An ecosystem-level perspective of allelopathy

Allelopathy is an interference mechanism by which plants release chemicals which affect other plants; while it has often been proposed as a mechanism for influencing plant populations and communities, its acceptance by plant ecologists has been limited because of methodological problems as well as difficulties of relating the results of bioassays used for testing allelopathy to vegetation patterns in the field. Here we argue that the concept of allelopathy is more appropriately applied at the ecosystem-level, rather than the traditional population/community level of resolution. Firstly, we consider the wide ranging effects of secondary metabolites (widely regarded as allelochemicals) on organisms and processes which regulate ecosystem function, including herbivory, decomposition and nutrient mineralization. It is apparent that plants with allelopathic potential against other organisms induce net changes in ecosystem properties, which may in turn impact upon the plant community in the longer term. We then illustrate these concepts using two contrasting examples of how invasive plant species with allelopathic potential may alter ecosystem properties through the production of secondary metabolites, i.e. Carduus nutans (nodding thistle) in New Zealand pastures and Empetrum hermaphroditum (crowberry) in Swedish boreal forests. In both cases the production of secondary metabolites by the invasive species induces important effects on other organisms and key processes, which help determine how the ecosystem functions and ultimately the structure of the plant community. These examples help demonstrate that the concept of allelopathy is most effectively applied at the ecosystem-level of resolution, rather than at the population-level (i.e. plant-plant interference).     

Alpine and Arctic tundra shrub populations show similar ontogenetic growth trends but differing absolute growth rates and lifespan

The majority of terrestrial primary production is performed by plants, the ontogenetic growth trends of which significantly influence biomass and carbon dynamics. Here, we present a study of ontogenetic trends in primary (apical) and secondary (stem thickening) growth of plants in Arctic (Svalbard, Norway) and alpine (Krkonoše, Czechia) populations of the black crowberry (Empetrum nigrum), the dominant plant species of certain tundra communities. The environmental conditions in alpine areas are more favourable for plant growth than those in the High Arctic, where temperatures are lower, there is less precipitation and soils are shallower, among other differences. These differences were clearly reflected in significant distinctions in absolute growth rates and shrub age between the populations under study. However, we found almost no divergence in ontogenetic growth trends between the populations (based on ring width measurements made from the base to the top of plants, known as serial sectioning). In both populations, primary and secondary stem base growth decrease over the course of ontogeny whereas secondary stem top growth and basal area increment increase. No significant differences in the slope of the trends were found in either primary or secondary stem base growth. Trends of the growth ratio between basal area increment and primary growth revealed neither absolute nor relative differences between the populations. Ontogenetic trends in the shrubs analysed were surprisingly stable despite the prominently dissimilar environmental conditions. Empetrum plants have adapted to the different environments by altering their absolute growth rate only. This adaptation has probably also resulted in the different longevity of plants constituting the study populations, confirming the theory that slower-growing plants live longer. Primary growth and secondary stem base growth seem to be more basic characteristics of plant growth compared to basal area increment and secondary growth at the apex because the latter two seem to be dependent on the absolute growth rate.