How are nitrogen availability,fine‐root mass,and nitrogen uptake related empirically? Implications for models and theory

Understanding the effects of global change in terrestrial communities requires an understanding of how limiting resources interact with plant traits to affect productivity. Here, we focus on nitrogen and ask whether plant community nitrogen uptake rate is determined (a) by nitrogen availability alone or (b) by the product of nitrogen availability and fine‐root mass. Surprisingly, this is not empirically resolved. We performed controlled microcosm experiments and reanalyzed published pot experiments and field data to determine the relationship between community‐level nitrogen uptake rate, nitrogen availability, and fine‐root mass for 46 unique combinations of species, nitrogen levels, and growing conditions. We found that plant community nitrogen uptake rate was unaffected by fine‐root mass in 63% of cases and saturated with fine‐root mass in 29% of cases (92% in total). In contrast, plant community nitrogen uptake rate was clearly affected by nitrogen availability. The results support the idea that although plants may over‐proliferate fine roots for individual‐level competition, it comes without an increase in community‐level nitrogen uptake. The results have implications for the mechanisms included in coupled carbon‐nitrogen terrestrial biosphere models (CN‐TBMs) and are consistent with CN‐TBMs that operate above the individual scale and omit fine‐root mass in equations of nitrogen uptake rate but inconsistent with the majority of CN‐TBMs, which operate above the individual scale and include fine‐root mass in equations of nitrogen uptake rate. For the much smaller number of CN‐TBMs that explicitly model individual‐based belowground competition for nitrogen, the results suggest that the relative (not absolute) fine‐root mass of competing individuals should be included in the equations that determine individual‐level nitrogen uptake rates. By providing empirical data to support the assumptions used in CN‐TBMs, we put their global climate change predictions on firmer ground.     

Embryo donation and receipt in Australia: views on the meanings of embryos and kinship relations

Research on embryo donation and receipt continues to grow, highlighting how specific national contexts shape views and experiences. The present article reports on a qualitative study on embryo donation and receipt in Australia. Interviews were conducted with 15 participants: embryo donors and those seeking to donate (6), embryo recipients and those seeking donors (3), people with embryos in storage or previously in storage (5), and egg donors where resulting embryos were donated to a third party (1). A deductive thematic analysis identified four key themes: understandings of embryos as cells, potential children, and/or children; a focus on relationships between “siblings”; importance of language and “family words” in discussing relationships; and extended family members having difficulty understanding the concept of embryo donation. The article concludes with a consideration of the implications of the findings in terms of the practice of embryo donation and the policies that surround it.     

Increase in nonnative understorey vegetation cover after nonnative conifer removal and passive restoration

Nonnative conifers are widespread in the southern hemisphere, where their use as plantation species has led to adverse ecosystem impacts sometimes intensified by invasion. Mechanical removal is a common strategy used to reduce or eliminate the negative impacts of nonnative conifers, and encourage native regeneration. However, a variety of factors may preclude active ecological restoration following removal. As a result, passive restoration – unassisted natural vegetation regeneration – is common following conifer removal. We asked, ‘what is the response of understorey cover to removal of nonnative conifer stands followed by passive restoration?' We sampled understorey cover in three site types: two‐ to ten‐year‐old clearcuts, native forest and current plantations. We then grouped understorey species by origin (native/nonnative) and growth form, and compared proportion and per cent cover of these groups as well as of bare ground and litter between the three site types. For clearcuts, we also analysed the effect of time since clearcut on the studied variables. We found that clearcuts had a significantly higher average proportion of nonnative understorey vegetation cover than native forest sites, where nonnative vegetation was nearly absent. The understorey of clearcut sites also averaged more overall vegetation cover and more nonnative vegetation cover (in particular nonnative shrubs and herbaceous species) than either plantation or native forest sites. Notably, 99% of nonnative shrub cover in clearcuts was the invasive nonnative species Scotch broom (Cytisus scoparius). After ten years of passive recovery since clearcutting, the proportion of understorey vegetation cover that is native has not increased and remains far below the proportion observed in native forest sites. Reduced natural regeneration capacity of the native ecosystem, presence of invasive species in the surrounding landscape and land‐use legacies from plantation forestry may inhibit native vegetation recovery and benefit opportunistic invasives, limiting the effectiveness of passive restoration in this context. Abstract in Spanish is available with online material.     

Endemic and cosmopolitan fungal taxa exhibit differential abundances in total and active communities of Antarctic soils

Our understanding of the diversity and community dynamics of soil fungi has increased greatly through the use of DNA-based identification. Community characterization of metabolically active communities via RNA sequencing has previously revealed differences between ‘active’ and ‘total’ fungal communities, which may be influenced by the persistence of DNA from nonactive components. However, it is not known how fungal traits influence their prevalence in these contrasting community profiles. In this study, we coextracted DNA and RNA from soil collected from three Antarctic islands to test for differences between total and active soil fungal communities. By matching these geographically isolated fungi against a global dataset of soil fungi, we show that widely dispersed taxa are often more abundant in the total community, whilst taxa restricted to Antarctica are more likely to have higher abundance in the active community. In addition, we find that active communities have lower richness, and show a reduction in the abundance of the most dominant fungi, whilst there are consistent differences in the abundances of certain taxonomic groups between the total and active communities. These results suggest that the views of soil fungal communities offered by DNA- and RNA-based characterization differ in predictable ways.     

Quantitative real-time PCR for rapid and accurate titration of recombinant baculovirus particles

We describe the use of quantitative PCR (QPCR) to titer recombinant baculoviruses. Custom primers and probe were designed to gp64 and used to calculate a standard curve of QPCR derived titers from dilutions of a previously titrated baculovirus stock. Each dilution was titrated by both plaque assay and QPCR, producing a consistent and reproducible inverse relationship between C(T) and plaque forming units per milliliter. No significant difference was observed between titers produced by QPCR and plaque assay for 12 recombinant viruses, confirming the validity of this technique as a rapid and accurate method of baculovirus titration.     

Canonical Wnt signaling is required for ophthalmic trigeminal placode cell fate determination and maintenance

Cranial placodes are ectodermal regions that contribute extensively to the vertebrate peripheral sensory nervous system. The development of the ophthalmic trigeminal (opV) placode, which gives rise only to sensory neurons of the ophthalmic lobe of the trigeminal ganglion, is a useful model of sensory neuron development. While key differentiation processes have been characterized at the tissue and cellular levels, the signaling pathways governing opV placode development have not. Here we tested in chick whether the canonical Wnt signaling pathway regulates opV placode development. By introducing a Wnt reporter into embryonic chick head ectoderm, we show that the canonical pathway is active in Pax3+ opV placode cells as, or shortly after, they are induced to express Pax3. Blocking the canonical Wnt pathway resulted in the failure of targeted cells to adopt or maintain an opV fate, as assayed by the expression of various markers including Pax3, FGFR4, Eya2, and the neuronal differentiation markers Islet1, neurofilament, and NeuN, although, surprisingly, it led to upregulation of Neurogenin2, both in the opV placode and elsewhere in the ectoderm. Activating the canonical Wnt signaling pathway, however, was not sufficient to induce Pax3, the earliest specific marker of the opV placode. We conclude that canonical Wnt signaling is necessary for normal opV placode development, and propose that other molecular cues are required in addition to Wnt signaling to promote cells toward an opV placode fate.