title: Following plant community assembly in semi-natural European grasslands by analyzing environmental factors vs. history effects
Creators: name: Plückers, Christine
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Community assembly
subject: semi-natural European grasslands
subject: dry acidic grasslands
subject: filter effects
subject: history effects
subject: priority effects
subject: legumes
subject: intraspecific trait variation
subject: 570

description: Understanding plant community assembly and succession has long preoccupied ecologists, with a plethora of different theories (stochastic, deterministic and an intermediate situation of both) put forward over time. Currently, the role of historical contingency in forming alternative vegetation states is attracting increasing attention, with priority effects caused by order of arrival of different species producing long-term and significant effects on ecosystem functioning and diversity. The role of nutrient availability in modulating the strength of priority effects is an important consideration, since significant effects of order of arrival on communities may depend strongly on how many nutrients are available in the soil. The range and the effect size of these key drivers of assembly (historical contingency, nutrients) along temporal, spatial and resource related gradients are rarely addressed. The main underlying goal is to understand community assembly better to gain knowledge that can be applied in the restoration of species-rich grasslands, so that specific desired goals of an ecosystem can be met. The topic of this dissertation is the investigation of priority effects in relation to soil nutrient availability over time during assembly of semi-natural European grasslands. The focus is on both community traits and species-specific traits (including intraspecific responses of traits). The main grassland field experiment (Manuscript 2) showed that priority effects do play a role in community assembly of dry acidic grasslands, but the stability over time depended on the variables measured, with stable priority effects being found in relation to community aboveground biomass and plant functional group composition but not for community composition or species richness. The low nutrient availability negatively influenced the establishment of target species and the breadth of the priority effect was not as widely found in mesic grasslands, which suggested the important role of abiotic factors on community assembly. These results supported the findings of Sutherland (1974), that the vegetation in some ways reached a stable state in terms of the plant functional group relative composition but not in terms of species richness of species composition. Furthermore it could be shown that not the species richness of the initial seed mixture was the determining factor, but functional composition especially the plant functional group of legumes have an abiding positive effect on community traits (aboveground productivity and cover) in these grasslands, which could be also demonstrated using a new automated field measurement system (Manuscript 1). In Manuscript 3 the positive priority effect of the plant functional group of legumes on productivity found in the field, could be confirmed in a pot experiment using more nutrient-rich potting soil. In terms of productivity and functional group composition the simultaneously sown controls and the legume first treatment developed similarly. Which also underlines the importance of legumes as keystone species for facilitation or complementary effects in species-rich grasslands. In general it could be shown that the priority effect was stronger than the sowing interval or sowing density effect. Legumes seemed to be better able to get a head-start and grow quickly, compete well and allow facilitation for neighbors (hence a combination of asymmetric competition but also functional complementarity allowing for good community and population performance when legumes arrived early). The exact mechanisms of this priority effect of legumes first, which has been since confirmed in other experiments, is not yet known, but could be related to changes in root traits and exudation and hence plant-plant interactions underground. Manuscript 4 studied the intraspecific trait variation of one target forb species Plantago lanceolata across two different soil nutrient levels and in interaction with other plant species in community assembly. In mesocosms limited soil nutrients were the main factor driving variation in intraspecific traits, but it also depended on which trait one focused on (again). Intriguingly, the traits converged over time when comparing between the two soil nutrient treatments. In the field soil fertility had less influence on trait differences. This showed that the multivariate factors, which act simultaneously on the species plant trait in the field, led to a more stable trait variation, and especially legume neighborhood (again) played a role in creating a positive facilitation and complementarity effect. The results of this thesis show us the importance of also focusing on plant functional groups, especially legumes in interaction with other groups, when investigating priority effects in community assembly. Whether one finds a priority effect or not and how stable it is may depend on which parameters one measures. In the future we need more knowledge of the mechanisms and prevalence, relevance and stability of priority effects, so that we can then hopefully steer communities in desired directions in terms of ecosystem functions (higher hay biomass, more carbon storage or other ecosystem services).