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Saatkamp, A., Cochrane, A., Commander, L., Guja, L., Jimenez-Alfaro, B., Larson, J., Nicotra, A., Poschlod, P., Silveira, F.A.O., Cross, A., Dalziell, E.L., Dickie, J., Erickson, T.E., Fidelis, A., Fuchs, A., Golos, P.J., Hope, M., Lewandrowski, W., Merritt, D.J., Miller, B.P., Miller, R., Offord, C.A., Ooi, M.K.J., Satyanti, A., Sommerville, K.D., Tangney, R., Tomlinson, S., Turner, S., Walck, J.L.

Woody-plant encroachment is a global phenomenon that has been affecting the southwestern United States since the late 1800s. Drought, overgrazing, herbivory, and competition between grasses and shrub seedlings have been hypothesized as the main drivers of shrub establishment. However, there is limited knowledge about the interactions among these drivers. Using a rainfall manipulation system and various herbivore exclosures, we tested hypotheses about how precipitation (PPT), competition between grasses and shrub seedlings, and predation affect the germination and first-year survival of mesquite (Prosopis glandulosa), a shrub that has encroached in Southern Great Plains and Chihuahuan Desert grasslands. We found that mesquite germination and survival (1) increased with increasing PPT, then saturated at about the mean growing season PPT level, (2) that competition between grasses and shrub seedlings had no effect on either germination or survival, and (3) that herbivory by small mammals decreased seedling establishment and survival, while ant granivory showed no effect. In addition to its direct positive effect on survival, PPT had an indirect negative effect via increasing small mammal activity. Current models predict a decrease in PPT in the southwestern United States with increased frequency of extreme events. The non-linear nature of PPT effects on Mesquite recruitment suggests asymmetric responses, wherein drought has a relatively greater negative effect than the positive effect of wet years. Indirect effects of PPT, through its effects on small mammal abundance, highlight the importance of accounting for interactions between biotic and abiotic drivers of shrub encroachment. This study provides quantitative basis for developing tools that can inform effective shrub management strategies in grasslands and savannas.

The application of biostimulants from natural origin represents one of the most innovative and promising strategies to promote plant growth and improve crop productivity. The biostimulants have the potential to release dormancy and enhance seed germination of many plant species by altering the physiological processes in seeds even under abiotic stress. The seed application of biostimulants has been considered as a simple, beneficial and sustainable technique to enhance crop productivity. An understanding of non-microbial biostimulants-induced seed germination, dormancy release and seedling establishment mechanisms may help to improve crop productivity. However, the impending mechanisms involved in the regulation of seed germination and seedling establishment by non-microbial biostimulants are still elusive. In this review, the possible mode of action of various non-microbial biostimulants in seed germination and the seedling establishment under optimal and sub-optimal conditions has been discussed. Furthermore, the gaps in utilizing the full potential of these biostimulants are also addressed.

1. Despite the disproportionate influence that propagule production, dispersal, seed-to-seedling recruitment and vegetative reproduction can have on plant population and community dynamics, progress has been slow in the directed collection of regeneration traits to inform community assembly outcomes. 2. While seed mass is globally available and linked to growth and reproductive output, there are limits to its explanatory ability. In this essay, we call for expanded efforts to integrate a more diverse set of regeneration traits into community assembly models. 3. First, we extend an existing community assembly framework to conceptualize regeneration as a series of transitional processes whose outcomes are influenced by abiotic filters, biotic interactions and species traits. We then briefly review the literature, highlighting filters and traits of demonstrated or theorized importance for each transition. Finally, we place regeneration in the context of existing and emerging modelling approaches in trait-based community assembly, summarizing key areas of progress needed to integrate regeneration traits into these efforts. 4. Synthesis. By incorporating influential regeneration traits into empirical studies and global data bases, we can begin to disentangle regenerative mechanisms underlying community assembly outcomes and enhance rapidly developing models of species' abundances, distributions and responses to environmental change.

Soybean seeds contain higher concentrations of oil (triacylglycerol) and fatty acids than do cereal crop seeds, and the oxidation of these biomolecules during seed storage significantly shortens seed longevity and decreases germination ability. Here, we report that diethyl aminoethyl hexanoate (DA-6), a plant growth regulator, increases germination and seedling establishment from aged soybean seeds by increasing fatty acid metabolism and glycometabolism. Phenotypic analysis showed that DA-6 treatment markedly promoted germination and seedling establishment from naturally and artificially aged soybean seeds. Further analysis revealed that DA-6 increased the concentrations of soluble sugars during imbibition of aged soybean seeds. Consistently, the concentrations of several different fatty acids in DA-6-treated aged seeds were higher than those in untreated aged seeds. Subsequently, quantitative PCR analysis indicated that DA-6 induced the transcription of several key genes involved in the hydrolysis of triacylglycerol to sugars in aged soybean seeds. Furthermore, the activity of invertase in aged seeds, which catalyzes the hydrolysis of sucrose to form fructose and glucose, increased following DA-6 treatment. Taken together, DA-6 promotes germination and seedling establishment from aged soybean seeds by enhancing the hydrolysis of triacylglycerol and the conversion of fatty acids to sugars.

Robust tree regeneration following high-severity wildfire is key to the resilience of subalpine and boreal forests, and 21st century climate could initiate abrupt change in forests if postfire temperature and soil moisture become less suitable for tree seedling establishment. Using two widespread conifer species, lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii var. glauca), we conducted complementary experiments to ask (1) How will projected early-to mid-21st-century warming and drying affect postfire tree seedling establishment and mortality? (2) How does early seedling growth differ between species and vary with warming and drying? With a four-year in situ seed-planting experiment and a one growing season controlled-environment experiment, we explored effects of climate on tree seedling establishment, growth, and survival and identified nonlinear responses to temperature and soil moisture. In our field experiment, warmer and drier conditions, consistent with mid-21ˢᵗ-century projections, led to a 92% and 76% reduction in establishment of lodgepole pine and Douglas-fir. Within three years, all seedlings that established under warmer conditions died, as might be expected at lower elevations and lower latitudes of species' ranges. Seedling establishment and mortality also varied with aspect; approximately 1.7 times more seedlings established on mesic vs. xeric aspects, and fewer seedlings died. In the controlled-environment experiment, soil temperatures were 2.0°–5.5°C cooler than the field experiment, and warming led to increased tree seedling establishment, as might be expected at upper treeline or higher latitudes. Lodgepole pine grew taller than Douglas-fir and produced more needles with warming. Douglas-fir grew longer roots relative to shoots, compared with lodgepole pine, particularly in dry soils. Differences in early growth between species may mediate climate change effects on competitive interactions, successional trajectories, and species distributions. This study demonstrates that climate following high-severity fire exerts strong control over postfire tree regeneration in subalpine conifer forests. Climate change experiments, such as those reported here, hold great potential for identifying mechanisms that could underpin fundamental ecological change in 21st-century ecosystems.

Plant species with broad climatic ranges might be more vulnerable to climate change than previously appreciated due to intraspecific variation in climatic stress tolerance. In tropical forests, drought is increasingly frequent and severe, causing widespread declines and altering community dynamics. Yet, little is known about whether foundation tropical trees vary in drought tolerance throughout their distributions, and how intraspecific variation in drought tolerance might contribute to their vulnerability to climate change. We tested for local adaptation in seedling emergence and establishment with a full‐factorial reciprocal transplant experiment including 27 populations and 109,350 seeds along a 3,500 mm precipitation gradient for a widespread tropical foundation tree, Metrosideros polymorpha, in Hawaii. To more precisely relate seedling performance to soil moisture, we conducted a complementary greenhouse experiment to test responses of the same focal populations to simulated drought. In the reciprocal transplant experiment, we observed significant variation among populations and sites in germination and seedling establishment rates. Overall, there was a significant link between historical rainfall of populations and their performance under current rainfall at the study sites consistent with local adaptation. In particular, populations from historically wet sites demonstrated lower germination rates in currently dry sites compared to wet field sites, while populations from historically dry sites germinated well across all sites, with particularly high germination in dry sites. In the greenhouse, seedlings from wet populations survived fewer days without water, and succumbed at wetter soil conditions than populations from historically dry sites, corroborating results from the field experiment. Synthesis. While climate change models project the greatest drying trends for historically dry areas in Hawaii, even moderate drying of wet sites could significantly reduce Metrosideros polymorpha recruitment given the sensitivity of seedlings to very slight changes in water regimes. Thus, although M. polymorpha demonstrates high seedling drought tolerance in some populations, providing evidence of resilience at the species‐scale, there are nonetheless vulnerable populations that will likely decline under climate change. Our approach demonstrates that even trees with high dispersal abilities can show significant clines in drought tolerance, and suggests that similar intraspecific variation might be an important consideration for other tropical foundational tree species. Intraspecific variation in drought tolerance is likely to influence species resilience to climate change. Using an experimental approach, we found that historical rainfall predicts seedling performance in Hawaii’s most widespread tree, Metrosideros polymorpha, such that seedlings from historically wet sites germinate less in dry sites, have greater mortality under drought, last fewer days without water, and succumb at higher soil water contents than seedlings from historically dry sites. Photo Caption: Metrosideros polymorpha seedling four weeks after transplant at start of simulated drought treatment in greenhouse experiment. Photo by Kasey E. Barton.

• Emergent diseases are an increasing problem in forests worldwide. Exotic pathogens are now threatening forests where pathogens have not traditionally been considered to be major ecological drivers of tree demography, such as water-limited Mediterranean forests. However, how pathogens might limit regeneration in invaded forests is largely unknown. • Here we used fungicide to analyse the impact of soil-borne oomycete pathogens on seedling establishment at community level in Mediterranean forests invaded by the exotic oomycete Phytophthora cinnamomi. Fungicide effects were modelled as a function of the tree neighbourhood composition, the seed mass of the target species, and the abiotic environment. • Fungicide application had positive effects on seedling performance that varied in magnitude and spatial structure among coexisting species. Seed mass predicted fungicide effects on seedling emergence, but not on survival or growth. Positive fungicide effects were modulated by levels of abiotic resources, mainly water, increasing with soil moisture. • Our results support a novel role for soil-borne oomycete pathogens as one more axis of the regeneration niche of woody species in water-limited forests. Given the increasing numbers of exotic oomycete pathogens worldwide, more research is needed to understand the role of this relevant microbial group as a factor shaping seedling establishment.

Establishment and persistence are central to community assembly and are determined by how traits interact with the environment to determine performance (trait–environment interactions). Community assembly studies have rarely considered such trait–environment interactions, however, which can lead to incorrect inferences about how traits affect assembly. We evaluated how functional traits, environmental conditions, and trait–environment interactions structure plant establishment, as a measure of performance. Within 12 prairie restorations created by sowing 70 species, we quantified environmental conditions and counted individuals of each seeded species to quantify first-year establishment. Three trait–environment interactions structured establishment. Leaf nitrogen interacted with herbivore pressure, as low leaf nitrogen species established relatively better under higher herbivory than species with high leaf nitrogen. Soil moisture interacted with root mass fraction (RMF), with low-RMF species establishing better with low soil moisture and higher-RMF species better on wetter soils. Specific leaf area (SLA) interacted with light availability, as low-SLA species established better under high light conditions and high-SLA species under low light conditions. Our work illustrates how community assembly can be better described by trait–environment interactions than correlating traits or environment with performance. This knowledge can assist species selection to maximize restoration success.