Nitrogen addition promotes terrestrial plants to allocate more biomass to aboveground organs: A global meta-analysis

A significant increase in reactive nitrogen (N) added to terrestrial ecosystems through agricultural fertilization or atmospheric deposition is considered to be one of the most widespread drivers of global change. Modifying biomass allocation is one primary strategy for maximizing plant growth rate, survival, and adaptability to various biotic and abiotic stresses. However, there is much uncertainty as to whether and how plant biomass allocation strategies change in response to increased N inputs in terrestrial ecosystems. Here, we synthesized 3516 paired observations of plant biomass and their components related to N additions across terrestrial ecosystems worldwide. Our meta-analysis reveals that N addition (ranging from 1.08 to 113.81 g m⁻² year⁻¹) increased terrestrial plant biomass by 55.6% on average. N addition has increased plant stem mass fraction, shoot mass fraction, and leaf mass fraction by 13.8%, 12.9%, and 13.4%, respectively, but with an associated decrease in plant reproductive mass (including flower and fruit biomass) fraction by 3.4%. We further documented a reduction in plant root-shoot ratio and root mass fraction by 27% (21.8%–32.1%) and 14.7% (11.6%–17.8%), respectively, in response to N addition. Meta-regression results showed that N addition effects on plant biomass were positively correlated with mean annual temperature, soil available phosphorus, soil total potassium, specific leaf area, and leaf area per plant. Nevertheless, they were negatively correlated with soil total N, leaf carbon/N ratio, leaf carbon and N content per leaf area, as well as the amount and duration of N addition. In summary, our meta-analysis suggests that N addition may alter terrestrial plant biomass allocation strategies, leading to more biomass being allocated to aboveground organs than belowground organs and growth versus reproductive trade-offs. At the global scale, leaf functional traits may dictate how plant species change their biomass allocation pattern in response to N addition.     

Seed size,number and strategies in annual plants: a comparative functional analysis and synthesis

Background and AimsPlants depend fundamentally on establishment from seed. However, protocols in trait-based ecology currently estimate seed size but not seed number. This can be rectified. For annuals, seed number should simply be a positive function of vegetative biomass and a negative function of seed size.MethodsUsing published values of comparative seed number as the ‘gold standard’ and a large functional database, comparative seed yield and number per plant and per m² were predicted by multiple regression. Subsequently, ecological variation in each was explored for English and Spanish habitats, newly calculated C-S-R strategies and changed abundance in the British flora.Key ResultsAs predicted, comparative seed mass yield per plant was consistently a positive function of plant size and competitive ability, and largely independent of seed size. Regressions estimating comparative seed number included, additionally, seed size as a negative function. Relationships differed numerically between regions, habitats and C-S-R strategies. Moreover, some species differed in life history over their geographical range. Comparative seed yield per m² was positively correlated with FAO crop yield, and increasing British annuals produced numerous seeds. Nevertheless, predicted values must be viewed as comparative rather than absolute: they varied according to the ‘gold standard’ predictor used. Moreover, regressions estimating comparative seed yield per m² achieved low precision.ConclusionsFor the first time, estimates of comparative seed yield and number for >800 annuals and their predictor equations have been produced and the ecological importance of these regenerative traits has been illustrated. ‘Regenerative trait-based ecology’ remains in its infancy, with work needed on determinate vs. indeterminate flowering (‘bet-hedging’), C-S-R methodologies, phylogeny, comparative seed yield per m² and changing life history. Nevertheless, this has been a positive start and readers are invited to use estimates for >800 annuals, in the Supplementary data, to help advance ‘regenerative trait-based ecology’ to the next level.     

Independent evolution of song diversity and song motor performance in canaries,goldfinches and allies indicates clade-specific trade-offs in birdsong

The diversity and the motor performance of birdsongs can both be sexually selected. In wood warblers, most species with high motor performance sing a greater proportion of trills, presumably to advertise performance, and thus have lower syllable diversity. We tested if this trade-off between motor performance and syllable diversity extends to canaries, goldfinches and allies, a clade with much longer and more varied songs. We assembled a molecular phylogeny and inferred song motor performance based on the speed of frequency modulation either in trills or in within-song intervals. The two metrics of performance were positively, but only mildly, related across species. While performance evaluated in intervals had high phylogenetic signal, performance evaluated in trills changed independently of phylogeny and was constrained by body size. Species in densely vegetated habitats sang fewer trills, but did not differ in motor performance. Contrary to wood warblers, song motor performance did not predict the proportion of trilled syllables nor within-song syllable diversity, perhaps because large differences in the song duration of canaries, goldfinches and allies prevent trills from severely compromising syllable diversity. Opposed results in wood warblers and in these finches indicate the existence of clade-specific trade-offs in the evolution of birdsong.     

A design-constraint trade-off underpins the diversity in ecologically important traits in species Escherichia coli

Bacterial species are internally diverse in genomic and multi-locus gene comparisons. The ecological causes of phenotypic and genotypic diversity within species are far less well understood. Here, we focus on the competitive fitness for growth on nutrients within Escherichia coli, an internally rich species. Competition experiments in nutrient-limited chemostats revealed that members of the ECOR collection exhibited a wide continuum of competitive abilities, with some fitter and some less fit than the lab strain MG1655. We observed an inverse relationship between competitiveness and the resistance of strains to detergent and antibiotic, consistent with the notion that membrane permeability and competitive fitness are linked by a trade-off between self-preservation and nutritional competence (SPANC); high permeability has a postulated cost in antibacterial sensitivity whereas a low permeability has a cost in nutrient affinity. Isolates moved along the markedly nonlinear trade-off curve by mutational adaptation; an ECOR strain sensitive to antibacterials and a good competitor was easily converted by mutation into a mutant with higher resistance but poorer competition in the presence of low antibiotic concentrations. Conversely, a resistant ECOR strain changed into a better competitor after a short period of selection under nutrient limitation. In both directions, mutations can affect porin proteins and outer membrane permeability, as indicated by protein analysis, gene sequencing and an independent assay of outer membrane permeability. The extensive, species-wide diversity of E. coli in ecologically important traits can thus be explained as an evolutionary consequence of a SPANC trade-off driven by antagonistic pleiotropy.     

Trade-offs in interspecific comparisons in plant ecology and how plants overcome proposed constraints

Comparative studies on plant species have not distinguished between two inherently different applications of the idea of a trade-off. In the first case, the theoretical trade-off between two variables leads to a trend line, about which there is a degree of scatter. Any two species in the study are expected, in theory, to show a true trade-off, i.e. feature A must decrease for feature B to increase. This I call the ‘trend line’ type of plot. In the second case, the species are expected to fill a considerable part of the space defined by two axes and the theoretical trade-off leads to a boundary line which limits the extent of the cloud of points. In this case, many interspecific comparisons are not expected to show evidence of a trade-off, but those on the boundary line are expected to do so. This I call the ‘boundary line’ type of plot. In both types of plot, species within a given clade may show a true trade-off, while large numbers of unrelated species do not.

First, I undertake to show, by means of examples, the reality of the distinction made above, and to demonstrate that statistically significant and ecologically important negative correlations between features A and B in the ‘trend line’ type of plot can be accompanied by huge variation from the trend line. Second, for plots of the ‘boundary line’ type, I undertake to show that authors have not always tested sufficiently rigorously the constraint lines concerning combinations of supposedly incompatible tolerances, and have not allowed for the absence of certain combinations of unfavourable conditions in the field and consequent lack of selection pressure for the evolution of species with a combination of extreme tolerances.

I review two examples of each kind of constrained evolution. First, I take the worldwide leaf economics spectrum and the negative correlation between growth rate in high light and survival rate in deep shade. Second, I review the supposed incompatibilities between shade tolerance and drought tolerance, and between waterlogging tolerance and drought tolerance.

I conclude that the term ‘trade-off’ is used too loosely. The common lack of true trade-offs has made possible the species richness of present-day vegetation. Also, small numbers of species have evolved combinations of tolerances that might seem improbable. Not enough research is being aimed at understanding the mechanistic basis of variation about trend lines, and the crossing of supposed boundary lines.