Explore the vast range of titles available.

Treefall gaps are the \"engines of regeneration\" in tropical forests and are loci of high tree recruitment, growth, and carbon accumulation. Gaps, however, are also sites of intense competition between lianas and trees, whereby lianas can dramatically reduce tree carbon uptake and accumulation. Because lianas have relatively low biomass, they may displace far more biomass than they contribute, a hypothesis that has never been tested with the appropriate experiments. We tested this hypothesis with an 8-yr liana removal experiment in central Panama. After 8 years, mean tree biomass accumulation was 180% greater in liana-free treefall gaps compared to control gaps. Lianas themselves contributed only 24% of the tree biomass accumulation they displaced. Scaling to the forest level revealed that lianas in gaps reduced net forest woody biomass accumulation by 8.9% to nearly 18%. Consequently, lianas reduce whole-forest carbon uptake despite their relatively low biomass. This is the first study to demonstrate experimentally that plant-plant competition can result in ecosystem-wide losses in forest carbon, and it has critical implications for recently observed increases in liana density and biomass on tropical forest carbon dynamics.

Eastern deciduous forests are changing in species composition and diversity outside of classical successional trajectories. Three disturbance mechanisms appear central to this phenomenon: fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. Which factor is most responsible is a matter of great debate and remains unclear, at least partly because few studies have simultaneously investigated more than one process. We conducted a large-scale experiment in mesophytic forests of West Virginia, USA, to test three key hypotheses: (1) the fire hypothesis (fire suppression limits diversity to few shade-tolerant, fire-intolerant species that replace and suppress many fire-tolerant species); (2) the gap hypothesis (small gaps typical of today's forests promote dominance of a few shade-tolerant species); and (3) the browsing hypothesis (overbrowsing by deer limits diversity to a few unpalatable species). We tested these hypotheses using a factorial experiment that manipulated surface fire, large canopy gap formation (gap size ∼255 m 2 ), and browsing by deer, and we followed the fates of >28 000 seedlings and saplings for five years. Understory tree communities in control plots were dominated (up to 90%) by Fagus grandifolia , averaging little more than two species, whereas overstories were diverse, with 10-15 species. Fire, large canopy gaps, and browsing all dramatically affected understory composition. However, our findings challenge views that fire and large canopy gaps can maintain or promote diversity, because browsers reduced the benefits of gaps and created depauperate understories following fire. Consequently, two major disturbances that once promoted tree diversity no longer do so because of browsing. Our findings appear to reconcile equivocal views on the role of fire and gaps. If browsers are abundant, these two disturbances either depress diversity or are less effective. Alternatively, with browsers absent, these disturbances promote diversity (three- to fivefold). Our results apply to large portions of eastern North America where deer are overabundant, and we provide compelling experimental evidence that historical disturbance regimes in combination with low browsing regimes typical of pre-European settlement forests could maintain high tree species diversity. However, restoring disturbances without controlling browsing may be counterproductive.

The mechanistic basis underpinning forest succession is the gap-phase paradigm in which overstory disturbance interacts with seedling and sapling shade tolerance to determine successional trajectories. The theory, and ensuing simulation models, typically assume that understory plants have little impact on the advance regeneration layer's composition. We challenge that assumption by reviewing over 125 papers on 38 species worldwide that form dense and persistent understory canopies. Once established, this layer strongly diminishes tree regeneration, thus altering the rate and direction of forest succession. We term these dense strata recalcitrant understory layers. Over half of the cases reviewed were linked to increases in canopy disturbance and either altered herbivory or fire regimes. Nearly 75% of the studies declared that competition and allelopathy were the likely interference mechanisms decreasing tree regeneration, yet only 25% of the studies used manipulative field experiments to test these putative mechanisms. We present a conceptual model that links the factors predisposing the formation of recalcitrant understory layers with their interference mechanisms and subsequent impacts on succession. We propose that their presence constricts floristic diversity and argue for their explicit inclusion in forest dynamics theory and models. Finally, we offer management suggestions to limit their establishment and mitigate their impacts.

Salvage logging following windthrow is common throughout forests worldwide even though the practice is often considered inimical to forest recovery. Because salvaging removes trees, crushes seedlings, and compacts soils, many warn this practice may delay succession, suppress diversity, and alter composition. Here, over 8 yr following windthrow, we experimentally evaluate how salvaging affects tree succession across 11 gaps in Eastern deciduous forests of Pennsylvania, wherein each gap was divided into salvaged and control (unsalvaged) halves. Our gaps vary in size and windthrow severity, and we explicitly account for this variation as well as variation in soil disturbance (i.e., scarification) resulting from salvaging so that our results would be generalizable. Salvage logging had modest and ephemeral impacts on tree succession. Seedling richness and density declined similarly over time in both salvaged and unsalvaged areas as individuals grew into saplings. The primary impact of salvaging on succession occurred where salvaging scarified soils. Here, salvaging caused 41 to 82% declines in sapling abundance, richness, and diversity, but these differences largely disappeared within 5 yr. Additionally, we documented interactions between windthrow severity and scarification. Specifically, low‐severity windthrow and scarification combined reinforced dominance by shade‐tolerant and browse‐tolerant species (Acer pensylvanicum, Fagus grandifolia). In contrast, high windthrow severity and scarification together reduced the density of a fast‐growing pioneer tree (Prunus pensylvanica) and non‐tree vegetation cover by 75% and 26%, respectively. This reduction enhanced the recruitment of two mid‐successional tree species, Acer rubrum and Prunus serotina, by 2 and 3‐fold, respectively. Thus, our findings demonstrate that salvaging creates novel microsites and mitigates competing vegetation, thereby enhancing establishment of important hardwoods and promoting tree species coexistence. Our results, coupled with an assessment of 27 published post‐windthrow salvage studies, suggest short‐term studies may overestimate the impact of salvaging on regeneration. We conclude that the ecological costs and benefits of salvaging depend upon the variation in canopy and soil disturbance severity as well as the timescale at which effects are evaluated. Thus, our findings are inconsistent with the view that salvaging inexorably undermines plant diversity; rather we suggest salvaging can promote tree species coexistence within various contexts.

1. Seed dispersal and subsequent recruitment dynamics play a crucially important role in regulating species coexistence and structuring tree diversity in diverse forests. Wildlife, which can dually shape the tree recruitment process by simultaneously functioning as natural enemies and seed dispersers, are undergoing widespread changes in population and behaviour due to the detrimental effects of expanding global road networks. However, the impact of these changes on recruitment dynamics through the alteration of seed dispersal processes remains understudied. 2. Here, we assessed how roads affect animal-mediated recruitment dynamics from the seed to adult stages of trees using a ubiquitous vertebrate-seed model: rodents and acorns. To quantify the degree to which proximity to a road alters seed dispersal and regulates subsequent recruitment dynamics, we conducted seed dispersal and prédation experiments, and investigated the natural recruitment of a dominant tree species (Quercus aliena) from September 2009 to July 2012 in a subtropical forest of central China. 3. Roads caused a decreased seed dispersal distance and increasing larder hoarding, demonstrating a weakened contribution of rodents to seed dispersal services. These patterns were stronger in the masting year than in the non-masting year. Correspondingly, seedlings, saplings and adult trees had higher densities near roads than far from roads. Near roads, recruit transition rates (i.e. seedling-to-sapling and sapling-to-juvenile) were low, and the influential strength of conspecific adult density was weakened over these recruit transitions, indicating that space limitation and lottery competition, not conspecific negative density-dependent effects, approximately determined the near-road recruitment process. Furthermore, most adults near roads were young, and their ages matched the road ages. 4. Synthesis, Roads diminished animal-mediated seed dispersal services and disrupted subsequent recruitment dynamics in Quercus aliena populations, thus weakening the key process promoting diversity in forest ecosystems. Given that these animal-regulated roles in recruitment dynamics are widespread stabilizing forces for tree coexistence, our findings suggest that the ubiquity of roads and their continued expansion will contribute to the increase in forests dominated by a relatively low number of species. Consequently, the growing expansion of road networks will ultimately cause profound changes in community composition and structure across diverse forests worldwide.

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.

Feedbacks between soil communities and plants may determine abundance and diversity in plant communities by influencing fitness and competitive outcomes. We tested the core hypotheses of soil community feedback theory: plant species culture distinct soil communities that alter plant performance and the outcome of interspecific competition. We applied this framework to inform the repeated dominance of Solidago canadensis in old-field communities. In glasshouse experiments, we examined the effects of soil communities on four plant species' performance in monoculture and outcomes of interspecific competition. We used terminal restriction fragment length polymorphism (TRFLP) analysis to infer differences in the soil communities associated with these plant species. Soil community origin had strong effects on plant performance, changed the intensity of interspecific competition and even reversed whether plant species were limited by conspecifics or heterospecifics. These plant–soil feedbacks are strong enough to upend winners and losers in classic competition models. Plant species cultured significantly different mycorrhizal fungal and bacterial soil communities, indicating that these feedbacks are likely microbiotic in nature. In old-fields and other plant communities, these soil feedbacks appear common, fundamentally alter the intensity and nature of plant competition and potentially maintain diversity while facilitating the dominance of So. canadensis.

Disruptions to historic disturbance and herbivory regimes have altered plant assemblages in forests worldwide. An emerging consensus suggests that these disruptions often result in impoverished forest biotas. This is particularly true for eastern U.S. deciduous forests where large gaps and understory fires were once relatively common and browsers were far less abundant. Although much research has focused on how disturbance and browsers affect tree diversity, far less attention has been devoted to forest understories where the vast majority (>75%) of the vascular species reside. Here we test the hypothesis that the reintroduction of disturbances resembling historic disturbance regimes and moderate levels of ungulate browsing enhance plant diversity. We explore whether once‐common disturbances and their interaction with the top‐down influence of browsers can create conditions favorable for the maintenance of a rich herbaceous layer in a region recognized as a temperate biodiversity hotspot in West Virginia, USA. We tested this hypothesis via a factorial experiment whereby we manipulated canopy gaps (presence/absence) of a size typically found in old‐growth stands, low‐intensity understory fire (burned/unburned), and deer browsing (fenced/unfenced). We tracked the abundance and diversity of more than 140 herb species for six years. Interactions among our treatments were pervasive. The combination of canopy gaps and understory fire increased herbaceous layer richness, cover, and diversity well beyond either disturbance alone. Furthermore, we documented evidence that deer at moderate levels of abundance promote herbaceous richness and abundance by preferentially browsing fast‐growing pioneer species that thrive following co‐occurring disturbances (i.e., fire and gaps). This finding sharply contrasts with the negative impact browsers have when their populations reach levels well beyond those that occurred for centuries. Although speculative, our results suggest that interactions among fire, canopy gaps, and browsing provided a variable set of habitats and conditions across the landscape that was potentially capable of maintaining much of the plant diversity found in temperate forests.

Intraspecific phytochemical variation across a landscape can cascade up trophic levels, potentially mediating the composition of entire insect communities. Surprisingly, we have little understanding of the processes that regulate and maintain phytochemical variation within species, likely because these processes are complex and operate simultaneously both temporally and spatially. To assess how phytochemistry varies within species, we tested the degree to which resource availability, contrasting soil type, and herbivory generate intraspecific chemical variation in growth and defense of the tropical shrub, Piper imperiale (Piperaceae). We quantified changes in both growth (e.g., nutritional protein, above- and below-ground biomass) and defense (e.g., imide chemicals) of individual plants using a well-replicated fully factorial shade-house experiment in Costa Rica. We found that plants grown in high light, nutrient- and richer old alluvial soil had increased biomass. High light was also important for increasing foliar protein. Thus, investment into growth was determined by resource availability and soil composition. Surprisingly, we found that chemical defenses decreased in response to herbivory. We also found that changes in plant protein were more plastic compared to plant defense, indicating that constitutive defenses may be relatively fixed, and thus an adaptation to chronic herbivory that is common in tropical forests. We demonstrate that intraspecific phytochemical variation of P. imperiale is shaped by resource availability from light and soil type. Because environmental heterogeneity occurs over small spatial scales (tens of meters), herbivores may be faced with a complex phytochemical landscape that may regulate how much damage any individual plant sustains.

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.