Moisture conditions and the presence of bryophytes determine fescue species abundance in a dry calcareous grassland

Festuca ovina is the abundant matrix-forming species and F. rubra a subordinate species in shallow-soil calcareous grasslands. F. pratensis is a transient species, occurring sparsely in this community. We hypothesised that the different abundances of these three species are primarily due to the differential effect of moisture conditions on their germination and early establishment, and that the effect of the pattern of rainfall intensity depends on the presence or absence of a bryophyte layer. We studied the dependence of the germination and establishment of the three fescue species on the moisture conditions both in the laboratory and in the patches of intact grassland community (microcosms). In a laboratory germination experiment, F. pratensis showed the highest, F. rubra , the intermediate and F. ovina, the lowest drought tolerance. In microcosms, the establishment of F. ovina was the highest. At the same time, the annual mortality of seedlings of F. ovina was the lowest. All three species responded positively to an increasing irrigation level. Differently from F. ovina, F. rubra showed a positive response only in plots from which the bryophyte layer had been removed, while F. pratensis responded positively to both irrigation and bryophyte removal. We conclude that moisture conditions have a differential effect on the three fescue species mainly in the seedling establishment, not in the germination phase. For the successful establishment of F. rubra and F. pratensis, the coincidence of high rainfall and local disturbance, removing bryophytes, is required. The presence or absence of bryophytes had no effect on establishment in dry years, while in rainy years the removal of bryophytes has a clear positive effect.     

Festuca campestris density and defoliation regulate abundance of the rhizomatous grass Poa pratensis in a fallow field

Invasion by the rhizomatous grass Kentucky bluegrass (Poa pratensis) is a global phenomenon, including into foothills rough fescue (Festuca campestris) grasslands of southwestern Alberta, Canada. In order to better understand the competitive relationships between these species, we conducted a fallow field study where rough fescue bunchgrass tussocks were transplanted at one of three planting densities (15, 30, or 45 cm spacing), and then subject to various treatments in a factorial design, including one‐time intensive summer defoliation and seeding of bluegrass into adjacent bare soil. Rough fescue plants exhibited marked intraspecific competition, as high planting densities increased tussock mortality, while decreasing plant tiller counts and relative inflorescence production, together with plant and tiller‐specific mass. However, high densities of the bunchgrass also reduced the cover and biomass of encroaching bluegrass, coincidental with reduced resource (soil moisture and light) availability in mid‐summer. Although summer defoliation increased rough fescue tiller counts, this disturbance reduced plant and tiller mass, and also increased Kentucky bluegrass. We conclude that while high densities of nondefoliated stands of rough fescue may increase resistance to bluegrass encroachment, a reduction in either fescue plant density or vigor via defoliation can increase the risk of bluegrass invasion within northern temperate grassland.     

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.     

Using structural equation modelling to test the passenger,driver and opportunist concepts in a Poa pratensis invasion

The passenger, driver, and opportunist models are conceptual models of the invasion process used to describe alternative invasion scenarios. In the passenger model, both the invasive species and native community respond independently to environmental changes. In the driver model, changes to the native community are driven by the invasive species, while in the opportunist model invasion occurs in response to changes in the native community. In any given invasion scenario, however, it is possible that the relationships between the invasive, the native community, and the environment correspond to some combination of these invasion models acting simultaneously. We study invasion by Poa pratensis in a grassland in Alberta, Canada. Poa pratensis is a non‐native plant implicated with loss of plant diversity in the region. In a three year field experiment, we manipulate the environment though defoliation, water addition, and nitrogen addition, and measure responses of P. pratensis cover, and cover and richness of the native community. We use structural equation modelling to describe the relationships between the invasive, the native community, and the environmental changes, and then interpret these relationships using the three invasion models. We found that P. pratensis predominantly invaded via the driver model, with subsequent reductions in native plant cover, but not in species richness. Positive effects of the environmental changes on P. pratensis also aided its ability to drive native cover. As well, we found some involvement of the opportunist model, through a negative relationship between the native community and the invasive. As invasion mainly proceeded via the driver model, management actions to limit invasion should focus on efforts to control abundance of P. pratensis itself.     

Flooding and grazing promote germination and seedling establishment in the perennial grass Paspalum dilatatum

Seed germination and seedling emergence are key processes for population recruitment. Flooding and grazing are disturbances forming gaps that may strongly influence recruitment patterns in space and time, but their combined effects and action mechanisms have rarely been addressed. In this study we analysed the effects of microhabitat conditions associated with winter flooding and spring‐summer defoliation on seed germination and seedling establishment of Paspalum dilatatum, a dominant perennial C₄ grass in native grasslands of the Flooding Pampa, Argentina. The dynamics of seedling emergence from natural seed banks and buried seeds was studied in a factorial experiment with flooding and defoliation treatments applied to soil monoliths (mesocosms) collected from natural grassland. Additional laboratory experiments were applied to investigate seed germination under different combinations of temperature, light quality and simulated flooding. Seed germination and seedling emergence of P. dilatatum were promoted by flooding and high intensity defoliation. Gaps generated by flooding were maintained by high intensity defoliation exercising a synergistic effect on survival seedlings. Flooding resulted in the breaking of seed dormancy and higher germination rates associated with alternating temperature and the activation of the phytochrome system. Our results indicate that microhabitat conditions associated with the disturbances forming gaps, such as flooding and heavy grazing, synergistically promote the recruitment process of this dominant grass species.     

Factors Affecting Seed Germination and Seedling Establishment of Fen-Meadow Species

Availability of seeds and provision of “safe sites” for seedling recruitment are essential for successful restoration of seminatural grassland communities. Inability to provide species‐specific conditions for seedling recruitment appears to be a major factor limiting establishment of fen‐meadow species on restoration sites. This contention was tested in the field for both germination and establishment conditions for a selection of fen‐meadow species. A Cirsio‐Molinietum fen meadow and an agriculturally semi‐improved species‐poor grass dominated rush pasture were used. Seeds of Carex ovalis, Cirsium dissectum, Molinia caerulea, Succisa pratensis, and Holcus lanatus were sown onto treatments comprising either irrigation or no irrigation, presence or absence of existing vegetation canopy, and presence or absence of soil disturbance. Germination of all except H. lanatus was higher in the fen meadow than in the rush pasture. The fen‐meadow site was less susceptible to drought, provided more light to the seed environment, and showed a stronger day–night variation in relative humidity compared with the rush pasture. All the fen‐meadow species responded strongly to the experimental treatments, whereas H. lanatus showed only a small response. Soil disturbance was the major factor that increased germination. Removal of the vegetation canopy improved germination only in S. pratensis. Conditions affecting survival of seedlings were different from those affecting seed germination. Seedling survival was greater on the fen‐meadow site than on the rush pasture. Canopy presence was the major factor that reduced seedling survival. Few seedlings survived in the presence of the rush pasture canopy. Irrigation and soil disturbance were of minor importance for seedling survival on both sites. Safe sites for seed germination and seedling establishment of fen‐meadow species existed on the fen meadow even without soil disturbance and gap creation. Safe sites for seedling recruitment were not present in the rush pasture. The need for species‐specific definition of safe site characteristics at the two stages of seedling recruitment (i.e., for seed germination and for seedling survival) was demonstrated. The implications of these findings for restoration of seminatural grasslands are discussed.     

Mycorrhiza and seedling establishment in a subarctic meadow: Effects of fertilization and defoliation

Abstract. Question: How does changing resource availability induced by fertilization and defoliation affect seedling establishment and mycorrhizal symbiosis in a subarctic meadow? Location: 610 m a.s.l., Kilpisjärvi (69°03’N, 20°50’E), Finland. Methods: A short‐term full‐factorial experiment was established, with fertilization and defoliation of natural established vegetation as treatments. Seeds of two perennial herbs Solidago virgaurea and Gnaphalium norvegicum were sown in natural vegetation and their germination and growth followed. At the final harvest we measured the response in terms of arbuscular mycorrhizal (AM) colonization, biomass and nitrogen concentration of the seedlings and the established vegetation. Results: Germination rate was negatively affected by defoliation in the unfertilized plots. The shoot biomass of S. virgaurea seedlings was reduced by the defoliation and fertilization treatments, but not affected by their interaction. In G. norvegicum, the germination rate and the seedling shoot biomass were negatively correlated with moss biomass in the plots. In the established plants the arbuscular colonization rate was low and defoliation and fertilization treatments either increased or did not affect the colonization by AM fungi. In the seedlings, the colonization rate by AM fungi was high, but it was not affected by treatments. Both seedlings and established plants were colonized by dark septate fungi. Conclusions: Reduction of plant biomass by herbivores can have different effects on seedling growth in areas of high and low soil nutrient availability. The weak response of AM colonization to defoliation and fertilization suggests that AM symbiosis is not affected by altering plant resource availability under the conditions employed in this study.     

Effects of elevated CO2 on development and larval food-plant preference in the butterfly Coenonympha pamphilus (Lepidoptera, Satyridae)

The objective of this study was to determine how increasing atmospheric CO₂ change plant tissue quality in four native grassland grass species (Agrostis stolonifera, Anthoxanthum odoratum, Festuca rubra, Poa pratensis) which are all larval food‐plants of Coenonympha pamphilus (Lepidoptera, Satyridae). We assessed the effect of these changes on the performance and larval food‐plant preference of C. pamphilus in a greenhouse experiment. Furthermore, we tested the interactive effects of elevated CO₂ and soil nutritional availability in F. rubra and its effect an larval development of C. pamphilus. In general, elevated CO₂ decreased leaf water concentration, nitrogen concentration and specific leaf area (SLA), while leaf starch concentration was increased in all grass species. A species‐specific reaction to elevated CO₂ was only found for foliar starch concentration. P. pratensis did not increase its starch concentration under elevated CO₂ conditions, whereas the other three species did. Fertilisation, investigated only for F. rubra, increased leaf nitrogen concentration and amplified the CO₂‐induced decrease in leaf nitrogen. Development time of C. pamphilus was on the average prolonged by two days under elevated CO₂ and the prolongation differed from 0.7 to 5.3 days among food‐plant species. Pupal fresh weight differed marginally between CO₂ treatments. Fertilisation of the larval food‐plant F. rubra shortened development time by one day and significantly increased pupal and adult fresh weights. C. pamphilus larvae showed a clear food‐plant preference among grass species at the age of 36 h or older. Additionally, a change of food‐plant preference under elevated CO₂ was found. Larvae at ambient CO₂ preferred Agrostis stolonifera and F. rubra, while under elevated CO₂Anthoxanthum odoratum and P. pratensis were preferred. The present study demonstrates that larval development of C. pamphilus is affected by food‐plant species and CO₂ induced changes in foliar chemistry. Although we found some species‐specific reactions to elevated CO₂ for foliar chemistry, no such CO₂ by species interaction was found for insect development. The change in food‐plant preference of larvae under elevated CO₂ implies potential changes in selection pressure for grass species and might therefore affect evolutionary processes.     

Pinus monophylla establishment in an expanding Pinus‐Juniperus woodland: Environmental conditions,facilitation and interacting factors

Abstract. The tree species comprising Pinus‐Juniperus woodlands are rapidly expanding into shrub‐grasslands throughout their range. Observational studies indicate that establishment is facilitated by nurse plants, but little information exists on the mechanisms involved. I examined both abiotic and biotic factors influencing Pinus monophylla establishment in Artemisia tridentata steppe with expanding populations of P. monophylla and Juniperus osteosperma. I determined soil water contents, temperatures, and nutrient characteristics for the primary establishment microhabitats, i.e. under Pinus, under Juniperus, under Artemisia, tree interspace and sage interspace, and evaluated the emergence and survival response of two seedling cohorts over a 3‐yr period for the different microhabitats. I also examined the effects of seed burial and predation on seedling establishment. Microhabitats under trees and shrubs had higher extractable P and K, higher organic matter, total nitrogen and cation exchange capacity than interspace microhabitats. Soil water contents (0–15 cm) were lower in interspaces than under shrubs or trees due to dry surface (0–5 cm) soils. Soil temperatures (at 1 and 15 cm) were lowest under trees, intermediate under shrubs, and highest in interspaces. Timing and rate of seedling emergence were temperature dependent with the order of emergence paralleling mean growing season temperatures: tree interspace = shrub interspace > under shrub > under Juniperus under Pinus. Seed burial was required for rooting and the highest emergence occurred from depths of 1 and 3 cm indicating that caching by birds and rodents is essential and that animals bury seeds at adequate if not optimal depths for emergence. Seedlings required microenvironmental modification for survival; all seedlings, including those that emerged from seeds and transplants, died within the first year in interspace microhabitats. Survival in under‐tree or under‐shrub microhabitats depended on soil water availability and corresponded closely to soil water contents over the 3‐yr study. Under‐shrub microhabitats had more favourable soil and micro‐environmental characteristics than under‐tree microhabitats and had the highest seedling life spans for the first‐year seedling cohort. Predation of Pinus seedlings by rodents was a significant cause of mortality with caged transplants exhibiting life spans that were 74% longer overall than uncaged transplants. Emergence and survival of P. monophylla within the expanding woodland were dependent upon a complex set of interacting factors including growing season conditions, microhabitat characteristics, and animal species.     

Bottlenecks and spatiotemporal variation in the sexual reproduction pathway of perennial meadow plants

Sexual reproduction is important for the growth of populations and the maintenance of genetic diversity. Several steps are involved in the sexual reproduction pathway of plants: the production of flowers, the production of seeds and the establishment of seedlings from seeds. In this paper we quantify the relative importance and spatiotemporal variability of these different steps for four grassland perennials: Centaurea jacea, Cirsium dissectum, Hypochaeris radicata and Succisa pratensis. We compared undisturbed meadows with meadows where the top soil layer had been removed as a restoration measure. Data on the number of flower heads per flowering rosette, the numbers of flowers and seeds per flower head, and the seedling establishment probabilities per seed were collected by field observations and experiments in several sites and years. Combination of these data shows that H. radicata and S. pratensis have higher recruitment rates (1.9 and 3.3 seedlings per year per flowering rosette, respectively) than the more clonal C. dissectum and C. jacea (0.027 and 0.23, respectively). Seedling establishment is the major bottleneck for successful sexual reproduction in all species. Large losses also occurred due to failing seed set in C. dissectum. Comparison of the coefficients of variation per step in space and time revealed that spatiotemporal variability was largest in seedling establishment, followed closely by flower head production and seed set.     

Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var.glandulosa) on sites differing in herbaceous biomass and grazing history

Summary Emergence and survival of honey mesquite (Prosopis glandulosa var.glandulosa Torr.) seedlings was quantified on sites with contrasting grazing histories: long-term continuous grazing (LTG) and long-term protection (LTP) from grazing by cattle. On each site, different levels of heroaceous defoliation were imposed at monthly intervals (no defoliation=ND, moderate=MD and heavy=HD). The two weeks following seed dissemination appeared to be the most critical toProsopis establishment on LTP-ND plots. Openings in the herbaceous layer created by moderate defoliation of grasses on the LTP site increased germination and/or survival 7-to 8-fold during this period. However, increasing the degree of defoliation from moderate to heavy did not stimulate additional emergence on either the LTP or LTG site. Emergence from scarified seed placed in cattle dung (17 to 30%) was lower than that of bare seed placements in various microhabitats (43–60%). However, deposition of scarifiedProsopis seed in dung in conjunction with graminoid defoliation may be the most likely combination of events when livestock are present. Emergence from seeds transported into grasslands by other fauna likely would be low, unless seeds were deposited in areas where grasses had been defoliated.Prosopis survival was comparably high in dung and bare seed placements after one growing season. survival of seedlings present two weeks after seed dissemination ranged from 74 to 97% at the end of the second growing season. Seedling survival and shoot development (biomass, leaf area and height) were similar on LTP and LTG sites, regardless of the level of herbaceous defoliation or seed placement. In addition, the magnitude and patterns of net photosynthesis, stomatal conductance and xylem water potential were comparable among one-year-old seedtings on ND, MD and HD plots, even though differences in herbaceous species composition and above- and below-ground biomass between these treatments were substantial. Such data suggest competition for soil resources between grasses andProsopis may be minimal early in the life cycle ofProsopis. High rates ofProsopis emergence and establishment on LTP-MD plots are counter to the widespread assumption that long-term and/or heavy grazing is requisite forProsopis encroachment into grasslands. Results are discussed with regard to factors contributing to the recent, widespread invasion of this woody legume into grasslands of southwestern North America.Abbreviations LTG long-term grazed - LTP long-term protected from grazing - ND non-defoliated - MD moderate defoliation - HD heavy defoliation     

Phylogeographical history of the widespread meadow fescue (Festuca pratensis Huds.) inferred from chloroplast DNA sequences

Aim Here we explore the variation in chloroplast DNA (cpDNA) in a widespread Eurasian diploid forage grass, meadow fescue (Festuca pratensis Huds.), to address its phylogeographical history. In particular, we aim to answer whether the post‐glacial migration routes of meadow fescue are associated with the spread of agriculture or concurrent with well‐documented natural migration pathways from glacial refugia. Location A total of 56 Eurasian accessions of F. pratensis were analysed, representing the entire native distribution area as well as non‐native areas in northernmost Europe. Methods Based on initial sequencing of 10 non‐coding cpDNA regions, three regions were sequenced for all F. pratensis accessions. For reference, three closely related species [the diploid Lolium perenne L. and the polyploids Festuca arundinacea Schreb. and Festuca gigantea (L.) Vill.] were also sequenced, as well as the more distantly related Festuca ovina L. Divergence times were estimated assuming a simple molecular clock, calibrated using a previously published estimate of 9 Myr for the divergence between fine‐leaved (F. ovina) and broad‐leaved fescues (F. pratensis, F. arundinacea and F. gigantea). Results Limited, but geographically structured, cpDNA variation was observed in F. pratensis. Three haplotypes, estimated to have diverged 0.16 Ma, were identified: one western European (A), one with a wide eastern distribution from central‐eastern Europe into Asia (B) and one Caucasian (C). The haplotypes of the polyploids and L. perenne were estimated to have diverged from haplotype A in F. pratensis 0.8–1.3 Ma. Main conclusions We found no definite evidence for migration of the diploid F. pratensis associated with the spread of agriculture from the Fertile Crescent after the last glaciation. The distinct geographical structuring of the present‐day variation in cpDNA can rather be explained by northwards expansion of the western haplotype from an Iberian refugium, expansion of the eastern haplotype from an unlocated (south‐)eastern refugium and glacial survival without subsequent expansion from a Caucasian refugium. The high level of cpDNA divergence observed between this diploid and the polyploids which have probably been derived from it may suggest that the very low level of cpDNA variation in the diploid is caused by a recent bottleneck. Today, F. pratensis is widespread in the open agricultural landscape but appears otherwise confined to naturally open habitats such as river banks, and its populations may have been decimated when dense forests dominated in the previous interglacial.     

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.     

Time of germination and establishment success after fire of three obligate seeders in a Mediterranean shrubland of central Spain

Aim Determining how differences in time of germination can affect plant establishment in plant communities that, after a disturbance, must reestablish from seeds under climatic conditions subject to extremes, such as the Mediterranean. Although early germination may be beneficial for survival in summer, when drought is severe, this may expose the seedlings to winter extremes, thus to higher mortality. Understanding how sensitive is the establishment of different species to temporal patterns of germination will help to understand the factors that control species distribution and community stability in disturbance‐prone environments, as well as its sensitivity to changes in weather patterns as climate changes. Methods An experimental fire was made in early fall in an old Cistus–Erica shrubland in Toledo (central Spain). After fire, germination, survival and growth of the three dominant seeder species (Cistus ladanifer, Erica umbellata and Rosmarinus officinalis) were monitored during the first 3 years after fire. Seedlings were tagged to identify their time of emergence, and divided into cohorts according to their month of germination. Differences in survival of the various cohorts were evaluated by means of a Wilcoxon (Gehan) statistic. Height of surviving, tagged plants was compared among cohorts by means of a Kolmogorov–Smirnov test. Results The year following fire was one of the driest on record, while the next one was one of the wettest. Germination was more abundant during the first than during the second year. Establishment was mainly from first‐year germination, as the majority of second‐year germinated seedlings died. Temporal patterns of germination within a year and between years varied between species. Seedling mortality was highest immediately following germination, not in summer. Mortality was related to time of germination: during a given period of time, the mortality of younger seedlings was higher than that of older ones. However, survival was not highest for the first cohorts. In general, the earlier the seedlings germinated the more vigourous they became, more clearly so for Cistus than for Rosmarinus, but differences tended to disappear with time. Overall, time of germination varied between species and affected differently seedling survival and vigour of the various species. Rosmarinus and Cistus had sufficient survivors to reestablish the initial population. Erica, despite abundant germination, suffered a strong population reduction. Main conclusions Mediterranean shrub species differ in their temporal patterns of germination and survival after fire. The effect of time of germination is complex: germinating early is advantageous since old seedlings fared better than younger ones when confronted with the same rigours. However, germinating early might expose the seedlings to greater hazards and the first cohort might not survive best. The temporal window for establishment is narrow and mainly restricted to the first year after fire. Second year seedlings, irrespective of most favourable conditions, survived very little. Missing the window of establishment might virtually lead to a population collapse, despite having very high germination, as found for Erica.     

Restoring montane cloud forest: establishment of three Fagaceae species in the old fields of central Veracruz,Mexico

Tropical montane cloud forest is rapidly disappearing and our knowledge of how to restore this system is limited. In a cloud forest of central Veracruz, Mexico, we studied seedling survival, growth, and causes of mortality (microenvironment and herbivory) of three native tree species, Fagus grandifolia, Quercus germana, and Q. xalapensis, transplanted into abandoned pastures (<1 year old) and secondary forests (9–17 years old). Microenvironment differed between the two habitats, temperature and photosynthetically active radiation were higher, and humidity was lower in the abandoned pastures than in the secondary forests. Seedling survival was greater in secondary forests than in pastures: F. grandifolia, 94 and 64%; Q. germana, 88 and 68%; and Q. xalapensis, 61 and 57%, respectively. The cause of mortality differed between habitats, with gophers (24.2% mortality) and mice (4.8%) killing the most seedlings in pastures, and damping‐off (16%) was the most important cause in secondary forests. The relative growth rate in height and basal area was significantly higher in abandoned pastures than in secondary forests; Q. xalapensis had the highest growth rate, followed by Q. germana and F. grandifolia. The environmental conditions in this mountainous cloud forest region seem less stressful to planted seedlings than the conditions of other lowland systems, as frequent clouds favor their establishment even in open sites. We conclude that Fagaceae species can successfully establish in abandoned pastures in mesic environments. Thus, the species studied can be used to speed cloud forest regeneration in the same area at different successional stages.     

Photosynthetic performance of invasive Pinus ponderosa and Juniperus virginiana seedlings under gradual soil water depletion

Changes in climate, land management and fire regime have contributed to woody species expansion into grasslands and savannas worldwide. In the USA, Pinus ponderosa P.&C. Lawson and Juniperus virginiana L. are expanding into semiarid grasslands of Nebraska and other regions of the Great Plains. We examined P. ponderosa and J. virginiana seedling response to soil water content, one of the most important limiting factors in semiarid grasslands, to provide insight into their success in the region. Photosynthesis, stomatal conductance, maximum photochemical efficiency of PSII, maximum carboxylation velocity, maximum rate of electron transport, stomatal limitation to photosynthesis, water potential, root‐to‐shoot ratio, and needle nitrogen content were followed under gradual soil water depletion for 40 days. J. virginiana maintained lower L_s, higher A, g_s, and initial F_v/F_m, and displayed a more gradual decline in V_cmax and J_max with increasing water deficit compared to P. ponderosa. J. virginiana also invested more in roots relative to shoots compared to P. ponderosa. F_v/F_m showed high PSII resistance to dehydration in both species. Photoinhibition was observed at ～30% of field capacity. Soil water content was a better predictor of A and g_s than Ψ, indicating that there are other growth factors controlling physiological processes under increased water stress. The two species followed different strategies to succeed in semiarid grasslands. P. ponderosa seedlings behaved like a drought‐avoidant species with strong stomatal control, while J. virginiana was more of a drought‐tolerant species, maintaining physiological activity at lower soil water content. Differences between the studied species and the ecological implications are discussed.     

Differential effect of drought regimes on the seedling performance of six floodplain grassland species

• The performance of seedlings is crucial for the survival and persistence of plant populations. Although drought frequently occurs in floodplains and can cause seedling mortality, studies on the effects of drought on seedlings of floodplain grasslands are scarce. We tested the hypotheses that drought reduces aboveground biomass, total biomass, plant height, number of leaves, leaf area and specific leaf area (SLA), and increases root biomass and root‐mass fraction (RMF) and that seedlings from species of wet floodplain grasslands are more affected by drought than species of dry grasslands.
• In a greenhouse study, we exposed seedlings of three confamilial pairs of species (Pimpinella saxifraga, Selinum carvifolia, Veronica teucrium, Veronica maritima, Sanguisorba minor, Sanguisorba officinalis) to increasing drought treatments. Within each plant family, one species is characteristic of wet and one of dry floodplain grasslands, confamilial in order to avoid phylogenetic bias of the results.
• In accordance with our hypotheses, drought conditions reduced aboveground biomass, total biomass, plant height, number of leaves and leaf area. Contrary to our hypotheses, drought conditions increased SLA and decreased root biomass and RMF of seedlings. Beyond the effects of the families, the results were species‐specific (V. maritima being the most sensitive species) and habitat‐specific. Species indicative of wet floodplain grasslands appear to be more sensitive to drought than species indicative of dry grasslands.
• Because of species‐ and habitat‐specific responses to reduced water availability, future drought periods due to climate change may severely affect some species from dry and wet habitats, while others may be unaffected.

     

Dry matter allocation and nitrogen productivity explain growth responses to photoperiod and temperature in forage grasses

The mechanisms responsible for fluctuations in species composition of semi-natural grassland are not well understood. To identify plant traits that determine the poor competitive ability of Festuca pratensis compared to Dactylis glomerata especially during summer, the growth of both grasses was monitored over time and at different temperatures and photoperiods. Plants of both grasses were grown from seed with non-limiting nutrient supply at three day/night temperatures (11/6, 18/13 and 25/20°C) and two photoperiods (16 and 12 h). F. pratensis had a significantly lower relative growth rate than D. glomerata, mainly due to its lower specific leaf area and reduced nitrogen productivity. At high temperature, F. pratensis had a considerably lower root weight ratio than D. glomerata leading to substantially slower root growth. F. pratensis responded to a shorter photoperiod with an increase in the net assimilation rate, whereas D. glomerata responded with an increase in specific leaf area. The low competitive ability of F. pratensis compared to D. glomerata was mainly associated with its lower specific leaf area and nitrogen productivity. The stronger decline of its competitive ability during summer was probably related to the decreased allocation of dry matter to the roots at higher temperatures which leads to slower root growth compared to D. glomerata. Received: 7 September 1998 / Accepted: 29 July 1999     

Supplement to the hepatic flora of the Shetland Islands

Background: Grasslands have only a few dominants and most of the diversity consists of subdominants. Because dominants differ widely in phenology and resource use, dominants may control the recruitment and establishment of other species.

Aims: To explore the relationship between the identity of the dominant species and successional vegetation changes in grassland communities.

Methods: The compositional change over 23 years in 1900 permanent plots dominated by four grasses (Andropogon gerardii, Elymus repens, Poa pratensis and Schizachyrium scoparium) was examined within 19 old fields in Minnesota. Fields were abandoned 1–56 years before sampling. Rate of directional change and degree of compositional dissimilarity were determined.

Results: Non-natives, P. pratensis and E. repens, were associated with either no or a slow directional change. Elymus repens was associated with high dissimilarity and P. pratensis with intermediate dissimilarity. Natives, A. gerardii and S. scoparium, were associated with compositional change that followed expectations based on field age. The rate of directional change and degree of dissimilarity between sampling intervals was lower for A. gerardii relative to S. scoparium, the only species to be associated with strong directional change.

Conclusions: Dominance by non-native grass species may impede traditional successional processes and result in a community composition quite dissimilar from native prairies.     

Aggregated recruitment patterns under adult crowns in Photinia glabra,a bird‐dispersed tree species

To clarify recruitment patterns of Photinia glabra, which is an evergreen, broad‐leaved, bird‐dispersed tree species, we analyzed spatial distribution in P. glabra recruits at each growth stage and demography of current‐year seedlings with respect to distributions of adults in a warm‐temperate secondary forest, western Japan. Although individuals ≥ 5 cm diameter at breast height (DBH) that had nearly produced fruits showed a random distribution, seedlings (≥ 1 year old, < 10‐cm stem length [SL]), small saplings (10 ≤ SL < 30 cm) and large saplings (≥ 30‐cm SL, < 5‐cm DBH) were clumped and associated with reproductive adults at approximately 2–3‐m scales, nearly equal to their average crown radius. Based on monitoring the demography of current‐year seedlings, emerged seedling density profoundly decreased, and no seedlings survived at longer than an adult's crown scales, with distance‐dependent mortality as a result of disease and herbivory not greatly affecting the current‐year seedling mortality. Thus, aggregated seed dispersal under the crown of adult P. glabra would directly influence the distribution of recruits for P. glabra in this forest. Of the bird‐dispersed tree species in this forest, P. glabra produced the highest amount of fruits during large crop years, and their fruits ripened during the late seasonal period (early January), suggesting that birds might be strongly attracted to these species, in turn leading to seeds being deposited mostly under the tree crowns. We propose that dispersal limitation would occur, even in a bird‐dispersed tree species such as P. glabra, owing to plant–bird interactions in the forest.