Relative growth rate (RGR) is currently the most commonly used method for measuring and comparing species' intrinsic growth potential. Comparative studies have, for example, revealed that small-seeded species have higher RGR, leading to the common belief that small-seeded species possess physiological adaptations for rapid growth that would allow them to outgrow large-seeded species, given sufficient time. We show that, because RGR declines as individual plants grow, it is heavily biased by initial size and does not measure the size-corrected growth potential that determines the outcome of competition in the long term. We develop a daily growth model that includes a simple mechanistic representation of aboveground and belowground growth and its dependency on plant size and environmental factors. Intrinsic growth potential is encapsulated by the size-independent growth coefficient, G. We parameterized the model using repeated-harvest data from 1724 plants of nine species growing in contrasting nutrient and temperature regimes. Using information-theoretic criteria, we found evidence for interspecific differences in only three of nine model parameters: G, aboveground allocation, and frost damage. With other parameters shared between species, the model accurately reproduced aboveand belowground biomass trajectories for all nine species in each set of environmental conditions. In contrast to conventional wisdom, the relationship between G and seed size was positive, despite a strong negative correlation between seed size and average RGR, meaning that large-seeded rather than small-seeded species have higher size-corrected growth potential. Further, we found a significant positive correlation between G and frost damage that, according to simulations, causes rank reversals in final biomass under daily temperature changes of ±5°C. We recommend the wider use of this new kind of plant growth analysis as a better way of understanding underlying differences in species' physiology; but we recognize that RGR is still a useful metric if considering the potential rate of population increase in empty habitats. Abstract. Relative growth rate (RGR) is currently the most commonly used method for measuring and comparing species' intrinsic growth potential. Comparative studies have, for example, revealed that small-seeded species have higher RGR, leading to the common belief that small-seeded species possess physiological adaptations for rapid growth that would allow them to outgrow large-seeded species, given sufficient time. We show that, because RGR declines as individual plants grow, it is heavily biased by initial size and does not measure the size-corrected growth potential that determines the outcome of competition in the long term. We develop a daily growth model that includes a simple mechanistic representation of aboveground and belowground growth and its dependency on plant size and environmental factors. Intrinsic growth potential is encapsulated by the size-independent growth coefficient, G. We parameterized the ...
Summary• Most plants suffer some degree of herbivore attack and many actively defend themselves against such an event. However, while such defence is generally assumed to be costly, it has sometimes proved difficult to demonstrate the costs of defensive compounds.• Here, we present a method for analysing growth rates which allows the effects of variation in initial plant size to be properly accounted for and apply it to 30 lines from a recombinant inbred population of Arabidopsis thaliana. We then relate different measures of relative growth rate (RGR) to damage caused by a specialist lepidopteran insect and to levels of putative defensive compounds measured on the same lines.• We show that seed size variation within the recombinant inbred population is large enough to generate differences in RGR, even when no other physiological differences exist. However, once size-standardized, RGR was positively correlated with herbivore damage (fast-growing lines suffered more damage) and was negatively correlated with the concentration of several glucosinolate compounds.• We conclude that defensive compounds do have a growth cost and that the production of such compounds results in reduced herbivore damage. However, size standardization of RGR was essential to uncovering the growth costs of defensive compounds.
The study suggests that delayed development of key primary developmental features of the stem in this ecotype of Arabidopsis results in a 'short and flexible' rather than a 'short and rigid' strategy for maintaining upright axes in conditions of severe mechanical perturbation. The mechanism is comparable with more general phenomena in plants where changes in developmental rate can significantly affect the overall growth form of the plant in both ecological and evolutionary contexts.
Small-seeded plant species are often reported to have high relative growth rate or RGR. However, because RGR declines as plants grow larger, small-seeded species could achieve higher RGR simply by virtue of their small size. In contrast, size-standardized growth rate or SGR factors out these size effects. Differences in SGR can thus only be due to differences in morphology, allocation, or physiology. We used nonlinear regression to calculate SGR for comparison with RGR for 10 groups of species spanning a wide range of life forms. We found that RGR was negatively correlated with seed mass in nearly all groups, but the relationship between SGR and seed mass was highly variable. We conclude that smallseeded species only sometimes possess additional adaptations for rapid growth over and above their general size advantage.
Competition and disturbance are potent ecological forces that shape evolutionary trajectories. These forces typically work in opposition: when disturbance is infrequent, densities are high and competition is intense. In contrast, frequent disturbance creates a low-density environment in which competition is weak and good dispersal essential. We exploited recent advances in genomic research to quantify the response to selection by these powerful ecological forces at the phenotypic and molecular genetic level in experimental landscapes. We grew the annual plant Arabidopsis thaliana in discrete patches embedded in a hostile matrix and varied the number and size of patches and the intensity of disturbance, by creating both static and dynamic landscapes. In static landscapes all patches were undisturbed, whereas in dynamic landscapes all patches were destroyed in each generation, forcing seeds to disperse to new locations. We measured the resulting changes in phenotypic, genetic, and genotypic diversity after five generations of selection. Simulations revealed that the observed loss of genetic diversity dwarfed that expected under drift, with dramatic diversity loss, particularly from dynamic landscapes. In line with ecological theory, static landscapes favored good competitors; however, competitive ability was linked to growth rate and not, as expected, to seed mass. In dynamic landscapes, there was strong selection for increased dispersal ability in the form of increased inflorescence height and reduced seed mass. The most competitive genotypes were almost eliminated from highly disturbed landscapes, raising concern over the impact of increased levels of human-induced disturbance in natural landscapes.Arabidopsis thaliana | competition | disturbance | evolution | genomics N atural selection should lead to adaptive evolution in response to ecological forces. However, in spatially structured landscapes, competition and disturbance can exert conflicting selection pressures. For example, high disturbance rates lead to reduced densities and reduced competition but favor those traits that confer good dispersal ability. In contrast, low disturbance rates lead to high density and intense competition, favoring traits that confer good competitive ability (1-3). However, although competition and disturbance both lead to exclusion and loss of diversity, coexistence can theoretically occur in multispecies communities at some intermediate disturbance level (4-7).Although the conditions for coexistence have been a subject of great debate (7), the relative strengths of disturbance and competition as forces shaping the phenotypic and genetic composition of plant communities have rarely been examined in an experimental setting. Here we used communities composed of multiple genotypes of Arabidopsis thaliana (L.) Heynh. to address this question. We considered this to be an appropriate model for an ecological community because natural populations of Arabidopsis thaliana are >95% self-fertilizing (8); hence, like a group of species, coo...
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