Effects of delayed hay removal on the nutrient balance of roadside plant communities

1. Mass losses and nutrient losses from fresh roadside cuttings were studied in the field during a 6-week period. Large amounts (over 50%) appear to be lost from the cuttings. The losses were positively related to initial nutrient concentrations during this short-term study. Mass and nitrogen losses were best explained by the initial C:N ratio, phosphorus and potassium losses by the initial phosphorus concentration.
2. For potassium the losses were particularly large (up to 90%). For this element only, the observed relationship between loss rate and initial chemical composition could not be established significantly. It is concluded that potassium is mainly lost by leaching whereas the major nitrogen, phosphorus and mass losses are most probably caused by rapid microbial decomposition of readily soluble substances.
3. Using existing data on chemical composition of other roadside cuttings, nutrient losses after different hay removal delay times were modelled for different plant communities. All or most of the losses were assumed to return to the soil system.
4. When soil impoverishment is aimed for, cuttings should be removed within 1 or 2 weeks in most plant communities. If removal is delayed longer, the amounts of nutrients removed will often fall below the annual atmospheric input. In plant communities where annual above-ground production of nitrogen and phosphorus are lower than the annual atmospheric deposition already, rapid removal of the cuttings may be the only way to maintain at least potassium at a limiting level.
5. The main effect of hay-making on the soil nutrient status most likely consists of a reduction of the potassium availability, at least on sandy soils with a low cation exchange capacity and provided there is little delay in hay removal.     

Effects of vegetation management by mowing on ground-dwelling arthropods

Species-rich grasslands are rare in the Netherlands and need consistent vegetation management to retain their characteristic biodiversity. Roadside verges are important refuges for grassland plants since the mowing management no longer aims at traffic safety only but also strives for botanical diversity. Although arthropods are highly abundant in roadside verges, the effect of different mowing practices on this group is largely unknown. During 4 years, we studied ground beetles, weevils, ants and ground-dwelling spiders with pitfall traps in experimental plots in roadside verges with five different mowing treatments: (i) no management, (ii) and (iii) mowing once a year with and without hay removal, (iv) and (v) mowing twice a year with and without hay removal. This was done in a plant productivity gradient; the experiment was repeated in low-, medium- and high-productive verges. In the low-productive site, the effect of management on the arthropods only existed in a higher abundance in plots mown twice per year with hay removal. In the medium- and high-productive sites, mowing twice a year with hay removal resulted not only in highest abundances but also in highest arthropod species richness. Mowing twice without hay removal and mowing once with removal showed intermediate values, while mowing once per year without removal and particularly the absence of management resulted in low diversity and low abundance. To promote ground-dwelling arthropods in medium-to-high-productive grassland verges, we recommend a management of mowing twice a year with the removal of hay. It is reasoned that some form of rotational management, aiming at leaving some vegetation refuges intact after mowing events, may further promote arthropod survival. However, caution should be taken that these refuges are not too large, as overall suitability for ground-dwelling arthropod decreases rapidly in such patches. Out of several studied vegetation characteristics, the number of flowering plant species (medium-productive verge) and total flower abundance (high-productive verge) appeared to represent suitable, and easily monitored, proxies that significantly mirror arthropod diversity.     

Temporal effects of agri-environment schemes on ditch bank plant species

Many of the agri-environment schemes (AES) implemented in the Western Peat District of the Netherlands have as their objective the conservation of the diversity of ditch bank plants. We investigated the effects of AES on ditch bank species in this area, using a dataset collected by 377 farmers who managed and monitored ditch banks during a 10-year period. We found that species richness has increased minimally over the last 10 years in ditch banks. Yet, we found no differences in increases in time between ditch banks with and without AES. In both ditch bank types plant species composition changed to species with higher nitrogen tolerance. Furthermore, species that disperse over long distances by water increased, whereas species with no capacity to disperse over long distances declined in both ditch bank types. This indicates that changes in vegetation composition in ditch banks are affected by other factors than AES.     

How ants find each other; temporal and spatial patterns in nuptial flights

Reproduction is a key factor in understanding population ecology and therefore species occurrence. However, patterns in reproductive behaviour for distinct ant species remain insufficiently known. In this paper strategies in mate finding are studied for six ant species (Lasius niger, Lasius umbratus, Temnothorax nylanderi, Myrmica rubra, Myrmica ruginodis, Stenamma debile) in a forest – forest edge – agricultural field gradient. Using window traps, we studied whether these species had a restricted nuptial flight season, displayed swarming behaviour, and whether the alates aggregated at the forest edge. The flight season was limited to one month or less for L. niger, T. nylanderi, M. rubra, M. ruginodis and S. debile. Swarming behaviour occurred in all but one (L. umbratus) species. Although none of the six species seemed to have highest nest density at the forest edge, three of them, M. rubra, M. ruginodis and S. debile, showed male aggregations there, indicating this to be the main reproduction site. This last finding could be due to a more suitable micro-climate, but most likely, edges are conspicuous land marks which are used by ants to meet mates. The behavioural patterns of ant sexuals at the forest edge can influence dispersal possibilities in fragmented landscapes, reproductive success and nest densities. Received 27 November 2007; revised 27 March; accepted 5 April 2008.     

Occurrence of carpophores of ectomycorrhizal fungi in selected stands ofPinus sylvestris in the Netherlands in relation to stand vitality and air pollution

The relation between the occurrence of carpophores of mycorrhizal fungi, tree vitality, and air pollution in ‘young’ (5–10 years) and ‘old’ (50–80 years) stands ofPinus sylvestris L. was investigated. In the Netherlands 21 homogeneous plots of 1000 m² were selected on dry, sandy soils. Tree vitality was assessed in the summer of 1985 and during the autumn the plots were searched for carpophores of mycorrhizal fungi. In the young plots 3 times more species of ectomycorrhizal fungi and 20 times more carpophores per plot were found than in the old plots. In the old plots, the number of carpophores and the number of species showed significant positive correlations with the average number of needleyears of the trees. The number of carpophores and the number of species were significantly negative correlated with the concentrations of SO₂ and O₃ in the air and the amount of NH₃ emission. The number of needleyears, the crown density and the overall tree vitality were found to be significantly negative correlated with the amount of NH₃ emission, and the number of needleyears also with the concentration of O₃ in the air. Such correlations were not found for the young plots. No differences were found in soil chemical properties between the plots. Evidence was obtained that the fructification of mycorrhizal fungi of old stands ofPinus sylvestris is influenced by air pollution either directly, or indirectly by way of tree vitality. It is hypothesized that the young plots offer more advantageous circumstances for fructification of the fungi because of the disturbance of the upper soil-layer at the time of planting.     

In situ annual nitrogen mineralization predicted by simple soil properties and short-period field incubation

A typical method to determine nitrogen mineralization is year-round field incubation of undisturbed soil cores. As this technique is very laborious, the aim of the present study was to establish less time consuming methods to assess the annual N mineralization. Options considered were reducing the period of subsequent field incubations to less than a full year, and prediction using simple (direct) soil measurements. In situ annual N mineralization was measured in 74 semi-natural, unfertilized sites under year-round vegetation cover by sequential incubation of undisturbed soil cores during 2 full years. The sites classify into 14 very different plant community types and encompass a wide range of edaphic conditions. A high correlation with the annual N mineralization was maintained when sequential incubations were restricted to the period early March–mid August (r=0.98). Shorter periods may also be used, as long as they lie within the period early March–mid August and cover 3 months at least. The average annual N mineralization could be predicted accurately using the soil mineral nitrogen pool (N_min), moisture content and pH as the predictor variables. In a multiple regression model based on log-transformed values, the R ² was 0.835. Samples (preferably bulked) are best taken in early March, just before the start of the growing season. Higher N_min contents (moisture and pH assumed constant) indicated higher annual N mineralization rates. Higher moisture contents (at constant N_min and pH) generally indicated lower annual rates. Soil pH showed a quadratic relation (at fixed N_min and moisture) with maximum annual mineralization occurring at a pH-CaCl₂ value of 5.5–5.6. N_min appeared to be the most important predictor variable. Even within plant community types (with a reduced range of conditions), its relationship with the annual N mineralization persisted. Frequently studied factors, such as total soil N, organic matter content, C:N ratio, soluble organic N and particle size composition, only showed low and insignificant correlations, although stronger relationships have sometimes been reported. It is assumed this discrepancy is caused by the smaller range of conditions usually studied. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reliability of Ellenberg indicator values for moisture,nitrogen and soil reaction: a comparison with field measurements

Abstract. Ellenberg indicator values for moisture, nitrogen and soil reaction were correlated with measured soil and vegetation parameters. Relationships were studied through between‐species and between‐site comparisons, using data from 74 roadside plots in 14 different plant communities in The Netherlands forming a wide range. Ellenberg moisture values correlated best with the average lowest moisture contents in summer. Correlations with the annual average groundwater level and the average spring level were also good. Ellenberg N‐values appeared to be only weakly correlated with soil parameters, including N‐mineralization and available mineral N. Instead, there was a strong relation with biomass production. We therefore endorse Hill & Carey's (1997) suggestion that the term N‐values be replaced by ‘productivity values'. For soil reaction, many species values appeared to need regional adjustment. The relationship with soil pH was unsatisfactory; mean indicator values were similar for all sites at pH > 4.75 because of wide species tolerances for intermediate pH levels. Site mean reaction values correlated best (r up to 0.92) with the total amount of calcium (exchangeable Ca²⁺ plus Ca from carbonates). It is therefore suggested that reaction values are better referred to as ‘calcium values'. Using abundance values as weights when calculating mean indicator values generally improved the results, but, over the wide range of conditions studied, differences were small. Indicator values for bryophytes appeared well in line with those for vascular plants. It was noted that the frequency distributions of indicator values are quite uneven. This creates a tendency for site mean values to converge to the value most common in the regional species pool. Although the effect on overall correlations is small, relationships tended to be less linear. Uneven distributions also cause the site mean indicator values at which species have their optimum to deviate from the actual Ellenberg values of these species. Suggestions for improvements are made. It is concluded that the Ellenberg indicator system provides a very valuable tool for habitat calibration, provided the appropriate parameters are considered.