The last half of the previous century has seen an explosion in publications on biocrusts, communities of eukaryotic and prokaryotic organisms inhabiting the uppermost surface of predominantly dryland soils. Much of the early work emanated from the western United States, yet there have been few attempts to document the breadth of this work and its contribution to our understanding of the ecosystem roles of biocrusts. We used a structured literature search to extract the 868 publications on biocrusts published between January 1900 and July 2024, and explored the trends in publications in 12 subject areas over that time. We found that almost half of the 868 publications focussed on the ecological and physiological effects of biocrusts and that more recent research explored emerging fields such as restoration, monitoring and climate change impacts. Five authors comprised about 10% of all authors on these publications, and 5.5% of publications had 10 or more authors. The number of authors per publication tended to increase over time. We identified three main periods of research ranging from basic ecology and exploration of ecological mechanisms pre-2000 to biocrust function, physiology and climatic drivers up to 2020. The post-2020 period was characterized by a greater emphasis on molecular approaches, restoration and climate change impacts. Our literature review identifies knowledge gaps associated with the need for more trained taxonomists, and greater education on biocrust ecology and function. Potential developments in biocrust research include a greater use and recognition of biocrust species traits, the establishment of a dedicated international biocrust society, and the development of a global research and monitoring network to coordinate methods and provide a framework to answer critical knowledge gaps.

Productivity and function of arid and semiarid ecosystems (drylands) are optimised when critical resources such as water, organic matter and nutrients are assembled into discrete patches known as fertile patches or fertile islands. The strength of these islands is greater when the focal island plants are larger, but it is unclear whether this size effect holds true within similar-sized island species or across gradients in productivity. We examined the strength of the fertile island effect beneath three shrub species of similar structure but varying size across a gradient in productivity from ranges (low productivity), to footslopes (intermediate) and plains (high productivity). We predicted that fertility effects would be greater for larger plants and under lower productivity. There was no evidence of a productivity effect, but the fertility island effect increased with shrub size, irrespective of species. Furthermore, the fertile island effect was stronger for plant-related attributes (plants and litter) than functions associated with either soil fertility or soil stability. Our results reinforce the notion that plant size is an important determinant of the extent to which perennial plants enhance their understorey environment, irrespective of productivity or species. This has important implications for restoration programmes involving shrub plantings where the aim is to enhance the functionality of degraded dryland systems.

Humans depend heavily on nature. Drylands are home to 2.5 billion people, but the extent to which nature contributes to people (NCP) in drylands has been little explored. We examined the global contribution of nature to people, aiming to compare drylands and non-drylands. We predicted a lower contribution in drylands than non-drylands, largely because of the sparser population densities (peoples’ needs) and more degraded status of natural resources (lower potential contribution). Consistent with expectation, nature’s contribution was about 30% lower in drylands, with significantly lower values for drylands in Asia, Oceania, Africa and South America, but no difference for Europe and North America. Differences were due mainly to lower contributions from material and regulating contributions, i.e., the regulation of air quality, climate, water quantity and flow, soil protection and the supply of woody material, and potentially, lower use by people in drylands. Predicted declines in rainfall and increasing temperature are likely to place increasing pressure on nature to contribute to human well-being in drylands. A better understanding of nature’s contributions to people would improve our ability to allocate limited resources and achieve sustainable development in drylands.

Massive stars are predominantly found in binaries and higher order multiples. While the period and eccentricity distributions of OB stars are now well established across different metallicity regimes, the determination of mass-ratios has been mostly limited to double-lined spectroscopic binaries. As a consequence, the mass-ratio distribution remains subject to significant uncertainties. Open questions include the shape and extent of the companion mass-function towards its low-mass end and the nature of undetected companions in single-lined spectroscopic binaries. In this contribution, we present the results of a large and systematic analysis of a sample of over 80 single-lined O-type spectroscopic binaries (SB1s) in the Milky Way and in the Large Magellanic Cloud (LMC). We report on the developed methodology, the constraints obtained on the nature of SB1 companions, the distribution of O star mass-ratios at LMC metallicity and the occurrence of quiescent OB+black hole binaries.

Rotation is one of the important parameters affecting the evolution and final fate of massive stars but the origin of fast rotators remains unclear (imprint of the star formation process, result of binary interactions). In this work, we aim at investigating the binary status, photometric variability, and runaway status of a statistically meaningful sample of Galactic fast-rotating O stars. We perform a comprehensive multi-epoch analysis of new high-quality spectroscopic observations gathered by the IACOB and OWN surveys. Notably, we find that the total percentage of spectroscopic binaries in the investigated sample range between 25 and 40%, in agreement with previous finding for the case of O-type stars with lower projected rotational velocities. On the contrary, the fraction of runaway stars among fast rotators (∼35–50%) is significantly higher than in the case of slow rotators (∼20–30%). By combining all these observational results we will evaluate each scenario about the origin of fast rotators.

We use the results of a supernova light-curve population synthesis to predict the range of possible supernova light curves arising from a population of single-star progenitors that lead to type IIP supernovae. We calculate multiple models varying the initial mass, explosion energy, nickel mass and nickel mixing and then compare these to type IIP supernovae with detailed light curve data and pre-explosion imaging progenitor constraints. Where a good fit is obtained to observations, we are able to achieve initial progenitor and nickel mass estimates from the supernova lightcurve that are comparable in precision to those obtained from progenitor imaging. For 2 of the 11 IIP supernovae considered our fits are poor, indicating that more progenitor models should be included in our synthesis or that our assumptions, regarding factors such as stellar mass loss rates or the rapid final stages of stellar evolution, may need to be revisited in certain cases. Using the results of our analysis we are able to show that most of the type IIP supernovae have an explosion energy of the order of log(Eexp/ergs) = 50.52 ± 0.10 and that both the amount of nickel in the supernovae and the amount of mixing may have a dependence on initial progenitor mass.

Observations of star-forming galaxies in the distant Universe have confirmed the importance of massive stars in shaping galaxy emission and evolution. Distant stellar populations are unresolved, and the limited data available must be interpreted in the context of stellar population models. Understanding these populations, and their evolution with age and heavy element content is key to interpreting processes such as supernovae, cosmic reionization and the chemical enrichment of the Universe. With the upcoming launch of JWST and observations of galaxies within a billion years of the Big Bang, the uncertainties in modelling massive stars - particularly their interactions with binary companions - are becoming increasingly important to our interpretation of the high redshift Universe. In turn, observations of distant stellar populations provide ever stronger tests against which to gauge the success of, and flaws in, current massive star models. Here we briefly review the current status binary stellar population synthesis.

We present results of a supernova lightcurve population synthesis, predicting the range of possible supernova lightcurves arising from a population of progenitor stars that include interacting binary systems. We show that the known diversity of supernova lightcurves can be interpreted as arising from binary interactions. Given detailed models of the progenitor stars, we are able to the determine what parameters within these stars determine the shape of their supernova lightcurve. The primary factors are the mass of supernova ejecta and the mass of hydrogen in the final progenitor. We find that there is a continuum of lightcurve behaviour from type IIP, IIL, to IIb supernovae related to the range of hydrogen and ejecta masses. Most type IIb supernovae arise from a relatively narrow range of initial masses from 10 to 15 M⊙. We also find a few distinct lightcurves that are the result of stellar mergers.

Considering as many as 70% of massive stars interact with a binary companion (Sana et al.2012, 2014), we created a new model of the optical nebular emission of HII regions by combining the results of the Binary Population and Spectral Synthesis (BPASS, Eldridge, Stanway et al.2017) code with the photoionization code (CLOUDY). This is discussed more in detail in Xiao et al.2018a. Then we use this model to explore a variety of emission-line diagnostics of CCSN host HII regions from the PMAS/PPAK Integral-field Supernova hosts COmpilation (PISCO, Galbany et al.2018). We determine the age, metallicity and gas parameters for H II regions associated with CCSNe, contrasting the above variables to distribution type II and type Ibc SNe. We find their nebular emission and CCSN progenitor types are largely determined by past and ongoing binary interactions, for example mass loss, mass gain and stellar mergers. However we note these two types SNe have little preference in their host environment metallicity measured by oxygen abundance or in progenitor initial mass, except that at lower metallicities supernovae are more likely to be of type II. The BPASS and nebular emission models are available from bpass.auckland.ac.nz and warwick.ac.uk/bpass.

The Binary Population and Spectral Synthesis suite of binary stellar evolution models and synthetic stellar populations provides a framework for the physically motivated analysis of both the integrated light from distant stellar populations and the detailed properties of those nearby. We present a new version 2.1 data release of these models, detailing the methodology by which Binary Population and Spectral Synthesis incorporates binary mass transfer and its effect on stellar evolution pathways, as well as the construction of simple stellar populations. We demonstrate key tests of the latest Binary Population and Spectral Synthesis model suite demonstrating its ability to reproduce the colours and derived properties of resolved stellar populations, including well-constrained eclipsing binaries. We consider observational constraints on the ratio of massive star types and the distribution of stellar remnant masses. We describe the identification of supernova progenitors in our models, and demonstrate a good agreement to the properties of observed progenitors. We also test our models against photometric and spectroscopic observations of unresolved stellar populations, both in the local and distant Universe, finding that binary models provide a self-consistent explanation for observed galaxy properties across a broad redshift range. Finally, we carefully describe the limitations of our models, and areas where we expect to see significant improvement in future versions.

Recent observational modelling of the atmospheres of hydrogen-free Wolf–Rayet stars have indicated that their stellar surfaces are cooler than those predicted by the latest stellar evolution models. We have created a large grid of pure helium star models to investigate the dependence of the surface temperatures on factors such as the rate of mass loss and the amount of clumping in the outer convection zone. Upon comparing our results with Galactic and LMC WR observations, we find that the outer convection zones should be clumped and that the mass-loss rates need to be slightly reduced. We discuss the implications of these findings in terms of the detectability of Type Ibc supernovae progenitors, and in terms of refining the Conti scenario.

We present a new model of the optical nebular emission from HII regions by combining the results of the Binary Population and Spectral Synthesis (bpass) code with the photoionization code cloudy (Ferland et al. 1998). We explore a variety of emission-line diagnostics of these star-forming HII regions and examine the effects of metallicity and interacting binary evolution on the nebula emission-line production. We compare the line emission properties of HII regions with model stellar populations, and provide new constraints on their stellar populations and supernova progenitors. We find that models including massive binary stars can successfully match all the observational constraints and provide reasonable age and mass estimation of the HII regions and supernova progenitors.

We analyzed spectra of all Wolf-Rayet stars in the Small Magellanic Cloud (SMC). We find that, unlike predicted, mass-transfer in binaries is not needed to explain their formation.

We have recently released version 2.0 of the Binary Population and Spectral Synthesis (BPASS) population synthesis code. This is designed to construct the spectra and related properties of stellar populations built from ~200,000 detailed, individual stellar models of known age and metallicity. The output products enable a broad range of theoretical predictions for individual stars, binaries, resolved and unresolved stellar populations, supernovae and their progenitors, and compact remnant mergers. Here we summarise key applications that demonstrate that binary populations typically reproduce observations better than single star models.