Plant community assembly during succession from arable fields to semi-natural grassland

Sammanfattning: Large areas of grazed, species rich semi-natural grassland were lost in Europe during the last two centuries and as a consequence led to decreases in grassland specialist species. Therefore, today the “restoration” of grassland on previously arable fields is recommended for the protection of grassland specialists. Even “substitute habitats” for lost grasslands are discussed as temporary alternative habitats for grassland specialists. During the “restoration” of grasslands on previously arable fields, the grazing management interrupts the progress of succession (to forest) at an intermediate stage, the semi-natural grassland which is maintained as an alternative endpoint of succession. Within this type of “truncated” arable-to-grassland succession the disturbance filter by grazing management may have a strong influence on plant communities, next to the dispersal, abiotic and biotic filters which may influence plant community assembly during succession. Ecological theory suggests that arable-to-grassland succession may be mainly driven by deterministic or by stochastic processes and that plant communities either diverge or converge during succession. Specialist species are expected to be most common in the final stage of succession. Spatial dispersal processes may be estimated by connectivity measures, but simple, biological meaningful measures are still missing. The overall aim of the thesis is to understand plant community assembly during a long-term (≥ 280 years) arable-to-grassland succession with a special focus on i) changes in different facets of diversity and ii) the underlying processes (assessed as filters of plant community assembly) leading to the changes. We further investigated how to improve the estimation of dispersal processes from old to young grasslands by incorporating basic movement patterns into connectivity measures and we examined changes in the species frequencies during succession and its implications for grassland specialist conservation. GIS analyses were used to detect grassland ages (t_5: 5-14, t_15: 15-49, t_50: 50-179 and t_280: ≥ 280 years old) and land-use changes over the last 280 years. The differently aged grasslands on previously arable fields were arranged to a chronosequence of succession. Null model analysis and hierarchical partitioning were used to identify processes influencing plant community assembly during the arable-to-grassland succession. Within an information-theoretic framework different IFM connectivity measures were compared with the help of specialist richness (actual connectivity) models to examine whether modification in the support area, the mean dispersal distance or the edge:area transformed the IFM connectivity into a simple functional connectivity measure. Finally ordination and indicator species analyses were used to investigate frequency-changes of typical grassland species during succession. At present-day most of the grasslands on previously arable fields are early-successional (5-49 years old) grasslands. Mid-successional (50-279 years old) grasslands are very rare (3% of the grasslands on previously arable fields). 22.5% of the grasslands are old semi-natural grasslands. Taxonomic and functional alpha diversity increase with grassland age, but show different patterns during the succession, whereas taxonomic and functional beta diversity decrease during succession. Taxonomic beta diversity was significantly associated with the disturbance (grazing), the dispersal (realised connectivity) and the abiotic (phosphorus, nitrogen) filters of plant community assembly during succession. A connectivity measure based on the “realised support area” including an edge:area ratio and a short-intermediate mean dispersal distance approximated actual connectivity best. Mid-successional grasslands host typical grassland species in intermediate frequencies and some of the so-called typical grassland species have even their highest within-plot frequencies in these transient grasslands. Our results suggest that plant communities converge in terms of their species composition and species frequencies during the arable-to-grassland succession. The disturbance filter seems to be the main driver during the succession. The disturbance by grazing animals influences plant community assembly directly and also indirectly via the dispersal and abiotic filter, because the grazing animal may also contribute to species dispersal and may remove nutrients from the grasslands. The dispersal filter in grazed grassland systems may, in a simple functional way, be estimated by a modified IFM connectivity measure that is based on a realised support area, reflecting where the main dispersal vector of plant species actually can move. The mean dispersal distance of the main dispersal vector seems to function as an estimate for the mean dispersal distance of multiple species. Our results further suggest that mid-successional grasslands may make a valuable contribution to the protection of typical grassland species and that they may function as “substitute habitat” for lost old semi-natural grasslands. Rather than focussing solely on high-quality old semi-natural grassland fragments, the incorporation of land-use dynamics in species protection schemes seems to be desirable, because some of the so-called typical grassland species seem to have found a temporal niche in the transient and slightly more nutrient rich mid-successional grasslands.