José M. Facelli

Rooting theories of plant community ecology in microbial interactions.

Predominant frameworks for understanding plant ecology have an aboveground bias that neglects soil micro-organisms. This is inconsistent with recent work illustrating the importance of soil microbes in terrestrial ecology. Microbial effects have been incorporated into plant community dynamics using ideas of niche modification and plant-soil community feedbacks. Here, we expand and integrate qualitative conceptual models of plant niche and feedback to explore implications of microbial interactions for understanding plant community ecology. At the same time we review the empirical evidence for these processes. We also consider common mycorrhizal networks, and propose that these are best interpreted within the feedback framework. Finally, we apply our integrated model of niche and feedback to understanding plant coexistence, monodominance and invasion ecology.

Plant Litter: Light Interception and Effects on an Old‐Field Plant Community

We studied the effects of litter ofthe annual grass Setariafaberii, the perennial herb Solidago spp. (mostly S. canadensis), and leaves of the hardwood tree Quercus alba on a successional plant community. We also assessed light interception by these litter types in the laboratory. Light extinction followed the Beer-Lambert exponential law. Solidago litter had the highest transmittance constant and Quercus litter the lowest. The three types of litter produced different light mosaics at the microsite (0.8 cm diameter) scale. In the field, all three litter types affected community structure, but the effect of Quercus was the strongest. Litter reduced the density of the two dominant grasses, Setaria faberii and Panicum dichotomiflorum. Quercus and Setaria litter resulted in biomass compensation (i.e., fewer but larger individuals) by S. faberii but not by P. dichotomiflorum, which probably was always outcompeted by S. faberii. Solanum carolinense, the main dicot in the community, was unaffected by litter addition. Setaria and Solidago litter enhanced the establishment of Erigeron annuus, but Quercus litter reduced it. Litter reduced the number of flowering individuals of S. faberii and the number of seeds per plot; Quercus litter increased the production of seeds per individual. We conclude that species-dependent effects of litter on plant populations may significantly alter interspecific interactions and change plant community structure through direct and indirect effects.

Multiple Indirect Effects of Plant Litter Affect the Establishment of Woody Seedlings in Old Fields

I studied the effects of oak leaf litter, herb competition, and insect herbivory on the establishment of seedlings of Ailanthus altissima in New Jersey, USA. I performed two experiments in an early-successional site in two consecutive years. In the first exper- iment I tested the effects of litter and competition, and their interaction, on establishment and growth of tree seedlings. In the second experiment I studied the effect of insecticide application in combination with litter and herb competition on seedling establishment and growth, and on arthropod abundance. In the absence of competition, litter had no significant effect on the biomass of woody seedlings, though it delayed seedling emergence. Herb competition reduced the growth of the woody seedlings, but the addition of leaf litter reduced the density and biomass of the herbs, indirectly improving the growth of the woody seedlings. Litter increased both invertebrate damage to seedling cotyledons and seedling mortality. The application of insecticide increased seedling emergence and reduced seedling mortality, and more so in the presence of litter. The total number of arthropods and the number of plant-chewing arthropods were increased by the addition of litter and reduced by application of insecticide. Thus, litter had, simultaneously, a positive indirect effect on individual biomass through reduced competition, and a negative indirect effect on the number of individuals through increased seed and/or seedling predation, and a negative direct effect on seedling mortality.

Global sampling of plant roots expands the described molecular diversity of arbuscular mycorrhizal fungi.

We aimed to enhance understanding of the molecular diversity of arbuscular mycorrhizal fungi (AMF) by building a new global dataset targeting previously unstudied geographical areas. In total, we sampled 96 plant species from 25 sites that encompassed all continents except Antarctica. AMF in plant roots were detected by sequencing the nuclear SSU rRNA gene fragment using either cloning followed by Sanger sequencing or 454-sequencing. A total of 204 AMF phylogroups (virtual taxa, VT) were recorded, increasing the described number of Glomeromycota VT from 308 to 341 globally. Novel VT were detected from 21 sites; three novel but nevertheless widespread VT (Glomus spp. MO-G52, MO-G53, MO-G57) were recorded from six continents. The largest increases in regional VT number were recorded in previously little-studied Oceania and in the boreal and polar climatic zones — this study providing the first molecular data from the latter. Ordination revealed differences in AM fungal communities between different continents and climatic zones, suggesting that both biogeographic history and environmental conditions underlie the global variation of those communities. Our results show that a considerable proportion of Glomeromycota diversity has been recorded in many regions, though further large increases in richness can be expected in remaining unstudied areas.

DIFFERENCES IN SEED BIOLOGY OF ANNUAL PLANTS IN ARID LANDS: A KEY INGREDIENT OF THE STORAGE EFFECT

We used a combination of field studies and laboratory experiments to characterize key ecological aspects of the seed biology and soil seed bank dynamics of annual plant communities in chenopod shrublands of South Australia. A sequential study of the soil seed bank demonstrated seasonal and between-year variability in numbers and composition of the soil seed bank. Soil samples incubated under different temperature and watering regimes produced different communities, indicating that species respond differentially to various environmental combinations. Emergence was extremely low at low water availability and at high temperatures, even in trays with ample water. A high percentage of seeds of four out of five species buried in the field remained viable for two years, while the fifth, Carrichtera annua, showed a sharp decline in seed viability, reaching nearly zero survivorship. Our results indicate that, in this system, annual plant communities result from germination of a fraction of seeds present in the soil seed bank, when autumn or winter rainfalls occur. Because different species have different responses to various combinations of environmental conditions, the community composition varies from year to year. This variability is likely to be a component of coexistence through the storage effect.

Forces that structure plant communities: quantifying the importance of the mycorrhizal symbiosis

John Klironomos, Martin Zobel, Mark Tibbett, William D. Stock, Matthias C. Rillig, Jeri L. Parrent, Mari Moora, Alexander M. Koch, Jose M. Facelli, Evelina Facelli, Ian A. Dickie and James D. Bever

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.

Underground friends or enemies: model plants help to unravel direct and indirect effects of arbuscular mycorrhizal fungi on plant competition.

*We studied the effects of two arbuscular mycorrhizal (AM) fungi, singly or together, on the outcome of competition between a host (tomato cultivar, wild-type (WT)) and a surrogate nonhost (rmc, a mycorrhiza-defective mutant of WT) as influenced by the contributions of the direct and AM phosphorus (P) uptake pathways to plant P. *We grew plants singly or in pairs of the same or different genotypes (inoculated or not) in pots containing a small compartment with (32)P-labelled soil accessible to AM fungal hyphae and determined expression of orthophosphate (P(i)) transporter genes involved in both AM and direct P uptake. *Gigaspora margarita increased WT competitive effects on rmc. WT and rmc inoculated with Glomus intraradices both showed growth depressions, which were mitigated when G. margarita was present. Orthophosphate transporter gene expression and (32)P transfer showed that the AM pathway operated in single inoculated WT, but not in rmc. *Effects of AM fungi on plant competition depended on the relative contributions of AM and direct pathways of P uptake. Glomus intraradices reduced the efficiency of direct uptake in both WT and rmc. The two-fungus combination showed that interactions between fungi are important in determining outcomes of plant competition.

Germination requirements and responses to leaf litter of four species of eucalypt.

We studied how the responses of four species of eucalypt to leaf litter related to their germination responses to light and water availability. Two of the species (Eucalyptus obliqua and E. baxteri) have a mesic distribution, while the other two (E. oleosa, and E. incrassata) are more xerophytic. We studied the effect of litter on emergence of the four species in a glasshouse experiment. Litter did not affect the emergence of E. incrassata and E. oleosa, but enhanced the emergence of E. obliqua and E. baxteri. Litter increased the seedling mortality of all four species. Germination responses to light and water availability were studied in growth cabinets under controlled conditions. The germination of E. obliqua and E. baxteri was substantially lower under fluorescent light than in darkness, but that of E. oleosa and E. incrassata was not affected by the light environment. The germination of E. obliqua and E. baxteri was significantly reduced by reduced water potential (ψa). Substantial germination of E. oleosa and E. incrassata occurred even at very low ψa (less than −1.05 MPa). We conclude that both the shade and the humid micro-environment provided by litter may have contributed to the emergence responses of the four species to litter, and these responses may correspond to ecological adaptations to the different environments in which they live.

The relationship between the diversity of arbuscular mycorrhizal fungi and grazing in a meadow steppe

AimsTo study the relationship between changes in soil properties and plant community characters produced by grazing in a meadow steppe grassland and the composition and diversity of spore-producing arbuscular mycorrhizal fungi (AMF).MethodsA field survey was carried out in a meadow steppe area with a gradient of grazing pressures (a site with four grazing intensities and a reserve closed to grazing). The AMF community composition (characterized by spore abundance) and diversity, the vegetation characters and soil properties were measured, and root colonization by AMF was assessed.ResultsAMF diversity (richness and evenness) was higher under light to moderate grazing pressure and declined under intense grazing pressures. Results of multiple regressions indicated that soil electrical conductivity was highly associated with AMF diversity. The variation in AMF diversity was partially associated to the density of tillers of the dominant grass (Leymus chinensis), the above and below-ground biomass and the richness of the plant community.ConclusionsWe propose that the relationship between plants and AMF is altered by environmental stress (salinity) which is in turn influenced by animal grazing. Direct and indirect interactions between vegetation, soil properties, and AMF community need to be elucidated to improve our ability to manage these communities.