Resource and non-resource root competition among trees of different successional status

1. This study assessed the effects of resource (i.e. nutrients) and non-resource (i.e. interference for space) competition from fine roots of competing grasses on the growth, morphology and architecture of fine roots of four tree species of varying successional status: Populus deltoides × P. balsamifera (a\ud hybrid), Betula papyrifera, Acer saccharum and Fraxinus americana. We tested the general hypothesis that tree fine-roots are affected by both below-ground resource and non-resource competition from non-self plants, and the more specific hypothesis that this effect is stronger in early- successional tree species.\ud 2. The experiment was conducted in split-containers where half of the roots of tree seedlings experienced either below-ground resource competition or non-resource competition, or both, by grasses while the other half experienced no competition.\ud 3. The late-successional tree species A. saccharum and\ud F. americana were mostly affected by resource competition, whereas the early-successional P. deltoides × balsamifera\ud and B. papyrifera were strongly affected by both resource and non-resource competition. Non-resource competition reduced fine-root growth, root branching over root length (a measure of root architecture) and specific root length (a measure of root morphology) of both early-successional species.\ud 4. Synthesis. This study suggests that early-successional tree species have been selected for root avoidance or segregation and late-successional tree species for root tolerance of competition as mechanisms to improve below-ground resource uptake in their particular environments. It also\ud contradicts recent studies showing perennial and annual grasses tend to overproduce roots in the presence of non-self conspecific plants. Woody plants, required to grow and develop for long periods in the presence of other plants, may react differently to non-self root competition than perennial or\ud annual grasses that have much shorter lives

Association between Opuntia species invasion and changes in land-cover in the Mediterranean region

In Mediterranean regions, biological invasions pose a major threat to the conservation of native species and the integrity of ecosystems. In addition, changes in land-cover are a widespread phenomenon in Mediterranean regions, where an increase in urban areas and major changes from agricultural abandonment to shrub encroachment and afforestation are occurring. However, the link between biological invasions and changes in land-cover has scarcely been analyzed. We conducted a regional survey of the distribution of the two alien prickly-pear cacti Opuntia maxima and O. stricta in Cap de Creus (Catalonia, Spain) and related patterns of invasion to spatially explicit data on land-cover/change from 1973 to 1993 to test the hypotheses that the two Opuntia species invade areas that have experienced large land-cover transformations. We found that Opuntia invasion is particularly high in shrublands and woodlands located near urban areas. O. maxima are over-represented in the shrublands and O. stricta in the woodlands that were former crops. Crop coverage has dropped by 71% in this 20-year period. This study highlights the role of past land-cover in understanding the present distribution of plant invasions

Mobile DNA can drive lineage extinction in prokaryotic populations

Natural selection ultimately acts on genes and other DNA sequences. Adaptations that are good for the gene can have adverse effects at higher levels of organization, including the individual or the population. Mobile genetic elements illustrate this principle well, because they can self-replicate within a genome at a cost to their host. As they are costly and can be transmitted horizontally, mobile elements can be seen as genomic parasites. It has been suggested that mobile elements may cause the extinction of their host populations. In organisms with very large populations, such as most bacteria, individual selection is highly effective in purging genomes of deleterious elements, suggesting that extinction is unlikely. Here we investigate the conditions under which mobile DNA can drive bacterial lineages to extinction. We use a range of epidemiological and ecological models to show that harmful mobile DNA can invade, and drive populations to extinction, provided their transmission rate is high and that mobile element-induced mortality is not too high. Population extinction becomes more likely when there are more elements in the population. Even if elements are costly, extinction can still occur because of the combined effect of horizontal gene transfer, a mortality induced by mobile elements. Our study highlights the potential of mobile DNA to be selected at the population level, as well as at the individual level

Out of Sight but Not out of Mind: Alternative Means of Communication in Plants

Current knowledge suggests that the mechanisms by which plants communicate information take numerous forms. Previous studies have focussed their attention on communication via chemicals, contact and light; other methods of interaction between plants have remained speculative. In this study we tested the ability of young chilli plants to sense their neighbours and identify their relatives using alternative mechanism(s) to recognised plant communication pathways. We found that the presence of a neighbouring plant had a significant influence on seed germination even when all known sources of communication signals were blocked. Furthermore, despite the signalling restriction, seedlings allocated energy to their stem and root systems differently depending on the identity of the neighbour. These results provide clear experimental evidence for the existence of communication channels between plants beyond those that have been recognized and studied thus far

Is analysing the nitrogen use at the plant canopy level a matter of choosing the right optimization criterion?

Optimization theory in combination with canopy modeling is potentially a powerful tool for evaluating the adaptive significance of photosynthesis-related plant traits. Yet its successful application has been hampered by a lack of agreement on the appropriate optimization criterion. Here we review how models based on different types of optimization criteria have been used to analyze traits—particularly N reallocation and leaf area indices—that determine photosynthetic nitrogen-use efficiency at the canopy level. By far the most commonly used approach is static-plant simple optimization (SSO). Static-plant simple optimization makes two assumptions: (1) plant traits are considered to be optimal when they maximize whole-stand daily photosynthesis, ignoring competitive interactions between individuals; (2) it assumes static plants, ignoring canopy dynamics (production and loss of leaves, and the reallocation and uptake of nitrogen) and the respiration of nonphotosynthetic tissue. Recent studies have addressed either the former problem through the application of evolutionary game theory (EGT) or the latter by applying dynamic-plant simple optimization (DSO), and have made considerable progress in our understanding of plant photosynthetic traits. However, we argue that future model studies should focus on combining these two approaches. We also point out that field observations can fit predictions from two models based on very different optimization criteria. In order to enhance our understanding of the adaptive significance of photosynthesis-related plant traits, there is thus an urgent need for experiments that test underlying optimization criteria and competing hypotheses about underlying mechanisms of optimization

Missing domesticated plant forms: can artificial selection fill the gap?

In the course of their evolution, the angiosperms have radiated into most known plant forms and life histories. Their adaptation to a recently created habitat, the crop field, produced a novel form: the plant that allocates an unprecedented 30–60% of its net productivity to sexual structures. Long-lived trees, shrubs and vines of this form evolved, as did annual herbs. Perennial herb forms with increased allocation to asexual reproduction evolved, but there are no examples of perennial herbs with high sexual effort. We suggest that sowing seed into annually tilled fields favored shorter-lived herbs because of trade-offs between first-year seed production and relative growth rate and/or persistence. By propagating cuttings, people quickly domesticated tuber crops and large woody plants. Perennial herbs were too small to be efficiently propagated by cuttings, and the association between longevity, allogamy and genetic load made rapid domestication by sexual cycles unlikely. Perennial grain crops do not exist because they could not have evolved under the original set of conditions; however, they can be deliberately developed today through artificial phenotypic and genotypic selection

Shaping 3D root system architecture

Plants are sessile organisms rooted in one place. The soil resources that plants require are often distributed in a highly heterogeneous pattern. To aid foraging, plants have evolved roots whose growth and development are highly responsive to soil signals. As a result, 3D root architecture is shaped by myriad environmental signals to ensure resource capture is optimised and unfavourable environments are avoided. The first signals sensed by newly germinating seeds — gravity and light — direct root growth into the soil to aid seedling establishment. Heterogeneous soil resources, such as water, nitrogen and phosphate, also act as signals that shape 3D root growth to optimise uptake. Root architecture is also modified through biotic interactions that include soil fungi and neighbouring plants. This developmental plasticity results in a ‘custom-made’ 3D root system that is best adapted to forage for resources in each soil environment that a plant colonises