Soil nitrogen availability under grasses of different palatability in a temperate semi‐arid rangeland of central Argentina

Abstract Grazing by domestic livestock is frequently associated with the replacement of high‐nutrient palatable species with low‐nutrient unpalatable species, which may have a substantial effect on nutrient cycling. The objective of the present study was to compare soil N availability and net N mineralization in soils under Poa ligularis (palatable grass) with those in soils under Stipa tenuissima (unpalatable grass) in a temperate semi‐arid rangeland of central Argentina. Nitrogen availability and net mineralization under laboratory and field conditions were measured. Soil N availability under P. ligularis was higher than or similar to soil N availability under S. tenuissima. In situ net N mineralization in the soil under P. ligularis was lower than or similar to net N mineralization in the soil under S. tenuissima. Potential net N mineralization was greater in the soil under P. ligularis than in the soil under S. tenuissima. Our results suggest that the replacement of palatable grasses by unpalatable grasses in the temperate semi‐arid rangelands of central Argentina may imply a reduction in the rate of nutrient cycling.     

Effect of competition intensity on recruitment of palatable and unpalatable grasses

In this study, we made an attempt to reveal how competition intensity from established plants impacts on palatable and unpalatable grass seedlings recruitment, in a natural mesic grassland of central Argentina. Our objective was to assess the seedling recruitment of a palatable species (Chascolytrum subaristatum) and an unpalatable species (Nassella trichotoma) in microsites differing in competition intensity from established plants. Identity (C. subaristatum and N. trichotoma) and defoliation severity were used as surrogate for competition intensity. In March 2017, we permanently marked established individuals of N. trichotoma and C. subaristatum and placed two circular plots adjacent to each individual. In one plot we added seeds of N. trichotoma and in the other seeds of C. subaristatum. After seeding, established plants were randomly assigned to one of three level of defoliation: without defoliation, low defoliation severity and high defoliation severity. From April to November 2017 (i.e. over a complete annual growing cycle), we measured seedling density, recruitment and growth. Our results supported the hypothesis that seedlings of palatable grasses are more competitive than seedlings of unpalatable grasses. Seedling of the palatable grass C. subaristatum recruited successfully regardless the intensity of competition from established plants, whereas seedlings of the unpalatable grass N. trichotoma recruited better under low competitive pressure from established plants. Our results suggest that the availability of microsites with low competitive pressure from the established vegetation, created by selective grazing of palatable grasses, promotes the recruitment of unpalatable grass seedlings. This mechanism may contribute to the species replacement process commonly observed in heavy grazed grasslands.     

Post‐fire shrub recruitment in a semi‐arid grassland: the role of microsites

Question: (1) Which factors regulate post‐fire recruitment and spread of the shrub Senecio bracteolatus in Patagonian grasslands? (2) What is the role of the grass Stipa speciosa on S. bracteolatus establishment in the post‐fire succession? Location: Northwest Patagonia, Argentina. Methods: We studied the effect of fire on S. bracteolatus recruitment and density by comparing these variables between burned and unburned grasslands. In burned areas, we compared abiotic characteristics and seedling establishment under the canopy of grasses (S. speciosa) and in gaps (inter‐tussock areas). Post‐fire interactions between S. bracteolatus seedlings and S. speciosa were studied using field and greenhouse experiments. Results: Density of S. bracteolatus was higher in burned than in unburned areas. In burned sites, seedlings were more abundant under tussock grasses, whereas juveniles were more abundant in gaps. Tussocks generated more attenuated micro‐environmental conditions than gaps during stressful summers. Gaps were more abundant in burned sites, while “under tussock” microsites were more frequent in unburned sites. In burned areas, tussocks allowed higher establishment of seedlings (facilitation), but gaps allowed more seedling growth and higher persistence of juveniles. Conclusions: Fire promoted S. bracteolatus recruitment in Patagonian grasslands by increasing the availability of favourable gap microsites. Grass protection for shrub seedlings became negative with time, probably due to competition with grasses. Gaps led to better performance and persistence of shrub plants. Six years after fire, higher shrub recruitment and adult density (observed as a trend) in burned grassland provides an opportunity for potential S. bracteolatus invasion.     

Effects of adult neighbour and gap size on seedling emergence and early growth of <Emphasis Type="Italic">Bromus inermis</Emphasis> Leyss

A field study was conducted to determine the effects of neighbour root exclusion and gap size on the seedling emergence and early growth of Bromus inermis. Seeds of B. inermis were added to artificially created gaps in an improved shortgrass steppe in northern China. Neighbour root exclusion was accomplished using PVC tubes sunk in the soil of gaps. Emergence and survival of seedlings were greater in all gaps than in the control (0-cm diameter), but better growth performance (height of seedling, leaves, tillers and dry weight per seedling) was observed only in large gaps (20- and 40-cm diameter) with neighbour root present and gaps without neighbour root presence. Seedling growth performance was improved by reducing root and shoot competition. Neighbour root exclusion provided a favourable environment for seedling establishment. Differences between gaps in light levels and topsoil temperature can explain the patterns of germination. These results confirm that B. inermis is a gap-enhanced species. Our study strongly suggests that asymmetric competition by adult neighbour plants negatively influences the establishment of B. inermis.     

Establishing Native Grasses in a Big Sagebrush–Dominated Site: An Intermediate Restoration Step

Many semiarid rangelands in the Great Basin, U.S.A., are shifting dominance to woody species as a consequence of land degradation including intense livestock grazing and fire suppression. Whereas past rehabilitation efforts in Big sagebrush (Artemisia tridentata) steppes removed the shrub and added introduced forage grasses to successfully shift communities from shrublands to grasslands, current consensus is that native species should be included in restoration projects and that retention of some woody plants is desirable. We examined the potential for interseeding grasses into dense shrub communities as a precursor to thinning shrubs and releasing grasses from shrub interference. We compared seedling establishment of the native grass, Bluebunch wheatgrass (Pseudoroegneria spicata), with that of the Eurasia grass, Crested wheatgrass (Agropyron desertorum), in dense Ar. tridentata stands. Shrubs may play an important role as nurse plants for seedling establishment (reduced solar radiation, “island of fertility” effect) but result in highly contrasting light environments and root interference for seedlings. In experimental plots, we examined effects of Ar. tridentata shade levels (0, 40, 70, and 90% reduction of solar radiation) and initial root exclusion (present/absent) on the establishment and growth of P. spicata and Ag. desertorum seedlings. With this design we evaluated the interference effects of Ar. tridentata on the two grasses and identified the most beneficial microsites for grass restoration in Ar. tridentata–dominated communities. We predicted seedling survival and growth to be greater under moderate shade (40% reduction) and limited root competition than under no or strong shade conditions (0 and 90%) and unrestricted root interactions. Fifty to 85% of the P. spicata and Ag. desertorum seedlings survived the dry summer months of 1995 and 1996 and the intervening winter. Neither shading nor root exclusion from Ar. tridentata affected final seedling survival of either species. Seedling biomass of both grass species was negatively affected by initial root interactions with Ar. tridentata. However, the analysis of seedling biomass variability (coefficient of variation) indicated that in all shade and root‐exclusion treatments, some seedlings of both species developed to large individuals to survive in Ar. tridentata–dominated rangelands. Thus, the use of interseeding techniques shows promise for restoring herbaceous species in dense Ar. tridentata stands and should be given further consideration when shrub retention is an important consideration.     

Effects of Frequent Mowing on Survival and Persistence of Forbs Seeded into a Species-Poor Grassland

Many early attempts at tallgrass prairie reconstruction failed to achieve the high species diversity of remnant prairies, and instead consist primarily of C₄ grasses. We hypothesized that frequent mowing of established prairie grasses could create sufficient gaps in the aboveground and belowground environment to allow for the establishment of native forbs from seed. We studied forb seedling establishment in a 25‐year‐old prairie planting in northern Iowa that was dominated by native warm‐season grasses. In winter 1999, 23 species of native forbs were broadcast into the recently burned sod at a rate of 350 viable seeds/m². Treatment plots were mowed weekly for either one or two growing seasons, and control plots were unmowed. Mowed plots had greater light availability than controls, especially when warm‐season grasses began to flower. Overwinter seedling mortality was 3% in mowed treatments compared to 29% in the controls. Forbs in mowed plots had significantly greater root and shoot mass than those in control plots in the first and second growing seasons but were not significantly more abundant. By the fourth growing season, however, forbs were twice as abundant in the mowed treatments. No lasting negative impacts of frequent mowing on the grass population were observed. Mowing a second year influenced species composition but did not change total seedling establishment. Experimental evidence is consistent with the idea that mowing reduced competition for light from large established grasses, allowing forb seedlings the opportunity to reach sufficient size to establish, survive, and flower in the second and subsequent years.     

Nutrient resorption from senescing leaves in two Stipa species native to central Argentina

Abstract Nutrient resorption from senescing leaves enables plants to conserve and reuse nutrients. As such, it could be expected that plant species adapted to infertile soils have a higher nutrient resorption efficiency (percentage reduction of nutrients between green and senesced leaves) and/or higher nutrient resorption proficiency (absolute reduction of nutrients in senesced leaves) than those adapted to fertile soils. Our objective was to compare nitrogen (N) and phosphorous (P) resorption of two congener grasses that successfully occupy uplands of relatively low fertility (Stipa gynerioides) or lowlands of relatively high fertility (Stipa brachychaeta) in natural grasslands of central Argentina. The two Stipa species did not differ in N and P resorption efficiency, but S. gynerioides had a higher N and P resorption proficiency than S. brachychaeta. As a consequence, leaf‐level N and P use efficiency were higher in the species adapted to low fertility conditions than in the species adapted to high fertility conditions. The higher nutrient resorption proficiency of S. gynerioides was also associated with relatively low leaf‐litter decomposition and nutrient release rates found in a previous study.     

Differential effects of the crustose Diploschistes diacapsis and the squamulose Fulgensia bracteata on the establishment of a Mediterranean grass species

The interest of the scientific community in biological soil crusts has grown exponentially over the last decades. One of the scientific research interests is the study of the effect of these crusts on plant establishment. Findings in this topic have been controversial, and some differences were attributed to crust types. Biological soil crusts dominated by lichens are common components of Stipa tenacissima steppes in arid and semi‐arid environments of the southern Mediterranean. In the current study, we conducted growth chamber experiments to investigate the differential effects of two lichen species with continuous crustose thalli (Diploschistes diacapsis) and with squamulose semicontinuous thalli (Fulgensia bracteata) on seed germination, root penetration, shoot emergence and seed viability of the tussock grass species S. tenacissima. Our results showed that under laboratory conditions, two distinct lichen species had significantly different effects on the establishment of S. tenacissima. Our findings clearly demonstrated that D. diacapsis significantly decreased germination, root penetration and shoot emergence of S. tenacissima compared to F. bracteata. This can be related to differences in morphological and physiological characteristics between crustose and squamulose lichens. Overall, we suggest that D. diacapsis and crustose lichens generally can act as natural barrier to the establishment of S. tenacissima.     

Competitive interactions between palatable and unpalatable grasses native to a temperate semi-arid grassland of Argentina

The argument that selective grazing leads to competitive replacement of palatable grasses by unpalatable grasses is based upon the assumption that the competitive ability of the palatable species is higher than the one of unpalatable species in the absence of grazing. In order to test this hypothesis we have compared the competitive ability of Stipa clarazii (palatable) and S. trichotoma (unpalatable) under field conditions, and S. clarazii and S. gynerioides (unpalatable) under greenhouse conditions. The three species are native to a temperate semi-arid grassland of central Argentina. In the field experiment, plants of both species were grown either independently or in pairs (palatable + unpalatable), protected from grazing. Shoot and seed production were measured at the end of the growing seasons of 1993, 1994 and 1995. In the greenhouse experiment, plants of both species were grown in pots, either in monoculture or in mixture, under conditions of high and low water and mineral nutrient availability. Total biomass and seed production were measured at the end of the experimental period. In both experiments the presence of the unpalatable species did not affect (P < 0.05) the productive responses of the palatable species. On the contrary, the presence of the palatable species significantly reduced (P < 0.05) the productive responses of the unpalatable species. Our results support the assumption, on which most interpretations of floristic changes induced by grazing are based, that the competitive ability of palatable grasses is higher than the one of unpalatable grasses in the absence of grazing.     

The interaction of gap age and microsite for herb layer species in a near‐natural spruce forest

Question: To what degree does the regeneration of understorey forest species depend on gaps of different age and on gap‐induced and non‐gap‐induced microsites? Do species preferences for a specific microsite change with the developmental stage of the gap? How do different species in the understorey interact over time? Location: Near‐natural spruce forest on Mt. Brocken in the Harz National Park, Germany. Methods: We established 90 study plots, stratified according to different gap age classes and undisturbed forest, and including subplots with three different gap‐induced types of microsites (logs, stumps and root plates) and two non‐gap‐induced microsites (moss‐covered rocks and ordinary forest ground). Results: Significant interactions of species were encountered with gap age as well as with microsite type, light availability and competition. While shoot densities of Vaccinium myrtillus were highest at intermediate gap age, Calamagrostis villosa and Trientalis europaea showed highest densities in the oldest gaps. The species preferred different microsites but had higher densities on non‐gap‐induced microsites, and their preferences changed over time. Unexpectedly, species shoot densities were not always negatively affected by densities of competing species. Conclusion: The results confirmed the importance of gaps for regeneration of forest herb layer species, but pointed to a much higher importance of microsites that were not induced by gaps compared to gap‐induced microsites. Niche differentiation between different herb layer species can be conceived as species‐specific preferences for microsite types that change with gap age, as a result of light conditions, degree of decay of logs and root plates and presence of competitors.     

Variation in dormancy and germination in three co-occurring perennial forest herbs

Mesic deciduous forest herbs often disperse seed with morphophysiological dormancy (MPD) that prevents germination during unfavorable periods for seedling survival. However, for seeds of some species with MPD, seasonal separation of root and shoot emergence and variation in dormancy levels can complicate interpretation of seedling emergence timing in the field. We tested whether dormancy-break and germination requirements differed among co-occurring perennial forest herbs, Actaea racemosa, Hydrastis canadensis, and Sanguinaria canadensis, which are wild-harvested for their medicinal properties and known to have MPD. Seeds of all species exhibited a summer → autumn → winter requirement for seedling emergence in spring. However, species differed in seed-bank persistence due to variation in primary dormancy levels and stratification requirement of seeds. A. racemosa and H. canadensis can form short-term persistent seed bank, whereas S. canadensis can form a long-term persistent seed-bank, regardless of whether elaiosomes were removed from seeds prior to burial. A. racemosa seeds are dispersed in autumn with weak physiological dormancy, as seeds germinated to high rates at 15/6°C after 8 weeks. In contrast, most seeds of the summer dispersed species, H. canadensis and S. canadensis, require summer temperatures to overcome physiological dormancy. Consequently, seedling emergence is reduced and delayed by 1 year if seeds are not sown immediately following the period of natural dispersal. Seedling emergence was much lower in the field than in controlled conditions for all species, especially in the small-seeded A. racemosa. Interspecific variation in dormancy levels and germination traits must be considered when establishing populations for conservation purposes and in understanding recruitment limitation in perennial forest herbs.     

Influences of neighbouring plants on seedling establishment in limestone grassland

Abstract. Influences of neighbouring plants on seedling establishment of six dicotyledonous species was investigated in a nutrient-poor limestone grassland (Mesobrometum) in northern Switzerland. Microsites with different vegetational structure were created and seeds sown in them. The fate of emerging seedlings was followed for one or two years. A plant surviving its first growing season was regarded as established. Shelter by neighbouring plants appeared to be necessary for the establishment of Arabis hirsuta and Primula veris. Because of drought and frost heave, hardly any seedlings of these species established in gaps (4 - 21 %). In microsites with vegetation, their survival was significantly higher (40–57 %). Neighbouring plants slightly reduced the survival of Plantago lanceolata and Sanguisorba minor, but these species established well both in gaps (74 - 81 %) and in vegetated microsites (54 - 67 %). Medicago lupulina established well in all microsites in one year (71 - 79 %), but poorly in the next year (18–32 %). Linum catharticum emerged poorly in one year and was completely extinguished by a fungal pathogen the following year. The overall conclusion is, that gaps are of minor importance for recruitment of these species in this grassland. Physical hazards and pathogens control seedling establishment to a greater extent than competition by neighbouring plants. Some species are hardly able to establish without shelter of vegetation. Seed size is an important factor for success of establishment, especially in gaps. It is suggested, that the relatively low productivity and the absence of litter accumulation (due to mowing and biomass removal) are important conditions for the observed behaviour of the seedlings and juvenile plants in this community.     

Roles of cottony hairs in directed seed dispersal in riparian willows

Willows usually establish on wet substrates with fine sediments at sites that are created by large disturbances, but suitable microsites are spatially and temporally limited. Thus, we hypothesized that willow seeds are selectively dispersed to suitable microsites, such as those with a wet substrate, rather than unsuitable microsites, such as those with a dry substrate, with seedling establishment mediated by the cottony hairs attached to seeds (directed dispersal). To test our hypothesis, we compared several recruitment-related traits, including buoyancy, germination, and trapping at favorable microsites, in seeds of the riparian willows Salix sachalinensis and S. integra with and without cottony hairs in laboratory and field experiments. In both field and laboratory experiments, more seeds with cottony hairs were trapped in water and wet sand than in dry sand, in which no seeds of either species germinated. These results indicate that cottony hairs facilitate the recruitment of seeds to microsites favorable for seed germination and help seeds avoid unfavorable microsites. On the water surface, 17.6% of S. sachalinensis seeds and 68.0% S. integra seeds with cottony hairs floated for more than 6 days, whereas all seeds without cottony hairs sank immediately after being placed on the water surface. These results suggest that cottony hairs facilitate long-distance dispersal via flowing water and also help avoid germination under water, where willow seedlings fail to establish. Seeds of the two willow species were released from the cottony hairs and germinated immediately after the seeds were placed on wet sand, but not after placement on water or dry sand. These results suggest that the seeds are released from the cottony hairs when the hairs become wet and the seeds are striking to a suitable microsite for seedling establishment, such as wet sand. In riparian willows, the cottony hairs promote directed dispersal by moving seeds to discrete and predictable microsites where the seedling establishment is disproportionately high.     

Seedling emergence and survival in contrasting soil microsites in Patagonian Monte shrubland

Abstract. The potential of two perennial species (Larrea divaricata and Stipa tenuis) to colonize different soil microsites was analyzed in the Patagonian Monte shrubland. We hypothesize that the short-lived grass S. tenuis is more able to colonize the soil of microsites beneath vegetation patches where N-fertility is higher than those in bare soil, while the long-lived shrub L. divaricata colonizes different soil microsites irrespective of their N fertility. A greenhouse experiment was carried out to evaluate the emergence and survival of both species in different soil microsites at different water, inorganic N and seed densities. In all cases soil microsites were seed limited since the addition of viable seeds increased seedling emergence. Both species showed, however, different abilities to emerge and survive in different soil microsites. Microsites of bare soil were more favourable for seedling emergence and survival of L. divaricata than those beneath vegetation patches, independent of their water status. This ability of L. divaricata can not be explained on the basis of increased water or N availability, but probably because of lower salt content of bare soil. The addition of inorganic N reduced the survival of L. divaricata in both microsites but increased individual plant performance. The emergence and survival of S. tenuis was not different in both types of soil microsites but the addition of inorganic N increased seedling emergence and plant biomass. According to these results, emergence and plant performance of S. tenuis may be promoted during humid years by increased concentration of inorganic N. Since N mineralization occurs at a higher rate in soil microsites beneath vegetation patches than in those of bare soil, higher plant performance and probably establishment of S. tenuis is to be expected. These results are consistent with an existing conceptual model of plant dynamics under various grazing intensities in the Patagonian Monte shrubland based on previous field observations.     

Seedling recruitment of Succisella inflexa in fen meadows: Importance of seed and microsite availability

Abstract. The effects of different forms of land use on germination and establishment of the rare fen species Succisella inflexa were investigated in seed introduction experiments in a mown and an abandoned fen meadow in SE Germany. Treatments included abandonment, mowing in fall and mowing with creation of gaps in the moss and litter layer. Floating capacity of seeds was tested in order to estimate potential dispersal by water. On the mown meadow, gaps had a slightly positive effect on germination rates, while greatly increasing seedling survival until the next spring. At the abandoned site, litter inhibited germination, whereas mosses had a negative effect on germination and a positive effect on survival rates during the first year after germination. Both germination and seedling establishment were negatively affected by the presence of slug herbivores. On the abandoned site, no seedlings at all survived until the next spring. Even though seeds of Succisella inflexa were capable to float for several weeks and to germinate thereafter, the situation at the field sites indicates that longdistance dispersal is highly unlikely. Our results showed that not only direct effects of abandonment, such as accumulation of litter, may have led to poor germination and poor seedling establishment of the species. Additionally, indirect consequences of changes in land use, such as higher seedling herbivory by slugs and successional vegetation changes due to abandonment, were important in determining habitat quality and availability of microsites for seedling recruitment. Furthermore, early mowing imposed seed limitation on plant populations.     

Diversity of epicotyl dormancy among tropical montane forest species in Sri Lanka

• Fruiting season of many Sri Lankan tropical montane species is not synchronised and may not occur when conditions are favourable for seedling establishment. We hypothesised that species with different fruiting seasons have different seed dormancy mechanisms to synchronise timing of germination with a favourable season for establishment. Using six species with different fruiting seasons, we tested this hypothesis.
• Germination and imbibition of intact and manually scarified seeds were studied. Effect of GA₃ on germination was examined. Embryo length:seed length (E:S) ratio of freshly matured seeds and of those with a split seed coat was determined. Time taken for radicle and plumule emergence and morphological changes of the embryos were recorded.
• The radicle emerged from Ardisia missionis, Bheza nitidissima and Gaetnera walkeri seeds within 30 days, whereas it took >30 days in other species. Embryos grew in seeds of B. nitidissima and G. walkeri prior to radicle emergence but not in Microtropis wallichiana, Nothapodytes nimmoniana and Symplocos cochinchinensis. A considerable delay was observed between radicle and plumule emergence in all six species. Warm stratification and/or GA₃ promoted germination of all species.
• All the tested species have epicotyl dormancy. Seeds of B. nitidissima and G. walkeri have non‐deep simple morphophysiological epicotyl dormancy, and the other four species have non‐deep physiological epicotyl dormancy. Differences in radicle and epicotyl dormancy promote synchronisation of germination to a favourable time for seedling development. Therefore, information on dormancy‐breaking and germination requirements of both radicle and epicotyl are needed to determine the kind of dormancy of a particular species.

     

Allometric relationships between seed mass and seedling characteristics reveal trade-offs for neotropical gap-dependent species

A seed size–seed number trade-off exists because smaller seeds are produced in greater number but have a lower probability of establishment. This reduced establishment success of smaller-seeded species may be determined by biophysical constraints imposed by scaling rules. Root and shoot diameter, root growth extension rate (R _GER) and shoot length at death for dark-grown seedlings are predicted to scale with the cube root of seed embryo and endosperm mass (m). We confirmed this expectation for ten neotropical gap-dependent tree species with an embryo and endosperm dry mass >1 mg. However, for nine smaller seeded species (m < 1 mg) with photoblastic germination, root and shoot diameters were larger than expected, and consequently, R _GER was slower than expected. The maximum shoot thrust of seedlings from seeds with masses ≥1 mg was comparable to the estimated force required to displace overlying litter, supporting the hypothesis that photoblastic behaviour only occurs in seeds with insufficient shoot thrust to displace overlying leaves. Using the model soil water, energy and transpiration to predict soil drying in small and large gaps, we showed that: (1) gaps that receive a significant amount of direct sunlight will dry more quickly than small gaps that do not, (2) compared to the wet-season, soil that is already dry at depth (i.e. the dry-season) will dry faster after rainfall (this drying would most likely kill seedlings from small seeds) and (3) even during the wet-season, dry periods of a few days in large gaps can kill shallow-rooted seedlings. We conclude that the smaller the seed, the more vulnerable its seedling would be to both covering by litter and soil drying because it can only emerge from shallow depths and has a slow R _GER. Consequently, we suggest that these allometrically related factors contribute to the reduced establishment success of smaller-seeded species that underpins the seed size–seed number trade-off.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Effects of grazing intensity on growth,reproduction, and abundance of three palatable forbs in Kansas tallgrass prairie

The effects of different intensities of cattle grazing on theaboveground growth, reproduction, and abundances of three palatable forbs werestudied in native tallgrass prairie. Populations of Asterericoides, Ruellia humilis, andAmorpha canescens were sampled at peak flowering duringthe1993–1995 growing seasons in four annually-burned sites varying incattle stocking density [ungrazed, low, moderate, high]. The threeforbs exhibited reduced shoot growth and/or reproduction under moderate toheavy grazing, and in no case did grazing increase any measure of plantperformance. Ruellia showed reduced shoot height andbiomass, percentage of stems flowering, and reproductive biomass in response tograzing. Aster showed decreases in shoot biomass andheightwith grazing. Amorpha showed no change in shoot orreproductive biomass, but a decrease in percent of flowering stems and inreproductive allocation with grazing. Patterns in the percentage of stemsgrazedindicated generally high but variable palatability among these species. Bycontrast, the three species showed inconsistent population response to grazing.Abundance (frequency) of all three species indicated no short-term changebetween years in response to grazing intensity. Responses of these speciesdiffer considerably from those of most other perennial tallgrass prairie forbsthat are unpalatable, unconsumed, and increase in performance (e.g. size,abundance) due to release from competition from the dominant grasses underungulate grazing. The results demonstrate that immediate aboveground growth andreproductive responses of established adults to grazing are not good predictorsof grazer effects on population abundances in tallgrass prairie.     

Seed Bank Effects on Recovery after Disturbance in Reconstructed Tallgrass Prairies

Litter‐removing disturbances such as fire in grasslands temporarily increase available resources for plants, opening a window of opportunity for new establishment as communities recover. At this time, new individuals or species could be added to the community as a result of germination from the local seed bank. In reconstructed grasslands this may be problematic, as the seed bank may contain a suite of undesired species reflective of prior and surrounding land uses. In two, 25‐year‐old, low‐diversity reconstructed grasslands, we tested the effect of local seed bank establishment following litter‐removing disturbance using seedling removal plots (1 m²) and plots where natural seedling establishment was allowed. Following disturbance, the vegetation was either left intact or hayed to enhance seedling establishment (a common practice following inter‐seeding efforts). Although the seed bank and seedling community were dominated by resident grasses (Andropogon gerardii and Poa pratensis), recruitment from the seed bank increased species richness and reduced evenness through the addition of forb species (including Cirsium arvense) in one of the study sites. Haying temporarily altered the abundances of the dominant grasses, but did not consistently affect seedling recruitment. Disturbances that facilitate seed bank recruitment may promote establishment of undesired species within reconstructed grassland communities, and we need to take steps to better manage the contributions into and recruitment from the seed bank to reconstruct sustainable grasslands.