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1. Though not often examined together, both plant-soil feedbacks (PSFs) and functional traits have important influences on plant community dynamics and could interact. For example, seedling functional traits could impact seedling survivorship responses to soils cultured by conspecific versus heterospecific adults. Furthermore, levels of functional traits could vary with soil culturing source. In addition, these relationships might shift with light availability, which can affect trait values, microbe abundance, and whether mycorrhizal colonization is mutualistic or parasitic to seedlings. 2. To determine the extent to which functional traits mediate PSFs via seedling survival, we conducted a field experiment. We planted seedlings of four temperate tree species across a gradient of light availability and into soil cores collected beneath conspecific (sterilized and live) and heterospecific adults. We monitored seedling survival twice per week over one growing season, and we randomly selected subsets of seedlings to measure mycorrhizal colonization and phenolics, lignin, and NSC levels at three weeks. 3. Though evidence for PSFs was limited, Acer saccharum seedlings exhibited positive PSFs (i.e., higher survival in conspecific than heterospecific soils). In addition, soil microbes had a negative effect on A. saccharum and Prunus serotina seedling survival, with reduced survival in live versus sterilized conspecific soil. In general, we found higher trait values (measured amounts of a given trait) in conspecific than heterospecific soils and higher light availability. Additionally, A. saccharum survival increased with higher levels of phenolics, which were higher in conspecific soils and high light. Quercus alba survival decreased with higher AMF colonization. 4. We demonstrate that functional trait values in seedlings as young as three weeks vary in response to soil source and light availability. Moreover, seedling survivorship was associated with trait values for two species, despite both drought and heavy rainfall during the growing season that may have obscured survivorship-trait relationships. These results suggest that seedling traits could have an important role in mediating the effects of local soil source and light levels on seedling survivorship and thus plant traits could have an important role in PSFs., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Wood et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential., (© 2022. The Author(s).)

Despite its importance for forest regeneration, food webs, and human economies, changes in tree fecundity with tree size and age remain largely unknown. The allometric increase with tree diameter assumed in ecological models would substantially overestimate seed contributions from large trees if fecundity eventually declines with size. Current estimates are dominated by overrepresentation of small trees in regression models. We combined global fecundity data, including a substantial representation of large trees. We compared size-fecundity relationships against traditional allometric scaling with diameter and two models based on crown architecture. All allometric models fail to describe the declining rate of increase in fecundity with diameter found for 80% of 597 species in our analysis. The strong evidence of declining fecundity, beyond what can be explained by crown architectural change, is consistent with physiological decline. A downward revision of projected fecundity of large trees can improve the next generation of forest dynamic models., Competing Interests: Competing interest statement: C.J.C. and J.C. are coauthors on a 2020 article; S.J.W. and J.C. are coauthors on a 2019 article; J.R.P. and J.C. are coauthors on a 2020 article; M.U., J.K.Z., and J.C. are coauthors on two 2019 articles; and M. Silman, W.F.R., and J.C. are coauthors on 2018, 2019, and 2020 articles.

Plant-soil feedbacks (PSFs) are often involved in fundamental ecological processes such as plant succession and species coexistence. After a plant initiating PSFs dies, legacies of PSFs occurring as soil signatures that influence subsequent plants could persist for an unknown duration. Altered resource environments following plant death (especially light availability) could affect whether legacy effects manifest and persist. To evaluate PSFs and their legacies, we obtained soils from a chronosequence of Prunus serotina harvests. In a greenhouse experiment, we planted conspecific seedlings under two light levels in these soils of varying time since the influence of live Prunus serotina, and compared seed/seedling survival in soils from live trees, stumps, and surrounding forest matrix within each site and across the chronosequence. PSF legacies were measured as the difference between seedling performance in live tree and stump soils within a site. Negative PSF legacies of P. serotina were short-lived, lasting up to 0.5 years after tree removal. These effects occurred under 5% but not 30% full sun. PSFs and their legacies manifested in seed/seedling survival, but not biomass. Though restricted to low light, short-lived legacies of P. serotina PSFs could have lasting impacts on plant community dynamics during post-disturbance regeneration by disfavoring P. serotina regeneration in small tree-fall gaps.

In tropical forest communities, seedling recruitment can be limited by the number of fruit produced by adults. Fruit production tends to be highly unequal among trees of the same species, which may be due to environmental factors. We observed fruit production for ~2,000 trees of 17 species across 3 years in a wet tropical forest in Costa Rica. Fruit production was modeled as a function of tree size, nutrient availability, and neighborhood crowding. Following model selection, tree size and neighborhood crowding predicted both the probability of reproduction and the number of fruit produced. Nutrient availability only predicted only the probability of reproduction. In all species, larger trees were more likely to be reproductive and produce more fruit. In addition, number of fruit was strongly negatively related to presence of larger neighboring trees in 13 species; presence of all neighboring trees had a weak-to-moderate negative influence on reproductive status in 16 species. Among various metrics of soil nutrient availability, only sum of base cations was positively associated with reproductive status, and for only four species. Synthesis Overall, these results suggest that direct influences on fruit production tend to be mediated through tree size and crowding from neighboring trees, rather than soil nutrients. However, we found variation in the effects of neighbors and nutrients among species; mechanistic studies of allocation to fruit production are needed to explain these differences.

Experiments with plant species that can host both arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (EMF) are important to separating the roles of fungal type and plant species and understanding the influence of the types of symbioses on plant growth and nutrient acquisition. We examined the effects of mycorrhizal fungal type on the growth and tissue nutrient content of two tree species (Eucalyptus grandis and Quercus costaricensis) grown under four nutrient treatments (combinations of low versus high nitrogen (N) and phosphorus (P) with different N:P ratios) in the greenhouse. Trees were inoculated with unidentified field mixtures of AMF or EMF species cultivated on root fragments of AMF- or EMF-specific bait plants. In E. grandis, inoculation with both AMF and EMF positively affected belowground plant dry weight and negatively affected aboveground dry weight, while only inoculation with AMF increased tissue nutrient content. Conversely, Q. costaricensis dry weight and nutrient content did not differ significantly among inoculation treatments, potentially due to its dependence on cotyledon reserves for growth. Mineral nutrition of both tree species differed with the ratio of N to P applied while growth did not. Our results demonstrate that both tree species' characteristics and the soil nutrient environment can affect how AMF and EMF interact with their host plants. This research highlights the importance of mycorrhizal fungal-tree-soil interactions during early seedling growth and suggests that differences between AMF and EMF associations may be crucial to understanding forest ecosystem functioning.

In closed-canopy forests, gap formation and closure are thought to be major drivers of forest dynamics. Crown defoliation by insects, however, may also influence understory resource levels and thus forest dynamics. We evaluate the effect of a forest tent caterpillar outbreak on understory light availability, soil nutrient levels and tree seedling height growth in six sites with contrasting levels of canopy defoliation in a hardwood forest in northern lower Michigan. We compared resource levels and seedling growth of six hardwood species before, during and in the three years after the outbreak (2008-2012). Canopy openness increased strongly during the forest tent caterpillar outbreak in the four moderately and severely defoliated sites, but not in lightly defoliated sites. Total inorganic soil nitrogen concentrations increased in response to the outbreak in moderately and severely defoliated sites. The increase in total inorganic soil nitrogen was driven by a strong increase in soil nitrate, and tended to become stronger with increasing site defoliation. Seedling height growth increased for all species in the moderately and severely defoliated sites, but not in lightly defoliated sites, either during the outbreak year or in the year after the outbreak. Growth increases did not become stronger with increasing site defoliation, but were strongest in a moderately defoliated site with high soil nutrient levels. Growth increases tended to be strongest for the shade intolerant species Fraxinus americana and Prunus serotina, and the shade tolerant species Ostrya virginiana. The strong growth response of F. americana and P. serotina suggests that recurring forest tent caterpillar outbreaks may facilitate the persistence of shade intolerant species in the understory in the absence of canopy gaps. Overall, our results suggest that recurrent canopy defoliation resulting from cyclical forest insect outbreaks may be an additional driver of dynamics in temperate closed-canopy forests., Competing Interests: The authors have declared that no competing interests exist.

Establishing diverse mycorrhizal fungal communities is considered important for forest recovery, yet mycorrhizae may have complex effects on tree growth depending on the composition of fungal species present. In an effort to understand the role of mycorrhizal fungi community in forest restoration in southern Costa Rica, we sampled the arbuscular mycorrhizal fungal (AMF) community across eight sites that were planted with the same species ( Inga edulis, Erythrina poeppigiana, Terminalia amazonia, and Vochysia guatemalensis ) but varied twofold to fourfold in overall tree growth rates. The AMF community was measured in multiple ways: as percent colonization of host tree roots, by DNA isolation of the fungal species associated with the roots, and through spore density, volume, and identity in both the wet and dry seasons. Consistent with prior tropical restoration research, the majority of fungal species belonged to the genus Glomus and genus Acaulospora , accounting for more than half of the species and relative abundance found on trees roots and over 95% of spore density across all sites. Greater AMF diversity correlated with lower soil organic matter, carbon, and nitrogen concentrations and longer durations of prior pasture use across sites. Contrary to previous literature findings, AMF species diversity and spore densities were inversely related to tree growth, which may have arisen from trees facultatively increasing their associations with AMF in lower soil fertility sites. Changes to AMF community composition also may have led to variation in disturbance susceptibility, host tree nutrient acquisition, and tree growth. These results highlight the potential importance of fungal-tree-soil interactions in forest recovery and suggest that fungal community dynamics could have important implications for tree growth in disturbed soils.

Understanding processes that promote species coexistence is integral to diversity maintenance. In hyperdiverse tropical forests, local conspecific density (LCD) and light are influential to woody seedling recruitment and soil nutrients are often limiting, yet the simultaneous effects of these factors on seedling survival across time remain unknown. We fit species- and age-specific models to census and resource data of seedlings of 68 woody species from a Costa Rican wet tropical forest. In decreasing order of prevalence, seedling survivorship was related to LCD, soil base cations, irradiance, nitrogen, and phosphorus. Species-specific responses to factors did not covary, providing evidence that species life history strategies have not converged to one continuum of high-surviving stress tolerant to low-surviving stress intolerant species. Survival responses to all factors varied over the average seedling's lifetime, indicating seedling requirements change with age and conclusions drawn about processes important to species coexistence depend on temporal resolution., (© 2016 by the Ecological Society of America.)

Plants that store nonstructural carbohydrates (NSC) may rely on carbon reserves to survive carbon-limiting stress, assuming that reserves can be mobilized. We asked whether carbon reserves decrease in resource stressed seedlings, and if NSC allocation is related to species' relative stress tolerances. We tested the effects of stress (shade, drought, and defoliation) on NSC in seedlings of five temperate tree species (Acer rubrum Marsh., Betula papyrifera Marsh., Fraxinus americana L ., Quercus rubra L., and Quercus velutina Lam.). In a greenhouse experiment, seedlings were subjected to combinations of shade, drought, and defoliation. We harvested seedlings over 32-97 days and measured biomass and NSC concentrations in stems and roots to estimate depletion rates. For all species and treatments, except for defoliation, seedling growth and NSC accumulation ceased. Shade and drought combined caused total NSC decreases in all species. For shade or drought alone, only some species experienced decreases. Starch followed similar patterns as total NSC, but soluble sugars increased under drought for drought-tolerant species. These results provide evidence that species deplete stored carbon in response to carbon limiting stress and that species differences in NSC response may be important for understanding carbon depletion as a buffer against shade- and drought-induced mortality.

Although one of the most widely studied hypotheses for high tree diversity in the tropics, the Janzen-Connell hypothesis (JC), and the community compensatory trend upon which it is based, have conflicting support from prior studies. Some of this variation could arise from temporal variation in seedling survival of common and rare species. Using 10 years of data from La Selva Biological Station in Costa Rica, we analyzed annual seedling survival and found that negative density-dependence (negative DD) was significantly stronger for rare species than for common species in 2 years and was significantly stronger for common species than for rare species in 4 years. This temporal variation in survival was correlated with climatic variables: in warmer and wetter years, common species had higher negative DD than rare species. The relationship between climate and variation in JC effects on seedling survival of common and rare species could have important consequences for the maintenance of tree species diversity in Central America, which is predicted to experience warmer and wetter years as global change proceeds.

Plant growth responses to resources may be an important mechanism that influences species' distributions, coexistence, and community structure. Irradiance is considered the most important resource for seedling growth in the understory of wet tropical forests, but multiple soil nutrients and species have yet to be examined simultaneously with irradiance under field conditions. To identify potentially limiting resources, we modeled tree seedling growth as a function of irradiance and soil nutrients across five sites, spanning a soil fertility gradient in old-growth, wet tropical forests at La Selva Biological Station, Costa Rica. We measured an array of soil nutrients including total nitrogen (total N), inorganic N (nitrate [NO3-] and ammonium [NH4+]), phosphate (PO4-), and sum of base cations (SBC; potassium, magnesium, and calcium). Shade in the forest understory did not preclude seedling growth correlations with soil nutrients. Irradiance was a significant predictor of growth in 52% of the species, inorganic N in 54% (NO3- in 32%; NH4+ in 34%), total N in 47%, SBC in 39%, and PO4- in 29%. Overall, growth was correlated with both irradiance and soil nutrients in 45% of species and with soil nutrients only in an additional 48%; rarely was irradiance alone correlated with growth. Contrary to expectations, the magnitudes of growth effects, assessed as the maximum growth response to significant resources for each species, were similar for irradiance and most soil nutrients. Among species whose growth correlated with soil nutrients, the rank importance of nutrient effects was SBC, followed by N (total N, NO3-, and/or NH4+) and PO4-. Species' growth responsiveness (i.e., magnitudes of effect) to irradiance and soil nutrients was negatively correlated with species' shade tolerance (survival under 1% full sun). In this broad survey of species and resources, the nearly ubiquitous effects of soil nutrients on seedling growth challenge the idea that soil nutrients are less important than irradiance in the light-limited understory of wet tropical forests.

Density-dependent seedling mortality could increase with a species relative abundance, thereby promoting species coexistence. Differences among species in light-dependent mortality also could enhance coexistence via resource partitioning. These compatible ideas rarely have been considered simultaneously. We developed models of mortality as functions of irradiance and local conspecific density (LCD) for seedlings of 53 tropical woody species. Species varied in mortality responses to these factors, but mortality consistently increased with shading and LCD. Across species, density-dependent mortality on a per-neighbour basis was inversely related to species community abundance, but higher LCD in more common species resulted in a weak relationship between species abundance and density-dependent mortality scaled to species maximum LCD. Species mortality responses to shading and maximum LCD were strongly and positively correlated. Our results suggest that species differences in density-dependent mortality are more strongly related to physiologically based life-history traits than biotic feedbacks related to community abundance., (© 2011 Blackwell Publishing Ltd/CNRS.)

Under optimal partitioning theory (OPT), plants preferentially allocate biomass to acquire the resource that most limits growth. Within this framework, higher root mass under low nutrients is often assumed to reflect an allocation response to build more absorptive surface. However, higher root mass also could result from increased storage of total nonstructural carbohydrates (TNC) without an increase in non-storage mass or root surface area. To test the relative contributions of TNC and non-storage mass as components of root mass responses to resources, we grew seedlings of seven northern hardwood tree species (black, red, and white oak, sugar and red maple, American beech, and black cherry) in a factorial light x nitrogen (N) greenhouse experiment. Because root mass is a coarse metric of absorptive surface, we also examined treatment effects on fine-root surface area (FRSA). Consistent with OPT, total root mass as a proportion of whole-plant mass generally was greater in low vs. high N. However, changes in root mass were influenced by TNC mass in all seven species and were especially strong in the three oak species. In contrast, non-storage mass contributed to increased total root mass under low N in three of the seven species. Root morphology also responded, with higher fine-root surface area (normalized to root mass) under low vs. high N in four species. Although biomass partitioning responses to resources were consistent with OPT, our results challenge the implicit assumption that increases in root mass under low nutrient levels primarily reflect allocation shifts to build more root surface area. Rather, root responses to low N included increases in: TNC, non-storage mass and fine-root surface area, with increases in TNC being the largest and most consistent of these responses. The greatest TNC accumulation occurred when C was abundant relative to N. Total nonstructural carbohydrates storage could provide seedlings a carbon buffer when respiratory or growth demands are not synchronized with photosynthesis, flexibility in responding to uncertain and fluctuating abiotic and biotic conditions, and increased access to soil resources by providing an energy source for mycorrhizae, decomposers in the rhizosphere, or root uptake of nutrients.

The fate of nitrogen (N) in senescing fine roots has broad implications for whole-plant N economies and ecosystem N cycling. Studies to date have generally shown negligible changes in fine root N per unit root mass during senescence. However, unmeasured loss of mobile non-N constituents during senescence could lead to underestimates of fine root N loss. For N fertilized and unfertilized potted seedlings of Populus tremuloides Michx., Acer rubrum L., Acer saccharum Marsh. and Betula alleghaniensis Britton, we predicted that the fine roots would lose mass and N during senescence. We estimated mass loss as the product of changes in root mass per length and root length between live and recently dead fine roots. Changes in root N were compared among treatments on uncorrected mass, length (which is independent of changes in mass per length), calcium (Ca) and corrected mass bases and by evaluating the relationships of dead root N as a function of live root N, species and fertilization treatments. Across species, from live to dead roots, mass decreased 28-40%, N uncorrected for mass loss increased 10-35%, N per length decreased 5-16%, N per Ca declined 14-48% and N corrected for mass declined 12-28%. Given the magnitude of senescence-related root mass loss and uncertainties about Ca dynamics in senescing roots, N loss corrected for mass loss is likely the most reliable estimate of N loss. We re-evaluated the published estimates of N changes during root senescence based on our values of mass loss and found an average of 28% lower N in dead roots than in fine roots. Despite uncertainty about the contributions of resorption, leaching and microbial immobilization to the net loss of N during root senescence, live root N was a strong and proportional predictor of dead root N across species and fertilization treatments, suggesting that live root N alone could be used to predict the contributions of senescing fine roots to whole-plant N economies and N cycling.

Seedling limitation could structure communities, but often is evaluated with sampling units that are orders of magnitude smaller than mature plants. We censused seedlings for 5.5 years in five 1 x 200-m transects in a wet Neotropical forest. For 106 common species (> or = 10 seedlings in a transect), we calculated prevalence (occurrence of > or = 1 newly emerged seedlings per sampling unit) at 1 m(2) and at 1 m x mature crown diameter units by aggregating adjacent quadrats. For most species, prevalence was 2-25% at 1 m(2), but 20-92% at mature crown scales. Increased prevalence arose from broadly distributed seedlings within transects, with unoccupied segments generally shorter than crown diameters. At the landscape scale, 69% of 301 species were locally rare (< 10 seedlings) and only 16% were represented in all transects (maximally separated by 2.4 km). Nonetheless, for more common species, much lower estimates of seedling limitation at mature crown scales suggest weaker influence of seedling limitation on community dynamics than previously assumed.

A negative feedback between local abundance and natural enemies could contribute to maintaining tree species diversity by constraining population growth of common species. Soil pathogens could be an important mechanism of such noncompetitive distance and density-dependent (NCDD) mortality, but susceptibility to local pathogens may be ameliorated by a life history strategy that favors survivorship. In a shade-house experiment (1% full sun), we tested seedling life span, growth, and mass allocation responses to microbial extract filtered from conspecific-cultured soil in 21 tree species that varied in abundance and shade tolerance in a wet tropical forest (La Selva Biological Station, Costa Rica). Forty-three percent of the species had significant reductions, and 10% of the species had significant increases in life span, growth, root length, or root surface area when inoculated with microbial extract; 10% of the species experienced opposing reductions and increases in these characteristics. Contrary to expectation, species' local abundance was not related to species-specific responses to microbial extracts from cultured soils. Across species, seedling shade tolerance (survival at 1% full sun) was negatively correlated with susceptibility to the microbial. treatment for both survival and total mass accumulation, thereby exaggerating shade tolerance differences among species. Thus, soil pathogens may contribute to species coexistence through heightening niche differentiation rather than through negative density dependence in common species.

Interspecific differences in sapling growth responses to soil resources could influence species distributions across soil resource gradients. I calibrated models of radial growth as a function of light intensity and landscape-level variation in soil water and foliar N for saplings of four canopy tree species, which differ in adult distributions across soil resource gradients. Model formulations, characterizing different resource effects and modes of influencing growth, were compared based on relative empirical support using Akaike's Information Criterion. Contrary to expectation, the radial growth of species associated with lower fertility (Acer rubrum and Quercus rubra) was more sensitive to variation in soil resources than the high fertility species Acer saccharum. Moreover, there was no species tradeoff between growth under high foliar N versus growth under low foliar N, which would be expected if growth responses to foliar N mediated distributions. In general, there was functional consistency among species in growth responses to light, foliar N, and soil water availability, respectively. Foliar N influenced primarily high-light growth in F. grandifolia, A. rubrum, and Q. rubra (but was not significant for A. saccharum). In A. saccharum and A. rubrum, for which soil water availability was a significant predictor, soil water and light availability simultaneously limited growth (i.e., either higher light or water increased growth). Simple resource-based models explained 0.74-0.90 of growth variance, indicating a high degree of determinism. Results suggest that nitrogen effects on forest dynamics would be strongest in high-light early successional communities but that water availability influences growth in both early successional and understory environments.

We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species -Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina- with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots.