollen germinability with age, while the selfer's pollen germinability decline was non-significant and low (17%). CONCLUSIONS: Our results support the idea that the higher total cost of reproduction in outcrossers can deplete available resources more rapidly than in selfers, manifesting as a decline in male performance with plant age.

Phenological mismatch with trees reduces wildflower carbon budgets.

Interacting species can respond differently to climate change, causing unexpected consequences. Many understorey wildflowers in deciduous forests leaf out and flower in the spring when light availability is the highest before overstorey canopy closure. Therefore, different phenological responses by understorey and overstorey species to increased spring temperature could have significant ecological implications. Pairing contemporary data with historical observations initiated by Henry David Thoreau (1850s), we found that overstorey tree leaf out is more responsive to increased spring temperature than understorey wildflower phenology, resulting in shorter periods of high light in the understorey before wildflowers are shaded by tree canopies. Because of this overstorey-understorey mismatch, we estimate that wildflower spring carbon budgets in the northeastern United States were 12-26% larger during Thoreau's era and project a 10-48% reduction during this century. This underappreciated phenomenon may have already reduced wildflower fitness and could lead to future population declines in these ecologically important species.

Carbon gain phenologies of spring-flowering perennials in a deciduous forest indicate a novel niche for a widespread invader.

Strategies of herbaceous species in deciduous forests are often characterized by the timing of life history phases (e.g. emergence, flowering, leaf senescence) relative to overstory tree canopy closure. Although springtime photosynthesis is assumed to account for the majority of their annual carbon budgets, the 12-month photosynthetic trajectories of forest herbs have not been quantified. We measured the temporal dynamics of carbon assimilation for seven native herbaceous perennials and the biennial Alliaria petiolata, a widespread invader in eastern North American forests. We assessed the relative importance of spring, summer, and autumn to species-level annual carbon budgets. Spring-emerging species showed significant variation in carbon assimilation patterns. High spring irradiance before canopy closure accounted for 39-100% of species-level annual carbon assimilation, but summer and autumn accounted for large proportions of some species' carbon budgets (up to 58% and 19%, respectively). Alliaria was phenologically unique, taking advantage both autumn and spring irradiance. Although spring-emerging understory species are often expected to rely on early-season irradiance, our results highlight interspecific differences and the importance of mid-late season carbon gain. Phenological strategies of forest herbs are a continuum rather than discrete categories, and invasive species may follow strategies that are underrepresented in the native flora.

Shifts to earlier selfing in sympatry may reduce costs of pollinator sharing.

Coexisting plant congeners often experience strong competition for resources. Competition for pollinators can result in direct fitness costs via reduced seed set or indirect costs via heterospecific pollen transfer (HPT), causing subsequent gamete loss and unfit hybrid offspring production. Autonomous selfing may alleviate these costs, but to preempt HPT, selfing should occur early, before opportunities for HPT occur (i.e., "preemptive selfing hypothesis"). We evaluated conditions for this hypothesis in Collinsia sister species, C. linearis and C. rattanii. In field studies, we found virtually identical flowering times and pollinator sharing between congeners in sympatric populations. Compared to allopatric populations, sympatric C. linearis populations enjoyed higher pollinator visitation rates, whereas visitation to C. rattanii did not differ in sympatry. Importantly, the risk of HPT to each species in sympatry was strongly asymmetrical; interspecies visits comprised 40% of all flower-to-flower visits involving C. rattanii compared to just 4% involving C. linearis. Additionally, our greenhouse experiment demonstrated a strong cost of hybridization when C. rattanii was the pollen donor. Together, these results suggest that C. rattanii pays the greatest cost of pollinator sharing. Matching predictions of the preemptive selfing hypothesis, C. rattanii exhibit significantly earlier selfing in sympatric relative to allopatric populations.

A regional assessment of white-tailed deer effects on plant invasion.

Herbivores can profoundly influence plant species assembly, including plant invasion, and resulting community composition. Population increases of native herbivores, e.g. white-tailed deer (Odocoileus virginianus), combined with burgeoning plant invasions raise concerns for native plant diversity and forest regeneration. While individual researchers typically test for the impact of deer on plant invasion at a few sites, the overarching influence of deer on plant invasion across regional scales is unclear. We tested the effects of deer on the abundance and diversity of introduced and native herbaceous and woody plants across 23 white-tailed deer research sites distributed across the east-central and north-eastern USA and representing a wide range of deer densities and invasive plant abundance and identity. Deer access/exclusion or deer population density did not affect introduced plant richness or community-level abundance. Native and total plant species richness, abundance (cover and stem density) and Shannon diversity were lower in deer-access vs. deer-exclusion plots. Among deer-access plots, native species richness, native and total cover, and Shannon diversity (cover) declined as deer density increased. Deer access increased the proportion of introduced species cover (but not of species richness or stem density). As deer density increased, the proportion of introduced species richness, cover and stem density all increased. Because absolute abundance of introduced plants was unaffected by deer, the increase in proportion of introduced plant abundance is likely an indirect effect of deer reducing native cover. Indicator species analysis revealed that deer access favoured three introduced plant species, including Alliaria petiolata and Microstegium vimineum, as well as four native plant species. In contrast, deer exclusion favoured three introduced plant species, including Lonicera japonica and Rosa multiflora, and 15 native plant species. Overall, native deer reduced community diversity, lowering native plant richness and abundance, and benefited certain invasive plants, suggesting pervasive impacts of this keystone herbivore on plant community composition and ecosystem services in native forests across broad swathes of the eastern USA.

Effects of deer on the photosynthetic performance of invasive and native forest herbs.

Overabundant generalist herbivores can facilitate non-native plant invasions, presumably through direct and indirect modifications to the environment that affect plant performance. However, ecophysiological mechanisms behind ungulate-mediated plant invasions have not been well-studied. At a long-term Odocoileus virginianus (white-tailed deer) exclusion site in a temperate deciduous forest, we quantified deer-mediated ecophysiological impacts on an invasive biennial Alliaria petiolata (garlic mustard) and two palatable native herbaceous perennials, Maianthemum racemosum and Trillium grandiflorum. In mid-summer, we found that leaf-level light availability was higher in unfenced areas compared with areas fenced to exclude deer. Alliaria in unfenced areas exhibited 50 % higher mean maximum photosynthetic rates compared with fenced areas. Further, specific leaf area decreased by 48 % on average in unfenced areas, suggesting leaf structural responses to higher light levels. Similarly, Maianthemum had 42 % higher mean photosynthetic rates and 33 % decreased mean specific leaf area in unfenced areas, but these functional advantages were likely countered by high rates of deer herbivory. By contrast, Trillium exhibited significantly lower (26 %) maximum photosynthetic rates in unfenced areas, but SLA did not differ. Deer-mediated differences in light saturated photosynthetic rates for all three species were only significant during months with overstory tree canopy cover, when light availability in the herb layer was significantly lower in fenced areas. Alliaria's enhanced photosynthetic rates implicate overabundant deer, a situation that is nearly ubiquitous across its invaded range. Collectively, our results provide empirical evidence that generalist herbivores can alter non-native plant physiology to facilitate invasion.

Self-compatibility is over-represented on islands.

Because establishing a new population often depends critically on finding mates, individuals capable of uniparental reproduction may have a colonization advantage. Accordingly, there should be an over-representation of colonizing species in which individuals can reproduce without a mate, particularly in isolated locales such as oceanic islands. Despite the intuitive appeal of this colonization filter hypothesis (known as Baker's law), more than six decades of analyses have yielded mixed findings. We assembled a dataset of island and mainland plant breeding systems, focusing on the presence or absence of self-incompatibility. Because this trait enforces outcrossing and is unlikely to re-evolve on short timescales if it is lost, breeding system is especially likely to reflect the colonization filter. We found significantly more self-compatible species on islands than mainlands across a sample of > 1500 species from three widely distributed flowering plant families (Asteraceae, Brassicaceae and Solanaceae). Overall, 66% of island species were self-compatible, compared with 41% of mainland species. Our results demonstrate that the presence or absence of self-incompatibility has strong explanatory power for plant geographical patterns. Island floras around the world thus reflect the role of a key reproductive trait in filtering potential colonizing species in these three plant families.

Global biogeography of mating system variation in seed plants.

Latitudinal gradients in biotic interactions have been suggested as causes of global patterns of biodiversity and phenotypic variation. Plant biologists have long speculated that outcrossing mating systems are more common at low than high latitudes owing to a greater predictability of plant-pollinator interactions in the tropics; however, these ideas have not previously been tested. Here, we present the first global biogeographic analysis of plant mating systems based on 624 published studies from 492 taxa. We found a weak decline in outcrossing rate towards higher latitudes and among some biomes, but no biogeographic patterns in the frequency of self-incompatibility. Incorporating life history and growth form into biogeographic analyses reduced or eliminated the importance of latitude and biome in predicting outcrossing or self-incompatibility. Our results suggest that biogeographic patterns in mating system are more likely a reflection of the frequency of life forms across latitudes rather than the strength of plant-pollinator interactions.

Invader disruption of belowground plant mutualisms reduces carbon acquisition and alters allocation patterns in a native forest herb.

Invasive plants impose novel selection pressures on naïve mutualistic interactions between native plants and their partners. As most plants critically rely on root fungal symbionts (RFSs) for soil resources, invaders that disrupt plant-RFS mutualisms can significantly depress native plant fitness. Here, we investigate the consequences of RFS mutualism disruption on native plant fitness in a glasshouse experiment with a forest invader that produces known anti-fungal allelochemicals. Over 5 months, we regularly applied either green leaves of the allelopathic invader Alliaria petiolata, a nonsystemic fungicide to simulate A. petiolata's effects, or green leaves of nonallelopathic Hesperis matronalis (control) to pots containing the native Maianthemum racemosum and its RFSs. We repeatedly measured M. racemosum physiology and harvested plants periodically to assess carbon allocation. Alliaria petiolata and fungicide treatment effects were indistinguishable: we observed inhibition of the RFS soil hyphal network and significant reductions in M. racemosum physiology (photosynthesis, transpiration and conductance) and allocation (carbon storage, root biomass and asexual reproduction) in both treatments relative to the control. Our findings suggest a general mechanistic hypothesis for local extinction of native species in ecosystems challenged by allelopathic invaders: RFS mutualism disruption drives carbon stress, subsequent declines in native plant vigor, and, if chronic, declines in RFS-dependent species abundance.

The scope of Baker's law.

Baker's law refers to the tendency for species that establish on islands by long-distance dispersal to show an increased capacity for self-fertilization because of the advantage of self-compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.