Impacts of nitrogen deposition on herbaceous ground flora and epiphytic foliose lichen species in southern Ontario hardwood forests

Interactive effects of precipitation and above canopy nitrogen deposition on understorey vascular plants in a jack pine (Pinus banksiana) forest in northern Alberta, Canada

Elevated nitrogen (N) deposition in the bituminous sands region of northern Alberta, Canada is localized but expected to increase over time. Here we seek to determine the effects of above canopy N deposition on understorey vascular plants in a jack pine (Pinus banksiana) stand in a five-year experimental study. Aqueous N (ammonium nitrate) was applied four times annually (May through October) via helicopter above the canopy between 2011 and 2015 across a narrow but environmentally relevant N deposition gradient (0, 5, 10, 15, 20 and 25 kg N ha^-1 yr^-1). Changes in vascular plant species richness, diversity and total vascular cover were best explained by throughfall water flux, but the positive responses to precipitation decreased with increasing N application. Arctostaphylos uva-ursi and Maianthemum canadense showed positive cover increases in wet years; however, the positive cover expansion at ≥5 kg N ha^-1 yr^-1 treatments was suppressed relative to controls. Total cover expansion was muted in low precipitation years in treatments ≥10 kg N ha^-1 yr^-1. In contrast, Vaccinium vitis-idaea cover changes ≥10 kg N ha^-1 yr^-1 were consistently negative. There were no differences in soil net N mineralization rates, plant foliar N or NO₃^- leaching among treatments. We conjecture the extensive moss/lichen layer of the forest floor that accumulates most of incoming N in throughfall allows them to outcompete vascular plants for water during higher precipitation years, effectively reducing vascular cover expansion relative to controls. This work suggests the response of vascular plants in xeric jack pine ecosystems may interact with climate and these interactions should be considered in risk assessment studies.

NO2 air pollution drives species composition, but tree traits drive species diversity of urban epiphytic lichen communities

Lichens serve as important bioindicators of air pollution in cities. Here, we studied the diversity of epiphytic lichens in the urban area of Munich, Bavaria, southern Germany, to determine which factors influence species composition and diversity. Lichen diversity was quantified in altogether 18 plots and within each, five deciduous trees were investigated belonging to on average three tree species (range 1-5). Of the 18 plots, two were sampled in control areas in remote areas of southern Germany. For each lichen species, frequency of occurrence was determined in 10 quadrats of 100 cm² on the tree trunk. Moreover, the cover percentage of bryophytes was determined and used as a variable to represent potential biotic competition. We related our diversity data (species richness, Shannon index, evenness, abundance) to various environmental variables including tree traits, i.e. bark pH levels and species affiliation and air pollution data, i.e. NO₂ and SO₂ concentrations measured in the study plots. The SO₂ levels measured in our study were generally very low, while NO₂ levels were rather high in some plots. We found that the species composition of the epiphytic lichen communities was driven mainly by NO₂ pollution levels and all of the most common species in our study were nitrophilous lichens. Low NO₂ but high SO₂ values were associated with high lichen evenness. Tree-level lichen diversity and abundance were mainly determined by tree traits, not air pollution. These results confirm that ongoing NO₂ air pollution within cities is a major threat to lichen diversity, with non-nitrophilous lichens likely experiencing the greatest risk of local extinctions in urban areas in the future. Our study moreover highlights the importance of large urban green spaces for species diversity. City planners need to include large green spaces when designing urban areas, both to improve biodiversity and to promote human health and wellbeing.

Hyunsoonleella sp. HU1-3 Increased the Biomass of Ulva fasciata

Green algae are photosynthetic organisms and play an important role in coastal environment. The microbial community on the surface of green algae has an effect on the health and nutrition of the host. However, few species of epiphytic microbiota have been reported to play a role in promoting the growth of algae. In this study, 16S rDNA sequencing was used to study the changes of microbial composition on the surface of Ulva fasciata at different growth stages. Some growth promoting bacteria were identified. The possible growth-promoting behavior of the strains were verified by co-culture of pure bacteria obtained from the surface of U. fasciata with its sterile host. Among the identified species, a new bacterial species, Hyunsoonleella sp. HU1-3 (belonging to the family Flavobacteriaceae) significantly promoted the growth of U. fasciata. The results also showed that there were many genes involved in the synthesis of growth hormone and cytokinin in the genome of Hyunsoonleella sp. HU1-3. This study identified the bacterium Hyunsoonleella sp. HU1-3 for the first time, in which this bacterium has strong growth-promoting effects on U. fasciata. Our findings not only provide insights on the establishment of the surface microbiota of U. fasciata, but also indicate that Hyunsoonleella sp. HU1-3 is one of the important species to promote the growth of U. fasciata.

The invasive tree staghorn sumac affects soil N2 -fixing bacterial communities in north China

Soil N₂ -fixing bacterial communities (SNB) can enhance soil N availability and the invasiveness of invaders. Some invaders display different degrees of invasion across different climate regions. Given that bacterial communities may change with different climate regions, it is important to understand soil micro-ecological mechanisms driving the successful invasion of invaders across different climate regions. This study performed cross-site comparisons to comprehensively analyse effects of the invasive tree staghorn sumac (Rhus typhina L.) on the structure of SNB. In north China, we selected three sites within two sampling regions (a warm temperate region and a cold temperate region). Staghorn sumac invasion did not significantly affect soil physicochemical properties and the diversity and richness of SNB. LEfSe analysis showed that numerous SNB taxa changed significantly during staghorn sumac invasion. This may be attributed in part to the selective effects of allelochemicals released by staghorn sumac via leaf litter and/or root exudates. Consequently, staghorn sumac invasion may alter the structure, rather than the diversity and richness, of SNB to facilitate its invasion process by establishing a favourable soil microenvironment in the invaded habitats. The number of species and richness of SNB under staghorn sumac invasion were significantly lower in the warm temperate region than in the cold temperate region. A possible reason for the increased diversity and richness of SNB under staghorn sumac invasion in the cold temperate region may be because staghorn sumac in the cold temperate region can provide more nutrients into the soil sub-ecosystem, presumably to support a higher diversity and richness of SNB via the nutritional requirements of SNB. The changed structure of SNB under staghorn sumac invasion, especially across different climate regions, may play an important role in its successful invasion across most regions of north China.

The impact of pollutants from a major northern highway on an adjacent hardwood forest

Emissions of pollutants from highways can exert multiple stresses on adjacent ecosystems. In this study air concentrations of NO₂ and throughfall deposition of inorganic N (NO₃^- and NH₄⁺), SO₄^2-, Cl^-, base cations and several metals were measured in all four seasons along a 1.5km hardwood forest gradient extending away from a major highway (Highway 401) in southern Ontario, Canada. Soil and lichen chemistry and herbaceous plant and epiphytic lichen species composition were measured within the hardwood forest to evaluate impacts of these pollutants. Air concentrations of NO₂ and deposition of inorganic N, Cl^-, base cations and Cu and Zn in throughfall were significantly elevated within 100m of the road compared with the more distant sites. Concentrations of several pollutants including N (and δ¹⁵N), Na⁺, Al and Fe in epiphytic lichen tissue decreased with distance from the highway, and epiphytic lichen richness was lower at sites within 100m of the road. Despite high throughfall inputs of >15kgNha^-1y^-1 and 100kgNa⁺ha^-1y^-1 within 33m of the highway, for example, there was no significant difference in soil chemistry amongst sites. Plant community composition at sites within 80m of the highway differed from sites located further from the road, but it is unclear whether differences were due to highway emissions or were a result of natural forest edge effects.

Using change-point models to estimate empirical critical loads for nitrogen in mountain ecosystems

To protect ecosystems and their services, the critical load concept has been implemented under the framework of the Convention on Long-range Transboundary Air Pollution (UNECE) to develop effects-oriented air pollution abatement strategies. Critical loads are thresholds below which damaging effects on sensitive habitats do not occur according to current knowledge. Here we use change-point models applied in a Bayesian context to overcome some of the difficulties when estimating empirical critical loads for nitrogen (N) from empirical data. We tested the method using simulated data with varying sample sizes, varying effects of confounding variables, and with varying negative effects of N deposition on species richness. The method was applied to the national-scale plant species richness data from mountain hay meadows and (sub)alpine scrubs sites in Switzerland. Seven confounding factors (elevation, inclination, precipitation, calcareous content, aspect as well as indicator values for humidity and light) were selected based on earlier studies examining numerous environmental factors to explain Swiss vascular plant diversity. The estimated critical load confirmed the existing empirical critical load of 5-15 kg N ha^-1 yr^-1 for (sub)alpine scrubs, while for mountain hay meadows the estimated critical load was at the lower end of the current empirical critical load range. Based on these results, we suggest to narrow down the critical load range for mountain hay meadows to 10-15 kg N ha^-1 yr^-1.