Positive interactions and interdependence in the organization of plant communities
Positive interactions among plants, or facilitation, occur when the presence of one plant enhances the growth, survival, or reproduction of a neighbour. Historically, plant ecologists overwhelmingly accepted the notion that the distributions of plant species, and their organization into groups or communities, is determined individualistically, that is by the adaptation of each species in a “community” to a particular abiotic environment and competition. But in the last 30 years, hundreds of peer-reviewed papers have been published on facilitation by plants on other plants. This explicitly challenges the adequacy of a strict definition of the theory of individualistic communities. If the presence of one species can increase another species’ fitness, or the probability that another species will occur in a particular place, plant communities cannot be individualistic. I will discuss the balance of competition and facilitation in specific interactions, local to regional scale effects of foundation species on biodiversity, experiments conducted along environmental gradients, and the potential evolutionary roles of facilitation in the context of exploring the fundamental nature of communities.
Species pools in cultural landscapes: Niche construction, ecological opportunity and niche shifts
Niche conservatism, the idea that species maintain their realized niches when subjected to environmental change, is a dominating view in efforts to predict future changes in species distributions. However, evolutionary shifts in niche relationships are well-known, and there is evidence that species may alter their realized niche also in a shorter time scale. I discuss whether species may alter their niche relationships as a result of ongoing transformation of plant communities, due to species redistribution, climate and land use change. Niche construction is the process by which species may create niche space for themselves and for other species. I suggest a conceptual model for how human-mediated niche-construction creates ecological opportunity for niche shifts, thus affecting the build-up of species pools in cultural landscapes. My focus is on historical landscapes, but I will also discuss whether this model may be applied to current development of novel plant communities.
Hans de Kroon
Vegetation dynamics and species coexistence: inspiration from below
There is increasing interest in explaining species coexistence from the operation of soil biota. The soil biota that roots of species accumulate can be beneficial or antagonistic and may have profound differential effects on plant growth. In this talk I ask the question whether the root responses that we observe to biotic and abiotic factors are consistent with stabilizing mechanisms promoting coexistence. I report a number of experimental results and will attempt to scale them up to expected vegetation dynamics. Next, I will review results on fine-scaled vegetation dynamics from the literature and ask whether the patterns are consistent with results on plant-soil feedback and root responses. I will discuss the discrepancies between these different fields of study and the work required to bridge existing gaps.
Extremes versus the mean: Are plant traits driven by average climatic conditions, or by extreme events?
The strategies plants use vary hugely across different parts of the world – from the lush tall forests of the tropics to the tough little plants of arctic tundra. However, we know surprisingly little about global patterns in fundamental plant traits, and even less about which environmental variables drive these patterns. When ecologists include climatic variables in their analyses, they tend to focus on variables that give information about climate averages, such as mean annual precipitation and mean annual temperature. These variables do capture information about the conditions that the plants experience most often. However, one intriguing possibility is that it may not be the “average” days that impose the strongest selection pressure on plant populations, but rather the extreme events such as floods, droughts, heatwaves and intense storms. In this seminar, I will present the first test of the hypothesis that extreme climatic conditions are better predictors of plant traits than are mean climate variables.
Plant invasions: the role of recipient habitats
How much a plant community or habitat is invaded depends on a number of interacting factors and environmental settings, among which propagule pressure, climate and traits of invading species are traditionally considered most important. Recent studies have shown, however, that the identity of a habitat with its resource dynamics and disturbance regimes plays a key role in determining how much plant communities it harbours will be invaded. When assessing habitat invasions, distinction needs to be made between invasiveness as an inherent property of the habitat, and the actual level of invasion that we observe. Other issues that need to be taken into account are habitat species pools in both the source and target geographical regions as some habitat deliver disproportionally high or low numbers of successful invaders. Integrating these issues into a coherent framework would improve opportunities for risk assessment and for prediction of future invasions.
Competition and clonal connections
Clonal growth in plants is thought to reflect the parasitizing of older individuals (mothers) by younger individuals (daughters), allowing expansion into harsh environments. Evidence for this comes from short-term experiments on herbaceous species. We examined clones of an invasive tree expanding from forest into adjacent nutrient-poor grassland. Over two growing seasons, mothers accumulated ten times more experimentally-added nutrient isotope than did daughters, regardless of whether mothers or daughters were donors. The magnitude of nutrient accumulation by mothers closely matched their leaf mass relative to daughters, suggesting that flow direction is driven by demand. Nutrient flow from daughters in resource-poor grassland to mothers in resource-rich forest is a novel biogeochemical mechanism for resource flux between ecosystems. Further, nutrient accumulation by mothers in forests is a previously unreported mechanism of competition between grasses and trees that may contribute to tree invasion, and the concomitant losses of biodiversity and sequestered soil carbon. Most importantly, this field experiment showed that daughter ramets play the role of helpers, not parasites, by moving nutrients to, not from, mother ramets.
Community ecology of absent species
Community ecologists have so far focused mainly on species identified at a site. I suggest that we can understand better patterns and their underlying processes in ecological communities if we explore those species absent from the community. First, traditional sampling might be insufficient to detect dormant or rare species. Fortunately, modern DNA-based techniques can help us to find “hidden” species. Second, most species do not actually belong to the species pool of the community: their ecological habitat requirements differ or their distribution area is elsewhere. Such species are usually not relevant to a particular community but might form novel ecosystems. Third, a large part of the habitat-specific species pool is likely absent and constitutes dark diversity. If we know both observed and dark diversity, we can estimate community completeness and infer those processes that “divide” species into observed and dark diversity.
Mycorrhiza and plant communities – is there a link?
The possible impact of mycorrhizal fungi on plant community diversity and composition has been long discussed, but the actual role of mycorrhiza remains uncertain. Symbiotic fungi influence growth and reproduction of experimental plants, and affect experimental plant communities, but field evidence is limited. I review the current state of affairs and present original results on three issues: How do plant and arbuscular mycorrhizal (AM) fungal communities co-vary in nature? Are interactions between plants mediated by mycorrhiza? What is the overall effect of mycorrhizal fungi on plant community structure? I conclude that the role of mycorrhiza might be more important than earlier assumed.