Seed storage influences the seedling emergence and growth of Tocoyena formosa, a native species from Brazilian Savannah?

During community assembly, early arriving exotic species might suppress other species to a greater extent than do native species. Because most exotics were intentionally introduced, we hypothesize there was human selection on regeneration traits during introduction. This could have occurred at the across- or within-species level (e.g. during cultivar development). We tested these predictions by seeding a single species that was either native, exotic 'wild-type' (from their native range), or exotic 'cultivated' using 28 grassland species in a glasshouse experiment. Priority effects were assessed by measuring species' effect on establishment of species from a seed mix added 21 d later. Exotic species had higher germination and earlier emergence dates than native species, and differences were found in both 'wild' and 'cultivated' exotics. Exotic species reduced biomass and species diversity of later arriving species much more than native species, regardless of seed source. Results indicate that in situations in which priority effects are likely to be strong, effects will be greater when an exotic species arrives first than when a native species arrives first; and this difference is not merely a result of exotic species cultivation, but might be a general native-exotic difference that deserves further study.

Environmental constraints to native woody species recruitment in invaded mountain woodlands of central Argentina

Plant invasions are often implicated in declines of native plant species. However, common experimental designs have received criticism questioning the assumption that invasive plants are the primary cause for ecosystem deterioration. We used a combination of field observations and a transplant experiment to investigate the influence of an exotic invasive shrub, Ligustrum sinense (Chinese Privet) on native plant species in Piedmont floodplain forests of South Carolina, USA. We conducted vegetation surveys of 12 floodplain forests documenting abundance and cover of all herbaceous and woody plant species. Additionally, we established an experimental garden to compare survival and growth of L. sinense and four common native species transplanted into a mature L. sinense stand and an adjacent uninvaded area over two growing seasons. The vegetation survey demonstrated a strong negative relationship between L. sinense presence and herbaceous vegetation. As L. sinense cover increased, herbaceous cover and height, plant abundance, and native species richness decreased. In our transplant experiment we found drastic effects of L. sinense on native plant seedling survival and growth. Survival for all native species was lower under the L. sinense canopy and native seedling growth was substantially reduced. Results from both the vegetation survey and transplant experiment show that invasion of L. sinense suppresses herbaceous understory and prevents regeneration of native species by reducing seedling survival and growth. With an approach that combines multiple field sites and local site-specific investigations our research provides strong evidence that L. sinense is an agent of change in floodplain forests.

Growth, biomass allocation, and photosynthetic characteristics of seedlings of five invasive non-indigenous and four native species grown under different light regimes were studied to help explain the success of invasive species in Hawaiian rainforests. Plants were grown under three greenhouse light levels representative of those found in the center and edge of gaps and in the understory of Hawaiian rainforests, and under an additional treatment with unaltered shade. Relative growth rates (RGRs) of invasive species grown in sun and partial shade were significantly higher than those for native species, averaging 0.25 and 0.17 g g-1 week-1, respectively, while native species averaged only 0.09 and 0.06 g g-1 week-1, respectively. The RGR of invasive species under the shade treatment was 40% higher than that of native species. Leaf area ratios (LARs) of sun and partial-shade-grown invasive and native species were similar but the LAR of invasive species in the shade was, on average, 20% higher than that of native species. There were no differences between invasive and native species in biomass allocation to shoots and roots, or in leaf mass per area across light environments. Light-saturated photosynthetic rates (Pmax) were higher for invasive species than for native species in all light treatments. Pmax of invasive species grown in the sun treatment, for example, ranged from 5.5 to 11.9 μmol m-2 s-1 as compared with 3.0-4.5 μmol m-2 s-1 for native species grown under similar light conditions. The slope of the linear relationship between Pmax and dark respiration was steeper for invasive than for native species, indicating that invasive species assimilate more CO2 at a lower respiratory cost than native species. These results suggest that the invasive species may have higher growth rates than the native species as a consequence of higher photosynthetic capacities under sun and partial shade, lower dark respiration under all light treatments, and higher LARs when growing under shade conditions. Overall, invasive species appear to be better suited than native species to capturing and utilizing light resources, particularly in high-light environments such as those characterized by relatively high levels of disturbance.

Testing techniques to reduce weed infestation is a crucial step in developing direct tree seeding systems. The use of pre-emergence herbicides may be an alternative to manual weeding techniques, but so far, information on how they affect the seeds of native tree species is scarce. We established a greenhouse experiment to evaluate the effects of four pre-emergence herbicides (atrazine, diuron, isoxaflutole and oxyfluorfen) on weed suppression and seedling emergence and early growth of seven tropical forest tree species (Annona coriacea Mart., Citharexylum myrianthum Cham., Cordia ecalyculata Vell., Peltophorum dubium (Spreng.) Taub., Psidium guajava L., Pterogyne nitens Tul. and Schinus terebinthifolia Raddi). The experimental design was a randomized complete block design with five treatments and five replicates. The treatments consisted of a single dose of each pre-emergence herbicide and a control. Throughout the 60 days after sowing we evaluated weed cover and seedling emergence and early growth of tree species. Overall, our results suggest that all tested herbicides reduced weed cover; however, they also negatively affected tree species seedling emergence. Of the four herbicides tested, atrazine and diuron showed the greatest effects on tree seedling emergence, oxyfluorfen was least aggressive towards native species and isoxaflutole was most effective in weed control. Native tree species varied in their responses to herbicides, indicating that future experiments should increase the number of species tested as well as investigate how seed traits can affect the species responses to different herbicides.

Effects of Eucalyptus litter and roots on the establishment of native tree species in Eucalyptus plantations in South China

Fire tree (Morella faya) has invaded extensive areas of wet and mesic forest on the Island of Hawai’i, forming nearly monospecific stands. Our objective was to identify a method of controlling M. faya, which would allow native plants to establish while minimizing establishment by invasive plants. Treatments (logging all trees, trees left standing but girdled, and incremental girdling over 20 months) were selected to kill M. faya stands at different rates, thereby creating different conditions for species establishment. Leaf litter was either removed or left in place; seeds and seedlings of three native pioneer species, three native forest species, and three alien invasive species were then added to determine their ability to establish. Native pioneer species established best in the log and girdle treatments, whereas seedling emergence of native forest species was higher in the girdle and incremental girdle treatments. Seedlings of invasive species emerged faster than the natives, but each of them responded differently to the stand treatments. Leaf litter reduced seedling emergence for all species, with small‐seeded species (<1 mg/seed) affected most under low light conditions. No single method eliminated all invaders, but girdling of M. faya provided suitable conditions for most native species. If combined with selective removal of the most disruptive alien species and native seed additions, girdling could be an effective general strategy for restoring native forests that have been overwhelmed by woody invaders.

Early growth performances of 11 native tree species were investigated in three different sites in Mindanao, Philippines, to evaluate their adaptability and potential for plantation development. Three alien species were added to assess how native species could potentially compete with these alien species based on survival rate, diameter growth rate (DGR), and height growth rate (HGR). A year after planting, the native species common to all sites that obtained >80% survival rate were Casuarina equisetifolia L. Alstonia macrophylla Wall. ex. G.Don., Alstonia scholaris (L.) R.Br., and Parkia javanica (Lam.) Merr. and were comparable to an alien species Acacia mangium Willd. The species with the lowest survival rate (30%) across all sites was Duabanga moluccana Blume. Native species P. javanica, Eucalyptus deglupta Blume, and A. macrophylla all had a DGR of 34 mm year−1 and were comparable with the alien species Schizolobium parahyba (Vell.) S.F.Blake with 38 mm year−1. However, the HGR of native species E. deglupta (245 cm year−1) and Melia dubia Cav. (230 cm year−1) were higher than an alien species S. parahyba (222 cm year−1). No native species can compete with the DGR (52 mm year−1) and HGR (384 cm year−1) of A. mangium. Rainfall significantly explained 13%–97% of DGR, HGR and survival rate of >70% of the species while air temperature explained about 17%–96% of the variations of similar variables. This early assessment provides a strong basis to better predict the early performances of native species in the Philippines. Through this, appropriate silvicultural intervention can be recommended towards improving the growth and survival of the native seedlings as alternative industrial tree plantation species in the country.

Non‐native invasive grasses are driving intense fires across the globe but the impacts of native versus invader‐fuelled fires on community assemblages are poorly understood. By increasing fire intensity, grass invasions might increase below‐ground mortality of heat‐sensitive seeds and buds, thereby shifting community composition. We compared fuel loads in native and non‐native invasive (cogongrass, Imperata cylindrica) plant‐dominated areas of pine savannas in Florida. Then, we conducted a field experiment to examine how fuel loads and native and invasive fuel types affected soil heating and seedling emergence or resprouting of native and invasive plant species. Average fuel loads in invaded communities were 152% greater than that in native communities. Soil heating, including heating duration >60°C, maximum temperature and heat flux >60°C, increased, and seedling emergence and resprouting decreased with greater fuel loads; these relationships were similar across the overlapping range of native and invasive fuel loads. However, longer durations of soil heating at the higher average fuel loads of invaded communities resulted in 23% lower predicted probability of seedling emergence compared to average fuel loads of native communities. Invasive cogongrass resprouting was not affected by fuel loads, indicating that cogongrass tolerates the intense fires it generates. In contrast, seedling emergence and resprouting of most other species was reduced by greater fuel loads. Synthesis and applications. By increasing fuel loading and soil heating, grass invasions may alter post‐fire community assemblages and facilitate invasive grass dominance at the expense of native species via an invasion‐fire cycle. Fuel loads can be used to predict soil heating duration and depth, and these data, combined with information on species tolerances to heating, can be used to forecast the impacts of invasions on post‐fire community composition. To maintain fire regimes that promote native communities and resist invader dominance, it is critical to manage invasive species that increase fuel loads.

Morphology and Growth in Seedlings of Several C4, Perennial Grasses

Species-rich native grasslands in western Victoria, Australia, are often small, have a high perimeter to area ratio and are surrounded by non-native species. Few non-native species, however, have invaded them. A feature of species-rich grasslands is the presence of a bryophyte mat (composed of mosses and liverworts) that carpets the intertussock spaces. I assessed the role of these mats in plant invasions by sowing three non-native species (Briza maxima, Hypochoeris radicata, Plantago lanceolata) in replicated disturbed (mats removed) and undisturbed (mats intact) microsites at three grassland remnants (two recently burnt, one unburnt for 3 years) and followed seedling emergence, survival and growth for 5 months. Three native species were also sown for comparison. The rate of germination and total percent germination of non-native species were significantly enhanced at both burnt sites when the mat was disturbed. The large-seeded Briza maxima failed to germinate at both burnt sites in the absence of soil disturbance. The native species generally did not show a strong germination or growth response to soil disturbance in burnt areas. At the unburnt site, where monthly percent soil moisture was highest, final percent germination of the non-native and native species was greatest of any site in both microsites, and germination was not significantly affected by soil disturbance. Differences in the seed morphology of native and non-native species may play an important role in their ability to establish on bryophyte mats in moisture-limiting environments. Any activity that disrupts the mats in the frequently burnt, species-rich grassland remnants is likely to significantly enhance the germination and subsequent growth by non-natives. However, where burning is infrequent, germination of some non-native species may be expected, regardless of disturbance, although growth will likely be favoured in disturbed areas.

Effects of Wollastonia biflora expansion on the soil seed bank in native forest communities on a tropical coral island

Poor seedling emergence often limits the success of direct seeding in ecological restoration. New techniques for maximising seed use efficiency and seedling emergence are needed to help meet global targets for nature repair in the UN Decade on Restoration. Extruded pellets are widely used in agriculture and represent a promising advancement in seed‐based restoration. However, extruded pellets must be optimised for diverse suites of native species that possess a range of seed sizes and morphotypes. We investigated how seed mass affects the performance of native plant seeds (total % seedling emergence) when encapsulated in extruded pellets designed for revegetation. Two glasshouse trials were undertaken using seeds from 30 native Australian plant species. In Trial 1, we encapsulated seeds in the centre of pellets and determined the relationship between seed mass and emergence. In Trial 2, we encapsulated seeds nearer the periphery of pellets and determined whether the position of seeds (central vs. peripheral) affected emergence for a subset of 10 small‐seeded species. In both trials, emergence from pellets was compared to an optimal, bare‐seeded control to identify any barriers to seed encapsulation under well‐watered conditions. In Trial 1, when seeds were centrally encapsulated, emergence was generally higher for bare‐seeded controls relative to pelleted seeds. However, seed mass predicted emergence when seeds were encapsulated in the pellet centre (R2 = 0.32, p = 0.002), such that larger‐seeded species tended to have higher emergence than smaller‐seeded species. In Trial 2, encapsulating seeds nearer the pellet periphery (relative to the centre) resulted in an average 28‐fold increase in emergence for all 10 small‐seeded species. For half of the small‐seeded species trialled, emergence from the pellet periphery was equivalent to that of bare‐seeded controls. Synthesis and applications: Collectively, our results demonstrate: (1) a positive relationship between seed mass and emergence under central‐encapsulation, and (2) that emergence can be significantly improved for small‐seeded species when seeds are positioned nearer the pellet periphery. Translation of these findings into practice can help optimise emergence outcomes for native species with different sized seeds and nuanced germination requirements.

To evaluate factors limiting vegetation regeneration, studies of seed rain, seedling emergence, and survival were carried out on different phases of three different-aged quarries in Hong Kong. Seed-rain results showed that 1,741 seeds from 35 woody species were collected from 160 traps in a 1-year period. Older sites had higher seed number and species than younger sites. In terms of number, 68 % of seeds were dispersed by birds, whereas 30 and 2 % were dispersed by wind and civets, respectively. In situ seedling emergence experiment showed that most of the 12 woody species had higher emergence under field conditions, which implied that seedling emergence was not a limitation of vegetation regeneration. Most emergent seedlings finally died, which had significant correlations with the adverse microhabitat conditions. It was concluded that although seed rain contained some native woody species, it may still be insufficient in order to accelerate natural vegetation regeneration at the later successional stages on our local quarries. Management strategies should take into account the possibility of enhancing seed dispersal and improving microhabitat conditions. Using native species with fleshy fruits to attract birds by enrichment planting is strongly recommended, and postplanting care is essential to quality development of the rehabilitated ecosystem.

The weed Chromolaena odorata has negative impacts on invaded ecosystems. Canopy of its aerial parts and allelochemicals released by the weed can suppress the growth and survival of native species. Field assessment of native trees Aegle marmelos and Senegalia catechu evidenced that a declining trend of their seedlings under higher canopy of C. odorata. Experiments carried out in pots revealed the negative effects of the weed’s leachate and shade on growth and development of A. marmelos. The leachate increased proline levels in A. marmelos seedlings by ca. 33% in shade and 43% in light. Specific leaf area and secondary roots were decreased significantly under both light and shade conditions by leachate. In the light condition, leachate reduced seedling biomass by ca. 26% and root length by 16%. Shade alone decreased overall seedling growth, including leaf area and biomass with poor root growth and increased specific leaf area. Results showed that A. marmelos is susceptible to shade and C. odorata leachate during its early stage of growth and development. In addition to other factors contributing to the decline of A. marmelos population in nature, the invasion of C. odorata intensifies the challenge. Our study clarifies that the invasion of C. odorata in native habitat has further contributed to the population decline of native species alongside other contributing factors in nature. Hence, there is an urgent necessity to control and manage C. odorata to protect native species. Removal of C. odorata from the invaded site will be beneficial for approaching light for native seedlings as well as preventing the leaching substances into the soil.