Microclimate variability in alpine ecosystems as stepping stones for non‐native plant establishment above their current elevational limit

Alpine environments are currently relatively free from non‐native plant species, although their presence and abundance have recently been on the rise. It is however still unclear whether the observed low invasion levels in these areas are due to an inherent resistance of the alpine zone to invasions or whether an exponential increase in invasion is just a matter of time. Using a seed‐addition experiment on north‐ and south‐facing slopes (cf. microclimatic gradient) on two mountains in subarctic Sweden, we tested the establishment of six non‐native species at an elevation above their current distribution limits and under experimentally enhanced anthropogenic pressures (disturbance, added nutrients and increased propagule pressure). We found a large microclimatic variability in cumulative growing degree days (GDD) (range = 500.77°C, SD = 120.70°C) due to both physiographic (e.g. aspect) and biophysical (e.g. vegetation cover) features, the latter being altered by the experimental disturbance. Non‐native species establishment and biomass production were positively correlated with GDD along the studied microclimatic gradient. However, even though establishment on the north‐facing slopes caught up with that on the south‐facing slopes throughout the growing season, biomass production was limited on the north‐facing slopes due to a shorter growing season. On top of this microclimatic effect, all experimentally imposed anthropogenic factors enhanced non‐native species success. The observed microclimatic effect indicates a potential for non‐native species to use warm microsites as stepping stones for their establishment towards the cold end of the gradient. Combined with anthropogenic pressures this result suggests an increasing risk for plant invasion in cold ecosystems, as such stepping stones in alpine ecosystems are likely to be more common in a future that will combine a warming climate with persistent anthropogenic pressures.     

Habitat properties and plant traits interact as drivers of non‐native plant species’ seed production at the local scale

To answer the long‐standing question if we can predict plant invader success based on characteristics of the environment (invasibility) or the invasive species (invasiveness), or the combination of both, there is a need for detailed observational studies in which habitat properties, non‐native plant traits, and the resulting invader success are locally measured. In this study, we assess the interaction of gradients in the environmental and trait space on non‐native species fitness, expressed as seed production, for a set of 10 invasive and noninvasive non‐native species along a wide range of invaded sites in Flanders. In our multidimensional approach, most of the single environmental gradients (temperature, light availability, native plant species diversity, and soil fertility) and sets of non‐native plant traits (plant size, photosynthesis, and foliar chemical attributes) related positively with invader seed production. Yet correlation with seed production was much stronger when several environmental gradients were assessed in interaction, and even more so when we combined plant traits and habitat properties. The latter increased explanatory power of the models on average by 25% for invasive and by 7% for noninvasive species. Additionally, we report a 70‐fold higher seed production in invasive than in noninvasive species and fundamentally different correlations of seed production with plant traits and habitat properties in noninvasive versus invasive species. We conclude that locally measured traits and properties deserve much more attention than they currently get in invasion literature and thus encourage further studies combining this level of detail with the generality of a multiregion and multispecies approach across different stages of invasion.     

Light Dependent Reduction of Pertechnetate (TcO(4)) by Broken Chloroplasts

Arguments are given for a ferredoxin-mediated reduction of TcO₄⁻, preponderantly into extractable Tc(V) complexes, by illuminated, broken chloroplasts. Photosynthetic O₂- and NADP-reduction competitively inhibit Tc incorporation. As for O₂, the reaction can be stimulated by the auto-oxidizable electron acceptor methyl viologen. Furthermore TcO₄⁻ can function as terminal acceptor in the diaphorase reaction, with NADPH as electron donor.     

GABAergic signaling in the pulmonary neuroepithelial body microenvironment: functional imaging in GAD67-GFP mice

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates, but has also been reported in multiple cell types outside the CNS. A GABAergic system has been proposed in neuroepithelial bodies (NEBs) in monkey lungs. Pulmonary NEBs are known as complex intraepithelial sensory airway receptors and are part of the NEB microenvironment. Aim of the present study was to unravel a GABAergic signaling system in the NEB microenvironment in mouse lungs, enabling the use of genetically modified animals for future functional studies. Immunostaining of mouse lungs revealed that glutamic acid decarboxylase 65/67 (GAD65/67), a rate-limiting enzyme in the biosynthesis of GABA, and the vesicular GABA transporter (VGAT) were exclusively expressed in NEB cells. In GAD67-green fluorescent protein (GFP) knock-in mice, all pulmonary NEBs appeared to express GFP. For confocal live cell imaging, ex vivo vibratome lung slices of GAD67-GFP mice can be directly loaded with fluorescent functional probes, e.g. a red-fluorescent calcium dye, without the necessity of time-consuming prior live visualization of NEBs. RT-PCR of the NEB microenvironment obtained by laser microdissection revealed the presence of both GABA_A and GABA_B (R1 and R2) receptors, which was confirmed by immunostaining. In conclusion, the present study not only revealed the presence of a GABAergic signaling pathway, but also the very selective expression of GFP in pulmonary NEBs in a GAD67-GFP mouse model. Different proof of concept experiments have clearly shown that adoption of the GAD67-GFP mouse model will certainly boost future functional imaging and gene expression analysis of the mouse NEB microenvironment.     

Expression of mechanogated two-pore domain potassium channels in mouse lungs: special reference to mechanosensory airway receptors

Afferent activities arising from sensory nerve terminals located in lungs and airways are carried almost exclusively by fibres travelling through the vagus nerve. Based on electrophysiological investigations, intrapulmonary airway-related vagal afferent receptors have been classified into three main subtypes, two of which are myelinated and mechanosensitive, i.e., rapidly and slowly adapting receptors. To allow for a full functional identification of the distinct populations of airway receptors, morphological and neurochemical characteristics still need to be determined. Nerve terminals visualised using markers for myelinated vagal afferents seem to be almost uniquely associated with two morphologically well-formed airway receptor end organs, smooth muscle-associated airway receptors (SMARs) and neuroepithelial bodies (NEBs), localised in airway smooth muscle and epithelium, respectively. Due to the lack of a selective marker for SMARs in mice, no further neurochemical coding is available today. NEBs are extensively innervated diffusely spread groups of neuroendocrine cells in the airway epithelium, and are known to receive at least two separate populations of myelinated vagal afferent nerve terminals. So far, however, no evidence has been reported for the expression of channels that may underlie direct sensing and transduction of mechanical stimuli by the receptor terminals in NEBs and SMARs. This study focused on the expression of mechanogated two-pore domain K⁺ (K_2P) channels, TREK-1 and TRAAK, in mouse airways and more particular in the NEB micro-environment and in SMARs by multiple immunostaining. TREK-1 could be detected on smooth muscle cells surrounding intrapulmonary airways and blood vessels, while TRAAK was expressed on myelinated vagal afferents terminating both in SMARs and in the NEB micro-environment. Co-stainings with known markers for subpopulations of myelinated vagal afferents and general neuronal markers revealed that all identified SMARs exhibit TRAAK immunoreactivity, and that at least three subpopulations exist in mouse airways. Also, the intraepithelial terminals of both subpopulations of NEB-associated myelinated vagal sensory nerve fibres were shown to express TRAAK. In conclusion, the present study finally characterised an intrinsically mechanosensitive ion channel, the K_2P channel TRAAK, on the terminals of identified myelinated vagal nodose airway afferents, organised as SMARs and as components of the innervation of NEBs. These data support the hypothesis that both SMARs and NEBs harbour the morphological counterparts of electrophysiologically identified myelinated vagal airway mechanoreceptors. TRAAK appears to be strongly involved in regulating airway mechanosensing since it was found to be expressed on the terminals of all subpopulations of potential vagal mechanosensors.     

Anthropogenic factors overrule local abiotic variables in determining non-native plant invasions in mountains

Biological Invasions - The factors that determine patterns of non-native species richness and abundance are context dependent in both time and space. Global change has significantly boosted plant...     

Neurochemical pattern of the complex innervation of neuroepithelial bodies in mouse lungs

As best characterized for rats, it is clear that pulmonary neuroepithelial bodies (NEBs) are contacted by a plethora of nerve fiber populations, suggesting that they represent an extensive group of multifunctional intraepithelial airway receptors. Because of the importance of genetically modified mice for functional studies, and the current lack of data, the main aim of the present study was to achieve a detailed analysis of the origin and neurochemical properties of nerve terminals associated with NEBs in mouse lungs. Antibodies against known selective markers for sensory and motor nerve terminals in rat lungs were used on lungs from control and vagotomized mice of two different strains, i.e., Swiss and C57-Bl6. NEB cells were visualized by antibodies against either the general neuroendocrine marker protein gene-product 9.5 (PGP9.5) or calcitonin gene-related peptide (CGRP). Thorough immunohistochemical examination of NEB cells showed that some of these NEB cells also exhibit calbindin D-28 k (CB) and vesicular acetylcholine transporter (VAChT) immunoreactivity (IR). Mouse pulmonary NEBs were found to receive intraepithelial nerve terminals of at least two different populations of myelinated vagal afferents: (1) Immunoreactive (ir) for vesicular glutamate transporters (VGLUTs) and CB; (2) expressing P2X₂ and P2X₃ ATP receptors. CGRP IR was seen in varicose vagal nerve fibers and in delicate non-vagal fibers, both in close proximity to NEBs. VAChT immunostaining showed very weak IR in the NEB-related intraepithelial vagal sensory nerve terminals. nNOS- or VIP-ir nerve terminals could be observed at the base of pulmonary NEBs. While a single NEB can be contacted by multiple nerve fiber populations, it was clear that none of the so far characterized nerve fiber populations contacts all pulmonary NEBs. The present study revealed that mouse lungs harbor several populations of nerve terminals that may selectively contact NEBs. Although at present the physiological significance of the innervation pattern of NEBs remains enigmatic, it is likely that NEBs are receptor–effector end-organs that may host complex and/or multiple functional properties in normal airways. The neurochemical information on the innervation of NEBs in mouse lungs gathered in the present study will be essential for the interpretation of upcoming functional data and for the study of transgenic mice.     

Effects of species evenness can be derived from species richness – ecosystem functioning relationships

Although species richness effects on ecosystem functioning have been studied thoroughly in countless experiments, the effects of the other side of diversity – species evenness – remain less identified, especially at high species richness. Due to the large number of different model ecosystems that need to be created, the explanatory power of the experimental approach for evenness is indeed limited. We show here that experimental studies on the influence of species richness on ecosystem functions contain hidden information on the influence of species evenness. Both the effects of maximum and minimum evenness, and of a key set of intermediate evenness levels, can be derived from species richness – ecosystem function curves, and that for every richness level, by using communities with low species richness as the equivalent of highly uneven communities with higher richness. We show that evenness effects on ecosystem functioning have the same direction as richness effects, however with increasing effect sizes at higher richness levels. We validated our technique for a wide range of ecosystem functions and applied it to the species richness – community biomass data from an existing biodiversity experiment. Our approach could provide a fast and easy alternative to resource‐intensive experiments in which evenness is experimentally varied, as we can build on the elaborate existing literature on species richness to assess its effects.     

Shielded environments reduce stress in alien Asteraceae species during hot and dry summers along urban‐to‐rural gradients

Urban environments often host a greater abundance and diversity of alien plant species than rural areas. This is frequently linked to higher disturbance and propagule pressure, but could also be related to the additional establishment of species from warmer native ranges in cities, facilitated by the latter''s higher air temperatures and drier soils. A hitherto unresolved question is how stressful the urban environments become during climate extremes such as heatwaves and droughts. Do such episodes still favor alien plant species, or set them back? We used in situ measured phenotypic leaf and development trait responses of the six most widespread alien Asteraceae species from various native climates along Belgian urban‐to‐rural gradients, measured during two unusually warm and dry summers. Urbanization was characterized by three factors: the percentage of artificially sealed surfaces (urbanity, measured at three spatial scales from in situ to satellite‐based), the vegetation cover and the sky view factor (SVF, fraction of the hemisphere not blocked by buildings or vegetation). Across species, either from colder or warmer native climates, we found a predominant protective effect of shielded environments that block solar radiation (low SVF) along the entire urban‐to‐rural gradient. Such environments induced lower leaf anthocyanins and flavonols indices, indicating heat stress mitigation. Shielded environments also increased specific leaf area (SLA), a typical shade response. We found that vegetated areas had a secondary importance, increasing the chlorophyll content and decreasing the flavonols index, but these effects were not consistent across species. Finally, urbanity at the organism spatial scale decreased plant height, while broader‐scale urbanity had no significant influence. Our results suggest that sealed surfaces constrain alien Asteraceae during unusually warm and dry summers, while shielded environments protect them, possibly canceling out the lack of light. These findings shed new light on alien plant species success along urban‐to‐rural gradients in a changing climate.