Limiting Seedling Establishment Research Articles

Clonal propagation, scale dependent assembly, and nucleation drives natural regeneration in a restinga sandy coastal plain

AbstractOn the sandy coastal plains, environmental filters that limit seedling establishment and survival create a naturally heterogeneous landscape, formed by vegetation patches inserted in a matrix of bare sandy soil. The succession processes in these ecosystems depend on some pioneer species that facilitate the recruitment and survival of other species. Here, we evaluated the spatial pattern and changes of vegetation patches of restinga (i.e., coastal sandy plain vegetation) over one decade (2011–2021). The research was carried out in Alagoas State, Northeast of Brazil. We considered as patches the vegetation islands inserted in a matrix of bare sandy soil and applied methodologies of photointerpretation. Landscape metrics included patch area, edge, shape, and core area. To test the effect of landscape metrics on species occurrence and species richness per patch, we applied generalized linear models (GLM) with binomial and quasi‐Poisson distributions, respectively. We used the “betapart” package to evaluate the importance of turnover and nestedness as drivers of beta diversity among patches. Our data indicate that a key plant (Myrciaria floribunda), which can propagate by root suckers, initiates a succession process, creating regeneration nuclei, which seem to follow a deterministic succession pattern until the patch reaches 10 m2 in area. In the successional sequence, the patches grow and coalesce with neighboring patches. Species richness of patches presented an unimodal response to patch size. Overall, there is a greater contribution of species turnover to among‐patch beta diversity. However, for patches smaller than 10 m2, beta diversity was mostly driven by nestedness, reflecting a process of species addition to the growing patches. The succession pattern found in the study area should be considered for the formulation of nature‐based restoration models, expected to be more efficient and effective for ecosystems with similar environmental filters.

Tree species size class patterns portend compositional shifts and low resilience in managed northern hardwood forests

AbstractDeclining temperate forest biodiversity is a global challenge to future sustainability and resilience. Canopy tree diversity has declined in northern hardwood forests (NHFs) of the Great Lakes region since European colonization, with current tree regeneration patterns suggesting this trend may continue. However, regional (106 ha) patterns and drivers are underexplored. We used data from 141 managed NHF stands, Michigan, USA, to examine diversity indices across tree size classes (seedling, sapling, and canopy); stand‐level drivers of regeneration (seedling and sapling) diversity and individual species density; and patterns among species in relative abundances across size classes. Diversity was similar across size classes (mean asymptotic species richness of 6–7 species and <3 effective common species); however, some species contributing to sapling diversity are unlikely to contribute to future canopy diversity (Fraxinus americana and Fagus grandifolia, insect/pathogen limited; Ostrya virginiana, small maximum size). For the 11 most common species, conspecific canopy density (+, seedlings, significant for 10 species) and stand basal area (−, saplings, 3 species) most consistently associated with density; deer use, site quality, and substrate were less predictive. Patterns of relative density by size class among 18 species present on >10% of sites were consistent with species life history strategy and/or establishment/recruitment barriers. For seven species (including Tsuga canadensis, Betula alleghaniensis), relative density was lowest in the seedling layer, suggesting seedling establishment substrate and/or early shade mortality limitations. For two species (Acer saccharum, Quercus rubra), reduced sapling relative density compared with seedling or canopy strata suggests a sapling recruitment bottleneck from deer browsing/shade, whereas six species (e.g., F. grandifolia, O. virginiana, Pinus strobus) had highest relative density for saplings, suggesting sapling to canopy recruitment limitations. Lastly, seedlings were relatively most dense for Acer rubrum and Prunus spp., indicating disproportionally high seedling establishment, if not sapling or canopy recruitment, and for F. americana, consistent with recent pest‐related canopy mortality. Collectively our analyses suggest sustained low diversity and shifting composition are driven by limited local seed availability, limited seedling establishment substrate, deep shading, and abundant deer. Management aimed at overcoming these limitations may be necessary to promote future NHF resilience.

Seed traits inform the germination niche for biodiverse ecological restoration

Seeds are a primary source for generating plants for large-scale restoration and understanding the requirements for seed germination and establishment is fundamental to ecological restoration. Seed germination traits are central to defining the germination niche and identifying traits that may limit seedling establishment provides insights into how seeds may interact with the abiotic environment and the soil substrates specific to each restoration site and informs seed management practices. In this paper we review seed trait data derived from research to improve restoration practice across diverse ecosystems within Western Australia. We compile and evaluate seed trait data for up to 300 seed collections of 287 species that are used in restoration programmes to identify species, lifeform, and region-specific variation in seed dormancy, maximum germination, germination speed, base water potential, and germination niche breadth. Through a synthesis of this data, we outline our ecologically-guided approach to identifying key seed traits that support plant regeneration to improve the success of seed-based restoration across the biodiverse Western Australian landscape.

Seasonal temperature–moisture interactions limit seedling establishment at upper treeline in the Southern Rockies

AbstractOver recent decades, sharply rising temperatures without an accompanying increase in precipitation have created widespread heat‐induced drought stress, or hotter drought. Tree‐ring reconstructions have discovered significant declines in seedling establishment from hotter drought across montane and subalpine forest belts during this time, yet comparable studies at upper treeline are non‐existent. In this study, we reconstruct annual patterns of seedling establishment at upper treeline in the Southern Rocky Mountains to test the hypotheses that establishment is governed by temperature–moisture interactions and that slope aspect mediates the influence of hotter drought. To test these hypotheses, we destructively sampled seedlings along a network of six study sites to reconstruct annual patterns of establishment on opposite north‐facing and south‐facing slopes. Results from this research can be summarized into two main points with respect to the influence of climate on seedling establishment at upper treeline over approximately the last three decades (1991–2019). First, temperature–moisture interactions throughout the year play a crucial role in facilitating successful seedling establishment. Second, and perhaps most striking, is the complete lack of establishment at all sites over the past decade. This could signify that a threshold has been surpassed and conditions are now beyond the climatic optimum for successful seedling establishment above treeline moving forward. These results expand upon similar findings from forests at lower elevations, introducing the likelihood that seedling establishment along the entire mountain forest belt of the Southern Rocky Mountains is being impacted by hotter drought. This means that any declines in the rate of seedling establishment across montane and subalpine forests will not be offset by increased recruitment at treeline.

Survival of young, dense Betula ermanii stands after wildfire at top soil removal sites

Abstract A forest fire in a cool-temperate broadleaved forest in northern Japan, from 26 May to 19 June 2019, provided an opportunity to examine the post-fire dynamics of young, dense birch (Betula ermanii Cham.) stands that had previously undergone top soil removal through soil scarification. To characterize the post-fire responses (survival and resprouting) of birch trees, we set up two plots, 6 months after the fire. We investigated the trunk diameter at breast height and burn marks on the trunks (scorch height and bole charring percentage) of all B. ermanii trees in both plots. Survival and resprouting of each tree were monitored over a 10-month period (6 and 16 months after the fire). To quantify post-fire vegetation recovery of the forest floor, we manually mapped the dominant understory plant, dwarf bamboo (i.e. Sasa kurilensis (Rupr.) Makino et Shibata), from orthomosaic images obtained using an unmanned aircraft vehicle, and estimated the recovery rate in the second year after the fire. Additionally, the seedlings of woody species were counted in both plots. A logistic generalized linear model revealed that the size-dependent survival rates of the plants in each plots were similar in the first year after fire. All B. ermanii trees died without resprouting in the second year after the fire, indicating the lethal effects of fire on young birch trees in top soil removal sites. Moreover, a high recovery rate of dwarf bamboo over 2 years in both plots and limited woody seedling establishment of woody plants suggest that the fire resulted in the regeneration failure of young stands in the top soil removal sites. Based on these findings, we suggest that the vulnerability of young trees can lead to the rapid change in vegetation from young forest to dense dwarf bamboo cover after a fire.

High light-induced photoinhibition is not limiting seedling establishment at abrupt treeline ecotones in New Zealand.

Seedlings of New Zealand's treeline-forming Fuscospora cliffortioides (Hook.f.) perform poorly beyond the established canopy, limiting treeline advance. To test the long-standing assumption that photoinhibition impairs regeneration in the subalpine belt of New Zealand's Southern Alps, we assessed photosystem II (PSII) performance of seedling-sized individuals and microclimate variation. We performed diurnal, non-invasive chlorophyll-a-fluorescence measurements on exposed and canopy-sheltered individuals at two sites in New Zealand's Southern Alps during summer and winter. Diurnal recordings of the effective (ΦPSII) and optimal (Fv/Fm) photosynthetic quantum yield were supplemented with light response curves and micro-temperature recordings. ΦPSII returned to near-optimal values around 0.8 after 30min of shading, which rules out accumulative or long-term photoinhibition effects. The maximum electron transport rate derived from rapid light curves was significantly higher (+12%) in exposed compared with canopy-shaded individuals. Summer temperature fluctuated widely on the scree (-0.5 to 60.5°C) and near seedlings (-2 to 26.5°C). Our results revealed a remarkable level of light adaptation and contradict previous studies hinting at high light-induced photoinhibition as a treeline-limiting factor in the Southern Alps. By linking low ΦPSII on winter mornings, and large, sudden temperature drops in summer, we suspect that cold-induced photoinhibition might occur but the rapid recovery of ΦPSII seen across a wide temperature range makes lethal photo-oxidative damage rather unlikely. Given the demonstrably low summer frost tolerance of F. cliffortioides, cold-related damage resulting from frost events during the growing season or embolism induced by frost drought may offer more plausible explanations for the poor seedling establishment. Duration and frequency of these events could diminish with global warming, which may promote treeline advance.

Increased burning in a warming climate reduces carbon uptake in the Greater Yellowstone Ecosystem despite productivity gains

Abstract The effects of changing climate and disturbance on mountain forest carbon (C) stocks vary with tree species distributions and over elevational gradients. Warming can not only increase C uptake by stimulating productivity at high elevations but also enhance C release by increasing respiration and the frequency, intensity and size of wildfires. To understand the consequences of climate change for temperate mountain forests, we simulated interactions among climate, wildfire, tree species and their combined effects on regional C stocks in forests of the Greater Yellowstone Ecosystem, USA (GYE) with the LANDIS‐II landscape change model. Simulations used historical climate and future potential climate represented by downscaled projections from five general circulation models (GCMs) that bracket the range of variability under the representative concentration pathway (RCP) 8.5 emissions scenario. Total ecosystem C increased by 67% through 2100 in simulations with historical climate, and by 38%–69% with GCM climate. Differences in C uptake among GCMs resulted primarily from variation in area burned, not productivity. Warming increased productivity by extending the growing season, especially near upper tree line, but did not offset biomass losses to fire. By 2100, simulated area burned increased by 27%–215% under GCM climate, with the largest increases after 2050. With warming >3°C in mean annual temperature, the increased frequency of large fires reduced live C stocks by 4%–36% relative to the control, historical climate scenario. However, relative losses in total C were delayed under GCMs with large increases in summer precipitation and buffered by C retained in soils and the wood of fire‐killed trees. Increasing fire size limited seed dispersal, and reductions in soil moisture limited seedling establishment; both effects will likely constrain long‐term forest regeneration and C uptake. Synthesis. Forests in the GYE can maintain a C sink through the mid‐century in a warming climate but continued warming may cause the loss of forest area, live above‐ground biomass and, ultimately, ecosystem C. Future changes in C stocks in similar forests throughout western North America will depend on regional thresholds for extensive wildfire and forest regeneration and therefore, changes may occur earlier in drier regions.

Barriers for plant establishment in the abandoned tailings of Nacozari, Sonora, Mexico: the influence of compost addition on seedling performance and tailing properties

Past mining activities have left a legacy of abandoned mine tailing deposits whose metal contaminants poses serious risks to ecosystems and human health. While the development of a vegetated cover in mine tailings can help in mitigating these risks, the local factors limiting plant establishment in these sites are not well understood, restricting phytostabilization efforts. Here, we explore some of the barriers that limit seedling establishment of two species (Vachellia farnesiana and Prosopis velutina) in a mine tailing deposit located in Nacozari, Sonora, Mexico, and assess whether compost addition can help in overcoming these barriers in pot and field experiments. Our field observations found 20 times more carbon and at least 4 times more nitrogen concentration in areas under vegetated patches than in non-vegetated areas, while a previous study found no difference in metal concentrations and other physicochemical parameters. This suggests that organic matter and nutrients are a major limitation for plant establishment. In agreement with this, species failed to establish without compost addition in the field experiment. Compost addition also had a positive effect on biomass accumulation, pH and microbial activity, but increased the substrate soluble concentration of As, Cu, and Zn. Nonetheless, only Cu, K, and Mo in P. velutina accumulated in tissues at levels considered toxic for animal consumption. Our study documents that compost addition facilitated plant establishment for the phytostabilization of mine tailings and help to prevent the dispersion of most metal contaminants via animal consumption. We encourage the use of complementary strategies to minimize the risk of dispersion of metal contaminants.

Effect of the grass Leersia hexandra on the dispersal, seed germination, and establishment of Pachira aquatica seedlings

Abstract In wetlands, hydrochory is one of the main mechanisms of seed dispersal and there is often synchrony between propagule production and the flood season. Different sources of disturbance can prevent seed dispersal to suitable sites, and if environmental conditions are not adequate for germination and seedling establishment, recruitment will be limited, affecting succession. We worked in a disturbed tropical freshwater swamp where the native grass Leersia hexandra has dominated open areas, creating a grass matrix that surrounds patches of swamp forest. Leersia grows vigorously, forming cushions of dry matter that cover the soil, forming a potential obstacle to seed dispersal. We asked whether the vegetative growth of this grass prevents the entry and dispersal of seeds of the tropical swamp tree Pachira aquatica, thwarting seed germination and seedling establishment, and arresting succession. We set up transects in the grass matrix in two zones: close to the river (R) and bordering the tree patches or fragments (F). We quantified tree seed and seedling presence, survival and growth in situ and experimentally introduced seeds and seedlings in the field and monitored seed germination and the survival and growth of their seedlings, as well as that of transplanted seedlings. There was a negative relationship between the number of seeds and established seedlings, and the distance to river or fragment (r = −0.86, p < 0.001 for zone R; and r = −0.77, p < 0.001 for zone F) and with the grass cushion (r = −0.68, p = 0.005 for zone R; and r = −0.66, p = 0.007 for zone F); the grass creating a barrier to dispersal. When seeds were sown after clearing the grass cushion, germination success was high, so this stage is not limited. The transplanted seedlings had better survival and a greater final height than the seedlings of the sown seeds. Grass cover had a negative effect on both types of seedlings. Seedling survival rates were inversely related to grass cover, showing that seedlings overgrown by grass had low survival rates. Flooding is a stress factor for seedlings and produced mortality, in addition to the effects of the grass. Together, the field survey and the experiment show that succession is being arrested in two ways: (1) by limiting seed dispersal because the grass cushion slows the dispersal and penetration of seeds into the vegetation; and (2) by limiting seedling establishment because the grass competes for space and light. Our results show that even where the grass is native, slower growing, seed‐dependent species may struggle to compete and establish. If grass cover is increasing, these swamps are very vulnerable to a decrease in area because it is very difficult for them to regenerate naturally.

Identifying critical recruitment bottlenecks limiting seedling establishment in a degraded seagrass ecosystem

Identifying early life-stage transitions limiting seagrass recruitment could improve our ability to target demographic processes most responsive to management. Here we determine the magnitude of life-stage transitions along gradients in physical disturbance limiting seedling establishment for the marine angiosperm, Posidonia australis. Transition matrix models and sensitivity analyses were used to identify which transitions were critical for successful seedling establishment during the first year of seed recruitment and projection models were used to predict the most appropriate environments and seeding densities. Total survival probability of seedlings was low (0.001), however, transition probabilities between life-stages differed across the environmental gradients; seedling recruitment was affected by grazing and bioturbation prevailing during the first life-stage transition (1 month), and 4–6 months later during the third life-stage transition when establishing seedlings are physically removed by winter storms. Models projecting population growth from different starting seed densities showed that seeds could replace other more labour intensive and costly methods, such as transplanting adult shoots, if disturbances are moderated sufficiently and if large numbers of seed can be collected in sufficient quantity and delivered to restoration sites efficiently. These outcomes suggest that by improving management of early demographic processes, we could increase recruitment in restoration programs.

Extinction debt of a common shrub in a fragmented landscape

Fil: Gonzalez Varo, Juan P.. Consejo Superior de Investigaciones Cientificas. Estacion Biologica de Donana; Espana

Honey Mesquite (Prosopis glandulosa) Seedling Responses to Seasonal Timing of Fire and Fireline Intensity

Fire-resistant woody species have invaded many grassland and savanna ecosystems worldwide. Adults of many of these species are fire resistant because they resprout following fire; however, seedlings may be vulnerable to fire. Here we quantify effects of seasonal timing of fire and fireline intensity on seedling mortality response to fire in honey mesquite (Prosopis glandulosa Torr.), a native woody legume that has increased in abundance in southern Great Plains, USA grasslands. Scarified mesquite seeds were planted in plots with either mid-grass or tall-grass fuels in each of 2 years (Cohorts 1 and 2). Plots were burned in winter when seedlings were 10 or 22 months old or late summer when they were 17 months old. In the mid-grass fuel type, seedling mortality was greater (P ≤ 0.05) in summer-burned, 17-month-old seedlings (85%) than in winter-burned, 10-month-old seedlings (35%), suggesting summer fire may be important in limiting seedling establishment under lower-grass, fine-fuel conditions. The tall-grass fuel type yielded similar mortality responses to summer (89% and 79%) and winter (77% and 66%) fires in Cohorts 1 and 2, respectively (differences not significant at P ≤ 0.05). The study found a positive relationship between fireline intensity and seedling mortality, especially when only winter fires were included in the regression (r2 = 0.96). Summer fire may be most useful in lower-grass fuel amounts and may accomplish the same level of seedling mortality without the need for lengthy grazing deferral to accumulate sufficient grass fine fuel for a successful winter fire, as seedling mortality from winter fire depends on fireline intensity.

Granivory of invasive, naturalized, and native plants in communities differentially susceptible to invasion.

Seed predation is an important biotic filter that can influence abundance and spatial distributions of native species through differential effects on recruitment. This filter may also influence the relative abundance of nonnative plants within habitats and the communities' susceptibility to invasion via differences in granivore identity, abundance, and food preference. We evaluated the effect of postdispersal seed predators on the establishment of invasive, naturalized, and native species within and between adjacent forest and steppe communities of eastern Washington, USA that differ in severity of plant invasion. Seed removal from trays placed within guild-specific exclosures revealed that small mammals were the dominant seed predators in both forest and steppe. Seeds of invasive species (Bromus tectorum, Cirsium arvense) were removed significantly less than the seeds of native (Pseudoroegneria spicata, Balsamorhiza sagittata) and naturalized (Secale cereale, Centaurea cyanus) species. Seed predation limited seedling emergence and establishment in both communities in the absence of competition in a pattern reflecting natural plant abundance: S. cereale was most suppressed, B. tectorum was least suppressed, and P. spicata was suppressed at an intermediate level. Furthermore, seed predation reduced the residual seed bank for all species. Seed mass correlated with seed removal rates in the forest and their subsequent effects on plant recruitment; larger seeds were removed at higher rates than smaller seeds. Our vegetation surveys indicate higher densities and canopy cover of nonnative species occur in the steppe compared with the forest understory, suggesting the steppe may be more susceptible to invasion. Seed predation alone, however, did not result in significant differences in establishment for any species between these communities, presumably due to similar total small-mammal abundance between communities. Consequently, preferential seed predation by small mammals predicts plant establishment for our test species within these communities but not between them. Accumulating evidence suggests that seed predation can be an important biotic filter affecting plant establishment via differences in consumer preferences and abundance with important ramifications for plant invasions and in situ community assembly.

Restoration of Neotropical grasslands degraded by quarrying using hay transfer

AbstractAimThe campos rupestres are species‐rich tropical mountain grasslands in southeast Brazil that are threatened by mining and quarrying. Eight years following a strong disturbance (i.e. quarrying for gravel exploitation during the asphalting of highway MG‐010), spontaneous succession did not occur on degraded areas. This study was therefore designed to test the restoration technique of hay transfer as a means of enhancing seed dispersal from the reference ecosystems to the degraded areas.LocationCampo rupestre grasslands, Serra do Cipó, Minas Gerais, Brazil.MethodsNine degraded areas representing three kinds of substrate (latosol, sandy and stony) were selected. Hay transfer was carried out using hay collected in two kinds of grassland (i.e. campos arenosos and campos pedregosos) year‐round in 2010, in order to maximize the seed pool. The collected hay was distributed on 40 cm × 40 cm quadrats among the three types of substrate, with and without geotextile, according to the following protocol: hay from campo arenoso grasslands was spread on all types of substrate, while hay from campo pedregoso grasslands was used on stony substrate only.ResultsFew seedlings emerged within 14 mo of the treatment application, despite the large number of seeds contained in the hay, indicating that hay transfer may not be a useful method for restoring degraded areas of campos rupestres. The seedlings emerging on the degraded areas mainly belonged to ruderal species. Therefore, we suggest that, although poor soil conditions could limit seedling establishment, germination issues of the campo rupestre species are the first limitation for restoring degraded campos rupestres using hay transfer.ConclusionOur results highlight the inherent difficulty in restoring degraded areas of campos rupestres; their protection must be made a high conservation priority.

Species diversity and regeneration of native species in Pennisetum ciliare (buffelgrass) pastures from the thornscrub of Sonora, México

To examine the influence of buffelgrass land conversion and pasture management on native species diversity and regeneration patterns, we describe community attributes and population structure in four different active pastures in thornscrub vegetation from eastern Sonora, Mexico. We compare a relatively undisturbed thornscrub community with a contiguous five years old active pasture, to identify species able to regenerate under current management practices. Buffelgrass conversion has a significant influence on species diversity and other community attributes. Active pastures have lower species diversity, crown cover and basal area of native species than the studied native thornscrub. An active pasture subjected to heavy grazing and without management of the thorny legume, Acacia cochliacantha, showed the lower species diversity and an almost monospecific stand of this legume. It is likely that the reduction in species diversity is caused by the conversion process, inadequate management and regeneration barriers that limit seedling establishment in active pastures. Our data recorded eight native species (13%) that were able to regenerate in active pastures. However, the great majority of native species were unable to regenerate under pastures. This study shows that under current management, active buffelgrass pastures maintain only a small fraction of native species with regenerating populations in the thornscrub.

Plant community type and small‐scale disturbances, but not altitude, influence the invasibility in subarctic ecosystems

Little of our knowledge about invasibility comes from arctic and alpine ecosystems, despite increasing plant migration and invasion in those regions. Here, we examine how community type, altitude, and small-scale disturbances affect invasibility in a subarctic ecosystem. Over a period of 4 yr, we studied seedling emergence and establishment in 17 species sown in gaps or undisturbed vegetation in four subarctic community types (Salix scrub, meadow, rich heath, poor heath) along an elevation gradient. Invasibility was lowest in rich heath and highest in Salix scrub. Small disturbances significantly increased the invasibility in most communities, thereby showing the importance of biotic resistance to invasion in subarctic regions. Unexpectedly, invasibility did not decrease with increasing elevation, and it was also not related to summer temperature. Our data suggest that biotic resistance might be more important than abiotic stress for invasibility in subarctic tundra and that low temperatures do not necessarily limit seedling establishment at high altitudes. High elevations are therefore potentially more vulnerable to invasion than was originally thought. Changes in community composition as a result of species migration or invasion are most likely to occur in Salix scrub and meadow, whereas Empetrum-dominated rich heath will largely remain unchanged.

Seedling light limitation does not increase across a natural productivity gradient

Aims A key idea in plant community ecology is that the identity of the limiting resource shifts from soil nutrients in low productivity sites to light in high productivity sites. This idea, and its implications for plant community structure, has been tested many times in artificial productivity gradients (fertilization studies), but whether it applies to natural productivity gradients is unclear. Methods To test whether seedling light limitation would increase across a natural productivity gradient, I conducted a cross-site field experiment in southwest Michigan, USA. At each of six old fields naturally varying in productivity, I exposed transplanted seedlings of big bluestem (Andropogon gerardii) to a light addition (tie-back) treatment that increased light availability and measured their biomass after one and two growing seasons. Important Findings Seedlings responded positively to the tie-back treatment, but positive responses did not increase across the natural productivity gradient. These results suggest that although light does limit seedling establishment, the strength of light limitation does not depend on variation in productivity in natural systems. Instead, light limitation interacted with a variety of site differences to determine establishment. Although the general principle that light limitation increases with productivity is well established, these results indicate that it may not always occur in natural systems.

Facilitation by tussock‐forming species on seedling establishment collapses in an extreme drought year in a post‐mined Sphagnum peatland

AbstractQuestionsDoes the facilitation of herbaceous species seedling establishment by tussock‐forming species collapse in an extreme drought year? Do different germination phenologies, between the seedling species, influence the variation in facilitation?LocationA post‐mined Sphagnum peatland at Sarobetsu mire, Hokkaido, northern Japan.MethodsThe effects of tussock‐forming species, as potential facilitators, on seedling emergence and survival of five perennial herbs were examined during a 4‐yr seedling monitoring, which included an extreme drought. The general effects of tussocks on each seedling species during annual variations were simultaneously assessed using hierarchical Bayesian analysis. To examine the stress limiting seedling establishment, which was ameliorated by the tussocks, micro‐environments (i.e. light, temperature, water content and erosion) altered by morphological traits of the tussocks with litter cover were compared to those in open areas.ResultsIn general, tussocks facilitated seedling establishment of all species through a positive effect on emergence and no effect on survival; however, the facilitative effect declined for several species in an extreme drought year. Peat erosion was the limiting stress on seedling establishment in normal years, however, peat water content also limited seedling establishment in a drought year. During the drought year, the positive effect of tussocks on seedling emergence for species that germinated before or during the drought was weakened for one species, and a negative effect on seedling survival emerged in another species. In contrast, species that germinated after the drought did not exhibit annual variation in the effects of tussocks on seedling emergence and survival.ConclusionsOur results illustrate that collapse of facilitation arises under extremely severe conditions through addition of infrequently occurring stress (drought) on frequently occurring stress (peat erosion). Variation in seedling emergence and survival of the five study species in response to severe drought suggests that species‐specific traits must be considered when assessing how facilitation varies temporally. Further, these findings suggest that facilitation in temporally varying environments can drive plant community composition and dynamics.

Allelopathic potential of the neotropical dry-forest tree Acacia pennatula Benth.: inhibition of seedling establishment exceeds facilitation under tree canopies

Secondary succession after land abandonment in tropical dry forests has been suggested to be favoured by the facilitation effects for seedling establishment exerted by pioneer trees isolated in these savannah-like landscapes. However, it has also been noticed that these pioneer species may sometimes have an encroaching effect and arrest succession for several decades. We investigated in this study whether allelopathy can play a role in limiting seedling establishment of co-occurring tree species under the canopy of Acacia pennatula by means of lab bioassays and field experiments in north-west Nicaragua. Leaf extracts of A. pennatula did not affect seed germination but reduced the general growth and especially the development of the root compartment in seedlings, shifting their biomass allocation model to a reduced root/shoot ratio. Survival of planted seedlings under the canopy of A. pennatula was about 20–30% lower than outside, and this reduction was particularly pronounced as the dry season progressed, despite the milder conditions (e.g. higher soil moisture) being experienced in the inner positions under the canopy. Altogether, our results suggest that, rather than facilitating, A. pennatula may inhibit the establishment of seedlings under its canopy probably by means of an allelopathic interference in the development of the root system with critical negative consequences for young seedlings in terms of overcoming the dry season. This article warns about overemphasizing the nucleation effect that remnant and isolated trees may have to facilitate secondary succession in these highly disturbed savannah-like tropical dry forests.

First Report of Basal Stem Rot and Foliar Blight Caused by Pythium sylvaticum on Miscanthus sinensis in Illinois.

Miscanthus sinensis Anderss., a perennial grass, is native to eastern Asia. It has been widely grown as an ornamental in temperate regions of the world, including the United States, and recently has become an important component of public and private sector bioenergy feedstock Miscanthus selection programs. In August 2008, stem rot and blight was observed on M. sinensis plants in two irregular patches, ~2 to 2.5 × 1 to 1.5 m each in a trial plot that was preceded by corn, at the University of Illinois Energy Farm near Urbana, IL. At the time of the observation, most plants were dead and the wilted tillers had black, soft rotted basal stems. A few plants were stunted and the crowns of the tillers had black-to-brown soft rot. Some tillers' leaves were dead and others had turned light brown. Sample tissue fragments were surface disinfested in 0.5% NaOCl and plated on 1% water agar (WA). After 3 days of incubation in the dark at 23°C, colonies were transferred to corn meal agar (CMA), potato dextrose agar (PDA), or 10% V8 juice agar and incubated at 23°C under continuous white light for up to 2 weeks. Morphological characteristics of the isolates correspond to those originally described for Pythium sylvaticum W.A. Campb. & J.W. Hendrix (1). The mycelia grew and covered the 10-cm-diameter plates within 5 days. On PDA, the culture was a creamy white mycelial mat of coenocytic hyphae. The isolates produced only globose, terminal or intercalary hyphal swellings ranging from 28 to 48 μm in diameter, but no oogonia were produced on any of the three growth media. No zoospores were produced when agar blocks bearing mycelium were flooded with distilled water or 1% soil water. Sequence analysis was performed with the internal transcribed spacer (ITS) region of the rDNA amplified with primer pair ITS1/ITS4 (3) and the mitochondrially encoded cytochrome c oxydase subunit II (cox II) gene using primers FM58/FM66 (2). The resulting 871-bp ITS nucleotide sequence (Accession No. HM991706) was identical among all three isolates analyzed and 99% identical (100% coverage) to ITS sequences of multiple isolates of P. sylvaticum in GenBank. Likewise, the 544-bp cox II sequence (Accession No. HQ454429) was 99% identical (97% coverage) to cox II sequences of multiple isolates of P. sylvaticum. Six pots of M. sinensis seedlings were inoculated by placing two CMA plugs of a 2-week-old culture of isolate F71 at the crown. The control pots were mock inoculated with sterile CMA plugs. The plants were incubated at ~90% relative humidity (RH) and 25°C day and 22°C night for 3 days, and thereafter left on the greenhouse bench at ~65% RH with alternating 9 h of darkness and 15 h of light. Three weeks after inoculation, two of the inoculated seedlings wilted, others were stunted with leaves wilting from the tip downwards and the stems rotting from the crown upward. A thick mat of mycelia was seen on the rotted basal stems. No symptoms were observed in the control. P. sylvaticum was reisolated from both the rotted basal stems and the wilted foliage. To our knowledge, this is the first report of P. sylvaticum on M. sinensis. Infestation of farm soils with P. sylvaticum could limit M. sinensis biomass production significantly by limiting seedling establishment.