Potential Relative Growth Rate Research Articles

Plant Adaptation and Phosphorus Limitation in Tropical Forests: A Theoretical and Empirical Assessment

Ecosystem dynamics are shaped by plant adaptation to environmental stress, yet the conditions under which this occurs remain poorly understood. We developed a theoretical framework to predict how strategies used by tropical trees to cope with low-phosphorus (P) availability (that is, traits related to P uptake, and use) influence growth under P limitation. We then tested this framework against data on tree species in Borneo and a meta-analysis of results from pantropical nutrient addition experiments. Our theoretical framework predicts that plant traits associated with low-P environments, including enhanced P allocation to leaves, efficient P resorption, and root phosphatase activity, alleviate the negative effects of P scarcity more strongly for “inefficiently” growing plants, represented by large trees and old-growth forests, compared with saplings or secondary forests. In agreement with this prediction, changes in traits related to low-P environments increased the potential relative growth rate of large trees more than small trees in Borneo. Finally, theoretical expectation was supported by a meta-analysis which revealed stronger P limitation in saplings and secondary forests than in old-growth forests. Together, these findings provide a novel framework to interpret the relationship between resource constraints and plant performance and reinforce the importance of accounting for plant adaption to predict ecosystem responses to P limitation in tropical forests.

Stability and fitness cost associated with spirotetramat resistance in Oxycarenus hyalinipennis Costa (Hemiptera: Lygaeidae)

Dusky cotton bug, Oxycarenus hyalinipennis Costa (Hemiptera: Lygaeidae) is an important pest of cotton and causing economic losses to this crop. It also remains active round the year, infesting a number of host plants. Spirotetramat is a systemic insecticide and is effective against many sucking insect pests. A field collected population of O. hyalinipennis was reared in the laboratory under continuous spirotetramat selection pressure for 21 generations for the development of resistance to spirotetramat. The Spiro-Sel population was further reared for seven generations without insecticide exposure to assess the stability of spirotetramat resistance. Leaf dip method was used for the bioassays and selection. In this study, the impact of spirotetramat resistance on its stability and life history traits of Spiro-Sel, C1 (15 Spiro-Sel♀ × 15 UNSEL ♂) and C2 (15 Spiro-Sel♂ × 15 UNSEL ♀) O. hyalinipennis was assessed. Spiro-Sel (G21 ) population developed 2333-fold and 20.83-fold resistance compared with the susceptible and unselected (UNSEL) populations, respectively. Resistance to spirotetramat was unstable after seven generations (G28 ) when reared without exposure to any insecticide. A significant reduction in overall nymphal survival, fecundity, egg hatching and net reproductive rate of Spiro-Sel population was observed when compared with UNSEL population. Intrinsic rate of natural increase, biotic potential and mean relative growth rate were also lower in Spiro-Sel population compared to UNSEL population. The Spiro-Sel, C1 and C2 population had a relative fitness of 0.44, 0.51 and 0.44, respectively. Results of our study suggested that fitness cost is involved in the development of spirotetramat resistance. Unstable resistance and high fitness cost may provide great benefits to limit the evolution of resistance to spirotetramat in O. hyalinipennis. © 2021 Society of Chemical Industry.

The potential of indigenous rhizobacteria from areca nut rhizosphere in South Konawe regency as a plant growth promoter

Sustainable agriculture focuses on the applied of biological resources in agricultural cultivation process, therefore exploration and isolation of potential microbes as biological fertilizers need to be continuously developed. The research aim was to get the potential indigenous rhizobacteria isolates from the rhizosphere of areca nut plants as plant growth promotor. The experiment consists of 4 series based on the location of isolate collection, namely Angata 7 isolates, Landono 7 isolates, Palangga 6 isolates, and 4 isolates Wolasi, using a complete randomized design with three replications. The potential of these various isolates on a laboratory scale was evaluated via seed viability and vigour tests using upland rice as indicator plant. Inoculation of seeds using rhizobacteria isolates was able to improve the viability and vigour of rice seeds as shown by increasing potential development, germination percentage, vigour index, and relative growth rate as compared to control. The research found that 19 isolates which have the potential to promote plant growth were selected, namely AG1, AG2, AG3, AG4 and AG7 (Angata), LD1, LD2, LD3, LD4, LD5 and LD7 (Landono), PL1, PL2, PL5, PL6 (Palangga) and WL1, WL2, WL3 and WL4 (Wolasi). Field-scale experiments is required to evaluate these potential isolates as biofertilizers.

Effect of host plants on life history traits of Phenacoccus solenopsis (Homoptera: Pseudococcidae)

AbstractCotton mealybug,Phenacoccus solenopsisTinsley is an important polyphagous insect pest and causes severe losses to different crops worldwide. In the current study, we investigated the effect of different host plants, such asCaesalpinia pulcherrima,Plumeria rubra, Anthurium andraeanum, Jasminum sambac, andHibiscus rosasinensis, on the biological parameters ofP. solenopsis. The survival rate from crawler to adult, female nymphal duration, development time from crawler to female adult, and female adult weight were significantly different on the different hosts. Male nymphal duration, development time from crawler to male adult, pupal weight, emergence rate of male adults, and mean relative growth rate for male were similar on all the tested host plants. Pupal duration and generation time of male and female onH. rosasinensiswere significantly shorter than on the other hosts. Adult male and femaleP. solenopsislongevity was significantly shorter onH. rosasinensiscompared to other hosts. The fecundity was lower onC. pulcherrimaandA. andraeanumand hatchability was lower onC. pulcherrimathan on the other hosts. The net reproductive rate, the intrinsic rate of natural increase, and biotic potential and mean relative growth rate for female ofP. solenopsiswere significantly different on the tested hosts. Our results point to the role of host plants in increasing the populations ofP. solenopsisand could help to design cultural management strategies.

Spinosad resistance in an invasive cotton mealybug, Phenacoccus solenopsis: Cross-resistance, stability and relative fitness

Spinosad is new chemical insecticide, which acts on nicotinic acetylcholine receptor. Selection of Phenacoccus solenopsis with spinosad was carried out in the laboratory to characterize spinosad resistance. Cross resistance, stability and fitness cost of spinosad resistance were evaluated. Phenacoccus solenopsis showed a 282.45-fold resistance to spinosad after seven rounds of selection. Spinosad selected population (Spinosad-SEL pop) showed different levels of cross-resistance to profenofos, spinetoram and deltamethrin in comparison with unselected population. Spinosad resistance decreased significantly from 282.45 to 25.4-fold when resistant population was not exposed to the insecticide for up to six generations, indicating unstable resistance. Relative fitness of Spinosad-SEL pop was 0.51 with decrease in survival rate, fecundity, intrinsic rate of male and female, biotic potential and mean relative growth rate of male, whereas prolonged developmental time from egg to adult male and female, and generation time of both sexes compared with the unselected population. Resistance to spinosad indicates the need to rotate insecticides with others of differing modes of action and implementing alternative methods for P. solenopsis management.

Laboratory selection of chlorpyrifos resistance in an Invasive Pest, Phenacoccus solenopsis (Homoptera: Pseudococcidae): Cross-resistance, stability and fitness cost

The cotton mealybug, Phenacoccus solenopsis is an important polyphagous sucking pest of ornamentals, horticultural and fiber crops worldwide. Some P. solenopsis populations have developed insecticide resistance. This study evaluated cross-resistance, stability of insecticide resistance and life history traits affected by chlorpyrifos resistance in P. solenopsis. After nine generations selected with chlorpyrifos, P. solenopsis exhibited a 539.76-fold resistance level compared to an unselected population (UNSEL Pop). Chlorpyrifos selected population (Chlor-SEL Pop) displayed moderate cross-resistance to profenofos, nitenpyram and high cross-resistance to lambda-cyhalothrin. Biological parameters of P. Solenopsis were affected by chlorpyrifos resistance. The Chlor-SEL Pop had a significant reduction in fitness (relative fitness=0.10), along with significant decreases in pupal weight, fecundity, egg hatching %, intrinsic rate of natural population increase, biotic potential, and mean relative growth rate. It is concluded that selection with chlorpyrifos had marked effect on resistance development in P. solenopsis and upon removal of selection pressure chlorpyrifos resistance declined significantly indicating unstable resistance. Development of resistance led to high fitness costs for the chlorpyrifos-selected strain. These findings should be helpful for better and more successful resistance management of P. solenopsis.

Cross-resistance, the stability of acetamiprid resistance and its effect on the biological parameters of cotton mealybug,Phenacoccus solenopsis(Homoptera: Pseudococcidae), in Pakistan

Acetamiprid is a neonicotinoid insecticide that is effective against both soil and plant insects, including insects of the orders Lepidoptera, Coleoptera, Homoptera and Thysanoptera. In order to estimate the effects associated with insecticide exposure and devise better pest management tactics, a field population of Phenacoccus solenopsis was exposed to acetamiprid in the laboratory. Subsequently, cross-resistance and the effects of acetamiprid on the biological parameters of P. solenopsis were investigated. Following five rounds of selection with acetamiprid, P. solenopsis developed a 315-fold greater resistance to this chemical compared with an unexposed control population. The selected population also demonstrated very high to moderate cross-resistance to other tested insecticides. Furthermore, acetamiprid resistance remained unstable when the acetamiprid-selected population was not exposed for a further five generations. The acetamiprid-selected population had a relative fitness of 0.22, with significantly lower survival rate, pupal weight, fecundity, percentage hatching, net reproductive rate, intrinsic rate of natural increase, biotic potential and mean relative growth rate, with prolonged male and female nymphal duration, developmental time from egg to female adult and male and female longevity compared with the control population. P. solenopsis biological parameters are greatly affected by acetamiprid, and it is of significant cost for the insects to counter these effects. This study will be a valuable source of information for further understanding of acetamiprid resistance and for assisting the development of resistance management programmes.

A niche model to predict Microcystis bloom decline in Chaohu Lake, China

Cyanobacterial blooms occur frequently in lakes due to eutrophication. Although a number of models have been proposed to forecast algal blooms, a good and applicable method is still lacking. This study explored a simple and effective mathematical-ecological model to evaluate the growth status and predict the population dynamics of Microcystis blooms. In this study, phytoplankton were collected and identified from 8 sampling sites in Chaohu Lake every month from July to October, 2010. The niche breadth and niche overlap of common species were calculated using standard equations, and the potential relative growth rates of Microcystis were calculated as a weighted-value of niche overlap. In July, the potential relative growth rate was 2.79 (a.u., arbitrary units) but then rapidly declined in the following months to -3.99 a.u. in September. A significant correlation (R =0.998, P < 0.01) was found in the model between the net-increase in biomass of Microcystis in the field and the predicted values calculated by the niche model, we concluded that the niche model is suitable for forecasting the dynamics of Microcystis blooms. Redundancy analysis indicated that decreases in water temperature, dissolved oxygen and total dissolved phosphorus might be major factors underlying bloom decline. Based on the theory of community succession being caused by resource competition, the growth and decline of blooms can be predicted from a community structure. This may provide a basis for early warning and control of algal blooms.

Dissecting external effects on logistic-based growth: equations, analytical solutions and applications.

Characteristic growth patterns of individual organs are, to a large extent, determined by genetic factors, but can also be affected by intra-plant competition for resources and by environmental conditions. The current study proposes a dynamical system for modelling this dual control for logistic-based growth. The state of the system is defined by two state variables: size (s), and developmental age (α). The intrinsic properties of the system are represented by the potential relative growth rate as a function of α. This formulation allows dissection of the external effects on the system dynamics into two components: one that changes the duration of growth without affecting the final size and one that affects the final size without much effect on the duration. The former component determines the relationship between α and time, while the latter determines the effect on the system trajectory, s(α). The presented dynamical system is simpler and has a wider range of potential applications than the system proposed by Thornley and France (2005) for modelling logistic growth under resource limitation. The current approach can be also useful in ecology and in comparative studies of different genotypes and their responses to environmental conditions.

Ecosystem productivity can be predicted from potential relative growth rate and species abundance

We show that ecosystem-specific aboveground net primary productivity (SANPP, g g(-1) day(-1), productivity on a per gram basis) can be predicted from species-level measures of potential relative growth rate (RGRmax), but only if RGRmax is weighted according to the species' relative abundance. This is in agreement with Grime's mass-ratio hypothesis. Productivity was measured in 12 sites in a French Mediterranean post-agricultural succession, while RGRmax was measured on 26 of the most abundant species from this successional sere, grown hydroponically. RGRmax was only weakly correlated (r2 = 0.12, P < 0.05) with field age when species abundance was not considered, but the two variables were strongly correlated (r2 = 0.81, P < 0.001) when the relative abundance of species in each field was taken into account. SANPP also decreased significantly with field age. This resulted in a tight relationship (r2 = 0.77, P < 0.001) between productivity and RGRmax weighted according to species relative biomass contribution. Our study shows that scaling-up from the potential properties of individual species is possible, and that information on potential and realized species traits can be integrated to predict ecosystem functioning.

The relationship between relative growth rate and susceptibility to aphids in wild barley under different nutrient levels.

The Resource Availability Hypothesis (RAH) states that plants with a low Relative Growth Rate (RGR) and high levels of defence against herbivores or pathogens are favoured in habitats with low resource availability, whereas plants with a high potential RGR and low levels of defence are favoured in environments with high resource availability. High levels of defence are expected to result in lower reproduction and/or growth of the herbivores or pathogens. To test this hypothesis, four accessions of each of nine natural Hordeum spontaneum (wild barley) populations were grown in a climate chamber under two levels of nutrient supply. Susceptibility to Schizaphis graminum (greenbug) was quantified by placing a single adult greenbug on each plant and measuring its realised fecundity after 8 days. Data on potential RGR were available from a previous experiment. No support for the RAH was found. The correlation between potential RGR and greenbug reproduction was not significant, neither at the high nor at the low level of nutrient supply. Furthermore, on average plants grown under high and low nutrients did not differ in susceptibility. However, accessions-within-populations differed in the way susceptibility was affected by nutrient supply, and most accessions had a higher susceptibility under nutrient-poor conditions. It could be that these accessions differed in the spectrum of secondary metabolites they produced. Whatever the cause, the genetic variation for the reaction in susceptibility to nutrient supply suggests that selection could act in favour of more or less plasticity in plants without any apparent change in potential RGR.

Carbon cycling traits of plant species are linked with mycorrhizal strategy

Ecosystem carbon cycling depends strongly on the productivity of plant species and the decomposition rates of the litter they produce. We tested the hypothesis that classifying plant functional types according to mycorrhizal association explains important interspecific variation in plant carbon cycling traits, particularly in those traits that feature in a hypothesized feedback between vegetation productivity and litter turnover. We compared data from standardized 'screening' tests on inherent potential seedling relative growth rate (RGR), foliar nutrient concentrations, and leaf litter decomposability among 83 British plant species of known mycorrhizal type. There was important variation in these parameters between mycorrhizal plant types. Plant species with ericoid mycorrhiza showed consistently low inherent RGR, low foliar N and P concentrations, and poor litter decomposability; plant species with ectomycorrhiza had an intermediate RGR, higher foliar N and P, and intermediate to poor litter decomposability; plant species with arbuscular-mycorrhiza showed comparatively high RGR, high foliar N and P, and fast litter decomposition. Within the woody species subset, differentiation in RGR between mycorrhizal types was mostly confounded with deciduous versus evergreen habit, but the overall differentiation in litter mass loss between mycorrhizal types remained strong within each leaf habit. These results indicate that, within a representative subset of a temperate flora, ericoid and ectomycorrhizal strategies are linked with low and arbuscular-mycorrhizal species with high ecosystem carbon turnover. The incorporation of mycorrhizal association into current functional type classifications is a valuable tool in the assessment of plant-mediated controls on carbon and nutrient cycling.

Growth rate and biomass partitioning of wildtype and low‐gibberellin tomato (Solanum lycopersicum) plants growing at a high and low nitrogen supply

The growth‐promoting effects of gibberellins (GAs) on plants are well documented, but a complete growth analysis at the whole plant level on plants with an altered GA biosynthesis has never been reported. In the present work, the relative growth rate (RGR), biomass partitioning and morphological parameters of wildtype (Wt) tomato (Solanum lycopersicum L. cv. Moneymaker) plants were compared with those of isogenic (gib) mutants with a reduced biosynthesis of gibberellins. GA deficiency reduced RGR and specific leaf area (SLA, leaf area per unit leaf mass) and increased the net assimilation rate (NAR, the rate of biomass increment per unit leaf area). Despite the free access to nitrogen in the rooting medium, the low‐GA mutants had a much higher root mass ratio (RMR, the root mass per unit plant biomass) than the Wt, suggesting that the mutants were disturbed in their growth response to nitrate supply. The experiment was repeated at a low exponential nitrate supply, which forced all plants to grow at the same low RGR. The persistence of the differences in RMR at low N‐supply indicated that the high RMR of the mutants was a direct effect of low GA, which was independent of nitrate supply. Because the low N‐supply increased the RMRs of all genotypes to the same extent, the response of RMR to N‐supply does not seem to depend on GA. Although many of the traits of the slow growing GA mutants were very similar to those of inherently slow growing plant species from unproductive habitats, gibberellins are unlikely to be a main determinant of a plant's potential RGR.

Global versus local coupling models and theoretical stability analysis of size-structure dynamics in plant populations

A local coupling model for growth dynamics of plant populations is proposed, in which each individual occupies a square lattice point and follows logistic growth with potential maximum relative growth rate c 0 being reduced by the competitive effects from eight interacting neighbours. The competitive effects are given by the competition function W, which describes the degree of competitive asymmetry between individuals, β and the distance-dependent intensity of competition between individual i and j, α ij , as a coupling constant. Then a global coupling model corresponding to the local coupling model is proposed, in which every individual interacts with all the other individuals in the field. In the global coupling model, the coupling constant α as the same parameter for all the individuals was employed instead of α ij ’s in the local coupling model. α was given as an average of α ij ’s of eight interacting neighbours in the local coupling model. Both the coupling constants were normalized by the number of interacting individuals to make simulation results comparable between the models. Numerical simulations revealed that the local coupling model can be simulated by the corresponding global coupling model fairly well if population growth dynamics are continuous without deaths or new recruits. In both the models, multi-layered structure of size distribution was more likely to emerge under asymmetric and/or intense competition than under symmetric and/or weak competition. This conforms to the widely known phenomenon of size-structure dynamics in plant populations. Since theoretical analysis is impossible for the local coupling model, linear stability analysis of size-structure dynamics was made for the global coupling model. It was theoretically shown that if α< c 0/(1+ β), mono-layered size structure is stable; if α> c 0/(1+ β), multi-layered size structure is stable. As α and/or β increases (decreases), multi-layered (mono-layered) size structure gets stable. As c 0 (i.e. seedlings’ relative growth rate) increases, the domain of stable mono-layered (multi-layered) size structure becomes larger (smaller). Therefore, the above simulation results were supported by linear stability analysis of the dynamical systems. Ecological implications of these theoretical results are discussed concerning the relationship between the stability of stand size structure and the degree of competitive asymmetry (multi-layered versus mono-layered).

Effect of Changing Light Availability on Nutrient Foraging in Tropical Deciduous Tree-Seedlings

We evaluated the relationship between the capacity to respond to changes in light and the ability to search and exploit nutrient-rich soil patches (foraging). Seedlings of four tropical tree species (Caesalpinia platyloba, Celaenodendron mexicanum, Cordia alliodora and Heliocarpus pallidus) were exposed to a factorial combination of high (H) and low (L) photon flux density with fertilized (+ Fp) and unfertilized nutrient patches (- Fp) for 42 days. After this initial growth period, half of the plants from H were transferred to L (HL treatment), and half of the plants from L were transferred to H (LH) for another 42 days. When plants were transferred from high light to low light (HL), their productivity and RGR (relative growth rate) were lower than plants under high light, but when plants were transferred from low light to high light (LH), they reached similar or higher RGR (and sometimes biomass) compared to plants under continuously high light. The study species differed in their ability to explore and to benefit from the fertilized patch, depending on characteristics such as root length and root density in the fertilized patch and the potential relative growth rate of the species. The slow-growing species C. mexicanum showed no response to nutrient patches because it was unable to locate and produce roots in the patch. This species responded only to light, with slightly higher plant dry biomass and growth rate under HH and HL than under LH and LL, with thinner leaves (higher specific leaf area) under LL and LH. Fast-growing species had a higher ability than slow-growing species to forage for patchily distributed soil nutrients. This ability was maintained when plants experienced light increases (LH plants) but not when plants were growing under shaded conditions. In addition, the lack of the fertilized patch caused a strong reduction in RGR in the most productive light conditions (HH), so that HH plants had as low an RGR as LL and HL plants when grown without a fertilized patch. This response indicates a strong interaction between light capture and soil foraging behaviour.

Stem anatomy and relative growth rate in seedlings of a wide range of woody plant species and types.

Stem traits were analysed in laboratory-grown seedlings of 80 European woody and semiwoody species of known potential relative growth rate (RGR) and of similar ontogenetic phase. The objectives were, firstly, to assess the relation between stem structure and plant growth potential and, secondly, to explore how stem structure varies among species differing in life form and leaf habit. Hydraulic conductance was represented by the mean diameter of the widest xylem conduits (Dmax), and structural strength by the percentage of xylem tissue occupied by cell wall material (CWx) or stem tissue density (SD). Across all species RGR showed a weak positive correlation with Dmax and weak negative ones with CWx and SD, with slow-growers showing great dispersion of stem trait values. In the RGR-Dmax relationship this dispersion disappeared when trees were removed from the analysis. None of the relationships were significant among tree species alone. It was suggested that fast-growers require a xylem with wide conduits (high Dmax) to achieve high hydraulic conductivity, and "cheaply" constructed stems (low CWx and SD) to maximise allocation to leaves. However, the possession of such traits does not guarantee fast growth, as other factors may constrain RGR elsewhere in the plant. Deciduous seedlings showed higher Dmax and lower CWx than evergreens. Higher Dmax could reflect an innate higher tolerance of conductivity loss by freeze-induced embolism in deciduous plants, which are not burdened by the maintenance of foliage in winter. In contrast, life forms were differentiated most clearly by SD. For instance, shrub seedlings had less dense stem tissues than tree seedlings, possibly because they need less investment in long-term strength and stature.

A comparative study of leaf nutrient concentrations in a regional herbaceous flora.

Mineral nutrient concentrations were determined in leaves of 83 mostly herbaceous species collected from central England. Most samples were analysed for N, P, K, Ca, Mg, Na, Fe, Al, Mn, Cu and Zn. Concentrations of K, N and P showed similar levels of interspecific variability, with the highest concentrations being 6-9 times the lowest. Mg and (especially) Ca were much more variable, with the highest concentrations being 24 and 49 times the lowest respectively. Only in the case of P concentration was the majority of the variance in the data found at or below the species level. Most of the variance in Ca and Mg concentrations was between monocots and dicots. Concentrations of N and P were strongly positively correlated with each other. Only Ca and Mn were consistently associated with soil pH, positively and negatively respectively. Dicots tended to accumulate more Ca and Mn from high soil concentrations than did monocots. Concentration of P was significantly positively correlated with maximum potential relative growth rate. Plants of woodland and arable habitats contained high concentrations of P, and those of pasture and skeletal habitats contained low concentrations of P. The P: N ratio was higher in plants of arable habitats. Species with P-rich leaves tended to be currently increasing in abundance. The results suggest that plants with nutrient-rich foliage grow quickly, dominate nutrient-rich ecosystems and are generally increasing as a result of the eutrophication and disturbance arising from human exploitation.

Foliar macronutrient concentrations of forest understorey species in relation to Ellenberg's indices and potential relative growth rate

In order to investigate broad patterns of variation of the foliar mineral nutrient concentrations of herbaceous plant communities in the ground layer of W Europe forests, correlations were examined between Ellenberg's indices (N-index: mineral nitrogen availability, R-index: pH, F-index: soil moisture and L-index: light intensity) and literature values of macronutrient concentrations for 84 forbs and 39 graminoid species. Significant, positive correlations were found between the R-index and the plants' concentration of Ca and K (forbs only) and between the N-index and the plants' concentration of K, P (forbs only) and N (forbs and graminoids). Multiple regressions showed that the N-index was the best predictor of the plants' concentration of N (forbs and graminoids), P (forbs) and Ca (graminoids) and the R-index of the plants' concentration of Ca and K (forbs). The mineral nutrient concentrations of graminoids were lower and less strongly correlated with Ellenberg's indices than those of forbs. It is argued that the mineral nutrient concentrations in the plants match the availability of mineral nutrients in the soil for N, P (N-index) and Ca (R-index), but not for K and Mg. Significant, positive correlations were found between potential relative growth rate and the concentration of some elements (N, P, K and Ca in forbs, Ca and Mg in graminoids). This suggests that the increase in the concentration of these elements in plants along fertility gradients is due, at least partly, to genetically controlled alterations of leaf anatomy associated with increasing potential relative growth rate.

Comparative ecology of seed size and dispersal

Seed mass is correlated with a number of other plant traits, including dispersal mode, growth form and specific leaf area. Specific leaf area is the main determinant of potential relative growth rate and an indicator of the site quality to which a species is adapted. The relationships with dispersal mode and growth form have consistent form in five datasets from three continents, and each account for about 20-30 % of variation in log seed mass. Thus, there is also very substantial variation within growth form and dispersal categories. Much, but not all, of the 20-30% is associated with shifted family composition between growth forms or dispersal modes. Experiments have shown that seedlings of larger-seeded species are better able to survive hazards including deep shade, drought, physical damage and the presence of competing vegetation. If there is a common mechanism under these different hazards, it seemingly must be a ‘reserve effect’, whereby during deployment and early growth larger-seeded species hold a bigger percentage of seed reserves uncommitted to seedling structure and available to support respiration or repair damage. A reserve effect has not yet been demonstrated directly. It remains possible that different mechanisms operate under different hazards. Under a reserve effect, advantages of larger seed size should be temporary, and temporary advantage has indeed been observed with regard to seedling survival under dense shade. Although larger seed mass confers benefits on seedlings, larger seeds must necessarily be produced in smaller numbers per unit of resource allocated. Seed mass is presumed to have evolved as a compromise between these counterposed pressures. Yet there has proved to be surprisingly little difference in average seed mass between very different vegetation regions, at least in temperate climates. Rather, there is startlingly wide variation in seed mass among species growing interspersed with each other. Recent applications of game theory may be capable of accounting for this wide variation between coexisting species, but at present these models are driven by competition among seedling species (as opposed to between seedlings and adults). It remains unclear whether competition among seedlings is a decisive influence on species composition in most of the world’s vegetation types.

Intra-specific variation in relative growth rate: impact on competitive ability and performance of Lychnis flos-cuculi in habitats differing in soil fertility

Plant species from unproductive or adverse habitats are often characterized by a low potential relative growth rate (RGR). Although it is generally assumed that this is the result of selection for specific trait combinations that are associated with a low rate of net biomass accumulation, few studies have directly investigated the selective (dis-)advantage of specific growth parameters under a set of different environmental conditions. Aim of the present study was to quantify the impact of inherent differences in growth parameters among phenotypes of a single plant species, Lychnis flos-cuculi, on their performance under different soil nutrient conditions. Growth analysis revealed significant variation in RGR among progeny families from a diallel cross between eight genotypes originating from a single population. Differences in RGR were due to variation in both leaf area ratio (LAR) and in net assimilation rate (NAR). A genetic trade-off was observed between these two components of growth, i.e. progeny families with high investment in leaf area had a lower rate of net biomass accumulation per unit leaf area. The degree of plasticity in RGR to nutrient conditions did not differ among progeny families. Inherent differences in growth parameters among progeny families had a significant impact on their yield in competition with Anthoxanthum odoratum and Taraxacum hollandicum. In nutrient-rich conditions, progeny families with an inherently high leaf weight ratio (LWR) achieved higher yield in competition, but variation in this trait could not explain differences in competitive yield under nutrient-poor conditions. Inherent differences in growth parameters among progeny families were poorly correlated with differences in survival and average rosette biomass (a good predictor of fecundity) among these progeny families sown in four field sites along a natural gradient of soil fertility. In the more productive sites none of the growth parameters was significantly correlated with rosette biomass, but in the least productive site progeny families with an inherently high specific leaf area (SLA) tended to produce smaller rosettes than low-SLA families. These results are consistent with the view that a selective advantage may accrue from either high or low values of individual RGR components, depending on habitat conditions, and that the selective advantage of low trait values in nutrient-poor environments may result in indirect selection for low RGR in these habitats.