Ectomycorrhizal fungal community assembly on seedlings of a neotropical monodominant tree

Formation of ectomycorrhizae may facilitate early seedling survival of ectomycorrhizal tree species due to enhanced nutrient acquisition. This could be especially important in heavily shaded understories of tropical monodominant forests where host plant photosynthetic capacity is limited. Little information is available on ectomycorrhizal (ECM) fungal colonization or community development of monodominant seedling cohorts, which have high survival rates. Following a 2016 mast seeding event, we sequentially measured percent colonization and species composition of ECM fungi on live and recently dead seedlings of the tropical monodominant tree Dicymbe corymbosa. We also compared seedling ECM fungi to those of nearby adult conspecifics. Ectomycorrhizal fungal communities were remarkably different between seedling age classes, as well as between seedlings and adults. While the /russula-lactarius and /tomentella-thelephora lineages were species-rich throughout, there was 80–90% species turnover between 6- and 12-month-old seedlings. There was no difference in age-class fungal communities across sampling plots, indicating little spatial effect. Fungal colonization extent did not correlate with seedling age or differ markedly between live and dead seedlings. The number of ECM morphotypes increased with seedling age and tended to be greater on live versus dead seedlings. Interspecific competition between ECM fungi or soil nutrient fluxes may influence community assembly of ECM fungi in this tropical monodominant host tree

Dormancy-defense syndromes and tradeoffs between physical and chemical defenses in seeds of pioneer species

Seeds of tropical pioneer trees have chemical and physical characteristics that determine their capacity to persist in the soil seed bank. These traits allow seeds to survive in the soil despite diverse predators and pathogens, and to germinate and recruit even decades after dispersal. Defenses in seedlings and adult plants often are described in terms of tradeoffs between chemical and physical defense, but the interplay of defensive strategies has been evaluated only rarely for seeds. Here we evaluated whether classes of seed defenses were negatively correlated across species (consistent with tradeoffs in defense strategies), or whether groups of traits formed associations across species (consistent with seed defense syndromes). Using 16 of the most common pioneer tree species in a neotropical lowland forest in Panama we investigated relationships among four physical traits (seed fracture resistance, seed coat thickness, seed permeability, and seed mass) and two chemical traits (number of phenolic compounds and phenolic peak area), and their association with seed persistence. In addition, seed toxicity was assessed with bioassays in which we evaluated the activity of seed extracts against representative fungal pathogens and a model invertebrate. We did not find univariate tradeoffs between chemical and physical defenses. Instead, we found that seed permeability – a trait that distinguishes physical dormancy from other dormancy types – was positively associated with chemical defense traits and negatively associated with physical defense traits. Using a linear discriminant analysis and a hierarchical cluster analysis we found evidence to distinguish three distinct seed defense syndromes that correspond directly with seed dormancy classes (i.e., quiescent, physical, and physiological). Our data suggest that short and long‐term persistence of seeds can be achieved via two strategies: having permeable seeds that are well defended chemically, corresponding to the physiologically dormant defense syndrome; or having impermeable seeds that are well defended physically, corresponding to the physically dormant defense syndrome. In turn, transient seeds appear to have a lower degree of chemical and physical defenses, corresponding to the quiescent defense syndrome. Overall, we find that seed defense and seed dormancy are linked, suggesting that environmental pressures on seed persistence and for delayed germination can select for trait combinations defining distinct dormancy‐defense syndromes

Decadal survival of tropical pioneer seeds in the soil seedbank is accompanied by fungal infection and dormancy release

<p>Pioneer trees require high-light environments for successful seedling establishment. Consequently, seeds of these species often persist in the soil seed bank (SSB) for periods ranging from several weeks to decades. How they survive despite extensive pressure from seed predators and soilborne pathogens remains an intriguing question.</p> <p>This study aims to test the hypotheses that decades-old seeds collected from the SSB in a lowland tropical forest remain viable by i) escaping infection by fungi, which are major drivers of seed mortality in tropical soils, and/or ii) maintaining high levels of seed dormancy and seed coat integrity when compared to inviable seeds.</p> <p>We collected seeds of <em>Trema micrantha</em> and <em>Zanthoxylum ekmanii </em>at Barro Colorado Island, Panama, from sites where adult trees previously occurred in the past 30 years. We used carbon dating to measure seed age and characterized seed coat integrity, seed dormancy, and fungal communities.</p> <p>Viable seeds from the SSB ranged in age from 9 to 30 years for <em>T. micrantha</em>, and 5 to 33 years for <em>Z. ekmanii.</em> We found no evidence that decades-old seeds maintain high levels of seed dormancy or seed coat integrity. Fungi were rarely detected in fresh seeds (no soil contact), but phylogenetically diverse fungi were detected often in seeds from the SSB. Although fungal infections were more commonly detected in inviable seeds than in viable seeds, a lack of differences in fungal diversity and community composition between viable and inviable seeds suggested that viable seeds are not simply excluding fungal species to survive long periods in the SSB.</p> <p><em>Synthesis: </em>Our findings reveal the importance of a previously understudied aspect of seed survival, where the impact of seed-microbial interactions may be critical to understand long-term persistence in the SSB.</p><p>Funding provided by: National Science Foundation<br>Crossref Funder Registry ID: https://ror.org/021nxhr62<br>Award Number: DEB-1120205</p><p>Funding provided by: National Science Foundation<br>Crossref Funder Registry ID: https://ror.org/021nxhr62<br>Award Number: DEB-1119758</p><p>Funding provided by: Lawrence Livermore National Laboratory<br>Crossref Funder Registry ID: https://ror.org/041nk4h53<br>Award Number: DE-AC52-07NA27344</p><p>Seeds were extracted from soil cores by rinsing the soil with tap water under a series of sieves, with a minimum pore diameter of 2 mm for <em>Z. ekmanii</em> and 1 mm for <em>T. micrantha</em>. Seeds were identified to species and then partitioned for tests of i) seed viability measured using the tetrazolium test and seed-endophyte isolation frequency (692 and 593 seeds of <em>T. micrantha</em> and <em>Z. ekmanii,</em> respectively), ii) germination (% of germinated and dormant seeds) and carbon dating (a total of 239 and 196 seeds of <em>T. micrantha </em>and <em>Z. ekmanii</em>, respectively, where 9 viable seeds of each species were carbon dated), and iii) average seed fracture resistance as a measure of seed coat integrity (224 and 142 seeds of <em>T. micrantha </em>and <em>Z. ekmanii</em>, respectively).</p&gt

Does puberty mark a transition in sensitive periods for plasticity in the associative neocortex?

Postnatal brain development is studded with sensitive periods during which experience dependent plasticity is enhanced. This enables rapid learning from environmental inputs and reorganization of cortical circuits that matches behavior with environmental contingencies. Significant headway has been achieved in characterizing and understanding sensitive period biology in primary sensory cortices, but relatively little is known about sensitive period biology in associative neocortex. One possible mediator is the onset of puberty, which marks the transition to adolescence, when animals shift their behavior toward gaining independence and exploring their social world. Puberty onset correlates with reduced behavioral plasticity in some domains and enhanced plasticity in others, and therefore may drive the transition from juvenile to adolescent brain function. Pubertal onset is also occurring earlier in developed nations, particularly in unserved populations, and earlier puberty is associated with vulnerability for substance use, depression and anxiety. In the present article we review the evidence that supports a causal role for puberty in developmental changes in the function and neurobiology of the associative neocortex. We also propose a model for how pubertal hormones may regulate sensitive period plasticity in associative neocortex. We conclude that the evidence suggests puberty onset may play a causal role in some aspects of associative neocortical development, but that further research that manipulates puberty and measures gonadal hormones is required. We argue that further work of this kind is urgently needed to determine how earlier puberty may negatively impact human health and learning potential. This article is part of a Special Issue entitled SI: Adolescent plasticity