Norbert Hölzel

Restoration of a species-rich flood meadow by topsoil removal and diaspore transfer with plant material

Abstract In previous studies, limited dispersal was revealed to be the main obstacle to restoration of species-rich flood-meadows along the northern Upper Rhine in Germany. To overcome dispersal limitation we transferred freshly mown plant material from species-rich sources to a restoration site on a former arable field. Before plant material application, topsoil was removed to accelerate nutrient impoverishment and create favourable conditions for seedling recruitment. Topsoil removal led to a drastic reduction in organic matter and essential mineral nutrients to the level of target communities (P) or even below (N, K). At a removal depth of 30 cm content of the soil seed bank that comprised exclusively of annual arable weeds, ruderals and some common grassland species, declined by 60 – 80%, while at a removal depth of 50 cm the seed bank was almost completely eliminated. With few exceptions, all species recorded in source plant material were found established at the restoration site. However, the overall correlation between seed content in plant material and establishment success was not very high. Vegetation development at the restoration site was characterized by a rapid decline in arable weeds and ruderals, while resident grassland species and species transferred with plant material increased rapidly from the third year onwards. After four years as many as 102 species were established that could be exclusively attributed to plant material transfer, among them many rare and highly endangered plants. Establishment of species from plant material was most successful in regularly flooded plots, due to the suppression of competitors as well as the creation of favourable moisture conditions for seedling emergence. Diaspore transfer with plant material proved to be an extremely successful method in restoring species-rich grassland. However, high quality of plant material and suitable site conditions with low competition in early stages of succession seem to be essential prerequisites. Nomenclature: Wisskirchen & Haeupler (1998).

The impact of site conditions and seed dispersal on restoration success in alluvial meadows

Abstract We studied the restoration success of flood plain meadows in the northern Upper Rhine valley, where between 1988 and 1992, 35 ha of arable land was converted into grassland and subsequently managed for nature conservation. Remnant populations of typical alluvial meadow species were found in old meadows and along drainage ditches that dissect the whole area. We analysed the site conditions and phytosociological relevés in old and new meadows. Small differences in site parameters between old and new meadows contrasted with a clear floristic differentiation between the two meadow types. The vegetation of old meadows was much more differentiated along prevailing environmental gradients than the vegetation of new meadows. Despite the favourable site conditions for the re-establishment of species-rich meadows on the former arable land, restoration success was limited to the vicinity of remnant stands. In contrast to old meadows, indicator species of new grassland were still typical species of regularly disturbed ruderal and arable habitats, often capable of building up a persistent seed bank. The precise mapping of 23 target species revealed that even wind dispersal predominantly leads to re-establishment in the close circumference of parent plants. We found no indication that regular flooding, hay-making and autumnal grazing had an impact on recolonization of newly created grassland. Even under favourable conditions for the re-establishment of target species, restoration success in alluvial meadows proved to be strongly dispersal limited. We discuss the implications of our findings for future restoration management in grasslands. Nomenclature: Wisskirchen & Haeupler (1998).

Biodiversity at multiple trophic levels is needed for ecosystem multifunctionality

Many experiments have shown that loss of biodiversity reduces the capacity of ecosystems to provide the multiple services on which humans depend. However, experiments necessarily simplify the complexity of natural ecosystems and will normally control for other important drivers of ecosystem functioning, such as the environment or land use. In addition, existing studies typically focus on the diversity of single trophic groups, neglecting the fact that biodiversity loss occurs across many taxa and that the functional effects of any trophic group may depend on the abundance and diversity of others. Here we report analysis of the relationships between the species richness and abundance of nine trophic groups, including 4,600 above- and below-ground taxa, and 14 ecosystem services and functions and with their simultaneous provision (or multifunctionality) in 150 grasslands. We show that high species richness in multiple trophic groups (multitrophic richness) had stronger positive effects on ecosystem services than richness in any individual trophic group; this includes plant species richness, the most widely used measure of biodiversity. On average, three trophic groups influenced each ecosystem service, with each trophic group influencing at least one service. Multitrophic richness was particularly beneficial for ‘regulating’ and ‘cultural’ services, and for multifunctionality, whereas a change in the total abundance of species or biomass in multiple trophic groups (the multitrophic abundance) positively affected supporting services. Multitrophic richness and abundance drove ecosystem functioning as strongly as abiotic conditions and land-use intensity, extending previous experimental results to real-world ecosystems. Primary producers, herbivorous insects and microbial decomposers seem to be particularly important drivers of ecosystem functioning, as shown by the strong and frequent positive associations of their richness or abundance with multiple ecosystem services. Our results show that multitrophic richness and abundance support ecosystem functioning, and demonstrate that a focus on single groups has led to researchers to greatly underestimate the functional importance of biodiversity.

Land use intensification alters ecosystem multifunctionality via loss of biodiversity and changes to functional composition

Abstract Global change, especially land‐use intensification, affects human well‐being by impacting the delivery of multiple ecosystem services (multifunctionality). However, whether biodiversity loss is a major component of global change effects on multifunctionality in real‐world ecosystems, as in experimental ones, remains unclear. Therefore, we assessed biodiversity, functional composition and 14 ecosystem services on 150 agricultural grasslands differing in land‐use intensity. We also introduce five multifunctionality measures in which ecosystem services were weighted according to realistic land‐use objectives. We found that indirect land‐use effects, i.e. those mediated by biodiversity loss and by changes to functional composition, were as strong as direct effects on average. Their strength varied with land‐use objectives and regional context. Biodiversity loss explained indirect effects in a region of intermediate productivity and was most damaging when land‐use objectives favoured supporting and cultural services. In contrast, functional composition shifts, towards fast‐growing plant species, strongly increased provisioning services in more inherently unproductive grasslands.

The impact of flooding regime on the soil seed bank of flood‐meadows

. We assessed the significance of flooding for the floristic composition of seed banks in flood-meadows of the northern valley of the Upper Rhine. We compared three hydrological compartments of the alluvial plain, consisting of the regularly flooded land between the river and low summer dykes (functional flood-plain), the occasionally flooded land between summer dykes and high winter dykes (hybrid floodplain) and the land behind the winter dykes, which is now only submerged by ascending groundwater (fossil flood-plain). Due to their different flooding regime, the three compartments should differ with respect to the prevailing conditions of diaspore input. The seed density of soil samples increased with the duration of flooding in the three compartments, while species richness and the proportion of species not occurring in the vegetation was constant. The increase in seed density can be largely attributed to an increase of disturbance indicators, which are present in the above-ground vegetation and capable of forming a long-term persistent seed bank. No effects of flooding on the composition of seed banks in the three flood-plain compartments were found. The differences in seed bank composition can be largely explained by corresponding differences in above-ground vegetation and former and present-day meadow management. Seeds of species absent from above-ground vegetation can be attributed to the local species pool present in the immediate vicinity of the study plots. We discuss consequences of the results for the restoration of species-rich flood-plain meadows.

Interannual variation in land-use intensity enhances grassland multidiversity

Significance Land-use intensification is a major threat to biodiversity. So far, however, studies on biodiversity impacts of land-use intensity (LUI) have been limited to a single or few groups of organisms and have not considered temporal variation in LUI. Therefore, we examined total ecosystem biodiversity in grasslands varying in LUI with a newly developed index called multidiversity, which integrates the species richness of 49 different organism groups ranging from bacteria to birds. Multidiversity declined strongly with increasing LUI, but changing LUI across years increased multidiversity, particularly of rarer species. We conclude that encouraging farmers to change the intensity of their land use over time could be an important strategy to maintain high biodiversity in grasslands. Although temporal heterogeneity is a well-accepted driver of biodiversity, effects of interannual variation in land-use intensity (LUI) have not been addressed yet. Additionally, responses to land use can differ greatly among different organisms; therefore, overall effects of land-use on total local biodiversity are hardly known. To test for effects of LUI (quantified as the combined intensity of fertilization, grazing, and mowing) and interannual variation in LUI (SD in LUI across time), we introduce a unique measure of whole-ecosystem biodiversity, multidiversity. This synthesizes individual diversity measures across up to 49 taxonomic groups of plants, animals, fungi, and bacteria from 150 grasslands. Multidiversity declined with increasing LUI among grasslands, particularly for rarer species and aboveground organisms, whereas common species and belowground groups were less sensitive. However, a high level of interannual variation in LUI increased overall multidiversity at low LUI and was even more beneficial for rarer species because it slowed the rate at which the multidiversity of rare species declined with increasing LUI. In more intensively managed grasslands, the diversity of rarer species was, on average, 18% of the maximum diversity across all grasslands when LUI was static over time but increased to 31% of the maximum when LUI changed maximally over time. In addition to decreasing overall LUI, we suggest varying LUI across years as a complementary strategy to promote biodiversity conservation.

Assessing soil seed bank persistence in flood-meadows: The search for reliable traits

Abstract To assess seed bank persistence of target species in endangered flood-meadows (alliances Cnidion and Molinion), we investigated established vegetation and soil seed bank of 46 plots for 3 yr and 2 yr, respectively. As traits of seed persistence we calculated various continuous indices that refer to the frequency and abundance of species in above-ground vegetation and at different soil depths. Furthermore, we tested the significance and soundness of easily observed traits such as maximum seed density per plot and seed attributes (mass, size and shape) as predictors of soil seed bank features. In linear regression, SAI, the seed accumulation index, showed the best agreement (R2 = 0.64) with the seed longevity index that was derived from the database by Thompson et al. (1997) for a set of 115 species. The second best predictor (R2 = 0.39) of the seed longevity index was maximum seed density per plot in the lower soil layer (5–10 cm). Depth distribution indices and seed attributes showed weaker but still significant relations. The dynamic character of flood-meadows was reflected by a large proportion of species with a strong tendency to accumulate seeds in the soil relative to their importance in above-ground vegetation. Most of these species have a ruderal strategy, exploiting gaps after flood disturbances, while the dominants of flood-meadows tended to have short-lived seed banks. Compared to other grassland types, a relatively large proportion of rare and endangered target species can be expected to form long-term persistent seed banks. However, only under marginal conditions that facilitate seed survival in the soil (e.g. fallow) are these persistent seed banks likely to contribute to restoration. We conclude that easily observed traits of persistence such as seed weight, size and shape do not meet the accuracy needed in scientific and practical applications. Thus, there is a crucial demand for further seed bank studies in poorly investigated habitats and of rare species. Nomenclature: Wisskirchen & Haeupler (1998). Abbreviation: SAI = Seed accumulation index.

Low investment in sexual reproduction threatens plants adapted to phosphorus limitation

Plant species diversity in Eurasian wetlands and grasslands depends not only on productivity but also on the relative availability of nutrients, particularly of nitrogen and phosphorus. Here we show that the impacts of nitrogen:phosphorus stoichiometry on plant species richness can be explained by selected plant life-history traits, notably by plant investments in growth versus reproduction. In 599 Eurasian sites with herbaceous vegetation we examined the relationship between the local nutrient conditions and community-mean life-history traits. We found that compared with plants in nitrogen-limited communities, plants in phosphorus-limited communities invest little in sexual reproduction (for example, less investment in seed, shorter flowering period, longer lifespan) and have conservative leaf economy traits (that is, a low specific leaf area and a high leaf dry-matter content). Endangered species were more frequent in phosphorus-limited ecosystems and they too invested little in sexual reproduction. The results provide new insight into how plant adaptations to nutrient conditions can drive the distribution of plant species in natural ecosystems and can account for the vulnerability of endangered species.

Ecological significance of seed germination characteristics in flood-meadow species

Summary In climate chamber and outdoor experiments we analysed germination traits of 42 vascular plant species typical of floodmeadows of the alliance Cnidion. In Central Europe such flood-meadows and their highly endangered character species are confined to large lowland river corridors with a dry and warm climate. Due to the prevailing ecological conditions, particularl y the extremely high variability in soil moisture potential, it was hypothesised that flood-meadow species exhibit specific strat egies in terms of germination phenology, temperature requirements and primary dormancy to avoid constraints on seedling recruitment imposed by flooding, drought and competition from established vegetation. Furthermore, we expected that germination characteristics could be a key for the causal understanding of range size, confinement to large river corridors, recent rarification processes, seed bank persistence and establishment success in restoration projects. PCA ordination of germination traits reflected a continuous gradient of increasing primary dormancy levels ranging from species with a high capacity for fast and almost complete germination over a wide range of temperatures to those with high and narrow temperature requirements and/or a delayed and asynchronous emergence. Many of the studied species germinated only at relatively high temperature in the year of shedding, but the primary conditional dormancy disappeared in most cases after a period of cold wet stratification. Requirements for chilling and/or high germination temperature were revealed to be the most common strategies for avoidance of harmful autumn and winter germination. The majority of the studied species tends to exploit particularly favourable regeneration niches in early spring. Surprisingly, many species with large long-term persistent soil se ed banks exhibited relatively low dormancy levels in light ; this was correlated with small seed size and a consequent higher probability of burial. We found no relationship between germination characteristics and river corridor confinement or Central European range size. However, there was a significant trend in endangered species towards higher temperature requirements and delayed, asynchronous germination. This is presumably disadvantageous under the environmental conditions of subcontinental flood-meadows.

Seedling recruitment in flood-meadow species: The effects of gaps, litter and vegetation matrix

Question: Is the failure of establishment of rare flood-meadow species in habitat restoration primarily due to seed or microsite limitation? How do sown species respond to artificially created gaps and added litter at two neighbouring sites with similar physical conditions but contrasting vegetation matrix (young arable fallow field vs species-poor meadow sward)? Location: Upper Rhine valley, southwestern Germany, 85 m a.s.l. Methods: Seeds of six typical flood-meadow species were sown in four treatment combinations of the factors gap creation and litter addition. Seedling recruitment was monitored for three years. Results: Five of the six species established successfully at both sites largely irrespective of treatments, indicating seed limitation. Only in the small-seeded Arabis nemorensis, which was revealed to be strictly gap-dependent at the meadow site, could an obvious microsite limitation be shown. The nonsignificance of gap treatments in all other species at the relatively high productive meadow site is probably due to biomass removal by mowing in early summer. Only at the extremes of the seed size spectrum did the results meet predictions of plant ecological theory, such as the strict gap dependence of small-seeded species in closed swards or the positive to neutral response of large-seeded species to litter layers. Conclusions: Species identity was revealed to be the major factor influencing differences in recruitment. Due to the lack of a general trend in the response towards treatments the results support conceptual models that describe the interplay of facilitation and interference as a highly dynamic equilibrium, driven by variable abiotic and biotic marginal conditions.