Non-sown Species Research Articles

Seasonal Shifts in Diversity and Composition of a Tallgrass Prairie Restoration Have Implications for Sampling Time

<h3>ABSTRACT</h3> Restorations change across the growing season. Because of this, the point in the season that a restoration is sampled may affect the conclusions reached based on the sample. In this study, we explore seasonal changes in a prairie restoration experiment in eastern Kansas and investigate how these changes affect observed composition, biodiversity, and the effects of seeding density treatment on the plant community based on when, and how completely, vegetation is sampled. Free State Prairie was established in 2014 to test the effects of forb seeding density on forb establishment, diversity, and restoration success. We compared absolute cover data collected in early June and early September 2019 to each other and to combined data. We found changes in both composition and biodiversity from early-to-late in the season. Sown forbs decreased in cover and richness, while sown grasses increased in cover and richness. Nonsown species did not change in cover but decreased in richness. Neither individual sample fully represented the overall composition or biodiversity of the community. We detected a significant negative effect of forb seeding density on diversity in June, and with combined data, but not in September. As sampling time can affect both broad patterns of composition and diversity and observed results of establishment and management techniques, sampling multiple times in a year will provide the fullest and most accurate picture of the community. When multiple samples are impractical, sampling time should be selected carefully based on the phenology of the restoration and the variables of interest.

Super‐abundant C4 grasses are a mixed blessing in restored prairies

Forbs comprise most of the plant diversity in North American tallgrass prairie and provide vital ecosystem services, but their abundance in prairie restorations is highly variable. Restoration practitioners typically sow C4 grasses in high abundances because they are inexpensive, provide fuel for prescribed fires, can dominate reference sites, and suppress weeds that suppress sown forbs. However, C4 grasses can also suppress sown forbs, calling this practice into question. We evaluated how C4 grasses influence the abundance and diversity of sown forbs in 78 restored prairies across Illinois, Indiana, and Michigan. We found that the direct negative effects of C4 grasses on sown forbs outweighed indirect positive effects that occurred as C4 grasses suppressed nonsown species, which in turn suppressed sown forbs. This pattern was especially strong for the C4 grass big bluestem (Andropogon gerardii). Therefore, strategies to promote big bluestem and other C4 grasses would not promote sown forbs. Although C4 grass cover was not strongly related to two hypothesized drivers (time since fire or site age), seeding density of C4 grasses increased their cover. Sown forb cover also increased with forb seeding density, increased indirectly with fire (through its negative effect on nonsown species), and decreased indirectly with soil water‐holding capacity (through its positive effect on nonsown species). These results highlight the complex interplay of species groups during grassland restoration and show how managers can promote sown forbs in restored prairies: increasing forb seeding density and reducing time since fire and the abundance of C4 grasses and weeds.

Biodiversity-ecosystem functioning relationships in a long-term non-weeded field experiment.

Many grassland biodiversity experiments show a positive relationship between biodiversity and ecosystem functioning, however, in most of these experiments plant communities are established by sowing and natural colonization is prevented by selective weeding of non-sown species. During ecosystem restoration, for example on abandoned fields, plant communities start on bare soil, and diversity is often manipulated in a single sowing event. How such initial plant diversity manipulations influence plant biodiversity development and ecosystem functioning is not well understood. We examined how relationships between taxonomic and functional diversity, biomass production and stability develop over 16yr in non-weeded plots sown with 15 species, four species, or that were not sown. We found that sown plant communities become functionally similar to unsown, naturally colonized plant communities. However, initial sowing treatments had long-lasting effects on species composition and taxonomic diversity. We found only few relationships between biomass production, or stability in biomass production, and functional or taxonomic diversity, and the ones we observed were negative. In addition, the cover of dominant plant species was positively related to biomass production and stability. We conclude that effects of introducing plant species at the start of secondary succession can persist for a long time, and that in secondary succession communities with natural plant species dynamics diversity-functioning relationships can be weak or negative. Moreover, our findings indicate that in systems where natural colonization of species is allowed effects of plant dominance may underlie diversity-functioning relationships.

Flower-strip agri-environment schemes provide diverse and valuable summer flower resources for pollinating insects

The diversity and abundance of insect pollinators are declining. This decline reduces the potential ecosystem services of pollination for wild and cultivated plants. Specific agri-environment schemes (AES) are subsidised to support and conserve biodiversity in farmlands. In Belgium, the pollinator flower-strips AES, strips of flower-rich hay meadows, has been promoted as a potential scheme to increase pollinator abundance and diversity, even if their effectiveness has not been locally evaluated. The main objective of this research is to assess the capacity of pollinator-strip AES to provide flower-resources to diverse pollinators. During 2 years, we monthly measured the availability of flower resources (pollen and nectar) produced on four flower-strips surrounded by intensive farming in Belgium. We counted and identified insects that visited these flowers, and we constructed the plant–insect interactions networks. The pollinator-strip AES presented a mix of both sown and spontaneous plant species. The ten sown plant species were all present, even after 8 years of strip settings. Three of them, Centaurea jacea, Lotus corniculatus, and Daucus carota were mainly visited for nectar collection, and a spontaneous non-sown species, Trifolium repens, had a key role in providing high-quality pollen to insects. Most of the observed flower-visiting insects belonged to common species of Hymenoptera and Diptera. All are considered highly efficient pollinators. The Belgian pollinator flower-strips are effective AES that provide flower resources to pollinators, mainly during summer and support pollination services. Nevertheless, spring and autumn flower resources remain poor and could reduce the strips’ effectiveness for supporting long-term insect diversity.

Sowing density effects and patterns of colonization in a prairie restoration

AbstractA cost‐effective approach in plant restorations could be to increase sowing density for species known to be challenging to establish, while reducing sowing density for species that easily colonize on their own. Sowing need not occur evenly across the site for rapidly dispersing species. We explored these issues using a prairie restoration experiment on a high‐school campus with three treatments: plots sown only to grasses (G plots), to grasses and forbs (GF1), and to grasses and forbs with forbs sown at twice the density (GF2). In year 2, GF1 and GF2 plots had higher diversity than G plots, as expected, but GF2 treatments did not have twice the sown forb cover. However, high forb sowing density increased forb richness, probably by reducing stochastic factors in establishment. Cover of nonsown species was highest in G plots and lowest in GF2 plots, suggesting suppressive effects of native forbs on weedy species. Colonization of G plots by two sown forbs (Coreopsis tinctoria and Rudbeckia hirta) was apparent after 2.5 years, providing evidence that these species are self‐sustaining. Colonization was greater in edges than in the central areas of G plots. Through construction of establishment kernels, we infer that the mean establishment distance was shorter for R. hirta (6.7 m) compared to C. tinctoria (21.1 m). Our results lead us to advocate for restoration practices that consider not only seed sowing but also subsequent dispersal of sown species. Furthermore, we conclude that restoration research is particularly amenable for outdoor education and university‐high school collaborations.

Biodiversity theory applied to the real world of ecological restoration

One of the perceived benefits of biodiversity is resistance to invasion by exotic species. This has relevance for vegetation restoration: according to theory, sowing more species of the desired type would help to exclude the invasion of undesired ones. Oakley & Knox (Applied Vegetation Science, this issue) tested this in a real restoration situation: the revegetation of bare compacted clay after construction or commercial activity. Higher sown diversity did indeed reduce the invasion of non-sown species and, of particular practical relevance, reduced the invasion of exotic species.

The effect of reseeding methods on secondary succession during cropland restoration in the Highveld region of South Africa

The Grassland biome is the most transformed biome in South Africa, with cultivation and other human impacts having the largest effect on pristine grasslands. Conversion of natural grassland to cropland agriculture destroys natural vegetation seed banks. When croplands on the South African Highveld are abandoned, secondary succession often leads to low diversity Hyparrhenia hirta dominated plant communities. To investigate the effect of reseeding on secondary succession and botanical diversity, various reseeding treatments of cropland restoration were established on a recently abandoned cropland at Suikerbosrand Nature Reserve. Plots combining two seed mixtures, two seeding rates and two sowing methods were monitored and evaluated. The sowing method (plough vs rip-line) had the largest influence on the suppression of relic weeds as well as possible establishment of local non-sown species. The rip-line plots, where more resources were available due to spaces between the rip-lines, initially experienced higher densities of relic weeds followed by an increase in local non-sown species densities. Hyparrhenia hirta was one of the non-sown perennials increasing in the rip-line plots. The growth of H. hirta and relic weeds was, however, controlled in the plough plots, where species densities and phytomass production of perennial sown-in species were the highest.

Farmers’ fears and agro-economic evaluation of sown grass strips in France

Since 2005, French farmers must set up sown grass strips along rivers in order to decrease pesticide levels and soil erosion. Farmers have thus parcelled out their fields, set aside 3% of their farm size and managed grass without herbicide. Consequently, this environmental policy may cause farmers’ fears due to economic losses and weed infestations of their field margins. Here, we studied farmers’ perception of sown grass strips. First, we interviewed 29 farmers in two French regions. Second, we evaluated the economic loss of gross margin when replacing crop by grass. Third, we evaluated the weed risk using flora surveys in sown grass strips. Our results showed that two thirds of interviewed farmers thought that sown grass strips affected their farm revenue and represented a weed risk. Concerning economy, we found that farmers loose from 358 to 853€/ha the year of installation and from 126 to 641€/ha next years. This economic loss is mainly due to the loss of crop production, with a minor impact of grass management cost. At the farm level, 3% of sown grass strips decreased the farm revenue by 7%. Concerning the weed risk, the farmers’ perception was linked with the presence of some competitive perennial weeds, e.g. Cirsium arvense, and wind-dispersing weeds, e.g. Asteraceae. Sown grass strips with high weed species richness of 26 species on average, or with dominance of non-sown species (16.7% of sown grass strips) did not affect the farmers’ perception. In our study, the economic loss was weak and acceptable at the farm level.

Evaluation of Dandelion as a Potential Forage Species in Mixed‐Species Swards

Recent research has indicated that increasing species diversity of the pastures might improve sward productivity and stability. The present experiment evaluates the effect of dandelion (Taraxacum officinaleWeber) as a potential pasture species on the performance of a perennial ryegrass (Lolium perenneL.)–white clover (Trifolium repensL.) mixture in terms of total herbage yield, weed infestation, and nitrogen (N) yield in a replacement‐series experiment over three harvest years. The three species were grown in monocultures and in all possible binary and three‐species mixtures under two N fertilization levels (0 and 200 kg N ha−1). Herbage yield and N yield of the three‐species mixture were not significantly different from the ryegrass–white clover mixture over the 3 yr. The contribution of dandelion to the total herbage yield increased over years, which improved yield stability. A competition analysis, performed by calculating relative yield and relative yield total, indicated that ryegrass had a competitive advantage over white clover in fertilized and unfertilized swards, but over dandelion only when the sward was fertilized. Nitrogen application reduced the nonsown species yield in all mixtures, except that of white clover–dandelion. Our results indicate that dandelion might have the potential of contributing to herbage yield, nutritive value, and resistance to invasions of ryegrass–white clover pastures.

Forage Mixture Productivity and Botanical Composition in Pastures Grazed by Dairy Cattle

Some producers believe that planting pastures to several forage species benefits sustainability of grazing systems. We conducted a grazing study to determine if forage species diversity in pastures affects herbage productivity and weed invasion. One‐hectare pastures were planted to four mixtures in August 2001 and then grazed with lactating dairy cattle during 2002 and 2003. The mixtures were two species [orchardgrass (Dactylis glomerata L.) and white clover (Trifolium repens L.)], three species [orchardgrass, white clover, and chicory (Cichorium intybus L.)], six species [orchardgrass, tall fescue (Festuca arundinacea Schreb.), perennial ryegrass (Lolium perenne L.), red clover (Trifolium pratense L.), birdsfoot trefoil (Lotus corniculatus L.), and chicory], and nine species [the six‐species mixture plus white clover, alfalfa (Medicago sativa L.), and bluegrass (Poa pratensis L.)]. When rainfall was plentiful (2003), there were no differences in herbage yield among the mixtures; all averaged 9800 kg ha−1 dry matter. During 2002, which was dry, the two‐species mixture produced less herbage than the other mixtures (4800 vs. 7600 kg ha−1 dry matter). The proportion of nonsown species in the sward was lower for the six‐ and nine‐species mixtures than the two‐and three‐species mixtures, indicating less weed invasion for these complex mixtures. Red clover and chicory proportions decreased by 80% after 2 yr, and orchardgrass dominated in all pastures by May 2004. We conclude that planting a mixture of grasses, legumes, and chicory will benefit herbage production during dry years and will reduce weed invasion for a few years after planting under management similar to ours. Producers would have to reestablish the chicory and legume components relatively frequently to maintain these benefits.

Seed limitation and the regulation of community structure in oak savanna grassland

Summary 1 We present results from a long-term sowing experiment conducted in nutrient-poor savanna grassland in eastern Minnesota. We examine the effects of a one-time seed addition of 23 grassland species on plant community dynamics and structure over eight growing seasons. 2 Our goals were to: (i) test the importance of seed availability in regulating plant colonization dynamics and species richness; (ii) assess both the initial effects of sowing on species diversity and community structure and whether these effects increased, persisted or dissipated over the long-term; and (iii) determine the long-term impacts of sown species on the structure and dynamics of the existing community, including effects on species diversity, the abundance of existing (non-sown) species, extinction rate and abundance hierarchy. 3 Sowing led to the successful establishment of several plant species that had not been present in the plots and to increased abundance of other species that were already present. 4 Sowing led to sustained, significant changes in community structure, including increased species richness, increased community evenness, and decreased absolute and relative abundance of non-sown species. Effects of sowing were large and significant 8 years after sowing, revealing the role of seed limitation in these grassland communities. 5 In total, the results suggest that dispersal limitations, species pools and local biotic processes interact to regulate plant community structure.

Effects of sowing and management on vegetation succession during grassland habitat restoration

The impact of sowing a seed mixture to recreate a semi-natural community in combination with six cutting and/or grazing treatments on the vegetation that developed on former agricultural land was examined over 6 years. Introducing seeds significantly increased the number and cover of sown species persisting. Summer grazing by cattle maintained the number, but not cover, of sown species. Few sown species persisted when grazed by sheep, although those that did maintained high cover. Sorenson’s qualitative similarity index (based solely on species presence or absence data) revealed that pairs of sown and non-sown plots within a management treatment did not appear to converge during succession. However, using Sorenson’s quantitative similarity measure (based on both species occurrence and abundance) pairs of plots became increasingly similar after the first year. The sown plots became less similar to each other using the qualitative similarity measure, but this was less marked using the quantitative measure. In contrast, the non-sown plots became less similar to each other with the quantitative measure, but no changes were observed with the qualitative measure. The vegetation in the sown plots became more like that in the non-sown plots as sown species failed to persist. In contrast, the non-sown plots became more like the sown plots as the sown grasses Agrostis capillaris and Festuca rubra increased in abundance. The exception to this was the cattle-grazed sown plots, which retained more sown species, however, succession in this treatment also converged towards the non-sown plots because the non-sown species Trifolium repens and Ranunculus repens increased in abundance in this treatment. The addition of seeds of a desired grassland community appeared to have less effect in directing the trajectory of succession than did the vegetation management.