Tropical forests are renowned for their high diversity, yet in many sites a single tree species accounts for the majority of the individuals in a stand. An explanation for these monodominant forests ...remains elusive, but may be linked to mycorrhizal symbioses. We tested three hypotheses by which ectomycorrhizas might facilitate the dominance of the tree, Oreomunnea mexicana, in montane tropical forest in Panama. We tested whether access to ectomycorrhizal networks improved growth and survival of seedlings, evaluated whether ectomycorrhizal fungi promote seedling growth via positive plant–soil feedback, and measured whether Oreomunnea reduced inorganic nitrogen availability. We found no evidence that Oreomunnea benefits from ectomycorrhizal networks or plant–soil feedback. However, we found three‐fold higher soil nitrate and ammonium concentrations outside than inside Oreomunnea‐dominated forest and a correlation between soil nitrate and Oreomunnea abundance in plots. Ectomycorrhizal effects on nitrogen cycling might therefore provide an explanation for the monodominance of ectomycorrhizal tree species worldwide.
Wood contains the majority of the nutrients in tropical trees, yet controls over wood nutrient concentrations and their function are poorly understood.
We measured wood nutrient concentrations in 106 ...tree species in 10 forest plots spanning a regional fertility gradient in Panama. For a subset of species, we quantified foliar nutrients and wood density to test whether wood nutrients scale with foliar nutrients at the species level, or wood nutrient storage increases with wood density as predicted by the wood economics spectrum.
Wood nutrient concentrations varied enormously among species from fourfold in nitrogen (N) to > 30-fold in calcium (Ca), potassium (K), magnesium (Mg) and phosphorus (P). Community-weighted mean wood nutrient concentrations correlated positively with soil Ca, K, Mg and P concentrations. Wood nutrients scaled positively with leaf nutrients, supporting the hypothesis that nutrient allocation is conserved across plant organs. Wood P was most sensitive to variation in soil nutrient availability, and significant radial declines in wood P indicated that tropical trees retranslocate P as sapwood transitions to heartwood. Wood P decreased with increasing wood density, suggesting that low wood P and dense wood are traits associated with tree species persistence on low fertility soils.
Substantial variation among species and communities in wood nutrient concentrations suggests that allocation of nutrients to wood, especially P, influences species distributions and nutrient dynamics in tropical forests.
Extending Plant Defense Theory to Seeds Dalling, James W; Davis, Adam S; Arnold, A. Elizabeth ...
Annual review of ecology, evolution, and systematics,
11/2020, Volume:
51, Issue:
1
Journal Article
Peer reviewed
Open access
Plant defense theory explores how plants invest in defenses against natural enemies but has focused primarily on the traits expressed by juvenile and mature plants. Here we describe the diverse ways ...in which seeds are chemically and physically defended. We suggest that through associations with other traits, seeds are likely to exhibit defense syndromes that reflect constraints or trade-offs imposed by selection to attract dispersers, enable effective dispersal, ensure appropriate timing of seed germination, and enhance seedling performance. We draw attention to seed and reproductive traits that are analogous to defense traits in mature plants and describe how the effectiveness of defenses is likely to differ at pre- and postdispersal stages. We also highlight recent insights into the mutualistic and antagonistic interactions between seeds and microbial communities, including fungi and endohyphal bacteria, that can influence seed survival in the soil and subsequent seedling vigor.
Lianas are a key component of tropical forests; however, most surveys are too small to accurately quantify liana community composition, diversity, abundance, and spatial distribution - critical ...components for measuring the contribution of lianas to forest processes. In 2007, we tagged, mapped, measured the diameter, and identified all lianas ≥1 cm rooted in a 50-ha plot on Barro Colorado Island, Panama (BCI). We calculated liana density, basal area, and species richness for both independently rooted lianas and all rooted liana stems (genets plus clones). We compared spatial aggregation patterns of liana and tree species, and among liana species that varied in the amount of clonal reproduction. We also tested whether liana and tree densities have increased on BCI compared to surveys conducted 30-years earlier. This study represents the most comprehensive spatially contiguous sampling of lianas ever conducted and, over the 50 ha area, we found 67,447 rooted liana stems comprising 162 species. Rooted lianas composed nearly 25% of the woody stems (trees and lianas), 35% of woody species richness, and 3% of woody basal area. Lianas were spatially aggregated within the 50-ha plot and the liana species with the highest proportion of clonal stems more spatially aggregated than the least clonal species, possibly indicating clonal stem recruitment following canopy disturbance. Over the past 30 years, liana density increased by 75% for stems ≥1 cm diameter and nearly 140% for stems ≥5 cm diameter, while tree density on BCI decreased 11.5%; a finding consistent with other neotropical forests. Our data confirm that lianas contribute substantially to tropical forest stem density and diversity, they have highly clumped distributions that appear to be driven by clonal stem recruitment into treefall gaps, and they are increasing relative to trees, thus indicating that lianas will play a greater role in the future dynamics of BCI and other neotropical forests.
A trade-off between growth and mortality rates characterizes tree species in closed canopy forests. This trade-off is maintained by inherent differences among species and spatial variation in light ...availability caused by canopy-opening disturbances. We evaluated conditions under which the trade-off is expressed and relationships with four key functional traits for 103 tree species from Barro Colorado Island, Panama. The trade-off is strongest for saplings for growth rates of the fastest growing individuals and mortality rates of the slowest growing individuals (
r
2
= 0.69), intermediate for saplings for average growth rates and overall mortality rates (
r
2
= 0.46), and much weaker for large trees (
r
2
≤ 0.10). This parallels likely levels of spatial variation in light availability, which is greatest for fast- vs. slow-growing saplings and least for large trees with foliage in the forest canopy. Inherent attributes of species contributing to the trade-off include abilities to disperse, acquire resources, grow rapidly, and tolerate shade and other stresses. There is growing interest in the possibility that functional traits might provide insight into such ecological differences and a growing consensus that seed mass (SM), leaf mass per area (LMA), wood density (WD), and maximum height (
H
max
) are key traits among forest trees. Seed mass, LMA, WD, and
H
max
are predicted to be small for light-demanding species with rapid growth and mortality and large for shade-tolerant species with slow growth and mortality. Six of these trait-demographic rate predictions were realized for saplings; however, with the exception of WD, the relationships were weak (
r
2
< 0.1 for three and
r
2
< 0.2 for five of the six remaining relationships). The four traits together explained 43-44% of interspecific variation in species positions on the growth-mortality trade-off; however, WD alone accounted for >80% of the explained variation and, after WD was included, LMA and
H
max
made insignificant contributions. Virtually the full range of values of SM, LMA, and
H
max
occurred at all positions on the growth-mortality trade-off. Although WD provides a promising start, a successful trait-based ecology of tropical forest trees will require consideration of additional traits.
Slower leaf litter decomposition rates for trees associated with ectomycorrhizal (ECM) fungi compared to arbuscular mycorrhizal (AM) fungi may lead to the development of conservative nitrogen cycling ...and accumulation of soil organic matter in surface soils of ECM-dominated forests. Slower decomposition is hypothesized to occur via two often-confounded mechanisms: production of lower quality litter by ECM-associated trees compared to AM-associated trees and competition for nitrogen between ECM and saprotrophic decomposers in ECM-dominated stands. To disentangle the effects of litter quality and stand mycorrhizal type on decomposition, we measured litter mass loss rates of two AM species higher and two AM species similar in chemical quality to two ECM species. Leaf litter was decomposed for two years in neighboring ECM- and AM-dominated stands of a wet tropical montane forest. Litter phosphorus (P) was the strongest predictor of decomposition rates across all species, with no effect of litter mycorrhizal type on decomposition rates. Only one species, which exhibited intermediate litter chemical quality and decomposition rate, decomposed significantly faster in AM- compared to ECM-dominated stands. Leaf litter decomposition rates cannot be predicted directly from litter mycorrhizal type or stand mycorrhizal type because litter chemical quality and environmental conditions mediate the manifestation of slower decomposition in ECM stands.
Wood is a major carbon input into aquatic ecosystems and is thought to decay slowly, yet surprisingly little terrestrial carbon accumulates in marine sediments. A better mechanistic understanding of ...how habitat conditions and decomposer communities influence wood decay processes along the river–estuary–ocean continuum can address this seeming paradox. We measured mass loss, wood element, and polymer concentrations, quantified invertebrate-induced decay, and sequenced fungal communities associated with replicate sections of Guazuma branch wood submerged in freshwater, estuarine, and near-shore marine habitats and placed on the soil surface in nearby terrestrial habitats in three watersheds in the tropical eastern Pacific. Over 15 months, we found that wood decayed at similar rates in estuarine, marine, and terrestrial sites, reflecting the combined activity of invertebrate and microbial decomposers. In contrast, in the absence of shipworms (Teredinidae), which accounted for ~40% of wood mass loss in the estuarine habitats, decay proceeded more slowly in freshwater. Over the experiment, wood element chemistry diverged among freshwater, estuarine, and marine habitats, due to differences in both nutrient losses (e.g., potassium and phosphorus) and gains (e.g., calcium and aluminum) through decay. Similarly, we observed changes in wood polymer content, with the highest losses of cellulose, hemicellulose, and lignin moieties in the marine habitat. Aquatic fungal communities were strongly dominated by ascomycetes (88–99% of taxa), compared to terrestrial communities (55% ascomycetes). Large differences in fungal diversity were also observed across habitats with threefold higher richness in terrestrial than freshwater habitats and twofold higher diversity in freshwater than estuarine/marine habitats. Divergent decay trajectories across habitats were associated with widespread order-level differences in fungal composition, with distinct communities found in freshwater, estuarine and marine habitats. However, few individual taxa that were significantly associated with mass loss were broadly distributed, suggesting a high level of functional redundancy. The rapid processing of wood entering tropical rivers by microbes and invertebrates, comparable to that on land, indicates that estuaries and coastal oceans are hotspots not just for the processing of particulate and dissolved organic carbon, but also for woody debris and for the breakdown of lignin, the most recalcitrant polymer in plant tissue.
1. Plant defence theory provides a robust framework for understanding interactions between plants and antagonists, and for interpreting broad patterns in the functional‐trait composition of plant ...communities. However, this framework has been built almost entirely on traits expressed by seedlings and mature plants. 2. No equivalent seed defence theory exists that recognizes the distinct suite of natural enemies that seeds encounter, and the unique constraints to their response. Furthermore, most attention has been paid to insect and vertebrate seed predators active above ground, whereas microbes in soil also have large effects on seed survival, particularly for plants that recruit from soil seed banks. 3. We suggest that concurrent selection on seed dormancy and resistance to microbial antagonists should result in distinct seed defence syndromes. We predict that species with physical seed dormancy will rely on physical defences to exclude predators and pathogens, and rapid seed germination to escape pathogens at the emergence stage. In contrast, species with physiological seed dormancy will deploy a continuum of physical and chemical defences, depending on soil pathogen pressure and duration of seed persistence. Finally, seeds of some species persist in the soil in a non‐dormant, imbibed state, and lack obvious chemical and physical defences. These seeds may be especially dependent upon protection from beneficial seed‐inhabiting microbes. 4. Framing a general ‘seed defence theory' may help to account for the distribution of seed dormancy types across ecosystems. We predict that physiological dormancy will be favoured in dry or well‐drained environments where pathogen pressure is relatively low, germination cues are most unpredictable, and seedling recruitment success is most variable. In contrast, physical dormancy should be favoured in warm and moist environments where pathogen pressure is high, and where germination cues are a stronger predictor of recruitment success. Persistent, non‐dormant seeds are restricted to relatively aseasonal environments where favourable conditions for recruitment can occur over most of the year. 5. Synthesis. Integrating seed defence and dormancy traits can provide new insights into selection on dormancy types, and will help elucidate major trends in seed ecology and evolution. Understanding how seeds are defended also may improve our ability to predict plant regeneration and help develop innovative management strategies for weedy and invasive species.
Background and aims
Bark contains a substantial fraction of the nutrients stored in woody biomass, however the degree of functional coordination of bark, wood, and foliar nutrient pools, and its ...relationship to soil nutrient availability remains poorly understood.
Methods
Bark thickness and nitrogen, phosphorus, potassium, calcium, and magnesium concentrations were measured in 23 tree species present in two premontane wet tropical forests in western Panama differing in soil nutrient availability. Bark data were combined with existing wood and leaf data from the same species.
Results
Bark nutrients were positively correlated with leaf and wood nutrients for all elements. The low fertility site had both lower bark nutrient concentrations and thicker bark, driven primarily by species compositional differences between sites, and secondarily by intraspecific variation. Across species, bark nutrient concentration varied 4 to 25 fold, with the highest variation for calcium. Overall, bark accounted for the largest percent of Ca in above-ground biomass nutrient pools (22–82%) and a large fraction of the other nutrients studied (N: 6–53%, P: 5–50%, K: 4–40%, and Mg: 2–35%).
Conclusions
Bark represents a substantial, and highly variable, pool of biomass nutrients. The functional role of bark nutrients, the causes and consequences of this variation, and its relation to other bark traits, including bark thickness, deserve further study.
Both habitat filtering and dispersal limitation influence the compositional structure of forest communities, but previous studies examining the relative contributions of these processes with ...variation partitioning have primarily used topography to represent the influence of the environment. Here, we bring together data on both topography and soil resource variation within eight large (24–50 ha) tropical forest plots, and use variation partitioning to decompose community compositional variation into fractions explained by spatial, soil resource and topographic variables. Both soil resources and topography account for significant and approximately equal variation in tree community composition (9–34% and 5–29%, respectively), and all environmental variables together explain 13–39% of compositional variation within a plot. A large fraction of variation (19–37%) was spatially structured, yet unexplained by the environment, suggesting an important role for dispersal processes and unmeasured environmental variables. For the majority of sites, adding soil resource variables to topography nearly doubled the inferred role of habitat filtering, accounting for variation in compositional structure that would previously have been attributable to dispersal. Our results, illustrated using a new graphical depiction of community structure within these plots, demonstrate the importance of small-scale environmental variation in shaping local community structure in diverse tropical forests around the globe.
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