Yzaak de Vries

Long‐term vegetation changes in experimentally grazed and ungrazed back‐barrier marshes in the Wadden Sea

Abstract Vegetation succession in three back‐barrier salt marshes in the Wadden Sea was studied using a data set comprising 25 years of vegetation development recorded at permanent quadrats. The effect of livestock grazing on succession was assessed by comparing quadrats where grazing was experimentally prevented or imposed. We studied changes at the species level as well as at the level of the plant community. Special attention is given to effects on plant species richness and community characteristics that are relevant for lagomorphs (hares and rabbits) and geese. Inundation frequency and grazing were most important in explaining the variation in species abundance data. The three marshes studied overlap in the occurrence of different plant communities and the observed patterns were consistent between them. Clear differences in frequency and abundance of plant species were observed related to grazing. Most plant species had a greater incidence in grazed treatments. Species richness increased with elevation, and was 1.5 to 2 × higher in the grazed salt marsh. Grazing negatively influenced Atriplex portulacoides and Elymus athericus, whereas Puccinellia maritima and Festuca rubra showed a positive response. The communities dominated by Elymus athericus, Artemisia maritima and Atriplex portulacoides were restricted to the ungrazed marsh. Communities dominated by Puccinellia maritima, Juncus gerardi and Festuca rubra predominantly occurred at grazed sites. As small vertebrate herbivores prefer these plants and communities for foraging, livestock grazing thus facilitates for them. Nomenclature:van der Meijden (1990) and Schaminée et al. (1998).

Long-term dynamics of the hemiparasite Rhinanthus angustifolius and its relationship with vegetation structure

Abstract Questions: 1. How are the long-term dynamics of the root hemiparasite Rhinanthus angustifolius related to vegetation structure, grassland management and climate? 2. Does R. angustifolius have a long-term impact on standing crop and community composition? Location: A formerly fertilized grassland, part of a larger brook valley system in the nature reserve ‘Drentsche Aa’, near Groningen, The Netherlands. Methods: Vegetation development has been monitored since 1973 in 54 permanent plots in nine management regimes without fertilizer application. Results: 1. The hemiparasite established when standing crop was less than 600 g.m−2 and performed best under annual haymaking using machinery. Since its appearance, the population fluctuated stochastically, with two peaks. Coinciding collapses in six adjacent grasslands and comparison with an integrated climatic index suggest that the population collapses are induced by spring drought. 2. We did not find a relationship between total standing crop and R. angustifolius cover. Cover of grasses was negatively related to the abundance of the hemiparasite in the same and the previous year. Forb cover tended to increase with the parasite. Conclusions: R. angustifolius shows stochastic population fluctuations, mainly determined by spring drought, to which this species is probably highly vulnerable because of its parasitic and annual life style without a persistent seed bank. The hemiparasite also shows long-term relationships with grass cover (negative) and forb cover (positive), but it seems to have no lasting impact on standing crop. Abbreviations: AC = Autocorrelation; CC = Cross-correlation coefficient; Loef S = Study site within Drentsche Aa nature reserve. Nomenclature: Zuidhoff et al. (1996).

Grassland cutting regimes affect soil properties, and consequently vegetation composition and belowground plant traits

Background and aimsMachine mowing, mimicking the traditional hand mowing, is often used as a successful management tool to maintain grassland biodiversity, but few studies have investigated the long-term effects of traditional versus mechanical mowing of plant communities. Machine mowing as opposed to hand mowing causes soil compaction and reduction of soil aeration. In response, we expected strong effects on below-ground plant traits: root aerenchyma formation and superficial root growth, and no specific effects on aboveground traits. Effects were expected to be more pronounced in soils vulnerable to compaction.MethodsWe evaluated the changes in above- and belowground plant traits in a long-term (38-year) experiment with annual hand-mowing and machine-mowing management regimes on two different soil types: a coarse structured sandy soil and a finer structured sandy-organic soilResultsOnly on the organic soil, long-term machine mowing leads to lower soil aeration (more compacted soil) and a marked change in the belowground trait distribution of the plant community. Here we find a higher cover of superficially rooting species and marginally significant lower cover of species without morphological adaptations to soil hypoxia, but no effect on species with a high capacity of forming aerenchyma.ConclusionMowing with heavy machines on soils vulnerable to compaction affect the vegetation according to changes in soil physical conditions. This is reflected in a shift towards communities with greater proportion of superficially rooting species. Our results illustrate the sensitivity of grasslands to slight changes in the management regime.