Modelling cyanobacteria in shallow coastal seas

There is an increasing need to describe cyanobacteria bloom dynamics using ecosystem models. We consider two fundamentally different ways how cyanobacteria are currently implemented: a simple approach without explicit consideration of the life cycle which assumes that cyanobacteria grow due to nitrogen fixation alone and an advanced approach that computes the succession of four different stages of the cyanobacteria life cycle based on internal quotas of energy and nitrogen. To qualitatively and quantitatively intercompare these different approaches and with observations, we use the Baltic Sea ecosystem model ERGOM coupled to the one-dimensional water column model GOTM. Four experiments are carried out: three, using the simple approach with either (a) a prescribed constant minimum production, (b) no minimum value or (c) a prescribed constant minimum concentration, and one with (d) the full predictive life cycle. The model data of 35 years (1970-2005) are analyzed for the timing of the bloom, the interannual variability, the annual mean nitrogen fixation rates and the effect of cyanobacteria on eukaryotic phytoplankton. The results show significant differences. In the climatological seasonal mean, only the advanced approach which resolves the life cycle produces a realistic bloom onset and duration. The interannual variability of blooms is unrealistically small in the experiments with a prescribed minimum value. Annual mean nitrogen fixation rates diverge by up to 30% between the four model solutions. Finally, the representation of the cyanobacteria also influences the seasonal cycle of eukaryotic phytoplankton, i.e., flagellates. This study demonstrates that the way how cyanobacteria are implemented in coupled biological-physical models strongly determines the fluxes into the system and between the individual compartments.     

Simulating regeneration and vegetation dynamics in Mediterranean coniferous forests

This study aims to provide a quantitative framework to model the dynamics of Mediterranean coniferous forests by integrating existing ecological data within a generic mathematical simulator. We developed an individual-based vegetation dynamics model, constrained on long-term field regeneration data, analyses of tree-rings and seed germination experiments. The simulator implements an asymmetric competition algorithm which is based on the location and size of each individual. Growth is parameterized through the analysis of tree-rings from more than thirty individuals of each of the three species of interest. A super-individual approach is implemented to simulate regeneration dynamics, constrained with available regeneration data across time-since-disturbance and light-availability gradients. The study concerns an insular population of an endemic to Greece Mediterranean fir (Abies cephalonica Loudon) on the island of Cephalonia (Ionian Sea) and two interacting populations of a Mediterranean pine (Pinus brutia Ten.) and a more temperate-oriented pine (Pinus nigra Arn. ssp. pallasiana) on the island of Lesbos (NE Aegean Sea), Greece. The model was validated against plot-level observations in terms of species standing biomass and regeneration vigour and adequately captured regeneration patterns and overall vegetation dynamics in both study sites. The potential effects of changing climatic patterns on the regeneration dynamics of the three species of interest were subsequently explored. With the assumption that a warmer future would probably cause changes in the duration of cold days, we tested how this change would affect the overall dynamics of the study sites, by focusing on the process of cold stratification upon seed germination. Following scenarios of a warmer future and under the current model parameterization, changes in the overall regeneration vigour controlled by a reduction in the amount of cold days, did not alter the overall dynamics in all plant populations studied. No changes were identified in the relative dominance of the interacting pine populations on Lesbos, while the observed reduction in the amount of emerging seedlings of A. cephalonica on Cephalonia did not affect biomass yield at later stages of stand development.     

Plant colonization of ex-arable fields from adjacent species-rich grasslands: The importance of dispersal vs. recruitment ability

Species-rich semi-natural grasslands have declined drastically in Europe over the last century and occur in modern landscapes as small and isolated fragments. Recreation of these species-rich plant communities and increasing connectivity of remaining grasslands are important goals for nature conservation. Ex-arable fields located adjacent to species-rich grasslands have been suggested as targets for such recreation, but we still lack knowledge about the processes determining colonization from grasslands to ex-arable fields. We examined transects from species-rich semi-natural grasslands to ex-arable fields in a nature reserve located in southeast Sweden, and analysed relationships between observed colonization of ex-arable fields and two characteristics of species: dispersal ability and recruitment ability. Colonization of ex-arable fields was not related to dispersal ability, assessed as adhesive potential to animal coat, wind dispersal potential and seed mass (related to seed production), but was positively related to recruitment ability, assessed by sowing experiment. The same relationships appeared when using a performance index from sowing experiments conducted in the UK. We conclude that recruitment ability is a key factor for colonization of ex-arable fields. In our study, around 50% of semi-natural grassland species were able to colonize adjacent ex-arable field margins spontaneously within a time window of less than 50 years. Seed sowing is however needed to increase the speed of the colonization process, because increasing seed density promotes colonization of species with poor recruitment in ex-arable fields. Actions to increase area and connectivity of species-rich grasslands should consider incorporating ex-arable fields.     

Impact of seed mixture and mowing on food abundance for farmland birds in set-asides

We studied the impact of seed mixtures (grass-clover, less competitive grass vs. meadow plant mixture) and mowing (annual mowing vs. no mowing) on the abundance of seed and insect food for farmland birds in set-aside in a long-term field experiment. In general, seed food was less affected by the treatments than insect food. The impact of seed mixture on the abundance of seed food was dependent on the study year: the highest level recorded in the first year, followed by a substantial decline in the following years. Mowing increased the biomass of seed food. The impact of treatments on insect food abundance differed between the total amount of insect food, and had significant interactions with year. Different insect groups dominated in each year as well as their response to the treatments. For the total amount, the meadow plant seed mixture appeared to be the most beneficial and a slight positive impact of mowing appeared in the last two years of the experiment. The results suggest that the value of set-asides in providing food differs according to food components and declines with increasing age of the set-aside.