Mycorrhizal fungi affect orchid distribution and population dynamics.

Symbioses are ubiquitous in nature and influence individual plants and populations. Orchids have life history stages that depend fully or partially on fungi for carbon and other essential resources. As a result, orchid populations depend on the distribution of orchid mycorrhizal fungi (OMFs). We focused on evidence that local-scale distribution and population dynamics of orchids can be limited by the patchy distribution and abundance of OMFs, after an update of an earlier review confirmed that orchids are rarely limited by OMF distribution at geographic scales. Recent evidence points to a relationship between OMF abundance and orchid density and dormancy, which results in apparent density differences. Orchids were more abundant, less likely to enter dormancy, and more likely to re-emerge when OMF were abundant. We highlight the need for additional studies on OMF quantity, more emphasis on tropical species, and development and application of next-generation sequencing techniques to quantify OMF abundance in substrates and determine their function in association with orchids. Research is also needed to distinguish between OMFs and endophytic fungi and to determine the function of nonmycorrhizal endophytes in orchid roots. These studies will be especially important if we are to link orchids and OMFs in efforts to inform conservation.

Drivers of vegetative dormancy across herbaceous perennial plant species.

Vegetative dormancy, that is the temporary absence of aboveground growth for ≥ 1 year, is paradoxical, because plants cannot photosynthesise or flower during dormant periods. We test ecological and evolutionary hypotheses for its widespread persistence. We show that dormancy has evolved numerous times. Most species displaying dormancy exhibit life-history costs of sprouting, and of dormancy. Short-lived and mycoheterotrophic species have higher proportions of dormant plants than long-lived species and species with other nutritional modes. Foliage loss is associated with higher future dormancy levels, suggesting that carbon limitation promotes dormancy. Maximum dormancy duration is shorter under higher precipitation and at higher latitudes, the latter suggesting an important role for competition or herbivory. Study length affects estimates of some demographic parameters. Our results identify life historical and environmental drivers of dormancy. We also highlight the evolutionary importance of the little understood costs of sprouting and growth, latitudinal stress gradients and mixed nutritional modes.

Cosmopolitan Species As Models for Ecophysiological Responses to Global Change: The Common Reed Phragmites australis.

Phragmites australis is a cosmopolitan grass and often the dominant species in the ecosystems it inhabits. Due to high intraspecific diversity and phenotypic plasticity, P. australis has an extensive ecological amplitude and a great capacity to acclimate to adverse environmental conditions; it can therefore offer valuable insights into plant responses to global change. Here we review the ecology and ecophysiology of prominent P. australis lineages and their responses to multiple forms of global change. Key findings of our review are that: (1) P. australis lineages are well-adapted to regions of their phylogeographic origin and therefore respond differently to changes in climatic conditions such as temperature or atmospheric CO2; (2) each lineage consists of populations that may occur in geographically different habitats and contain multiple genotypes; (3) the phenotypic plasticity of functional and fitness-related traits of a genotype determine the responses to global change factors; (4) genotypes with high plasticity to environmental drivers may acclimate or even vastly expand their ranges, genotypes of medium plasticity must acclimate or experience range-shifts, and those with low plasticity may face local extinction; (5) responses to ancillary types of global change, like shifting levels of soil salinity, flooding, and drought, are not consistent within lineages and depend on adaptation of individual genotypes. These patterns suggest that the diverse lineages of P. australis will undergo intense selective pressure in the face of global change such that the distributions and interactions of co-occurring lineages, as well as those of genotypes within-lineages, are very likely to be altered. We propose that the strong latitudinal clines within and between P. australis lineages can be a useful tool for predicting plant responses to climate change in general and present a conceptual framework for using P. australis lineages to predict plant responses to global change and its consequences.

Symbiont abundance can affect host plant population dynamics.

PREMISE OF THE STUDY: Symbioses are almost universal, but little is known about how symbiont abundance can affect host performance. Many orchids undergo vegetative dormancy and frequent and protracted dormancy have been associated with population declines. If mycorrhizal fungi affect host plant performance, those effects are likely to alter patterns of vegetative dormancy. The goal of this study was to determine whether the abundance of mycorrhizal fungi is related to the likelihood of entering dormancy and whether fungal abundance varied with dormancy duration in the federally listed threatened orchid Isotria medeoloides. METHODS: We studied three populations of the threatened North American terrestrial orchid Isotria medeoloides using long-term emergence data and evaluated the relationship between the abundance of associated mycorrhizal fungi (Russulaceae) and orchid dormancy and emergence. Mycorrhizal fungi in soil adjacent to orchids were quantified in two ways. First, ectomycorrhizal (ECM) fungi on adjacent root tips were identified using DNA sequencing to determine their phylogenetic relationship to fungi that are known to form mycorrhizae with I. medeoloides. Second, we extracted DNA from soil samples and used quantitative real-time PCR to estimate the abundance of Russulaceae hyphae adjacent to each orchid. KEY RESULTS: We found that the abundance of Russulaceae, both in the soil and on nearby ECM root tips, was significantly related to orchid prior emergence. Both abundance and prior emergence history were predictive of future emergence. CONCLUSIONS: These results suggest that the abundance of mycorrhizal fungi can influence orchid population dynamics and is an essential component of orchid conservation.

Biotic resistance, disturbance, and mode of colonization impact the invasion of a widespread, introduced wetland grass.

Disturbance and biotic resistance are important factors driving plant invasions, but how these factors interact for plants with different modes of colonization (i.e., sexual and asexual) is unclear. We evaluated factors influencing the invasion of nonnative Phragmites australis, which has been rapidly expanding in brackish tidal wetlands in Chesapeake Bay. We conducted a survey of naturally occurring small-scale disturbances (removal of vegetation and/or sediment deposition) across four plant communities; determined the effects of small-scale disturbance and biotic resistance on P. australis seedling and rhizome emergence; and tested the effects of size and frequency of small-scale disturbances on seedling emergence and survival of transplanted seedlings. The results of our study demonstrate that the invasion window for seeds is in disturbed areas in high-marsh plant communities that flood less frequently; seedling emergence in undisturbed areas was negligible. Establishment of shoots from rhizome segments was low in all plant communities. Disturbance size and frequency had no significant impact on seed germination and seedling survival. Our findings provide evidence that small-scale within-wetland disturbances are important for the invasion of the nonnative lineage of P. australis by seeds in brackish tidal wetlands in Chesapeake Bay. Efforts to reduce disturbances, large and small, in wetlands can be used to limit P. australis invasion by seed, but invasion by rhizome is still likely to occur across many plant communities irrespective of the presence of disturbance.

Phragmites australis management in the United States: 40 years of methods and outcomes.

Studies on invasive plant management are often short in duration and limited in the methods tested, and lack an adequate description of plant communities that replace the invader following removal. Here we present a comprehensive review of management studies on a single species, in an effort to elucidate future directions for research in invasive plant management. We reviewed the literature on Phragmites management in North America in an effort to synthesize our understanding of management efforts, identify gaps in knowledge and improve the efficacy of management. Additionally, we assessed recent ecological findings concerning Phragmites mechanisms of invasion and integrated these findings into our recommendations for more effective management. Our overall goal is to examine whether or not current management approaches can be improved and whether they promote reestablishment of native plant communities. We found: (i) little information on community-level recovery of vegetation following removal of Phragmites; and (ii) most management approaches focus on the removal of Phragmites from individual stands or groups of stands over a relatively small area. With a few exceptions, recovery studies did not monitor vegetation for substantial durations, thus limiting adequate evaluation of the recovery trajectory. We also found that none of the recovery studies were conducted in a landscape context, even though it is now well documented that land-use patterns on adjacent habitats influence the structure and function of wetlands, including the expansion of Phragmites. We suggest that Phragmites management needs to shift to watershed-scale efforts in coastal regions, or larger management units inland. In addition, management efforts should focus on restoring native plant communities, rather than simply eradicating Phragmites stands. Wetlands and watersheds should be prioritized to identify ecosystems that would benefit most from Phragmites management and those where the negative impact of management would be minimal.

Effects of increased summer flooding on nitrogen dynamics in impounded mangroves.

Mangroves are important for coastal protection, carbon sequestration and habitat provision for plants and animals in the tropics and subtropics. Mangroves are threatened by habitat destruction and sea level rise, but management activities such as impounding for mosquito control can also have negative effects. We studied the effects of Rotational Impoundment Management (RIM) on nitrogen dynamics in impoundments dominated by three types of Black mangrove (Avicennia germinans) stands along the Indian River Lagoon (Florida). RIM, designed for noxious insect control, involves pumping estuarine water into impoundments in this area during spring and summer to raise water levels by 30 cm. We compared aspects of the nitrogen cycle before and after the start of the RIM and measured the same variables in an impoundment without RIM management. RIM led to the accumulation of ammonium in the substrate which coincided with a lowering of nitrification rates and decreased denitrification rates. Salt pan habitats dominated by dwarf mangroves became less saline following RIM initiation. Shoot growth of mangroves increased in response to higher nitrogen availability and lower pore water salinity. Mangrove responses were greatest in areas with dwarf and sparse mangrove cover. Overall, RIM resulted in lower nitrification and denitrification leading to lower nitrogen losses and increased Black mangrove growth, all benefits of RIM beyond those associated with noxious insect control.

Changes in community composition of ammonia-oxidizing betaproteobacteria from stands of Black mangrove (Avicennia germinans) in response to ammonia enrichment and more oxic conditions.

In flooded and non-flooded impounded forests of Black mangrove (Avicennia germinans), the community structure of the ammonia-oxidizing betaproteobacteria (ß-AOB) differed among distinct mangrove vegetation cover types and hydrological regimes. This had been explained by a differential response of lineages of ß-AOB to the prevailing soil conditions that included increased levels of moisture and ammonium. To test this hypothesis, slurries of soils collected from a flooded and a non-flooded impoundment were subjected to enhanced levels of ammonium in the absence and presence of additional shaking. After a period of 6 days, the community composition of the ß-AOB based on the 16S rRNA gene was determined and compared with the original community structures. Regardless of the incubation conditions and the origin of the samples, sequences belonging to the Nitrosomonas aestuarii lineage became increasingly dominant, whereas the number of sequences of the lineages of Nitrosospira (i.e., Cluster 1) and Nitrosomonas sp. Nm143 declined. Changes in community structure were related to changes in community sizes determined by quantitative PCR based on the amoA gene. The amoA gene copy numbers of ß-AOB were compared to those of the ammonia-oxidizing archaea (AOA). Gene copy numbers of the bacteria increased irrespective of incubation conditions, but the numbers of archaea declined in the continuously shaken cultures. This observation is discussed in relation to the distribution of the ß-AOB lineages in the impounded Black mangrove forests.

The Distribution of Ammonia-Oxidizing Betaproteobacteria in Stands of Black Mangroves (Avicennia germinans).

The distribution of species of aerobic chemolitho-autotrophic microorganisms such as ammonia-oxidizing bacteria are governed by pH, salinity, and temperature as well as the availability of oxygen, ammonium, carbon dioxide, and other inorganic elements required for growth. Impounded mangrove forests in the Indian River Lagoon, a coastal estuary on the east coast of Florida, are dominated by mangroves, especially stands of Black mangrove (Avicennia germinans) that differ in the size and density of individual plants. In March 2009, the management of one impoundment was changed to a regime of pumping estuarine water into the impoundment at critical times of the year to eliminate breeding sites for noxious insects. We collected soil samples in three different Black mangrove habitats before and after the change in management to determine the impacts of the altered hydrologic regimes on the distribution of 16s rRNA genes belonging to ammonia-oxidizing betaproteobacteria (ß-AOB). We also sampled soils in an adjacent impoundment in which there had not been any hydrologic alteration. At the level of 97% mutual similarity in the 16s rRNA gene, 13 different operational taxonomic units were identified; the majority related to the lineages of Nitrosomonas marina (45% of the total clones), Nitrosomonas sp. Nm143 (23%), and Nitrosospira cluster 1 (19%). Long-term summer flooding of the impoundment in 2009, after initiation of the pumping regime, reduced the percentage of N. marina by half between 2008 and 2010 in favor of the two other major lineages and the potential ammonia-oxidizing activity decreased by an average of 73%. Higher interstitial salinities, probably due to a prolonged winter drought, had a significant effect on the composition of the ß-AOB in March 2009 compared to March 2008: Nitrosomonas sp. Nm143 was replaced by Nitrosospira cluster 1 as the second most important lineage. There were small, but significant differences in the bacterial communities between the flooded and non-flooded impoundments. There were also differences in the community composition of the bacteria in the three Black mangrove habitats. N. marina was most dominant in all three habitats, but was partly replaced by Nitrosospira cluster 1 in sites dominated by sparsely distributed trees and by Nitrosomonas sp. Nm143 in sites characterized by taller, more densely distributed Black mangrove trees.

Limitations on orchid recruitment: not a simple picture.

Mycorrhizal fungi have substantial potential to influence plant distribution, especially in specialized orchids and mycoheterotrophic plants. However, little is known about environmental factors that influence the distribution of mycorrhizal fungi. Previous studies using seed packets have been unable to distinguish whether germination patterns resulted from the distribution of appropriate edaphic conditions or the distribution of host fungi, as these cannot be separated using seed packets alone. We used a combination of organic amendments, seed packets and molecular assessment of soil fungi required by three terrestrial orchid species to separate direct and indirect effects of fungi and environmental conditions on both seed germination and subsequent protocorm development. We found that locations with abundant mycorrhizal fungi were most likely to support seed germination and greater growth for all three orchids. Organic amendments affected germination primarily by affecting the abundance of appropriate mycorrhizal fungi. However, fungi associated with the three orchid species were affected differently by the organic amendments and by forest successional stage. The results of this study help contextualize the importance of fungal distribution and abundance to the population dynamics of plants with specific mycorrhizal requirements. Such phenomena may also be important for plants with more general mycorrhizal associations.

Restoration of the Mississippi Delta: lessons from Hurricanes Katrina and Rita.

Hurricanes Katrina and Rita showed the vulnerability of coastal communities and how human activities that caused deterioration of the Mississippi Deltaic Plain (MDP) exacerbated this vulnerability. The MDP formed by dynamic interactions between river and coast at various temporal and spatial scales, and human activity has reduced these interactions at all scales. Restoration efforts aim to re-establish this dynamic interaction, with emphasis on reconnecting the river to the deltaic plain. Science must guide MDP restoration, which will provide insights into delta restoration elsewhere and generally into coasts facing climate change in times of resource scarcity.

Hydrochory as a determinant of genetic distribution of seeds within Hibiscus moscheutos (Malvaceae) populations.

Seed dispersal is a major determinant of the spatial genetic structure of plant populations. In this study, we evaluated the role of distinct hydrologic regimes in determining the spatial genetic structure of the seed bank of the wetland plant Hibiscus moscheutos. We analyzed seeds in surface soil samples collected in the autumn and the following spring by determining their allozyme genotypes and estimated the pattern in seed movements during flooding. We selected study sites in nontidal and tidal wetlands with different flooding regimes. One nontidal site had no flooding, while the second nontidal site was inundated for most of the year. One tidal wetland site flooded with almost every tide, and a second tidal site was inundated at moderate frequency. Genetic makeup of the seed bank at the nonflooded site changed little between seasons. Secondary seed dispersal altered absolute allele frequencies at the other three sites, with the greatest change occurring at the two tidally influenced sites. This study demonstrates that secondary hydrochory influences the genetic composition of the seed bank and that hydrologic conditions play an important role in determining the local patterns in seed movements.

Orchid-fungus fidelity: a marriage meant to last?

The characteristics of plant-mycorrhizae associations are known to vary in both time and space, but the ecological consequences of variation in the dynamics of plant-fungus interactions are poorly understood. For example, do plants associate with single fungi or multiple fungi simultaneously, and do the associations persist through a plant's lifetime or do plants support a succession of different fungi? We investigated these and other questions related to plant-fungus interactions in Goodyera pubescens, an evergreen terrestrial orchid of the eastern United States, that interacts with closely related fungi in the genus Tulasnella. Unlike the mycorrhizal associations of other plants, orchid-mycorrhizal associations only benefit the orchid, based on current evidence. Many terrestrial orchids have been found to associate with specific groups of fungi. This characteristic could potentially limit orchids to relatively narrow ranges of environmental conditions and may be a contributing factor in the decline of many orchids in the face of changing environmental conditions. We found that G. pubescens protocorms (developing embryos prior to leaf production) and adults associated with only one fungal individual at a time. The orchid-fungus association persists for years, but during a drought period that was associated with the death of many plants, surviving plants were able to switch to new fungal individuals. These results suggest that G. pubescens interacts with the same fungal partner during periods of modest environmental variation but is able to switch to a different fungal partner. We hypothesize that the ability to switch fungi allows G. pubescens to survive more extreme environmental perturbations. However, laboratory experiments suggest that switching fungi has potential costs, as it increases the risk of mortality, especially for smaller individuals. Our findings indicate that it is unlikely that switching fungi is a common way to improve tolerance of less severe environmental fluctuations and disturbances. These findings may have important implications for plant responses to severe climatic events or to more gradual environmental changes such as global warming.

Watershed land use is strongly linked to PCBs in white perch in Chesapeake Bay subestuaries.

We related total PCBs (t-PCBs) in white perch (Morone americana), an abundant estuarine resident that supports a valuable recreational and commercial fishery in the mid-Atlantic region, to the amount and spatial arrangement of developed land in watersheds that discharge into 14 subestuaries of Chesapeake Bay. We considered the intensity of development in watersheds using four developed land-use measures (% impervious surface, % total developed land, % high-intensity residential + commercial [%high-res/comm], and % commercial) to represent potential source areas of PCBs to the subestuaries. We further evaluated the importance of source proximity by calculating three inverse-distance weighted (IDW) metrics of development, an approach that weighted developed land near the shoreline more heavily than developed land farther away. Unweighted percentages of each of the four measures of developed land explained 51-69% of the variance in t-PCBs. However, IDWs markedly improved the relationships between % developed land measures and t-PCBs. Percent commercial land, weighted by its simple inverse distance, explained 99% of the variance in t-PCBs, whereas the other three measures explained as much as 93-97%. PCBs historically produced or used in commercial and residential areas are apparently persisting in the environment atthe scale of the watersheds and subestuaries examined in this study, and developed land close to the subestuary has the greatest unit effect on t-PCBs in fish. These findings provide compelling evidence for a strikingly strong linkage between watershed land use and t-PCBs in white perch, and this relationship may prove useful for identifying unsampled subestuaries with a high risk of PCB contamination.

Nutrient and sediment removal by a restored wetland receiving agricultural runoff.

Few studies have measured removal of pollutants by restored wetlands that receive highly variable inflows. We used automated flow-proportional sampling to monitor the removal of nutrients and suspended solids by a 1.3-ha restored wetland receiving unregulated inflows from a 14-ha agricultural watershed in Maryland, USA. Water entered the wetland mainly in brief pulses of runoff, which sometimes exceeded the 2500-m3 water holding capacity of the wetland. Half of the total water inflow occurred in only 24 days scattered throughout the two-year study. Measured annual water gains were within 5% of balancing water losses. Annual removal of nutrients differed greatly between the two years of the study. The most removal occurred in the first year, which included a three-month period of decreasing water level in the wetland. In that year, the wetland removed 59% of the total P, 38% of the total N, and 41% of the total organic C it received. However, in the second year, which lacked a drying period, there was no significant (p > 0.05) net removal of total N or P, although 30% of the total organic C input was removed. For the entire two-year period, the wetland removed 25% of the ammonium, 52% of the nitrate, and 34% of the organic C it received, but there was no significant net removal of total suspended solids (TSS) or other forms of N and P. Although the variability of inflow may have decreased the capacity of the wetland to remove materials, the wetland still reduced nonpoint-source pollution.

Nitrogen limitation of growth and nutrient dynamics in a disturbed mangrove forest, Indian River Lagoon, Florida.

The objectives of this study were to determine effects of nutrient enrichment on plant growth, nutrient dynamics, and photosynthesis in a disturbed mangrove forest in an abandoned mosquito impoundment in Florida. Impounding altered the hydrology and soil chemistry of the site. In 1997, we established a factorial experiment along a tree-height gradient with three zones, i.e., fringe, transition, dwarf, and three fertilizer treatment levels, i.e., nitrogen (N), phosphorus (P), control, in Mosquito Impoundment 23 on the eastern side of Indian River. Transects traversed the forest perpendicular to the shoreline, from a Rhizophora mangle-dominated fringe through an Avicennia germinans stand of intermediate height, and into a scrub or dwarf stand of A. germinans in the hinterland. Growth rates increased significantly in response to N fertilization. Our growth data indicated that this site is N-limited along the tree-height gradient. After 2 years of N addition, dwarf trees resembled vigorously growing saplings. Addition of N also affected internal dynamics of N and P and caused increases in rates of photosynthesis. These findings contrast with results for a R. mangle-dominated forest in Belize where the fringe is N-limited, but the dwarf zone is P-limited and the transition zone is co-limited by N and P. This study demonstrated that patterns of nutrient limitation in mangrove ecosystems are complex, that not all processes respond similarly to the same nutrient, and that similar habitats are not limited by the same nutrient when different mangrove forests are compared.

Phenology of roots and mycorrhiza in orchid species differing in phototrophic strategy.

• The mycorrhiza of orchids represents an energy source that may replace or supplement photosynthesis. Dependency on mycotrophy in adult life stages would thus be expected to be inversely related to the prevalence of phototrophic structures. • The phenology of underground parts and mycorrhizal infection were monitored in five terrestrial species differing in leaf phenology (and thus in phototrophic strategy): Goodyera pubescens (evergreen), Tipularia discolor (wintergreen), Galearis spectabilis and Liparis lilifolia (summergreen) and Corallorhiza odontorhiza (chlorophyll deficient), growing sympatrically in a North American deciduous forest. • Mycorrhizal infection was extensive in T. discolor roots and C. odontorhiza rhizomes. Only the proximal part of roots was infected in G. pubescens, and mycorrhizal colonisation was patchy in roots and tubers of G. spectabilis and localized in the rhizome in L. lilifolia. Mycotrophic roots were long-lived (1.5-3 yrs) determinate structures and mycorrhizal infection reached maximum intensity 2-6 months after development. Mycotrophy appeared to be active all year round in mature organs. • The phenology of mycotrophic roots and patterns of mycorrhizal infection were not related to the leafy season. The hypothesis that phototrophic and mycotrophic nutrition alternate through the seasons could not be confirmed.

The effect of petal size manipulation on pollinator/seed-predator mediated female reproductive success of Hibiscus moscheutos.

The effects of petal-size manipulations on the behavior of pollinators and pollen/seed predators, and on pollen removal and deposition, were studied in Hibiscus moscheutos (Malvaceae) populations. The ultimate effects on the female reproductive success of flowers, such as fruit set, seed predation rate, and final seed set were also measured. We applied three levels of petal removal (100%, 50%, and 0% size reduction in radius) to flowers in natural populations. Two pollinators (Bombus pennsylvanicus and Ptilothrix bombiformis) ignored flowers without petals, suggesting that pollinators use petals as a visual cue to locate flowers. Consequently, 100% petal removal reduced female reproductive success considerably, mainly through a higher rate of fruit abortion due to failure of pollen deposition on stigmas. No significant differences between the 50% petal removal treatment and uncut control were detected in any components of female success examined. The results, therefore, suggest that differences in petal size have little influence on female reproductive success of Hibiscus flowers at our study site. Final seed set varied considerably depending on the larval densities of two coleopteran seed predators (Althaeus hibisci and Conotrachelus fissunguis). A. hibisci responded to petal size, and a higher density of adults was found in flowers in which petal size had not been reduced. Because Althaeus feed on pollen as adults and no effect of petal size on seed predation was detected, the preference of Althaeus for larger flowers may represent a foraging strategy for adult beetles and may exert counteracting selection pressure on petal size through male reproductive success of flowers.