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Understanding the mechanisms that promote the coexistence of hundreds of species over small areas in tropical forest remains a challenge. Many tropical tree species are presumed to be functionally equivalent shade tolerant species but exist on a continuum of performance trade‐offs between survival in shade and the ability to quickly grow in sunlight. These trade‐offs can promote coexistence by reducing fitness differences.Variation in plant functional traits related to resource acquisition is thought to predict variation in performance among species, perhaps explaining community assembly across habitats with gradients in resource availability. Many studies have found low predictive power, however, when linking trait measurements to species demographic rates.Seedlings face different challenges recruiting on the forest floor and may exhibit different traits and/or performance trade‐offs than older individuals face in the eventual adult niche. Seed mass is the typical proxy for seedling success, but species also differ in cotyledon strategy (reserve vs. photosynthetic) or other leaf, stem and root traits. These can cause species with the same average seed mass to have divergent performance in the same habitat.We combined long‐term studies of seedling dynamics with functional trait data collected at a standard life‐history stage in three diverse neotropical forests to ask whether variation in coordinated suites of traits predicts variation among species in demographic performance.Across hundreds of species in Ecuador, Panama and Puerto Rico, we found seedlings displayed correlated suites of leaf, stem, and root traits, which strongly correlated with seed mass and cotyledon strategy. Variation among species in seedling functional traits, seed mass, and cotyledon strategy were strong predictors of trade‐offs in seedling growth and survival. These results underscore the importance of matching the ontogenetic stage of the trait measurement to the stage of demographic dynamics.Our findings highlight the importance of cotyledon strategy in addition to seed mass as a key component of seed and seedling biology in tropical forests because of the contribution of carbon reserves in storage cotyledons to reducing mortality rates and explaining the growth‐survival trade‐off among species.Synthesis: With strikingly consistent patterns across three tropical forests, we find strong evidence for the promise of functional traits to provide mechanistic links between seedling form and demographic performance. [ABSTRACT FROM AUTHOR]

Summary: Among tropical forests, lianas are predicted to have a growth advantage over trees during seasonal drought, with substantial implications for tree and forest dynamics. We tested the hypotheses that lianas maintain higher water status than trees during seasonal drought and that lianas maximize leaf cover to match high, dry‐season light conditions, while trees are more limited by moisture availability during the dry season.We monitored the seasonal dynamics of predawn and midday leaf water potentials and leaf phenology for branches of 16 liana and 16 tree species in the canopies of two lowland tropical forests with contrasting rainfall regimes in Panama.In a wet, weakly seasonal forest, lianas maintained higher water balance than trees and maximized their leaf cover during dry‐season conditions, when light availability was high, while trees experienced drought stress. In a drier, strongly seasonal forest, lianas and trees displayed similar dry season reductions in leaf cover following strong decreases in soil water availability.Greater soil moisture availability and a higher capacity to maintain water status allow lianas to maintain the turgor potentials that are critical for plant growth in a wet and weakly seasonal forest but not in a dry and strongly seasonal forest. [ABSTRACT FROM AUTHOR]

Pre‐dispersal seed mortality caused by premature fruit drop is a potentially important source of plant mortality, but one which has rarely been studied in the context of tropical forest plants. Of particular interest is premature fruit drop triggered by enemies, which—if density dependent—could contribute to species coexistence in tropical forest plant communities.We used a long‐term (31 year) dataset on seed and fruit fall obtained through weekly collections from a network of seed traps in a lowland tropical forest (Barro Colorado Island, Panama) to estimate the proportion of seeds prematurely abscised for 201 woody plant species. To determine whether enemy attack might contribute to premature fruit drop, we tested whether plant species abscise more of their fruit prematurely if they (a) have attributes hypothesised to be associated with high levels of enemy attack and (b) are known to be attacked by one enemy group (insect seed predators). We also tested (c) whether mean rates of premature fruit drop for plant species are phylogenetically conserved.Overall rates of premature fruit drop were high in the plant community. Across all species, 39% of seeds were abscised before completing their development. Rates of premature seed abscission varied considerably among species and could not be explained by phylogeny. Premature seed abscission rates were higher in species which are known to host pre‐dispersal insect seed predators and species with attributes that were hypothesised to make them more susceptible to attack by pre‐dispersal enemies, namely species which (a) have larger seeds, (b) have a greater average height, (c) have temporally predictable fruiting patterns and (d) are more abundant at the study site.Synthesis. Premature fruit drop is likely to be a major source of seed mortality for many plant species on Barro Colorado Island. It is plausible that pre‐dispersal seed enemies, such as insect seed predators, contribute to community‐level patterns of premature fruit drop and have the potential to mediate species coexistence through stabilising negative density dependence. Our study suggests that the role of pre‐dispersal enemies in structuring tropical plant communities should be considered alongside the more commonly studied post‐dispersal seed and seedling enemies. [ABSTRACT FROM AUTHOR]

Nutrient addition experiments indicate that nitrogen and phosphorus limit plant processes in many tropical forests. However, the long‐term consequences for forest structure and species composition remain unexplored. We are positioned to evaluate potential long‐term consequences of nutrient addition in central Panama where we have maintained a factorial nitrogen–phosphorus–potassium fertilization experiment for 21 yr and an independent study quantified the species‐specific nutrient requirements of 550 local tree species. Here, we ask whether nutrients limit reproduction at the species and community levels. We also ask whether species‐specific reproductive responses to nutrient addition are stronger among species associated with naturally fertile soils, which could contribute to a shift in species composition. We quantified species‐level reproductive responses for 38 focal species in the 21st year of the experiment and community‐level reproductive litter production for the first 20 yr. Species‐level reproductive responses to nitrogen and potassium addition were weak, inconsistent across species, and insignificant across the 38 focal species. In contrast, species‐level responses to phosphorus addition were consistently and significantly positive across the 38 focal species but were unrelated to species‐specific phosphorus requirements documented independently for the same species. Community‐level reproductive litter production was unaffected by nutrient addition, possibly because spatial and temporal variation is large. We conclude that phosphorus limits reproduction by trees in our experiment but find no evidence that reproductive responses to phosphorus addition favor species associated with naturally phosphorus‐rich soils. [ABSTRACT FROM AUTHOR]

Tropical forest productivity represents an important global carbon sink, but many tropical forests grow on infertile soils. Efficient nutrient cycling by litterfall has long been assumed to maintain tropical tree growth, but there is no direct evidence that the nutrients cycled in litterfall are essential for tropical forest productivity.To test whether nutrient cycling by litterfall maintains tropical forest above‐ground productivity, we established large‐scale long‐term litter removal (L−) and litter addition (L+) treatments in a mature lowland tropical forest. We hypothesised that the removal of nutrients in litter would reduce tree growth, survival and litter production in L− plots. By contrast, the addition of nutrients in litter would enhance tree growth, survival and litter production in L+ plots. To test our hypotheses, we recorded tree growth and survival every 2 years, and measured litterfall monthly during 17 years of treatments.Tree growth and litterfall declined over time in L− plots, with consistently lower growth rates compared to controls after 8 years, and lower litter production after 4 years of treatments. By contrast, although litterfall was higher in the L+ plots relative to the controls, there was only a minor transient increase in tree growth immediately after the start of treatments. Tree survival declined over time in all treatments but was not affected by litter manipulation.The long‐term decline in tree growth and litterfall in the L− plots provides the first empirical evidence that nutrient cycling by litterfall plays a key role in maintaining above‐ground productivity in this tropical forest. By contrast, the transient increase in growth in the L+ plots can be attributed to the large inputs of nutrients with the addition of the entire litter standing crop at the start of treatments. The addition of nutrients in litter over the long term was nonetheless sufficient to enhance litter production, possibly by accelerating leaf turnover.Synthesis: Efficient nutrient cycling by litterfall makes an important contribution to the annual nutrient requirements of mature tropical forest trees, compensating for infertile soils. Disturbances that disrupt this finely balanced cycle could therefore reduce biomass carbon sequestration in tropical forests. [ABSTRACT FROM AUTHOR]

A Thomson Web of Science search for the topic "leaf age" AND potassium AND tropic* identified just one similar study, which reported a similar strong decline in foliar K with leaf age ([16]). This scientific commentary refers to 'Changes in leaf functional traits with leaf age: When do leaves decrease their photosynthetic capacity in Amazonian forests? Google Scholar Crossref Search ADS WorldCat 6 Kitajima K, Mulkey SS, Wright SJ (1997 a) Decline of photosynthetic capacity with leaf age in relation to leaf longevities for five tropical canopy tree species. The LDO model integrates seasonality in the timing of leaf production and abscission, total leaf area present and the contribution of expanding, mature and old leaves to successfully explain seasonality in gross ecosystem production. [Extracted from the article]

Abstract: We present a meta‐analysis of plant responses to fertilization experiments conducted in lowland, species‐rich, tropical forests. We also update a key result and present the first species‐level analyses of tree growth rates for a 15‐yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community‐level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low‐P soils would respond most slowly. The data did not support this prediction, species‐level tree growth responses to P addition were unrelated to species‐level soil P associations. The meta‐analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta‐analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta‐analysis found no support for the long‐standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species‐rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient‐poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected. [ABSTRACT FROM AUTHOR]

Producing and retaining leaves underlie the performance and survivorship of seedlings in deeply shaded tropical forests. These habitats are characterized by conditions ideal for foliar bacteria, which can be potent plant pathogens. Leaf production, retention and susceptibility to enemies may ultimately depend upon interactions among soil nutrients and foliar microbes, yet this has never been tested., We experimentally evaluated the degree that foliar bacteria and soil resource supply mediate leaf dynamics for five common tree species (five different families) in a Panamanian forest. We reduced foliar bacteria with antibiotics for 29 months and measured leaf production, retention and damage for seedlings nested within a replicated 15-yr factorial nutrient enrichment experiment (nitrogen, N; phosphorus, P; potassium, K)., Our results demonstrate that when we applied antibiotics, soil nutrients - particularly N - always regulated seedling leaf production (and to a lesser extent herbivore damage) for all five tree species. In addition, it was common for two macronutrients together to negate or completely reverse the impact of applying either one alone., Our findings of frequent plant-microbe-nutrient interactions are novel and suggest that these interactions may reinforce plant species-environment associations, thereby creating a fairly cryptic and fine-scale dimension of niche differentiation for coexisting tree species. [ABSTRACT FROM AUTHOR]

Multiple anthropogenic drivers affect every natural community, and there is broad interest in using functional traits to understand and predict the consequences for future biodiversity. There is, however, no consensus regarding the choice of analytical methods. We contrast species- and community-level analyses of change in the functional composition for four traits related to drought tolerance using three decades of repeat censuses of trees in the 50-ha Forest Dynamics Plot on Barro Colorado Island, Panama. Community trait distributions shifted significantly through time, which may indicate a shift toward more drought tolerant species. However, at the species level, changes in abundance were unrelated to trait values. To reconcile these seemingly contrasting results, we evaluated species-specific contributions to the directional shifts observed at the community level. Abundance changes of just one to six of 312 species were responsible for the community-level shifts observed for each trait. Our results demonstrate that directional changes in community-level functional composition can result from idiosyncratic change in a few species rather than widespread community-wide changes associated with functional traits. Future analyses of directional change in natural communities should combine community-, species-, and possibly individual-level analyses to uncover relationships with function that can improve understanding and enable prediction. [ABSTRACT FROM AUTHOR]

For species to coexist, performance must decline as the density of conspecific individuals increases. Although evidence for such conspecific negative density dependence (CNDD) exists in forests, the within-species spatial repulsion it should produce has rarely been demonstrated in adults. In this study, we show that in comparison to a null model of stochastic birth, death, and limited dispersal, the adults of dozens of tropical forest tree species show strong spatial repulsion, some to surprising distances of approximately 100 meters. We used simulations to show that such strong repulsion can only occur if CNDD considerably exceeds heterospecific negative density dependence—an even stronger condition required for coexistence—and that large-scale repulsion can indeed result from small-scale CNDD. These results demonstrate substantial niche differences between species that may stabilize species diversity. [ABSTRACT FROM AUTHOR]

Rare plant species often suffer stronger conspecific negative density dependence (CNDD) in studies that assess the impact of local conspecific density on individual survival. All else equal, this causes a relative disadvantage among rare species that appears inconsistent with the role of CNDD in coexistence. The resolution to this apparent paradox is for lower species abundance to decrease the frequency of conspecific interactions sufficiently to outweigh the disadvantage of stronger CNDD. Whether this occurs in natural systems is untested because existing metrics do not isolate demographic impacts of CNDD, and it is also uncertain for tropical forest trees because the greater spatial aggregation observed in rare species could cause higher frequency of conspecific interactions despite lower abundance on the landscape. We develop a new metric, effective density-dependent mortality (EDDM), to quantify the proportion of individuals that are killed by density-dependent effects. We apply EDDM to a long-term study of seed fall and recruitment at Barro Colorado Island, Panama. Rare species had stronger CNDD but lower conspecific densities, and EDDM increased with abundance. Lower abundance, thus, reduces the frequency of conspecific interaction and, consequently, mortality associated with CNDD. This mechanism allows rare species to avoid a disadvantage-when-rare that would, all else equal, result from stronger CNDD in rare species. Our work provides empirical support for a resolution to the apparently paradoxical findings that rare species experience stronger CNDD and may help reconcile contrasting findings for the relationship between the CNDD strength and abundance. [ABSTRACT FROM AUTHOR]

Contagious seed dispersal refers to the tendency for some sites to receive many dispersed seeds while other sites receive few dispersed seeds. Contagious dispersal can lead to interspecific associations in seed arrival, and this in turn might lead to interspecific associations in seedling recruitment. We evaluate the extent of spatially contagious seed arrival, the frequency of positive interspecific associations in seed arrival, and their consequences for seedling recruitment at the community level in a tropical moist forest. We quantified seed arrival to 200 passive seed traps for 28 yr of weekly censuses and seedling recruitment to 600 1-m2 quadrats for 21 yr of annual censuses on Barro Colorado Island, Panama. We assessed whether spatially contagious seed dispersal was more important among zoochorous species than among anemochorous species, increased in importance with similarity in fruiting times, and led to interspecific associations in seed arrival and seedling recruitment. We controlled adult seed source associations statistically to evaluate predicted relationships. We found that spatially contagious seed arrival was widespread among zoochorous species, but also occurred among anemochorous species when the strong, consistent trade winds were present. Significant interspecific associations in seed arrival were more likely for pairs of species with zoochorous seeds and similar fruiting times and persisted through seedling recruitment. Thus, interspecifically contagious seed dispersal affects local species composition and alters the mixture of interspecific interactions through the seed, germination, and early seedling stages in this forest. Future investigations should consider the implications of interspecific association at the regeneration stages documented here for later life stages and species coexistence. [ABSTRACT FROM AUTHOR]

Seed size and toughness affect seed predators, and size-dependent investment in mechanical defence could affect relationships between seed size and predation. We tested how seed toughness and mechanical defence traits (tissue density and protective tissue content) are related to seed size among tropical forest species. Absolute toughness increased with seed size. However, smaller seeds had higher specific toughness both within and among species, with the smallest seeds requiring over 2000 times more energy per gram to break than the largest seeds. Investment in mechanical defence traits varied widely but independently of the toughness-mass allometry. Instead, a physical scaling relationship confers a toughness advantage on small seeds independent of selection on defence traits and without a direct cost. This scaling relationship may contribute to seed size diversity by decreasing fitness differences among large and small seeds. Allometric scaling of toughness reconciles predictions and conflicting empirical relationships between seed size and predation. [ABSTRACT FROM AUTHOR]

Questions remain as to which soil nutrients limit primary production in tropical forests. Phosphorus (P) has long been considered the primary limiting element in lowland forests, but recent evidence demonstrates substantial heterogeneity in response to nutrient addition, highlighting a need to understand and diagnose nutrient limitation across diverse forests. Fine-root characteristics including their abundance, functional traits, and mycorrhizal symbionts can be highly responsive to changes in soil nutrients and may help to diagnose nutrient limitation. Here, we document the response of fine roots to long-term nitrogen (N), P, and potassium (K) fertilization in a lowland forest in Panama. Because this experiment has demonstrated that N and K together limit tree growth and P limits fine litter production, we hypothesized that fine roots would also respond to nutrient addition. Specifically we hypothesized that N, P, and K addition would reduce the biomass, diameter, tissue density, and mycorrhizal colonization of fine roots, and increase nutrient concentration in root tissue. Most morphological root traits responded to the single addition of K and the paired addition of N and P, with the greatest response to all three nutrients combined. The addition of N, P, and K together reduced fine-root biomass, length, and tissue density, and increased specific root length, whereas root diameter remained unchanged. Nitrogen addition did not alter root N concentration, but P and K addition increased root P and K concentration, respectively. Mycorrhizal colonization of fine roots declined with N, increased with P, and was unresponsive to K addition. Although plant species composition remains unchanged after 14 years of fertilization, fine-root characteristics responded to N, P, and K addition, providing some of the strongest stand-level responses in this experiment. Multiple soil nutrients regulate fine-root abundance, morphological and chemical traits, and their association with mycorrhizal fungi in a species-rich lowland tropical forest. [ABSTRACT FROM AUTHOR]

Defaunation alters trophic interactions between plants and vertebrates, which may disrupt trophic cascades, thereby favoring a subset of plant species and reducing diversity. If particular functional traits characterize the favored plant species, then defaunation may alter community-wide patterns of functional trait composition. Changes in plant functional traits occurring with defaunation may help identify the species interactions affected by defaunation and the potential for other cascading effects of defaunation. We tested the hypotheses that defaunation would (1) disrupt seed dispersal, thereby favoring species whose dispersal agents are not affected (e.g., small birds, bats, and abiotic agents), (2) reduce seed predation, thereby favoring larger-seeded species, and (3) reduce herbivory, thereby favoring species with lower leaf mass per area (LMA), leaf toughness, and wood density. We examined how these six traits responded to vertebrate defaunation caused by hunters or by experimental exclosures among more than 30 000 woody seedlings in a lowland tropical moist forest. Exclosures reduced terrestrial frugivores, granivores, and herbivores, while hunters also reduced volant and arboreal frugivores and granivores. The comparison of exclosures and hunting allowed us to parse the impacts of arboreal and volant species (reduced by hunters only) and terrestrial species (reduced by both hunters and exclosures). The loss of terrestrial vertebrates alone had limited effects on plant trait composition. The additional loss of volant and arboreal vertebrates caused significant shifts in plant species composition towards communities with more species dispersed abiotically, including lianas and low wood-density tree species, and fewer species dispersed by large vertebrates. In contrast to previous studies, community seed mass did not decline significantly in hunted sites. Our exclosure results suggest this is because reducing seed predators disproportionately benefits large-seeded species, partially compensating for the reduction of seed dispersers at hunted sites. Our results demonstrate that decreased seed dispersal and seed predation are important determinants of seedling community compositional change as a consequence of defaunation. Defaunation may also negatively impact carbon storage in tropical forests, by favoring lianas and low wood density tree species. [ABSTRACT FROM AUTHOR]

We reviewed the evidence on the extent and efficacy of conservation of tropical forest biodiversity for each of the classes of conservation action defined by the new International Union for Conservation of Nature (IUCN) classification. Protected areas are the most tested conservation approach, and a number of studies show they are generally effective in slowing deforestation. There is some documentation of the extent of sustainable timber management in tropical forest, but little information on other landscape-conservation tactics. The extent and effectiveness of ex situ species conservation is quite well known. Forty-one tropical-forest species now survive only in captivity. Other single-species conservation actions are not as well documented. The potential of policy mechanisms, such as international conventions and provision of funds, to slow extinctions in tropical forests is considerable, but the effects of policy are difficult to measure. Finally, interventions to promote tropical conservation by supporting education and livelihoods, providing incentives, and furthering capacity building are all thought to be important, but their extent and effectiveness remain poorly known. For birds, the best studied taxon, the sum of such conservation actions has averted one-fifth of the extinctions that would otherwise have occurred over the last century. Clearly, tropical forest conservation works, but more is needed, as is critical assessment of what works in what circumstances, if mass extinction is to be averted. [ABSTRACT FROM AUTHOR]

Summary: Tropical forest root characteristics and resource acquisition strategies are underrepresented in vegetation and global models, hampering the prediction of forest–climate feedbacks for these carbon‐rich ecosystems. Lowland tropical forests often have globally unique combinations of high taxonomic and functional biodiversity, rainfall seasonality, and strongly weathered infertile soils, giving rise to distinct patterns in root traits and functions compared with higher latitude ecosystems. We provide a roadmap for integrating recent advances in our understanding of tropical forest belowground function into vegetation models, focusing on water and nutrient acquisition. We offer comparisons of recent advances in empirical and model understanding of root characteristics that represent important functional processes in tropical forests. We focus on: (1) fine‐root strategies for soil resource exploration, (2) coupling and trade‐offs in fine‐root water vs nutrient acquisition, and (3) aboveground–belowground linkages in plant resource acquisition and use. We suggest avenues for representing these extremely diverse plant communities in computationally manageable and ecologically meaningful groups in models for linked aboveground–belowground hydro‐nutrient functions. Tropical forests are undergoing warming, shifting rainfall regimes, and exacerbation of soil nutrient scarcity caused by elevated atmospheric CO2. The accurate model representation of tropical forest functions is crucial for understanding the interactions of this biome with the climate. [ABSTRACT FROM AUTHOR]

The mission of the Smithsonian Tropical Research Institute (STRI) is to increase "...knowledge about the past, present and future of tropical biodiversity and its relevance to human welfare." Scientists pursue their own research interests within this broad mandate. This review concerns the history of STRI and recent ecological and applied research conducted at STRI, emphasizing research that extends across decades due to sustained efforts of single investigators or multiple investigators working across generations. STRI began as a rustic field station established in the forests of Barro Colorado Island (BCI), Panama in 1923 that prospered without outside funding setting the stage for the subsequent development of a major research center. Today, STRI employs 34 scientists, maintains nine field stations and six laboratories and hosts 1,200 visiting scientists and students each year. BCI provides examples of modern research being informed by results published up to 95 years earlier. Baselines recorded more systematically starting 50 years ago will be even more valuable in the future. The same will be true for each field station and research network described in this volume. As the natural world changes, data from these field stations and research networks will provide irreplaceable insights into how tropical forests and coral reefs once functioned and how function changed through time. [ABSTRACT FROM AUTHOR]

Summary: Nutrient limitation may constrain the ability of recovering and mature tropical forests to serve as a carbon sink. However, it is unclear to what extent trees can utilize nutrient acquisition strategies – especially root phosphatase enzymes and mycorrhizal symbioses – to overcome low nutrient availability across secondary succession.Using a large‐scale, full factorial nitrogen and phosphorus fertilization experiment of 76 plots along a secondary successional gradient in lowland wet tropical forests of Panama, we tested the extent to which root phosphatase enzyme activity and mycorrhizal colonization are flexible, and if investment shifts over succession, reflective of changing nutrient limitation. We also conducted a meta‐analysis to test how tropical trees adjust these strategies in response to nutrient additions and across succession.We find that tropical trees are dynamic, adjusting investment in strategies – particularly root phosphatase – in response to changing nutrient conditions through succession. These changes reflect a shift from strong nitrogen to weak phosphorus limitation over succession. Our meta‐analysis findings were consistent with our field study; we found more predictable responses of root phosphatase than mycorrhizal colonization to nutrient availability.Our findings suggest that nutrient acquisition strategies respond to nutrient availability and demand in tropical forests, likely critical for alleviating nutrient limitation. [ABSTRACT FROM AUTHOR]

Growing evidence suggests that liana competition with trees is threatening the global carbon sink by slowing the recovery of forests following disturbance. A recent theory based on local and regional evidence further proposes that the competitive success of lianas over trees is driven by interactions between forest disturbance and climate. We present the first global assessment of liana–tree relative performance in response to forest disturbance and climate drivers. Using an unprecedented dataset, we analysed 651 vegetation samples representing 26,538 lianas and 82,802 trees from 556 unique locations worldwide, derived from 83 publications. Results show that lianas perform better relative to trees (increasing liana‐to‐tree ratio) when forests are disturbed, under warmer temperatures and lower precipitation and towards the tropical lowlands. We also found that lianas can be a critical factor hindering forest recovery in disturbed forests experiencing liana‐favourable climates, as chronosequence data show that high competitive success of lianas over trees can persist for decades following disturbances, especially when the annual mean temperature exceeds 27.8°C, precipitation is less than 1614 mm and climatic water deficit is more than 829 mm. These findings reveal that degraded tropical forests with environmental conditions favouring lianas are disproportionately more vulnerable to liana dominance and thus can potentially stall succession, with important implications for the global carbon sink, and hence should be the highest priority to consider for restoration management. [ABSTRACT FROM AUTHOR]

I present a meta‐analysis of plant responses to 48 nutrient addition experiments conducted with native species in naturally growing tropical forests, exclusive of mangrove forests. The added nutrients include nitrogen (N) in 36 experiments, phosphorus (P) in 33 experiments, calcium and potassium in one experiment each, and various mixtures of essential nutrients in the remaining experiments. I evaluate the hypotheses that nutrients limit tropical forest plants, nutrient limitation is stronger in successional than in old‐growth forests, P but not N is limiting in lowland forests, and N is limiting in montane forests. Responses to the most complete nutrient mix used in each experiment were strong for plant functions that contribute to aboveground production (Hedges' g averages 0.87) and nonsignificant for fine root biomass. Responses to N addition and to P addition were strong for tissue concentrations of the added element (Hedges' g averages 0.75 and 1.4, respectively), moderate for fine litter production (0.64 and 0.65, respectively), moderate to weak for plant growth (0.46 and 0.37, respectively) and nonsignificant for fine root biomass. Growth responses were stronger in successional than in old‐growth forests. All responses were unrelated to elevation. The 48 experiments included 30 factorial nitrogen‐phosphorus experiments that enable additional direct tests of the widely cited hypotheses that P limitation is stronger than N limitation in lowland forests and vice versa in montane forests. Both hypotheses were rejected. The N × P interaction effect was nonsignificant across the factorial experiments. In conclusion, nutrients clearly limit tropical forest plants. Limitation by N is widespread in both lowland and montane forests, and the same is true for P. Single experiments identify limitation by calcium and potassium, and correlative studies suggest limitation by calcium, potassium, and magnesium. The available evidence is consistent with the possibility that most macronutrients limit tropical forest plants; however, experiments focus almost exclusively on N and P. The way forward will include taking fuller advantage of existing nutrient addition experiments, siting new experiments strategically, and developing cost‐effective methods to assay responses to all of the essential nutrients soils supply to plants. [ABSTRACT FROM AUTHOR]

Over the past two decades, liana density and basal area have been increasing in many tropical forests, which has profound consequences for forest diversity and functioning. One hypothesis to explain increasing lianas is elevated nutrient deposition in tropical forests resulting from fossil fuels, agricultural fertilizer, and biomass burning. We tested this hypothesis by surveying all lianas ≥1 cm in diameter (n = 3,967) in 32 plots in a fully factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment in a mature tropical forest in central Panama. We conducted the nutrient‐addition experiment from 1998 until present and we first censused lianas in 2013 and then again in 2018. After 20 yr of nutrient addition (1998–2018), liana density, basal area, and rarefied species richness did not differ significantly among any of the nutrient‐addition and control treatments. Moreover, nutrient addition in the most recent 5 yr of the experiment did not affect liana relative growth, recruitment, or mortality rates. From 2013 until 2018, liana density, basal area, and species richness increased annually by 1.6%, 1.4%, and 2.4%, respectively. Nutrient addition did not influence these increases. Our findings indicate that nutrient deposition does not explain increasing lianas in this tropical forest. Instead, increases in tree mortality and disturbance, atmospheric carbon dioxide, drought frequency and severity, and hunting pressure may be more likely explanations for the increase in lianas in tropical forests. [ABSTRACT FROM AUTHOR]

Flowering and fruiting phenology have been infrequently studied in the ever‐wet hyperdiverse lowland forests of northwestern equatorial Amazonía. These Neotropical forests are typically called aseasonal with reference to climate because they are ever‐wet, and it is often assumed they are also aseasonal with respect to phenology. The physiological limits to plant reproduction imposed by water and light availability are difficult to disentangle in seasonal forests because these variables are often temporally correlated, and both are rarely studied together, challenging our understanding of their relative importance as drivers of reproduction. Here we report on the first long‐term study (18 years) of flowering and fruiting phenology in a diverse equatorial forest, Yasuní in eastern Ecuador, and the first to include a full suite of on‐site monthly climate data. Using twice monthly censuses of 200 traps and >1000 species, we determined whether reproduction at Yasuní is seasonal at the community and species levels and analyzed the relationships between environmental variables and phenology. We also tested the hypothesis that seasonality in phenology, if present, is driven primarily by irradiance. Both the community‐ and species‐level measures demonstrated strong reproductive seasonality at Yasuní. Flowering peaked in September–November and fruiting peaked in March–April, with a strong annual signal for both phenophases. Irradiance and rainfall were also highly seasonal, even though no month on average experienced drought (a month with <100 mm rainfall). Flowering was positively correlated with current or near‐current irradiance, supporting our hypothesis that the extra energy available during the period of peak irradiance drives the seasonality of flowering at Yasuní. As Yasuní is representative of lowland ever‐wet equatorial forests of northwestern Amazonía, we expect that reproductive phenology will be strongly seasonal throughout this region. [ABSTRACT FROM AUTHOR]

Detection of flowering and quantification of flowering phenology are key to monitoring the reproduction of tropical trees and their response to global change. However, effective monitoring of flowering over various scales from individuals to forest ecosystem levels is lacking due to the relatively small sizes of flowers, diverse flowering strategies across species, and the short duration of flowering, making accurate flower detection difficult. Drone-based surveys require less time and human resources than traditional ground-based flower surveys and thus may be able to help address this in a cost-effective manner but remain underexplored in species-rich tropical forest ecosystems. Here, we developed a method that integrated the Residual Networks 50 (ResNet50) deep learning algorithm with high resolution imagery (c. 0.05 m) from monthly drone surveys done in a 50-ha tropical forest plot on Barro Colorado Island (BCI), Panamá, over 2018–2020 to detect a diversity of flowering species in this tree community and to track the timing of flowering throughout the year. We built a comprehensive training library of canopy components (flower, leaf, branch, and shade) from this forest plot throughout the study period, trained a single deep learning model across all drone imagery, and validated it using five-fold cross validation at the pixel scale. We further generated image- and tree-crown-specific supervised classifications to evaluate the deep learning model at the tree-crown scale. Our deep learning method accurately classified flowers (User's accuracy = 95.3 %, Producer's accuracy = 85.8 %) while maintaining high predictive power for the other three classes (Overall accuracy = 98.4 %). Our results also demonstrated high consistency against tree-crown-specific supervised classifications for flower (r2 = 0.85), leaf (r2 = 0.84), and branch (r2 = 0.92) components, with lower agreement observed for the shade component (r2 = 0.59). These results demonstrate the effectiveness of our method in advancing fine-scale flower monitoring in the tropics, with potential to be extended to other regions or other remote sensing platforms with frequent high-resolution monitoring. The method will allow us to better monitor flowering in tropical forests and improve our understanding of how phenology and reproductive success may be affected by climate change. [ABSTRACT FROM AUTHOR]

Summary: Terrestrial biosphere models (TBMs) include the representation of vertical gradients in leaf traits associated with modeling photosynthesis, respiration, and stomatal conductance. However, model assumptions associated with these gradients have not been tested in complex tropical forest canopies.We compared TBM representation of the vertical gradients of key leaf traits with measurements made in a tropical forest in Panama and then quantified the impact of the observed gradients on simulated canopy‐scale CO2 and water fluxes.Comparison between observed and TBM trait gradients showed divergence that impacted canopy‐scale simulations of water vapor and CO2 exchange. Notably, the ratio between the dark respiration rate and the maximum carboxylation rate was lower near the ground than at the top‐of‐canopy, leaf‐level water‐use efficiency was markedly higher at the top‐of‐canopy, and the decrease in maximum carboxylation rate from the top‐of‐canopy to the ground was less than TBM assumptions.The representation of the gradients of leaf traits in TBMs is typically derived from measurements made within‐individual plants, or, for some traits, assumed constant due to a lack of experimental data. Our work shows that these assumptions are not representative of the trait gradients observed in species‐rich, complex tropical forests. [ABSTRACT FROM AUTHOR]

Aim To evaluate how wood density relates to tree growth rates in simple models and two tropical forests. Location Barro Colorado Island, Panama; and Pasoh Forest Reserve, Malaysia. Time Period 1986-2010. Major Taxa Studied Trees. Methods We derived expected relationships of wood density with diameter growth at a given diameter under a null hypothesis that aboveground biomass growth is independent of wood density, and an alternative hypothesis that biomass growth scales with crown area, which itself increases with wood density. We tested these assumptions and predictions through analyses of interspecific relationships of wood density with height, crown area and diameter growth at constant diameter in two tropical forests. Results Height was unrelated to wood density, whereas crown areas showed a slightly positive relationship to wood density. Thus, the expected exponent of diameter growth with wood density was equal to minus one under the null hypothesis, and equal to the exponent of crown area with wood density minus one under the alternative hypothesis. Empirical relationships of diameter growth and biomass growth with wood density were broadly consistent with the null hypothesis that biomass growth is unrelated to wood density at both sites, except in trees < 13 cm in diameter at Barro Colorado Island, which showed more negative relationships. Main conclusions Although most previous analyses of growth with wood density have examined linear relationships, simple models suggest that both tree diameter growth and tree biomass growth are power functions of wood density. Analyses in two tropical forests showed that aboveground biomass growth was approximately constant with wood density, and thus, that diameter growth was inversely proportional to wood density, for most tree sizes, although confidence intervals on the scaling exponents were broad. More negative relationships of growth with wood density at small sizes might reflect differential environmental filtering, in which higher wood density trees are found in less favourable understorey environments. [ABSTRACT FROM AUTHOR]

Summary: The representation of stomatal regulation of transpiration and CO2 assimilation is key to forecasting terrestrial ecosystem responses to global change. Given its importance in determining the relationship between forest productivity and climate, accurate and mechanistic model representation of the relationship between stomatal conductance (gs) and assimilation is crucial.We assess possible physiological and mechanistic controls on the estimation of the g1 (stomatal slope, inversely proportional to water use efficiency) and g0 (stomatal intercept) parameters, using diurnal gas exchange surveys and leaf‐level response curves of six tropical broadleaf evergreen tree species.g1 estimated from ex situ response curves averaged 50% less than g1 estimated from survey data. While g0 and g1 varied between leaves of different phenological stages, the trend was not consistent among species. We identified a diurnal trend associated with g1 and g0 that significantly improved model projections of diurnal trends in transpiration.The accuracy of modeled gs can be improved by accounting for variation in stomatal behavior across diurnal periods, and between measurement approaches, rather than focusing on phenological variation in stomatal behavior. Additional investigation into the primary mechanisms responsible for diurnal variation in g1 will be required to account for this phenomenon in land‐surface models. [ABSTRACT FROM AUTHOR]

Topographic heterogeneity and lateral subsurface flow at the hillslope scale of ≤1 km may have outsized impacts on tropical forest through their impacts on water available to plants under water-stressed conditions. However, vegetation dynamics and finer-scale hydrologic processes are not concurrently represented in Earth system models. In this study, we integrate the Energy Exascale Earth System Model (E3SM) land model (ELM) that includes the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), with a three-dimensional hydrology model (ParFlow) to explicitly resolve hillslope topography and subsurface flow and perform numerical experiments to understand how hillslope-scale hydrologic processes modulate vegetation along water availability gradients at Barro Colorado Island (BCI), Panama. Our simulations show that groundwater table depth (WTD) can play a large role in governing aboveground biomass (AGB) when drought-induced tree mortality is triggered by hydraulic failure. Analyzing the simulations using random forest (RF) models, we find that the domain-wide simulated AGB and WTD can be well predicted by static topographic attributes, including surface elevation, slope, and convexity, and adding soil moisture or groundwater table depth as predictors further improves the RF models. Different model representations of mortality due to hydraulic failure can change the dominant topographic driver for the simulated AGB. Contrary to the simulations, the observed AGB in the well-drained 50 ha forest census plot within BCI cannot be well predicted by the RF models using topographic attributes and observed soil moisture as predictors, suggesting other factors such as nutrient status may have a larger influence on the observed AGB. The new coupled model may be useful for understanding the diverse impact of local heterogeneity by isolating the water availability and nutrient availability from the other external and internal factors in ecosystem modeling. [ABSTRACT FROM AUTHOR]

The response of tropical forests to anthropogenic climate change is critically important to future global carbon budgets, yet remains highly uncertain. Here, we investigate how precipitation, temperature, solar radiation and dry- and wet-season lengths are related to annual tree growth, flower production, and fruit production in three moist tropical forest tree species using long-term datasets from tree rings and litter traps in central Panama. We also evaluated how growth, flower, and fruit production were interrelated. We found that growth was positively correlated with wet-season precipitation in all three species: Jacaranda copaia ( r = 0.63), Tetragastris panamensis ( r = 0.39) and Trichilia tuberculata ( r = 0.39). Flowering and fruiting in Jacaranda were negatively related to current-year dry-season rainfall and positively related to prior-year dry-season rainfall. Flowering in Tetragastris was negatively related to current-year annual mean temperature while Trichilia showed no significant relationships of reproduction with climate. Growth was significantly related to reproduction only in Tetragastris, where it was positively related to previous year fruiting. Our results suggest that tree growth in moist tropical forest tree species is generally reduced by drought events such as those associated with strong El Niño events. In contrast, interannual variation in reproduction is not generally associated with growth and has distinct and species-specific climate responses, with positive effects of El Niño events in some species. Understanding these contrasting climate effects on tree growth and reproduction is critical to predicting changes in tropical forest dynamics and species composition under climate change. [ABSTRACT FROM AUTHOR]

As distinct community assembly processes can produce similar community patterns, assessing the ecological mechanisms promoting coexistence in hyperdiverse rainforests remains a considerable challenge. We use spatially explicit neighbourhood models of tree growth to quantify how functional trait and phylogenetic similarities predict variation in growth and crowding effects for the 315 most abundant tree species in a 25-ha lowland rainforest plot in Ecuador. We find that functional trait differences reflect variation in (1) species maximum potential growth, (2) the intensity of interspecific interactions for some species, and (3) species sensitivity to neighbours. We find that neighbours influenced tree growth in 28% of the 315 focal tree species. Neighbourhood effects are not detected in the remaining 72%, which may reflect the low statistical power to model rare taxa and/or species insensitivity to neighbours. Our results highlight the spectrum of ways in which functional trait differences can shape community dynamics in highly diverse rainforests. [ABSTRACT FROM AUTHOR]

Phosphorus (P) cycles rapidly in lowland tropical forest soils, but the process have been proven difficult to quantify. Recently it was demonstrated that valuable data on soil P transformations can be derived from the natural abundance of stable oxygen isotopes in phosphate (δ18OP). Here, we measured the δ18OP of soils that had received long-term nutrient additions (P, nitrogen, and potassium) or litter manipulations in lowland tropical forest in Panama and performed controlled incubations of fresh soils amended with a single pulse of P. To detect whether δ18OP values measured in the incubations apply also for soils in the field, we examined the δ18OP values after rewetting dry soils. In the incubations, resin-P δ18OP values converged to ∼3.5‰ above the expected isotopic equilibrium with soil water. This contrasts with extra-tropical soils in which the δ18OP of resin-P matches the expected equilibrium with soil water. Identical above-equilibrium resin-P δ18OP values were also found in field soils that did not receive P additions or extra litter. We suggest that the 3.5‰ above-equilibrium δ18OP values reflect a steady state between microbial uptake of phosphate (which enriches the remaining phosphate with the heavier isotopologues) and the release of isotopically equilibrated cell internal phosphate back to the soil. We also found that soil nutrient status affected the microbial turnover rate because in soils that had received chronic P addition, the original δ18OP signature of the fertilizer was preserved for at least eight weeks, indicating that the off-equilibrium δ18OP values produced during microbial phosphate turnover was not imprinted in these soils. Overall, our results demonstrate that ongoing microbial turnover of phosphate mediates its biological availability in lowland tropical soils. [ABSTRACT FROM AUTHOR]

As extreme climate events are predicted to become more frequent because of global climate change, understanding their impacts on natural systems is crucial. Tropical forests are vulnerable to droughts associated with extreme El Niño events. However, little is known about how tropical seedling communities respond to El Niño–related droughts, even though patterns of seedling survival shape future forest structure and diversity. Using long‐term data from eight tropical moist forests spanning a rainfall gradient in central Panama, we show that community‐wide seedling mortality increased by 11% during the extreme 2015–16 El Niño, with mortality increasing most in drought‐sensitive species and in wetter forests. These results indicate that severe El Niño–related droughts influence understory dynamics in tropical forests, with effects varying both within and across sites. Our findings suggest that predicted increases in the frequency of extreme El Niño events will alter tropical plant communities through their effects on early life stages. [ABSTRACT FROM AUTHOR]

Summary: The effects of climate change on tropical forests will depend on how diverse tropical tree species respond to drought. Current distributions of evergreen and deciduous tree species across local and regional moisture gradients reflect their ability to tolerate drought stress, and might be explained by functional traits.We measured leaf water potential at turgor loss (i.e. 'wilting point'; πtlp), wood density (WD) and leaf mass per area (LMA) on 50 of the most abundant tree species in central Panama. We then tested their ability to explain distributions of evergreen and deciduous species within a 50 ha plot on Barro Colorado Island and across a 70 km rainfall gradient spanning the Isthmus of Panama.Among evergreen trees, species with lower πtlp were associated with drier habitats, with πtlp explaining 28% and 32% of habitat association on local and regional scales, respectively, greatly exceeding the predictive power of WD and LMA. In contrast, πtlp did not predict habitat associations among deciduous species.Across spatial scales, πtlp is a useful indicator of habitat preference for tropical tree species that retain their leaves during periods of water stress, and holds the potential to predict vegetation responses to climate change. [ABSTRACT FROM AUTHOR]

Most tropical woody plants produce new leaves and flowers in bursts rather than continuously, and most tropical forest communities display seasonal variation in the presence of new leaves, flowers, and fruits. This patterning suggests that phonological changes represent adaptations to either biotic or abiotic factors. Biotic factors may select for either a staggering or a clustering of the phenological activity of individual plant species. We review the evidence for several hypotheses. The idea that plant species can reduce predation by synchronizing their phonological activity has the best support. However, because biotic factors are often arbitrary wit respect to the timing of these peaks, it is essential also to consider abiotic influences. A review of published studies demonstrates a major role for climate. Peaks in irradiance are accompanied by peaks in flushing and flowering except where water stress makes this impossible. Thus, in seasonally dry forests, many plants concentrate leafing and flowering around the start of the rainy season; they also tend to fruit at the same time, probably to minimize seedling mortality during the subsequent dry season. Phenological variation at the level of the forest community affects primary consumers who respond by dietary switching, seasonal breeding, changes in range use, or migration. During periods of scarcity, certain plant products, keystone resources, act as mainstays of the primary consumer community. [ABSTRACT FROM AUTHOR]

Tropical forests are a key determinant of the functioning of the Earth system, but remain a major source of uncertainty in carbon cycle models and climate change projections. In this study, we present an updated land model (LM3PPA‐TV) to improve the representation of tropical forest structure and dynamics in Earth system models (ESMs). The development and parameterization of LM3PPA‐TV drew on extensive datasets on tropical tree traits and long‐term field censuses from Barro Colorado Island (BCI), Panama. The model defines a new plant functional type (PFT) based on the characteristics of shade‐tolerant, tropical tree species, implements a new growth allocation scheme based on realistic tree allometries, incorporates hydraulic constraints on biomass accumulation, and features a new compartment for tree branches and branch fall dynamics. Simulation experiments reproduced observed diurnal and seasonal patterns in stand‐level carbon and water fluxes, as well as mean canopy and understory tree growth rates, tree size distributions, and stand‐level biomass on BCI. Simulations at multiple sites captured considerable variation in biomass and size structure across the tropical forest biome, including observed responses to precipitation and temperature. Model experiments suggested a major role of water limitation in controlling geographic variation forest biomass and structure. However, the failure to simulate tropical forests under extreme conditions and the systematic underestimation of forest biomass in Paleotropical locations highlighted the need to incorporate variation in hydraulic traits and multiple PFTs that capture the distinct floristic composition across tropical domains. The continued pressure on tropical forests from global change demands models which are able to simulate alternative successional pathways and their pace to recovery. LM3PPA‐TV provides a tool to investigate geographic variation in tropical forests and a benchmark to continue improving the representation of tropical forests dynamics and their carbon storage potential in ESMs. [ABSTRACT FROM AUTHOR]

Tropical forests challenge us to understand biodiversity, as numerous seemingly similar species persist on only a handful of shared resources. Recent ecological theory posits that biodiversity is sustained by a combination of species differences reducing interspecific competition and species similarities increasing time to competitive exclusion. Together, these mechanisms counterintuitively predict that competing species should cluster by traits, in contrast with traditional expectations of trait overdispersion. Here, we show for the first time that trees in a tropical forest exhibit a clustering pattern. In a 50‐ha plot on Barro Colorado Island in Panama, species abundances exhibit clusters in two traits connected to light capture strategy, suggesting that competition for light structures community composition. Notably, we find four clusters by maximum height, quantitatively supporting the classical grouping of Neotropical woody plants into shrubs, understory, midstory, and canopy layers. [ABSTRACT FROM AUTHOR]

Stomata regulate CO2 uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (gs) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water‐use‐efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψleaf), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Niño. The plant traits included wood density, leaf‐mass‐per‐area (LMA), leaf carboxylation capacity (Vc,max), leaf water content, the degree of isohydry, and predawn Ψleaf. We first investigated how the choice of four commonly used leaf‐level gs models with and without the inclusion of Ψleaf as an additional predictor variable influence the ability to predict gs, and then explored the abiotic (i.e. month, site‐month interaction) and biotic (i.e. tree‐species‐specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψleaf did not improve model performance and that the models that represent the response of gs to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each gs model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R2 = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of gs dynamics over seasonal drought, and identifies a practical, trait‐based approach to improve modeling of carbon and water exchange in tropical forests. [ABSTRACT FROM AUTHOR]

The crucial role of tropical forests in the global carbon balance is underpinned by their extraordinarily high biomass and productivity, even though the majority of tropical forests grow on nutrient-poor soils. Nutrient cycling by litterfall has long been considered essential for maintaining high primary productivity in lowland tropical forests but few studies have tested this assumption experimentally. We review and synthesise findings from the Gigante Litter Manipulation Project (GLiMP), a long-term experiment in lowland tropical forest in Panama, Central America, in which litter has been removed from or added to large-scale plots for 15 years. We assessed changes in soil and litter nutrient concentrations in response to the experimental treatments and estimated nutrient return and nutrient use efficiency to indicate changes in nutrient cycling. The soil concentrations of most nutrients increased with litter addition and declined with litter removal. Litter removal altered nitrogen, potassium, manganese and zinc cycling, demonstrating the importance of litter inputs for maintaining the availability of these elements to plants. By contrast, litter addition only altered nitrogen cycling and, despite low concentrations of available soil phosphorus, the effects of litter manipulation on phosphorus cycling were inconsistent. We discuss potential mechanisms underlying the observed changes, and we emphasise the importance of decomposition processes in the forest floor for retaining nutrient elements, which partially decouples nutrient cycling from the mineral soil. Finally, by synthesising GLiMP studies conducted during 15 years of litter manipulation, we highlight key knowledge gaps and avenues for future research into tropical forest nutrient cycling. [ABSTRACT FROM AUTHOR]

Seedlings in moist tropical forests must cope with deep shade and seasonal drought. However, the interspecific relationship between seedling performance in shade and drought remains unsettled. We quantified spatiotemporal variation in shade and drought in the seasonal moist tropical forest on Barro Colorado Island (BCI), Panama, and estimated responses of naturally regenerating seedlings as the slope of the relationship between performance and shade or drought intensity. Our performance metrics were relative height growth and first-year survival. We investigated the relationship between shade and drought responses for up to 63 species. There was an interspecific trade-off in species responses to shade versus species responses to dry season intensity; species that performed worse in the shade did not suffer during severe dry seasons and vice versa. This trade-off emerged in part from the absence of species that performed particularly well or poorly in both drought and shade. If drought stress in tropical forests increases with climate change and as solar radiation is higher during droughts, the trade-off may reinforce a shift towards species that resist drought but perform poorly in the shade by releasing them from deep shade. [ABSTRACT FROM AUTHOR]

The top‐down and indirect effects of insects on plant communities depend on patterns of host use, which are often poorly documented, particularly in species‐rich tropical forests. At Barro Colorado Island, Panama, we compiled the first food web quantifying trophic interactions between the majority of co‐occurring woody plant species and their internally feeding insect seed predators. Our study is based on more than 200 000 fruits representing 478 plant species, associated with 369 insect species. Insect host‐specificity was remarkably high: only 20% of seed predator species were associated with more than one plant species, while each tree species experienced seed predation from a median of two insect species. Phylogeny, but not plant traits, explained patterns of seed predator attack. These data suggest that seed predators are unlikely to mediate indirect interactions such as apparent competition between plant species, but are consistent with their proposed contribution to maintaining plant diversity via the Janzen–Connell mechanism. [ABSTRACT FROM AUTHOR]

Myxomycetes (plasmodial slime molds) are abundant protist predators that feed on bacteria and other microorganisms, thereby playing important roles in terrestrial nutrient cycling. Despite their significance, little is known about myxomycete communities and the extent to which they are affected by nutrient availability. We studied the influence of long‐term addition of N, P, and K on the myxomycete community in a lowland forest in the Republic of Panama. In a previous study, microbial biomass increased with P but not N or K addition at this site. We hypothesized that myxomycetes would increase in abundance in response to P but that they would not respond to the sole addition of N or K. Moist chamber cultures of leaf litter and small woody debris were used to quantify myxomycete abundance. We generated the largest myxomycete dataset (3,381 records) for any single locality in the tropics comprised by 91 morphospecies. In line with our hypothesis, myxomycete abundance increased in response to P addition but did not respond to N or K. Community composition was unaffected by nutrient treatments. This work represents one of very few large‐scale and long‐term field studies to include a heterotrophic protist highlighting the feasibility and value in doing so. [ABSTRACT FROM AUTHOR]

Patterns of seed dispersal and seed mortality influence the spatial structure of plant communities and the local coexistence of competing species. Most seeds are dispersed in proximity to the parent tree, where mortality is also expected to be the highest, because of competition with siblings or the attraction of natural enemies. Whereas distance‐dependent mortality in the seed‐to‐seedling transition was often observed in tropical forests, few studies have attempted to estimate the shape of the survival‐distance curves, which determines whether the peak of seedling establishment occurs away from the parent tree (Janzen–Connell pattern) or if the peak attenuates but remains at the parent location (Hubbell pattern). In this study, we inferred the probability density of seed dispersal and two stages of seedling establishment (new recruits, and seedlings 20 cm or taller) with distance for 24 tree species present in the 50‐ha Forest Dynamics Plot of Barro Colorado Island, Panama. Using data from seed traps, seedling survey quadrats, and tree‐census records spanning the 1988–2014 period, we fit hierarchical Bayesian models including parameters for tree fecundity, the shape of the dispersal kernel, and overdispersion of seed or seedling counts. We combined predictions from multiple dispersal kernels to obtain more robust inferences. We find that Hubbell patterns are the most common and Janzen–Connell patterns are very rare among those species; that distance‐dependent mortality may be stronger in the seed stage, in the early recruit stage, or comparable in both; and that species with larger seeds experience less overall mortality and less distance‐dependent mortality. Finally, we describe how this modeling approach could be extended at a community scale to include less abundant species. [ABSTRACT FROM AUTHOR]

Nonstructural carbohydrates (NSCs) are essential for maintenance of plant metabolism and may be sensitive to short‐ and long‐term climatic variation. NSC variation in moist tropical forests has rarely been studied, so regulation of NSCs in these systems is poorly understood. We measured foliar and branch NSC content in 23 tree species at three sites located across a large precipitation gradient in Panama during the 2015–2016 El Niño to examine how short‐ and long‐term climatic variation impact carbohydrate dynamics. There was no significant difference in total NSCs as the drought progressed (leaf P = 0.32, branch P = 0.30) nor across the rainfall gradient (leaf P = 0.91, branch P = 0.96). Foliar soluble sugars decreased while starch increased over the duration of the dry period, suggesting greater partitioning of NSCs to storage than metabolism or transport as drought progressed. There was a large variation across species at all sites, but total foliar NSCs were positively correlated with leaf mass per area, whereas branch sugars were positively related to leaf temperature and negatively correlated with daily photosynthesis and wood density. The NSC homoeostasis across a wide range of conditions suggests that NSCs are an allocation priority in moist tropical forests. Nonstructural carbohydrates (NSCs), the organic compounds that drive plant metabolism, have rarely been studied in moist tropical forests, so their regulation in these systems is poorly understood. We measured foliar and branch NSCs in canopy trees across a large precipitation gradient in Panama during the 2015–2016 El Niño to examine how short‐ and long‐term climatic variation impact carbohydrate dynamics. There was a large variation in NSCs across species; however, there was no change in total NSCs as the drought progressed nor across the rainfall gradient, suggesting that NSCs are an allocation priority in moist tropical forests. [ABSTRACT FROM AUTHOR]

Local tree species distributions in tropical forests correlate strongly with soil water availability. However, it is unclear how species distributions are shaped by demographic responses to soil water availability. Specifically, it remains unknown how growth affects species distributions along water availability gradients relative to mortality.We quantified spatial variation in dry season soil water potential (SWP) in the moist tropical forest on Barro Colorado Island, Panama, and used a hierarchical Bayesian approach to evaluate relationships between demographic responses of naturally regenerating seedlings to SWP (RGRs and first‐year mortality) and species distributions along the SWP gradient for 62 species. We also tested whether species that were more abundant at the wet or dry end of the gradient performed better (a) at their "home end" of the gradient ("best at home" hypothesis) and (b) "at home" compared to co‐occurring species ("home advantage" hypothesis).Four and five species responded significantly to SWP in terms of growth or mortality respectively. Growth (but not mortality) responses were positively related to species distributions along the SWP gradient; species with a more positive (negative) growth response to SWP were more abundant at higher (lower) SWP, that is, at wetter (drier) sites. In addition, wet distributed species grew faster on the wet end of the SWP gradient than on the dry end ("best at home") and grew faster on the wet end than dry distributed species ("home advantage"). Mortality rates declined with seedling size for all species. Thus, seedling growth responses to SWP indirectly shaped local species distributions by influencing seedling size and thereby mortality risk.Synthesis. By demonstrating how growth responses to spatial variation in soil water availability affect species distributions, we identified a demographic process underlying niche differentiation on hydrological gradients in tropical forests. Recognizing the role of these growth responses in shaping species distributions should improve the understanding of tropical forest composition and diversity along rainfall gradients and with climate change. This study shows that growth responses to soil moisture play an important role in shaping the distribution of tree seedlings along a local soil moisture gradient. Species that grew well on the wet end of the gradient were more abundant there because they became taller more quickly, reducing their mortality risk, and because they grew faster than dry distributed species. [ABSTRACT FROM AUTHOR]

Aims: Tropical tree and lianas in the understory are limited by soil nutrients despite growing in extremely low light. It is not known if nomadic vines are also limited by nutrients in low light conditions.Methods: We measured differences in root architecture and mycorrhizal colonization, and leaf nutrients of a nomadic vine, Philodendron fragrantissimum (Araceae), in nitrogen (N) and phosphorus (P) fertilization plots in a lowland tropical moist forest in central Panama to measure potential nutrient limitation.Results: Relative to plants in control plots, leaf P concentration was 54% higher and leaf N concentration was 10% higher for plants in the P- and N-addition treatments, respectively. The N:P of leaves suggested P-limitation in the N-addition treatment and the control but not in the P-addition treatment. Root branching was highest in the P-addition treatment, and P-addition reduced mycorrhizal colonization.Conclusions: The large effect of P fertilization suggests that, like many tropical plants, P. fragrantissimum has the potential to be P-limited. Although further study is needed, we suggest that nomadic vines be added to the growth forms that respond to nutrient addition in the forest understory and conclude that nutrient-limitation seems like the rule rather than the exception in the light-limited understory. [ABSTRACT FROM AUTHOR]

Abstract: Abiotic constraints and biotic interactions act simultaneously to shape communities. However, these community assembly mechanisms are often studied independently, which can limit understanding of how they interact to affect species dynamics and distributions. We develop a hierarchical Bayesian neighborhood modeling approach to quantify the simultaneous effects of topography and crowding by neighbors on the growth of 124,704 individual stems ≥1 cm DBH for 1,047 tropical tree species in a 25‐ha mapped rainforest plot in Amazonian Ecuador. We build multi‐level regression models to evaluate how four key functional traits (specific leaf area, maximum tree size, wood specific gravity and seed mass) mediate tree growth response to topography and neighborhood crowding. Tree growth is faster in valleys than on ridges and is reduced by neighborhood crowding. Topography and crowding interact to influence tree growth in ~10% of the species. Specific leaf area, maximum tree size and seed mass are associated with growth responses to topography, but not with responses to neighborhood crowding or with the interaction between topography and crowding. In sum, our study reveals that topography and neighborhood crowding each influence tree growth in tropical forests, but act largely independently in shaping species distributions. While traits were associated with species response to topography, their role in species response to neighborhood crowding was less clear, which suggests that trait effects on neighborhood dynamics may depend on the direction (negative/positive) and degree of symmetry of biotic interactions. Our study emphasizes the importance of simultaneously assessing the individual and interactive role of multiple mechanisms in shaping species dynamics in high diversity tropical systems. [ABSTRACT FROM AUTHOR]

Summary: The fate of tropical forests under climate change is unclear as a result, in part, of the uncertainty in projected changes in precipitation and in the ability of vegetation models to capture the effects of drought‐induced mortality on aboveground biomass (AGB). We evaluated the ability of a terrestrial biosphere model with demography and hydrodynamics (Ecosystem Demography, ED2‐hydro) to simulate AGB and mortality of four tropical tree plant functional types (PFTs) that operate along light‐ and water‐use axes. Model predictions were compared with observations of canopy trees at Barro Colorado Island (BCI), Panama. We then assessed the implications of eight hypothetical precipitation scenarios, including increased annual precipitation, reduced inter‐annual variation, El Niño‐related droughts and drier wet or dry seasons, on AGB and functional diversity of the model forest. When forced with observed meteorology, ED2‐hydro predictions capture multiple BCI benchmarks. ED2‐hydro predicts that AGB will be sustained under lower rainfall via shifts in the functional composition of the forest, except under the drier dry‐season scenario. These results support the hypothesis that inter‐annual variation in mean and seasonal precipitation promotes the coexistence of functionally diverse PFTs because of the relative differences in mortality rates. If the hydroclimate becomes chronically drier or wetter, functional evenness related to drought tolerance may decline. [ABSTRACT FROM AUTHOR]

Summary: Drought disproportionately affects larger trees in tropical forests, but implications for forest composition and carbon (C) cycling in relation to dry season intensity remain poorly understood. In order to characterize how C cycling is shaped by tree size and drought adaptations and how these patterns relate to spatial and temporal variation in water deficit, we analyze data from three forest dynamics plots spanning a moisture gradient in Panama that have experienced El Niño droughts. At all sites, aboveground C cycle contributions peaked below 50‐cm stem diameter, with stems ≥ 50 cm accounting for on average 59% of live aboveground biomass, 45% of woody productivity and 49% of woody mortality. The dominance of drought‐avoidance strategies increased interactively with stem diameter and dry season intensity. Although size‐related C cycle contributions did not vary systematically across the moisture gradient under nondrought conditions, woody mortality of larger trees was disproportionately elevated under El Niño drought stress. Thus, large (> 50 cm) stems, which strongly mediate but do not necessarily dominate C cycling, have drought adaptations that compensate for their more challenging hydraulic environment, particularly in drier climates. However, these adaptations do not fully buffer the effects of severe drought, and increased large tree mortality dominates ecosystem‐level drought responses. [ABSTRACT FROM AUTHOR]

Understanding variation in leaf functional traits-including rates of photosynthesis and respiration and concentrations of nitrogen and phosphorus-is a fundamental challenge in plant ecophysiology. When expressed per unit leaf area, these traits typically increase with leaf mass per area (LMA) within species but are roughly independent of LMA across the global flora. LMA is determined by mass components with different biological functions, including photosynthetic mass that largely determines metabolic rates and contains most nitrogen and phosphorus, and structural mass that affects toughness and leaf lifespan (LL). A possible explanation for the contrasting trait relationships is that most LMA variation within species is associated with variation in photosynthetic mass, whereas most LMA variation across the global flora is associated with variation in structural mass. This hypothesis leads to the predictions that (i) gas exchange rates and nutrient concentrations per unit leaf area should increase strongly with LMA across species assemblages with low LL variance but should increase weakly with LMA across species assemblages with high LL variance and that (ii) controlling for LL variation should increase the strength of the above LMA relationships. We present analyses of intra- and interspecific trait variation from three tropical forest sites and interspecific analyses within functional groups in a global dataset that are consistent with the above predictions. Our analysis suggests that the qualitatively different trait relationships exhibited by different leaf assemblages can be understood by considering the degree to which photosynthetic and structural mass components contribute to LMA variation in a given assemblage. [ABSTRACT FROM AUTHOR]

Abstract: Mounting evidence suggests that anthropogenic global change is altering plant species composition in tropical forests. Fewer studies, however, have focused on long‐term trends in reproductive activity, in part because of the lack of data from tropical sites. Here, we analyze a 28‐year record of tropical flower phenology in response to anthropogenic climate and atmospheric change. We show that a multidecadal increase in flower activity is most strongly associated with rising atmospheric CO2 concentrations using yearly aggregated data. Compared to significant climatic factors, CO2 had on average an approximately three‐, four‐, or fivefold stronger effect than rainfall, solar radiation, and the Multivariate ENSO Index, respectively. Peaks in flower activity were associated with greater solar radiation and lower rainfall during El Niño years. The effect of atmospheric CO2 on flowering has diminished over the most recent decade for lianas and canopy trees, whereas flowering of midstory trees and shrub species continued to increase with rising CO2. Increases in flowering were accompanied by a lengthening of flowering duration for canopy and midstory trees. Understory treelets did not show increases in flowering but did show increases in duration. Given that atmospheric CO2 will likely continue to climb over the next century, a long‐term increase in flowering activity may persist in some growth forms until checked by nutrient limitation or by climate change through rising temperatures, increasing drought frequency and/or increasing cloudiness and reduced insolation. [ABSTRACT FROM AUTHOR]