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Conspecific density may contribute to patterns of species assembly through negative density dependence (NDD) as predicted by the Janzen‐Connell hypothesis, or through facilitation (positive density dependence; PDD). Conspecific density effects are expected to be more negative in darker and wetter environments due to higher pathogen abundance and more positive in stressful, especially dry, environments (stress‐gradient hypothesis). For NDD to contribute to maintaining diversity, it should be apparent at the community‐wide scale as a negative correlation between seedling recruitment, growth or survival and conspecific adult abundance (community compensatory trend; CCT). We examined seedling survival in relation to con‐ and heterospecific adults within 10 m and con‐ and heterospecific seedlings within 1 m for 13 species within two 4‐ha permanent plots located in dry and wet forests in Hawaii. We also examined interactions between conspecific density and light and species’ commonness. For all species pooled, adult conspecific effects were positive (PDD) in both dry and wet forests, though they were stronger in the dry forest. In contrast, seedling conspecific effects were negative (NDD), though only significantly so in the wet forest. The strength and direction of density effects varied with understorey light such that seedlings had the highest survival where both adult conspecific density and light were high but the lowest survival where seedling conspecific density and light were high. In the wet forest, the most common species showed positive effects of adult conspecifics, but the less common species showed negative adult conspecific effects. We found mixed evidence for a CCT: seedling survival was positively correlated with basal area, but negatively correlated with tree density (stems ha⁻¹). Thus, it remains unclear whether NDD is a diversity‐maintaining mechanism in these forests. Synthesis. Overall, we found that positive conspecific effects influenced seedling mortality patterns more than negative interactions did, even in tropical wet forest where NDD is predicted to drive species’ abundances. Additionally, the strength and direction of density effects varied with forest type, PAR, and species’ abundance, underscoring the need to consider abiotic factors and species’ life‐history traits in tests of density dependence hypotheses.

Amazonian white-sand forests (campinarana) are highly specialized ecosystems characterized by nutrient-poor sandy soils and pronounced seasonal variation in water availability. Plant species inhabiting these environments are exposed to alternating periods of water deficit and soil saturation, which may strongly constrain recruitment and early establishment. Aldina heterophylla is an endemic tree species specialist in these habitats. This study evaluated seed germination, biomass allocation, anatomical traits, and early seedling responses of A. heterophylla under contrasting water regimes: control, partial flooding, and drought. Seedling performance was assessed after 50 and 100 days of treatment. After 50 days, flooded seedlings showed significantly greater root biomass than control plants, indicating short-term plastic adjustment to saturated soils. However, prolonged flooding induced chlorosis, necrosis, leaf abscission, and partial mortality. In contrast, drought-stressed seedlings developed leaf senescence but maintained 100% survival throughout the experiment. Morphological and physiological traits varied significantly over time among treatments, particularly leaf number, height, and chlorophyll dynamics. The results indicate that A. heterophylla seedlings are more tolerant to water deficit than to prolonged flooding, although they display adaptive responses to temporary soil saturation. These findings improve understanding of regeneration processes in Amazonian white-sand ecosystems and provide relevant information for conservation planning under increasing climatic extremes.

Agricultural land is being increasingly abandoned on a global scale, with over 200 million hectares recovering from agricultural use. Plant community regeneration differs greatly in structure and composition after agricultural impacts, yet the mechanisms underpinning these dramatic changes are poorly understood. It is critical to determine the relative importance of abiotic and biotic factors that limit plant establishment and success during the recovery process. In particular, below‐ground competition for resources in soils affected by former agricultural uses may play an important role in limiting plant establishment. Yet, below‐ground competition is generally studied less than above‐ground, especially in the context of land‐use history. We compare plant establishment with and without below‐ground competition in the context of a large‐scale experiment manipulating land‐use histories (i.e. with and without a history of agriculture) and restoration of historical vegetation structure (i.e. thinned and unthinned canopy trees) and determine how life stage and the local environment (e.g. soil water holding capacity, vegetation cover) influence this relationship. For three of our four target species, below‐ground competition strongly limited establishment success, but did not interact with land‐use history and canopy thinning directly. Instead, land‐use history and canopy thinning interacted to affect establishment during germination and survival in spring, while below‐ground competition limited growth during the summer. The strength of below‐ground competition was affected by local resources, but the directionality of this relationship depended on agricultural history and canopy thinning. Synthesis and applications. Because adding seeds increased establishment in all cases, we recommend confronting land‐use legacies by overcoming dispersal limitation with seed additions (even in degraded sites) and ensuring that below‐ground structures are managed during restoration, especially in summer. In addition, managers should consider how the relationship between local resources and below‐ground competition at individual sites might depend on land‐use history or canopy thinning. Because adding seeds increased establishment in all cases, we recommend confronting land‐use legacies by overcoming dispersal limitation with seed additions (even in degraded sites) and ensuring that below‐ground structures are managed during restoration, especially in summer. In addition, managers should consider how the relationship between local resources and below‐ground competition at individual sites might depend on land‐use history or canopy thinning.

Restoration of dryland ecosystems is often limited by low seedling establishment and survival. Defoliation caused by insects and small mammals could be an overlooked cause of seedling mortality. In the sagebrush steppe, we examined the effect of seedling defoliation on the survival of perennial grasses commonly used as restoration materials. Under field conditions, seedlings of three perennial bunchgrass species (non-native Agropyron cristatum, and native grasses Poa secunda and Pseudoroegneria spicata) were defoliated at two intensities (30 % and 70 % leaf length removal) and frequencies (one or two clippings) and compared to a non-defoliated control. Following emergence the first year, clippings occurred at the two-leaf stage; a second clipping occurred 1 month later for repeated defoliation treatments. We monitored seedling survival and tillering for 2 years. We expected higher defoliation intensity and frequency to reduce survival for all species, but only a few treatments reduced Po. secunda survival. Conversely, larger-statured Triticeae (wheatgrasses) benefited from some defoliation treatments. In both years, A. cristatum survival increased with repeated defoliation at both intensities. Defoliation did not affect Ps. spicata survival in the first year, but a single defoliation in the second year resulted in increased survival. In both A. cristatum and Ps. spicata, higher-intensity defoliation reduced the boost to survival resulting from defoliation frequency. Seedlings with more tillers had greater survival probabilities, but tiller number was unaffected by defoliation. Further research may elucidate mechanisms seedlings use to compensate for or benefit from defoliation. In the meantime, managers should aim to select defoliation-tolerant species if they anticipate herbivory will be problematic for restoration sites. Restoration of drylands is often limited by low seedling establishment. Seedling herbivory is suspected to limit plant establishment but is poorly understood. Under field conditions, we simulated seedling defoliation at different intensities and frequencies for grasses commonly seeded for revegetating degraded North American drylands. Individuals were monitored for 2 years. Higher defoliation intensity and frequency were expected to limit survival and vegetative growth; however, this only occurred for a native bluegrass. Overall, grass seedlings were largely resilient to defoliation, with wheatgrass survival greater under some defoliation treatments. Individuals with more tillers had greater survival but tillering was unaffected by defoliation.

• Premise of the study: Few studies have analyzed the physiological performance of different life stages and the expression of ontogenetic niche shifts in lianas. Here, we analyzed the photosynthetic and morphological acclimation of seedlings of Stigmaphyllon lindenianum, Combretum fruticosum, and Bonamia trichantha to distinctive light conditions in a tropical dry forest and compared their response with the acclimation response of adult canopy lianas of the same species. We expected acclimation to occur faster through changes in leaf photochemistry relative to adaptation in morphology, consistent with the life history strategies of these lianas.• Methods: Seedlings were assigned to the following light treatments: high light (HH), low light (LL), sun to shade (HL), and shade to sun (LH) in a common garden. After 40 d, HL and LH seedlings were exposed to opposite light treatments. Light response curves, the maximum photosynthetic rate in the field (Amax), and biomass allocation were monitored for another 40 d on leaves expanded before transfer.• Key results: Photosynthetic responses, Amax, and biomass of Stigmaphyllon and Combretum varied with light availability. Physiological characters were affected by current light environment. The previous light environment (carryover effects) only influenced Amax. Morphological characters showed significant carryover effects. Stigmaphyllon showed high morphological and physiological plasticity. Sun-exposed seedlings of this liana increased stem biomass and switched from self-supporting to climbing forms.• Conclusions: Acclimation in seedlings of these lianas is consistent with the response of adult lianas in the canopy in direction, but not in magnitude. There was no evidence for ontogenetic niche shifts in the acclimation response.

1 The role of seed limitation in tropical forests remains uncertain owing to the scarcity of experimental evidence. We performed seed addition experiments to assess seed limitation for 32 shade-tolerant tropical forest species and monitored the natural seed rain of 25 of these species for 17 years. 2 One, two or five seeds were sown into 0.0079-m2 plots for large- (n = 5 species), medium- (n = 5) and small-seeded species (n = 22), respectively. The experiment was replicated at 69 sites, placed in groups of three at 23 locations. Seedling establishment was evaluated after 1 and 2 years in paired seed addition and control plots. Natural seedling emergence and understorey plant density were also measured. 3 Median natural seed rain was 0.31 seeds m(-2) year(-1) per focal species. 4 Seed addition enhanced seedling establishment in 31 and 26 of the 32 species after 1 and 2 years, respectively. Mean number of focal species' seedlings after 2 years was 0.002 seedlings in control plots and 0.12, 0.37 and 0.60 seedlings in seed addition plots for large-, medium- and small-seeded species, respectively. 5 A 25 seeds added treatment increased seedling establishment by greater than or equal to 2.0-fold over the five seeds added treatment after 2 years. 6 Community-wide recruitment and understorey plant density were strongly seed-limited. The natural density of understorey plants averaged 12 plants m(-2) and was significantly less than for seedlings of the single focal species in plots with greater than or equal to 2 seeds added 2 years earlier. 7 The number of established seedlings per seed added was independent of seed size. 8 Treatment (adding zero or five seeds), species identity and location all affected seedling establishment for the 11 small-seeded species represented at all sites, with treatment and its interactions accounting for 86% of the explained variation. 9 Our results suggest that seed limitation plays a dominant role in seedling recruitment and understorey plant community assembly in tropical forests. Although strong seed limitation may set the stage for species-neutral community assembly, the species differences in seedling establishment rate and its spatial variation demonstrate an important role for species-specific processes.

Amazonian floodplain forests (known as várzea) are classified into high or low várzea depending on the spatial position on the plains. This topographic feature exposes the terrain over different time periods of inundation, causing a major limiting factor for tree seedling establishment. We hypothesize that, strategically, most of the seeds produced by trees in low várzea forests germinate faster and in synchrony (temporally concentrated germination), and that their seedlings tend to have cotyledons without reserve or foliaceous cotyledons (PEF). By contrast, seeds produced by high-várzea specialist trees exhibit slower and temporally scattered germination, and their seedlings tend to have reserve storage cotyledons (CHR). Generalist species may show no clear pattern or may be related to high-várzea species. To test this hypothesis, diaspores of 10 tree species were collected: five of low-várzea specialist trees, three of high-várzea specialist trees and two of generalist species Seedling emergence and morphology were monitored daily in a nursery for a period of 150 days of being subjected to non-flooded (sown directly in várzea soil) and flooded conditions (15 days in water before sowing in the same soil). The seedling emergence of low-várzea species showed an increase of 37% in germinability whereas high-várzea and generalist species exhibited a decrease of 38% and 35% of germinability, respectively. Foliaceous cotyledons were preferentially found in seedlings of low-várzea species, and storage cotyledons were more common in those of high-várzea species, indicating how cotyledon morphology may determine the amount and use of resources available to a seedling during the first stages of establishment and growth. Conservation plans aiming for the maintenance of ecosystem services must consider these strategies.

1. Acer platanoides (Norway maple) is an important non-native invasive canopy tree in North American deciduous forests, where native species diversity and abundance are greatly reduced under its canopy. We conducted a field experiment in North American forests to compare planted seedlings of A. platanoides and Acer saccharum (sugar maple), a widespread, common native that, like A. platanoides, is shade tolerant. Over two growing seasons in three forests we compared multiple components of seedling success: damage from natural enemies, ecophysiology, growth and survival. We reasoned that equal or superior performance by A. platanoides relative to A. saccharum indicates seedling characteristics that support invasiveness, while inferior performance indicates potential barriers to invasion. 2. Acer platanoides seedlings produced more leaves and allocated more biomass to roots, A. saccharum had greater water use efficiency, and the two species exhibited similar photosynthesis and first-season mortality rates. Acer platanoides had greater winter survival and earlier spring leaf emergence, but second-season mortality rates were similar. 3. The success of A. platanoides seedlings was not due to escape from natural enemies, contrary to the enemy release hypothesis. Foliar insect herbivory and disease symptoms were similarly high for both native and non-native, and seedling biomass did not differ. Rather, A. platanoides compared well with A. saccharum because of its equivalent ability to photosynthesize in the low light herb layer, its higher leaf production and greater allocation to roots, and its lower winter mortality coupled with earlier spring emergence. Its only potential barrier to seedling establishment, relative to A. saccharum, was lower water use efficiency, which possibly could hinder its invasion into drier forests. 4. The spread of non-native canopy trees poses an especially serious problem for native forest communities, because canopy trees strongly influence species in all forest layers. Success at reaching the canopy depends on a tree's ecology in previous life-history stages, particularly as a vulnerable seedling, but little is known about seedling characteristics that promote non-native tree invasion. Experimental field comparison with ecologically successful native trees provides insight into why non-native trees succeed as seedlings, which is a necessary stage on their journey into the forest canopy.

Differential stress tolerance among species is a central concept for understanding coexistence in diverse ecosystems. Although multiple co-occurring stressors are present in many ecosystems, relatively little is understood as to how communities are shaped by interacting stressors at multiple scales. We use a regression design to test the effects of two environmental stress gradients, flooding and shade, as well as herbivore activity, mechanical damage, on first-year transplanted seedling survival and growth in twenty-one common garden plots in secondary Amazonian floodplain forests. Among ten woody species varying in flood and shade tolerance, half of the seedlings were clipped at 5 cm aboveground, removing ∼50% of plant biomass. Damage reduced seedling survival by ∼50% and aboveground biomass by 94% of undamaged seedlings. Despite declines in relative growth rates with increasing flood duration, the survival of most flood-tolerant species was unaffected by variation in prolonged submergence of 3-6 months. Meanwhile, low-flood-tolerant species displayed clear thresholds in survival of flood duration <150 d. The effects of all three stressors were largely independent, with the exception of two species whereby light availability interacted with flood duration to enhance seedling survival during prolonged floods and one species whereby flood duration interacted with damage. Trade-offs in stress tolerance among species were not apparent. Rather, damage, shade, and flood tolerance among species were all positively correlated, suggesting that combined stresses favor the persistence of a few highly stress-tolerant species-characterized by slow growth rates, high root:shoot ratios, and short stems-in this disturbed ecosystem. This study highlights the importance of dry season stressors (e.g., light availability, herbivore activity, and drought) as strong selective forces for the establishment and persistence of tropical floodplain communities. Furthermore, in an ecosystem where relatively little is known as to how seedlings persist in forest understories, we demonstrate how biotic and abiotic stressors ultimately shape woody species composition and richness.

Shorea tumbuggaia is an endemic and globally endangered, red-listed, semi-evergreen tree species, restricted to the southern Eastern Ghats in Andhra Pradesh and Tamil Nadu. The flowering is ephemeral and also not an annual event. Massive blooming, drooping inflorescence with pendulous flowers, ample pollen production, gradual pollen release as a function of anther appendage, aerodynamic size of the pollen grains with reticulate exine and muri separated by lumina and strong protogyny – all contribute to anemophily. The plant is self-compatible but there appears to be abortion of fruits from selfed-flowers. The fruits are large, winged and anemochorous. The fruits are attacked by an unidentified bruchid beetle prior to dispersal. In healthy fruits, the seed has no dormancy and it germinates as soon as it falls from the tree. The study reveals that non-annual, massive flowering, short flowering period, partial flowering at tree level, seed predation, short-distance seed dispersal, absence of seed dormancy, low rate of seedling establishment and inability of seedlings to compete with other plants collectively contributed to the occurrence of a small population of S. tumbuggaia in a restricted area of the Eastern Ghats forests and interplay of all these factors might have led to the 'endangered' status of this species.