Effects of plant diversity on biomass production and substrate nitrogen in a subsurface vertical flow constructed wetland

Most biodiversity experiments have been conducted in grassland ecosystems with nitrogen limitation, while little research has been conducted on relationships between plant biomass production, substrate nitrogen retention and plant diversity in wetlands with continuous nitrogen supply. We conducted a plant diversity experiment in a subsurface vertical flow constructed wetland for treating domestic wastewater in southeastern China. Plant aboveground biomass production ranged from 20 to 3121 g m^?2 yr^?1 across all plant communities. In general, plant biomass production was positively correlated with species richness (P = 0.001) and functional group richness (P = 0.001). Substrate nitrate concentration increased significantly with increasing plant species richness (P = 0.046), but not with functional group richness (P = 0.550). Furthermore, legumes did not affect biomass production (P = 0.255), retention of substrate nitrate (P = 0.280) and ammonium (P = 0.269). Compared to the most productive of the corresponding monocultures, transgressive overyielding of mixed plant communities did not occur in most polycultures. Because greater diversity of plant community led to higher biomass production and substrate nitrogen retention, thus we recommend that plant biodiversity should be incorporated in constructed wetlands to improve wastewater treatment efficiency.     

What happens to the sown species if a biodiversity experiment is not weeded?

Studies in experimental grasslands have extensively documented the effects of sown plant diversity on the colonization of new species, but the responses of the sown plant combinations themselves have rarely been investigated. We established experimental grasslands differing in species richness (1, 2, 4, 8, and 16) and functional group number and composition (1–4; legumes, grasses, small herbs, tall herbs), and we studied the changes in the abundance of sown species (residents) in both weeded and non-weeded subplots over a period of five years after sowing. The accumulation of new species through spontaneous colonization in the non-weeded treatment did not affect the number of resident species, but had increasingly negative effects over time on the cover of resident species and their aboveground biomass production at community level. Temporal stability of resident populations was lower and year-to-year changes in resident species composition were larger in non-weeded than in weeded subplots. Compositional dissimilarity between weeded and non-weeded treatments increased through time. These negative effects of the colonization of new species on the abundances and stability of resident populations depended on resident species identity and not on additional variation between different functional groups. The colonization of new species did not change the number of resident species emerging from seeds, but reduced seedling densities of residents. Colonization did not affect the structure of resident communities as measured by species evenness, functional trait diversity and mean trait values suggesting that colonization can destabilize the species composition of residents in terms of abundance while leaving them unchanged in terms of functional characteristics. Generally, negative impacts of colonizing species on residents which accelerated through time decreased with an increasing number of sown species. Sowing more diverse grassland mixtures increases their predictability in terms of ecosystem characteristics, which is important for ecological restoration and sustainable agriculture.     

Effects of plant diversity on microbial biomass and community metabolic profiles in a full-scale constructed wetland

There has been less understanding of relations of microbial community patterns with plant diversity in constructed wetlands. We conducted a single full-scale subsurface vertical flow constructed wetland (SVFCW, 1000 m²) study focusing on domestic wastewater processing. This study measured the size and structure of microbial community using fumigation extraction and BIOLOG Ecoplate? techniques, to examine the effects of macrophyte diversity on microbial communities that are critical in treatment efficiency of constructed wetlands. We also determined the relationship of plant diversity (species richness) with its biomass production under disturbance of the same wastewater supply. Linear regression analysis showed that plant biomass production strongly correlated with plant species richness (R = 0.407, P < 0.001). Increase in plant species richness increased microbial biomass carbon and nitrogen (R = 0.494, P < 0.001; R = 0.465, P < 0.001) and utilization of amino acids on Ecoplates (R = 0.235, P = 0.03), but limited the utilization of amine/amides (R = ?0.338, P = 0.013). Principal components analysis (PCA) showed that the diversity and community-level physiological profiles (CLPP) of microbial community at 168 h of incubation strongly depended on the presence or absence of plant species in the SVFCW system, but not on the species richness. This is the first step toward understanding relations of plant diversity with soil microbial community patterns in constructed wetlands, but the effect of species diversity on microbial community should be further studied.     

Modeling growth and carbon allocation in two reed beds (Phragmites australis) in the Scheldt estuary

Common reed (Phragmites australis) is a prominent species in the upstream part of the eutrophic Scheldt estuary (Belgium, The Netherlands). From 1996 till 1998, seasonal growth dynamics of the species were studied in two monospecific stands subjected to different salinity regimes (seasonal means 1.6 and 13.3 PSU, respectively). We addressed the following questions: how are these reed vegetations affected by meteorological conditions and by the growth site, what are the important growth processes and what is the fate of the annually fixed carbon. A mathematical model was developed and calibrated using the data from the oligohaline site. Subsequent application of the model to the mesohaline stand required adaptation of parameters relating to the partitioning of resources and timing of growth initiation only. At their peak, the aboveground biomass was 587–1678 g DW m⁻² at the 13.3 PSU site and 1116–2179 g DW m⁻² (1.6 PSU); more than 60% of the biomass was located underground. In 1996, biomasses were 2–3 times lower than in the other 2 years, caused by a retarded growth initiation. Probably due to a lower temperature in early 1996, rhizome bud burst occurred more than 1 month later compared to the other years. In addition, growth initiation was several weeks later in the mesohaline site. This appeared mainly responsible for the large difference in maximal aboveground biomass between both stations. Architecture of the plants was also affected, with a higher shoot density (about 50% more shoots), better-developed root system (15% of total belowground biomass compared to 5%) and more, but smaller leaves at the higher salinity site. Notwithstanding large differences in aboveground biomass, annual growth was similar at both stations (154 and 132 mol C m⁻² per year at the oligo- and mesohaline station, respectively). Primary production accounted for about 80% of all growth processes, rhizome remobilization for almost 20%, translocation of mass before sloughing of leaves accounting about 3%. Within a year, some 44% (oligohaline) and 36% (mesohaline) of new assimilates produced by photosynthesis accumulated as dead litter. The other part was respired by the plant itself, either to provide the energy for growth (23%) or maintenance costs (33–41% at the oligo- and mesohaline station, respectively). Calculated annual turnover rates of aboveground biomass, rhizomes and roots were 100, 62 and 73%, respectively.     

Utilization of ground-based digital photography for the evaluation of seasonal changes in the aboveground green biomass and foliage phenology in a grassland ecosystem

We investigated the usefulness of a ground-based digital photography to evaluate seasonal changes in the aboveground green biomass and foliage phenology in a short-grass grassland in Japan. For ground-truthing purposes, the ecological variables of aboveground green biomass and spectral reflectance of aboveground plant parts were also measured monthly. Seasonal change in a camera-based index (rG: ratio of green channel) reflected the characteristic events of the foliage phenology such as the leaf-flush and leaf senescence. In addition, the seasonal pattern of the rG was similar to that of the aboveground green biomass throughout the year. Moreover, there was a positive linear relationship between rG and aboveground green biomass (R² = 0.81, p < 0.05), as was the case with spectra-based vegetation indices. On the basis of these results, we conclude that continuous observation using digital cameras is a useful tool that is less labor intensive than conventional methods for estimating aboveground green biomass and monitoring foliage phenology in short-grass grasslands in Japan.     

Removal of total nitrogen by Cyperus alternifolius from wastewaters in simulated vertical-flow constructed wetlands

The vertical-flow constructed wetland (VFCW) is a promising engineering technique for removal of excess nutrients and certain pollutants from wastewaters. This study investigated the uptake and removal of total nitrogen (TN) by Cyperus alternifolius from domestic wastewaters in simulated VFCWs. A total of eight simulated VFCW treatments, including two different substrates, two different wet-to-dry ratios, and with and without C. alternifolius species (2 × 2 × 2 = 8), were employed for an operation period of 2 years in this study. Results show that more TN was removed from the influent with the presence of C. alternifolius compared to without this plant species. A linear correlation existed between the aboveground biomass and its TN content. An increase in total biomass by 100 g resulted in an increase in TN accumulation in the aboveground biomass by 2.4 g. Large amounts of TN were removed by the aboveground biomass compared to those by substrate adsorption. Results suggest that C. alternifolius played a discernable role in removal of TN from wastewaters in the simulated VFCWs.     

Shape matters in sampling plant diversity: Evidence from the field

The identification of shape and size of sampling units that maximises the number of plant species recorded in multiscale sampling designs has major implications in conservation planning and monitoring actions. In this paper we tested the effect of three sampling shapes (rectangles, squared, and randomly shaped sampling units) on the number of recorded species. We used a large dataset derived from the network of protected areas in the Siena Province, Italy. This dataset is composed of plant species occurrence data recorded from 604 plots (10 m × 10 m), each divided in a grid of 16 contiguous subplot units (2.5 m × 2.5 m). Moreover, we evaluated the effect of plot orientation along the main environmental gradient, to examine how the selection of plot orientation (when elongated plots are used) influences the number of species collected. In total, 1041 plant species were recorded from the study plots. A significantly higher species richness was recorded by the random arrangement of 4 subplots within each plot in comparison to the ‘rectangle’ and ‘square’ shapes. Although the rectangular shape captured a significant larger number of species than squared ones, plot orientation along the main environmental gradient did not show a systematic effect on the number of recorded species. We concluded that the choice of whether or not using elongated (rectangular) versus squared plots should dependent upon the objectives of the specific survey with squared plots being more suitable for assessing species composition of more homogeneous vegetation units and rectangular plots being more suited for recording more species in the pooled sample of a large area.     

Response of ecosystem CO2 exchange to biomass productivity in a high yield grassland

We measured the biomass production and ecosystem carbon CO₂ exchange in a high yield grassland dominated by Miscanthus sinensis. The experimental grassland is managed by mowing once a year in winter every year and the harvested biomass on the ground is left to become the humus. The maximum aboveground and belowground biomasses were 1117 and 2803 g d.w. m^?2 in our grassland. Although the high potential of our grassland for biomass production led to higher carbon uptake than with other types of grassland, the large biomass contributed to a higher respired carbon loss. Biomass increase led to a linear increase in ecosystem respiration. Over the 3 years, RE₁₀ increased with increasing aboveground biomass. The potential gross primary production at a photosynthetic photon flux density of 2000 μmol m² s^?1 logarithmic increased with LAI. These responses of CO₂ exchange to biomass production suggest this grassland behaved as weak CO₂ sink or near carbon neutral (?78 and 17 g C m^?2 year^?1) in current management.     

Agri-environment incentive payments and plant species richness under different management intensities in mountain meadows of Switzerland

We investigated whether agri-environmental incentive payments help to maintain biodiversity. We studied the effect of agricultural management intensity on vascular plant species richness and plant assemblages of mountain meadows in Switzerland. Other factors such as slope, altitude or accessibility (distance from farmyard) were also taken into account. Vegetation sampling was conducted at 69 sites representing five different management types, differing with respect to nutrient input and soil moisture: (i) dry extensive meadows; (ii) extensive meadows; (iii) dry low-intensive meadows; (iv) low-intensive meadows; (v) intensive meadows. There was a significant negative relationship between plant species richness and management intensity: The mean number of plant species per management type declined markedly when management intensity increased, although dry sites harboured slightly more species regardless of management intensity (dry extensive > dry low intensive > extensive > low intensive >> intensive meadows). Species richness was clearly affected by management intensity, but not so by slope, altitude or accessibility. There was a gradual shift in plant assemblages among management types with only intensive meadows differing from the other four types of differently managed meadows. We therefore found, in contrast to many studies done in the European lowlands, positive effects of incentive payments on plant species richness.     

Is seed availability enough to ensure colonization success?: An experimental study in road embankments

We tested the hypothesis that seed availability is a limiting factor for plant colonization of road embankments under Mediterranean climate conditions. Experimental sowing on 10 road embankments was carried out to compare the colonization success of plants that successfully colonize the road embankment and species that appear only occasionally in the road embankments. After sowing, we measured plant establishment, biomass production, and reproductive capacity of the species.The species that appear only occasionally in the road embankments had lower emergence rates (l.l ± 0.3%) than species that were successful colonizers (18.8 ± 2.9%). None of the species of the former group survived or reproduced. The results did not support the hypothesis that seed availability was the main factor limiting plant colonization in the road embankments. We concluded that the arrival of seeds to road embankments under Mediterranean climate conditions was not enough to ensure colonization success of plants. Other factors, like hydric stress, appeared to affect seedling establishment and plant growth. Reclamation measures such as species selection should be taken in account to ensure revegetation success of road embankments.     

High plant species richness indicates management-related disturbances rather than the conservation status of forests

There is a wealth of smaller-scale studies on the effects of forest management on plant diversity. However, studies comparing plant species diversity in forests with different management types and intensity, extending over different regions and forest stages, and including detailed information on site conditions are missing. We studied vascular plants on 1500 20 m × 20 m forest plots in three regions of Germany (Schwäbische Alb, Hainich-Dün, Schorfheide-Chorin). In all regions, our study plots comprised different management types (unmanaged, selection cutting, deciduous and coniferous age-class forests, which resulted from clear cutting or shelterwood logging), various stand ages, site conditions, and levels of management-related disturbances. We analyzed how overall richness and richness of different plant functional groups (trees, shrubs, herbs, herbaceous species typically growing in forests and herbaceous light-demanding species) responded to the different management types. On average, plant species richness was 13% higher in age-class than in unmanaged forests, and did not differ between deciduous age-class and selection forests. In age-class forests of the Schwäbische Alb and Hainich-Dün, coniferous stands had higher species richness than deciduous stands. Among age-class forests, older stands with large quantities of standing biomass were slightly poorer in shrub and light-demanding herb species than younger stands. Among deciduous forests, the richness of herbaceous forest species was generally lower in unmanaged than in managed forests, and it was even 20% lower in unmanaged than in selection forests in Hainich-Dün. Overall, these findings show that disturbances by management generally increase plant species richness. This suggests that total plant species richness is not suited as an indicator for the conservation status of forests, but rather indicates disturbances.     

Synthetic sheep urine alters fungal community structure in an upland grassland soil

Agricultural improvement (fertilisation, liming, intensification of grazing) of acidic upland pastures results in loss of indigenous flora and notable changes in microbial community structure. Such practices have recently raised concerns regarding the possible impacts on natural ecosystem biodiversity and functioning. The effects of synthetic sheep urine (SSU) and plant species on fungal community structure in upland grassland microcosms were investigated. Plant species typical of agriculturally unimproved (Agrostis capillaris) and improved (Lolium perenne) pastures were treated with low, medium or high concentrations of SSU, with harvests carried out 10 d and 50 d after SSU application. Root biomass was negatively affected by SSU addition whereas shoot biomass did not display any significant change. Fungal richness (number of operational taxonomic units) was negatively correlated with SSU concentration (p < 0.001), and also with time (p < 0.001).Multi-dimensional scaling plots revealed significant changes in fungal community composition, depending on concentration of SSU and plant species type, while canonical correspondence analysis also emphasised the importance of interacting environmental variables. In addition, SIMPER analyses supported the finding that shifts in fungal community composition under different SSU and plant treatments had occurred. Overall, while SSU appeared to be influential in determining fungal community structure, community changes were largely driven by interacting environmental factors. This study contributes to our understanding of the potential implications of intensified farming, in particular increased pressure from grazing animals, on fungal community structure in semi-natural grassland systems.     

Cover as a simple predictor of biomass for two shrubs in Tibet

Knowledge of the quantitative relationship between plant cover and its corresponding biomass for shrubs is not well known, especially for those on the Tibetan Plateau. Based on investigations of 35 sites, 90 plots and 95 standard individuals for two typical shrub species (Rhododendron nivale Hook. f. and Sophora moorcroftiana (Benth.) Baker) across Tibet, we developed allometric models for biomass estimation from measurements of crown diameter and/or height. We found that the parameters of crown projection area (CPA), height and their product (volume) were all significantly (p < 0.01) correlated with dry mass of different organs for both species at individual level. The CPA rather than volume best predicted aboveground dry mass. This is because that the bulk density declined significantly with increasing plant height, leading to the inappropriateness for plant height itself being employed as a parameter in biomass estimation, especially for shrubs in smaller size groups. At community level, cover was tightly correlated with the aboveground, belowground and total biomass (R² = 0.97–0.99). Therefore, biomass for the two shrubs can be simply estimated by measuring plant cover, which enables rapid estimation of shrubland carbon stock at large scales by using satellite data and repeated experiments over time. This non-destructive method using cover to estimate shrub biomass can be applied not only in arid ecosystems but also in alpine or subalpine environment.     

Trace elements in Phragmites australis growing in constructed wetlands for treatment of municipal wastewater

Biomass of Phragmites australis growing in four constructed wetlands with horizontal sub-surface flow (HF CWs) designed for treatment of municipal sewage in the Czech Republic have been analyzed for 19 trace elements. The biomass was harvested during the peak standing crop in early September and divided into stems, leaves, flowers, roots and rhizomes. Concentrations of monitored elements in both aboveground and belowground plant tissues were similar to those found in plants growing in natural stands. The highest concentrations were recorded for Al, Fe, Mn, Ba and Zn while the lowest concentrations were those of Hg, U and Cd. Concentrations decreased in the order of roots > rhizomes > leaves > stems. The root/leaf ratio averaged 70 and varied between 1.4 for molybdenum and 392 for cobalt. The belowground/aboveground concentration ratio ranged between 0.9 and 69.5 with an average value of 19. Due to average aboveground/belowground biomass ratio > 1, the belowground/aboveground standing stock ratios were lower with six elements (Ba, Zn, Se, Hg, Mo, and Mn) having this ratio < 1.     

Typha × glauca dominance and extended hydroperiod constrain restoration of wetland diversity

Urban wetlands typically have few plant species. In wetlands designed to improve water quality, nutrient-rich water and highly variable water levels often favor aggressive, flood-tolerant plants, such as Typha × glauca (hybrid cattail). At Des Plaines River Wetlands Demonstration Site (Lake Co., IL), we assessed T. × glauca dominance and plant community composition under varying hydroperiods in a complex of eight constructed wetlands. Plots flooded for more than 5 weeks during the growing season tended to be dominated by T. × glauca, while plots flooded fewer days did not. Plots with high cover of T. × glauca had low species richness (negative correlation, R² = 0.72, p < 0.001). However, overall species richness of the wetland complex was high (94 species), indicating that wetlands in urbanizing landscapes can support many plant species where T. × glauca is not dominant. T. × glauca-dominated areas resisted the establishment of a native plant community. Removing T. × glauca and introducing native species increased diversity initially, but did not prevent re-invasion. Although 12 of the 24 species we seeded became established in our cleared plots, T. × glauca rapidly re-invaded. In year 1, T. × glauca regained an average of 11 ramets m⁻², and its density doubled in year 2. The likelihood of planted species surviving decreased as duration of inundation increased, and in both seeded and planted plots, graminoids had greater survivorship through year 2 than forbs across a range of water levels. Within 4 years, however, T. × glauca was the most common plant, present in 92% of the cleared plots. Simply removing T. × glauca and adding propagules to an urban wetland is not sufficient to increase diversity.     

Morphological variations,biomass and ion accumulation of the aboveground shoots of Desmostachya bipinnata (L.) Stapf

The present study aimed to evaluate the morphological characteristics and biomass of Desmostachya bipinnata and their relation to the environmental variables in three main habitats (canal banks, railway and roadside shoulders) where it is found in Egypt. In addition, the ability of this plant is evaluated to accumulate nutrients and heavy metals in its aboveground shoots. Twenty five quadrats (1 m × 1 m per quadrat) were selected along five sites representing the different habitats of D. bipinnata for this study. The aboveground shoots displayed considerable morphological variations, differing in the different habitats. The above ground biomass, number of spikes and leaves, rachis length and diameter, leaf length, width and area, leaf sheath length, and spike length and diameter were in the order: road sides > canal banks > railway shoulders. A regression equation: biomass = (67.37 × density) + 108.2, was used to estimate the shoot biomass from the plant growth density. Metal uptake capability from soil to grass is in the order Fe > Zn > Cu > Mn, and all of them are in safe concentration ranges. These heavy metals had a transfer factor more than unity, which indicates that D. bipinnata is a powerful accumulator for heavy metals. In addition, the plant shoots exhibit high accumulation of inorganic and organic nutrients.     

Experimental evidence for a delayed response of the above-ground vegetation and the seed bank to the invasion of an annual exotic plant in deciduous forests

Invasions by alien plants significantly affect native biodiversity and ecosystem functioning. We conducted a 5-year field experiment to investigate potential effects of the annual invasive plant Impatiens glandulifera on both the native above-ground vegetation and the soil seed bank in a deciduous forest in Switzerland. Eight years after the establishment of I. glandulifera, we set up plots in patches invaded by the alien plant, in plots from which the invasive plant had been manually removed and in plots which were not yet colonized by the invasive plant. We examined plant species richness, diversity and plant species composition in the above-ground vegetation and soil seed bank in all plots one year and five years after the initiation of the experiment. The 36 plots (3 plot types × 6 replicates × 2 sites) were equally distributed over two forest sites. Neither the native above-ground vegetation nor the soil seed bank was influenced by the presence of I. glandulifera one year after the start of the field experiment. After five years, however, plant species richness of both the above-ground vegetation and the soil seed bank was reduced by 25% and 30%, respectively, in plots invaded by the alien plant compared to plots from which I. glandulifera had been removed or uninvaded plots. Furthermore, plots invaded by the alien plant had a lower total seedling density (reduction by 60%) and an altered plant species composition in the soil seed bank compared to control plots. Our field experiment indicates that negative effects of the annual invasive plant on the native above-ground vegetation and soil seed bank of deciduous forests become visible with a delay of several years.     

Extinction thresholds and negative responses of Afrotropical ant-following birds to forest cover loss in oil palm and agroforestry landscapes

Afrotropical ant-following birds are vulnerable to forest loss and disturbance, but critical habitat thresholds regarding their abundance and species richness in human-dominated landscapes, including industrial oil palm plantations, have never been assessed. We measured forest cover through Landsat imagery and recorded species richness and relative abundance of 20 ant-following birds in 48 plots of 1-km², covering three landscapes of Southwest Cameroon: Korup National Park, smallholder agroforestry areas (with farms embedded in forest), and an industrial oil palm plantation. We evaluated differences in encounter frequency and species richness among landscapes, and the presence of critical thresholds through enhanced adaptive regression through hinges. All species were detected in Korup National Park and the agroforestry landscape, which had similar forest cover (>85%). Only nine species were found in the oil palm plantation (forest cover = 10.3 ± 3.3%). At the 1-km² scale, the number of species and bird encounters were comparable in agroforests and the protected area: mean species richness ranged from 12.2 ± 0.6 in the park and 12.2 ± 0.6 in the agroforestry matrix to 1.0 ± 0.4 in the industrial oil palm plantation; whereas encounters decreased from 34.4 ± 3.2 to 26.1 ± 2.9 and 1.3 ± 0.4, respectively. Bird encounters decreased linearly with decreasing forest cover, down to an extinction threshold identified at 24% forest cover. Species richness declined linearly by ca. one species per 7.4% forest cover lost. We identified an extinction threshold at 52% forest cover for the most sensitive species (Criniger chloronotus, Dicrurus atripennis, and Neocossyphus poensis). Our results show that substantial proportions of forests are required to sustain complete ant-following bird assemblages in Afrotropical landscapes and confirm the high sensitivity of this bird guild to deforestation after industrial oil palm development. Securing both forest biodiversity and food production in an Afrotropical production landscape may be best attained through a combination of protected areas and wildlife-friendly agroforestry.     

Granivory reduces biomass and lignin concentrations of plant tissue during grassland assembly

Small mammals can influence grassland assembly by selecting against palatable plants – the community can become dominated by the plants they avoid. This predation-based selection could have indirect effects on community biomass and tissue quality, especially given how untasty plants may have higher concentrations of recalcitrant carbon compounds including lignin. We tested small mammal effects on biomass and tissue quality of roots and shoots in a two-year-old 18 ha restored tallgrass prairie with established zones of high and low plant predation. We focused on the three dominant herbaceous functional groups of tallgrass prairie (perennial forbs, C₃ and C₄ grasses), and targeted the early stages of assembly given that plant predation by small animals can unfold quickly and is difficult to subsequently quantify. We predicted rodent predation to create communities with reduced biomass but an increased abundance of lignin-rich plants; we only observed the former. Rodents reduced aboveground biomass by 46% but preferentially targeted lignin-rich plants, with the latter result explained by the predominance of granivory over herbivory – there was no opportunity for selection based on tissue palatability. Based strictly on aboveground biomass, we estimated small mammals reduced standing stocks of recalcitrant carbon by 65 kg ha⁻¹, with reductions in belowground stocks almost certainly higher given that root:shoot ratios averaged 21:1. Given that the quantity and quality of plant production can affect ecosystem functions including decomposition and the regulation of soil carbon stocks, our work suggests that non-random plant predation may substantially affect rates of soil carbon accumulation in the early stages of grassland development.     

The establishment success of native versus non-native herbaceous seed mixes on a revegetated roadside in Central Texas

Revegetation is an essential component of roadside and building site construction and improvement. In the southern United States, non-native grass species are frequently included in revegetation seed mixes used by highway authorities. Non-native species are frequently selected for aggressive growth characteristics; however, these same traits also render them potentially invasive, and subsequently hazardous to adjacent plant communities. Although the use of pure native seed mixes has been rejected in the past due to perceived inferior establishment characteristics, there have been few comparative quantitative field studies that justify this belief. The establishment characteristics of three seed mixes—one containing non-native species and two with native grass and forb species only—were compared in a randomized-block design along a Texas roadside following spring and summer sowing. After 60 days following the spring sowing, the two native-only seed mixes demonstrated 180 and 560% (F = 10.18; P < 0.0001) higher seed densities than the recommended native/non-native mix. The summer sowing results were similar with seedling densities 180 and 330% (F = 9.20; P < 0.01) greater than the standard non-native seeding. Although an aggressive colonizer from vegetative tissue such as stolons and rhizomes, the non-native Bermudagrass (Cynodon dactylon) had a lower than expected establishment rate thought to be due to high water demand during the first week following sowing. Given the invasive characteristics of this common component of many recommended revegetation seed mixes, these results call into question the widespread recommended use of Bermudagrass for such projects. Although gathered during 1 year only, these data indicate that examination of suites of early- and late-successional native species can provide a highly effective mix for revegetation projects. Furthermore, this reduces the potential for negative ecological consequences and provides added benefits associated with wholly native plant communities.