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Air is a major conduit for the dispersal of organisms at the local and the global scale. Most research has focused on the dispersal of plants, vertebrates and human disease agents. However, the air represents a key dispersal medium also for bacteria, fungi and protists. Many of those represent potential pathogens of animals and plants and have until now gone largely unrecorded. Here we studied the turnover in composition of the entire aerobiome, the collective diversity of airborne microorganisms. For that we performed daily analyses of all prokaryotesand eukaryotes (including plants) using multi-marker high-throughput sequencing for a total of three weeks. We linked the resulting communities to local weather conditions, to assess determinants of aerobiome composition and distribution. We observed hundreds of microbial taxa, mostly belonging to spore-forming organisms including fungi, but also protists. Additionally, we detected many potential human- and plantpathogens. Community composition fluctuated on a daily basis and was linked to concurrent weather conditions, particularly air pressure and temperature. Using network analyses, we identified taxonomically diverse groups of organisms with correlated temporal dynamics. In part, this was due to co-variation with environmental conditions, while we could also detect specific host-parasite interactions. This study provides the first full inventoryof the aerobiome and identifies putative drivers of its dynamics in terms of taxon composition. This knowledge can help develop early warning systems against pathogens and improve our understanding of microbial dispersal.

The coastal vegetation of islands is expected to be affected by future sea-level rise and other anthropogenic impacts. The biodiverse coastal vegetation on the eastern part of the Dutch Wadden Island of Ameland has experienced land subsidence caused by gas extraction since 1986. This subsidence mimics future sea-level rising through increased flooding and raising groundwater levels. We studied the effects of this relative sea-level rise and other environmental factors (i.e. insect outbreaks, temperature and precipitation) on the population dynamics (i.e. cover and age structure and annual growth) of the shrub sea-buckthorn (Hippophae rhamnoides L.) in young (formed after 1950) and old (formed before 1950) dune areas over a period of 56 years (1959–2015). We found an increase in sea-buckthorn cover in the young dune areas since 1959, while over time the population in the old dunes decreased due to successional replacement by other species. With the increasing age of the young dunes, we found also a decrease in sea-buckthorn after 2009. However the sharp decrease indicated that other environmental factors were also involved. The most important determinant of annual shrub growth appeared to be five outbreaks of the brown-tail moth (Euproctis chrysorrhoea L.), in the last decade. Relative sea-level rise caused more frequent flooding and reduced growth at lower elevations due to inundation or soil water saturation. This study clearly indicates that sea-buckthorn is affected by relative sea-level rise, but that other ecological events better explain its variation in growth. Although shrub distribution and growth can be monitored with robust methods, future predictions of vegetation dynamics are complicated by unpredictable extreme events caused by (a)biotic stressors such as insect outbreaks.

Regular mapping of vegetation structure is important for biodiversity monitoring, and increasingly for tracking compliance with nature policy mandates. As such, the Netherlands uses vegetation structure mapping to monitor the Natura 2000 site on the Dutch Wadden Sea island of Ameland. Three decades of natural gas extraction here has caused soil subsidence, impacting vegetation and habitats on the island. In the Netherlands, vegetation structure mapping is typically done using conventional techniques, primarily field surveys combined with visual interpretation of aerial photographs. This procedure is time-consuming and often too inconsistent and inefficient for large areas. In the current study we exploited commonly shared and low-cost aerial photographs and LiDAR data for detailed vegetation structure mapping. Aerial photographs are not always easy to use in automatic classification procedures, as they often lack calibrated spectral reflectance values. Furthermore, pre-processing of aerial photographs to render them more attractive may skew the image so that it no longer accurately depicts the original scene anymore. Our aim was to determine if automatic or semi-automatic classification techniques could be applied to these readily available Dutch data to support mapping and monitoring of the vegetation structure of larger areas. We compared the effectiveness of two well-known classification methods, namely rule-based (RB) and random forest (RF). The RF algorithm was applied with its default settings, as supplied by eCognition software. Both classification methods performed well, with overall accuracies of 84.1% (RB) and 86.4% (RF). Each method, however, has its advantages and disadvantages, which are discussed. Overall, RF classification was preferred over RB classification, as it was better able to handle the complexity of the rules needed for distinguishing more classes. Provision of in situ training data, such as vegetation relevés, was not really problem in the

Foto’s van eenzelfde locatie uit verschillende jaren illustreren heel goed hoe het Nederlandse landschap in de loop der tijd verandert. Een tweetal foto’s van het Oerdswater op Ameland (uit 1962 en 1989) was de aanleiding om dezelfde plek opnieuw vast te leggen en de veranderingen die uit de foto’s naar voren komen te beschrijven.

Aim: Deposition of nitrogen is assumed to cause loss of botanical diversity, probably through increased production and exclusion of less competitive species. However, if production is (co-)limited by phosphorus, acceleration of the phosphorus cycle may be responsible for the diversity loss and, where that is the case, nitrogen emission reduction may turn out to be an ineffective mitigation strategy. Here we study the feasibility of this mechanism through adding potassium and phosphorus to grassland where nitrogen limitation is absent. Methods: We made vegetation relevés in a long-term agricultural fertilisation experiment where potassium, phosphorus and nitrogen were being added to grassland on drained peat where nitrogen availability was high, even in unfertilised plots. We applied a multivariate analysis to investigate the effect of additions of K, K + P and K + P + N on the species composition. Results: Unfertilised plots had a very low biomass production and were rich in plant species despite their high nitrogen availability. Addition of potassium led to a strongly increased production but did not result in a reduction of species numbers. Phosphorus in addition to potassium increased production still further and decreased species numbers, most notably the number of endangered species. Conclusions: Even under nitrogen rich conditions species richness may be high in grasslands where phosphorous provides a limitation to plant growth. Phosphorus limitation and phosphorus enrichment are both common in grassland, at least in north-western Europe. Part of the general decrease in species numbers that is commonly ascribed to nitrogen enrichment may therefore be due to phosphorus enrichment. If phosphorus and nitrogen are co-limiting (which is often the case) the current nitrogen emission reduction policies may be effective, but not sufficient to restore grassland diversity to its pre-industrial level.

Measurements were carried out on the island of Ameland (The Netherlands) to determine whether notches cut into foredunes stimulated the supply of fresh calcareous beach and dune sand into the white and grey dune habitats behind the dunes, increasing these habitats' biological quality. Sediment characteristics and dynamics (deposition flux and grain size properties) as well as aspects of the vegetation (occurrence, composition and cover density) were studied along six transects, three behind an intact foredune and three behind a foredune with a notch cut into it. Compared to an intact foredune, the notched foredune exhibited higher deposition and accumulation behind the dune. The extra supply of sand was small, however, and for the notches studied, limited to the zone within approximately 50–60 m of the foredune's crest. Farther away from the dune, the effect of the notches became negligible. The presence of a notch did affect the grain size composition of sediment deposited behind the foredune. For intact foredunes, the grain size composition behind the dune was similar to that on the dune itself. When a notch had been cut, the sediment was finer behind the foredune, gradually coarsening away from the dune. Sand spray (deposition of sand eroded from the dune and transported in modified saltation during heavy winds) explains these granulometric results. The effect of the notches on the vegetation in the grey dune habitat behind the foredune was small and, for the notches studied, limited to the first approximately 35 m of the grey dune area, between 30 and 65 m from the foredune's crest. The notches had a greater effect on the white dune habitat but – in the opinion of the authors – this remained disproportionately small relative to the effort required for notch excavation and maintenance.

This paper analyses the effect of observed vegetation characteristics on modelled wave heights. Detailed information on species composition, as well as on height, number of stems, and diameter of the plant species of a restored salt marsh on the Wadden barrier island of Terschelling was used to parameterize and apply the Simulating Waves Nearshore – Vegetation wave model to a schematized restored salt-marsh zone in front of the dike. The results indicate that wave damping by vegetated forelands is strongly related to vegetation heterogeneity and salt-marsh zonation. The modelling works suggest that at the study site under storm conditions with a frequency of 5–10 times/y, a vegetated foreland of some 90 m in width will dampen the wave height more than 80%, whereas under extreme conditions (1/2000 y) a foreland covered with dense vegetation will dampen the wave height up to 50%. These results imply that at the study site a vegetated foreland in front of the dike leads to reduced wave attack on the dike, which may result in changed requirements for both height and revetment of the dike while maintaining the required safety level. Although there are still many questions concerning dimensions, management, and performance, developing a vegetated foreland seems an interesting strategy to adapt existing flood protection works to the effects of climate change.

Habitats in the Wadden Sea, a world heritage area, are affected by land subsidence resulting from naturalgas extraction and by sea level rise. Here we describe a method to monitor changes in habitat types byproducing sequential maps based on point information followed by mapping using a multinomial logitregression model with abiotic variables of which maps are available as predictors.In a 70 ha study area a total of 904 vegetation samples has been collected in seven sampling roundswith an interval of 2–3 years. Half of the vegetation plots was permanent, violating the assumptionof independent data in multinomial logistic regression. This paper shows how this dependency can beaccounted for by adding a random effect to the multinomial logit (MLN) model, thus becoming a mixedmultinomial logit (MMNL) model. In principle all regression coefficients can be taken as random, butin this study only the intercepts are treated as location-specific random variables (random interceptsmodel). With six habitat types we have five intercepts, so that the number of extra model parametersbecomes 15, 5 variances and 10 covariances.The likelihood ratio test showed that the MMNL model fitted significantly better than the MNL modelwith the same fixed effects. McFadden-R2for the MMNL model was 0.467, versus 0.395 for the MNL model.The estimated coefficients of the MMNL and MNL model were comparable; those of altitude, the mostimportant predictor, differed most. The MMNL model accounts for pseudo-replication at the permanentplots, which explains the larger standard errors of the MMNL coefficients. The habitat type at a givenlocation-year combination was predicted by the habitat type with the largest predicted probability. Theseries of maps shows local trends in habitat types most likely driven by sea-level rise, soil subsidence,and a restoration project.We conclude that in environmental modeling of categorical variables using panel data, dependencyof repeated observations at permanent plots

Question: Do low stone dams built to prevent erosion and to restore salt marshes through increased sedimentation affect plant species composition? Location: Dutch Wadden Sea area (ca. 53°N 5°E). Methods: Relevés (N = 170) were made of the vegetation of two restored salt marsh sites on the barrier islands Terschelling (Grië) and Ameland (Neerlands Reid). Existing relevés of salt-marsh vegetation (N = 6198) made along the entire Dutch Wadden Sea coast (both the mainland and the barrier islands) were used as a reference. The vegetation of the two restored sites (Grië NLR data) was compared with the reference by (1) simple species-by-species analysis based on frequencies in both data sets, and by (2) ordination, where relevés of the restored sites were projected into a multivariate space defined by the species' abundances in the reference relevés. Results: Out of the 37 species that are common (i.e. have a frequency >5%) in either the Grië NLR data or the reference data, 31 have frequencies that differ by less than a factor of five, and 23 differ by less than a factor of two. Furthermore, the Grië NLR data occupy a space that is well in the centre of the ordination space defined by the reference data. Conclusions: There are no conspicuous differences between salt-marsh vegetation behind low dams and the vegetation that has naturally developed on unprotected mudflats. We conclude that measures targeting salt marsh development in view of flood protection do not interfere with nature conservation.

This paper explores the impact of erosion and restoration measures on habitat development and on wave damping by a small salt marsh nestled alongside a dike on the Wadden island of Terschelling. The aim is to advance knowledge about the benefits and possible side-effects of salt-marsh restoration. Analysis of a time series of aerial photographs from 1944 to 2010 indicates that the salt marsh decreased steadily in size after maintenance of accretion works was terminated. In the western part of the marsh, which is accessible to sheep, vegetation is low (5–15 cm) and dominated by Salicornia europaea and by Spartina anglica. In the most intensively grazed parts, vegetation is very scarce. The eastern, inaccessible part of the salt marsh is covered by dense patches of the shrubby perennial Atriplex portulacoides and Spartina anglica (15–25 cm in height). SWAN wave models show that wave height at this location is significantly affected by the areal extent of the salt marsh as well as by the vegetation. High or dense vegetation are in the models nearly as effective in damping waves (with an initial height of 0.15 and 0.5 m) as widening the salt-marsh area by 350 m. A low density of low plants, as observed in the grazed part of the marsh, has almost no wave-damping effect. Even under conditions of sea level rise, a broader salt marsh vegetated with high plants significantly affects modelled wave height. Therefore, salt-marsh restoration is an adaptation measure worth exploring, though an array of effect types must be considered.

The study uses a rather unusual method, dendrochronology, to investigate the growth and survival of Atriplex portulacoides L. and Artemisia maritima L. on salt marshes at two field sites on the Dutch North Sea barrier islands of Terschelling and Ameland. By providing information on longevity of these typical salt-marsh shrubs, dendrochronology offers an indirect way to investigate the influence of management regime – grazing in this case – on marsh quality and areal extent. Diminishment of salt marshes is a continuing concern in the northern Netherlands. The two shrub species studied here, A. portulacoides and A. maritima, are common to salt marshes. With their extensive roots and branches, they facilitate sedimentation and stabilize salt marshes. Using dendrochronology, this study found that annual growth rings could be identified to determine shrub age and growth. In A. portulacoides these rings took the form of a narrow band of terminal parenchyma. In A. maritima they were made up of unlignified marginal parenchyma together with higher vessel density at the beginning of the growing season. Growth rings indicated that intense grazing was clearly detrimental to the survival of A. portulacoides at the Terschelling site. However, grazing facilitated survival of A. maritima at the Ameland site by reducing light and nutrient competition from grasses. No growth trends could be found, however, as the lifespan for both species is short and many other influences on shrub growth could be identified.