Plant colonization of ex-arable fields from adjacent species-rich grasslands: The importance of dispersal vs. recruitment ability
Introduction
In most parts of northern and western Europe, there has been a drastic decline in the extent of unimproved species-rich grasslands during the last 100 years (Poschlod and Bonn, 1998, Eriksson et al., 2002). In many countries, including Sweden, Finland and the UK, less than 10% of the original extent of these grasslands remains (Fuller, 1987, Bernes, 1994, Vainio et al., 2001). Lost grasslands have mainly been transformed to arable fields or forests, on various time scales (Dahlström et al., 2006). Those semi-natural grasslands that remain often contain a very high species richness of plants (e.g. Eriksson and Eriksson, 1997, Öster et al., 2007), as well as many other groups of organisms, such as fungi (Öster, 2008), and insects and birds (Söderström et al., 2001). Considerable resources are allocated to maintain the still existing semi-natural grasslands, and to restore or recreate such grasslands.
One method to increase the area and connectivity of species-rich grasslands is to recreate such plant communities on former arable fields (henceforth ex-arable fields) that are presently used as pastures. Along with other methods to achieve this goal, e.g. sod cutting (Bakker and Berendse, 1999), sowing of seed mixtures of grassland species into ex-arable fields has received a lot of attention (e.g. Pywell et al., 2002, Pywell et al., 2003, Pywell et al., 2007, Walker et al., 2004, Lindborg, 2006). A general conclusion from these studies is that seed availability seems to be a key factor limiting re-assembly of species-rich grassland communities, at least in fields that are not too nutrient rich. A high phosphorous content in the soil may however constrain establishment of grassland communities (Janssens et al., 1998). The long history of management, and thus the long time available for colonization, may thus be one of the mechanisms that have contributed to build up the high species richness in species-rich grasslands (Eriksson and Ehrlén, 2001, Eriksson et al., 2002, Eriksson et al., 2006). A likely limiting factor for colonization of new grasslands, on ex-arable fields, is therefore lack of time.
Many studies on recreating species-rich grassland communities on ex-arable fields have focused on site specific factors, such as management (disturbance) and soil conditions (e.g. Bakker and Berendse, 1999, Römermann et al., 2005a, Pywell et al., 2007). However, as seed sowing experiments show that seed availability is limiting establishment on ex-arable fields, it is likely that the surrounding landscape will also affect colonization. If few source populations are available in the vicinity of the target area for recreation of species-rich grassland, the degree of seed limitation will be particularly strong. Even if source populations exist, some species with good establishment ability may nevertheless be strongly limited by dispersal, even on a small scale (Coulson et al., 2001). If seed sowing is not carried out, it has been suggested that management alone (e.g. grazing) is unlikely to result in recovery of high species richness on ex-arable fields (Critchley et al., 2003), although natural colonization slowly increases the similarity between ex-arable fields and grasslands (Hansson and Fogelfors, 1998). In Sweden, where species-rich grasslands are still relatively abundant, ex-arable fields located adjacent to species-rich grasslands, are particularly interesting for recreation of such grassland communities. If successful, such recreation would increase the total area of species-rich grassland, and increase connectivity of remaining grasslands. It is however poorly known to what extent adjacent species-rich grasslands contribute to enhance species richness on ex-arable fields, and which factors are most important in determining spread of species to ex-arable fields. Three major processes can be postulated: (i) species seed production on the species-rich grasslands, (ii) the ability to disperse from the adjacent species-rich grasslands to the ex-arable field, or (iii) the ability to establish on the ex-arable field provided that dispersal has occurred.
In line with an increasing recognition of a landscape level perspective on conservation of semi-natural grasslands (e.g. Lindborg and Eriksson, 2004, Cousins, 2006), studies have addressed the question how to construct economically sustainable grazing regimes, that may be profitable for farmers, perhaps even without dependence on subsidies. Kumm, 2003, Kumm, 2004 suggested that a solution may be to create large areas incorporating both the remaining grasslands (which presently are often kept isolated by fencing) and ex-arable fields. In such a management system it would be commonplace that a species-rich grassland is located adjacent to grassland with, at least initially, low species richness. Still, both types of vegetation would be grazed by the same animals, which would freely move across the borders between the grasslands.
With these considerations as a background, we examined three questions relevant for evaluating the potential conservation benefits of “large grazing areas” (sensu Kumm, 2003, Kumm, 2004). (1) Given a time window of less than 50 years, how large fraction of semi-natural grassland species is able to spontaneously colonize adjacent ex-arable fields? (2) Is there a positive relationship between actual colonization of ex-arable fields and aspects of the dispersal ability of the species? (3) Is there a positive relationship between actual colonization of ex-arable fields and the recruitment ability of the species? Dispersal here means the ability of seeds to become transported in space, and recruitment means the ability to germinate and survive the early life stages, given that seeds have been dispersed to the site. This study was done within a relatively limited area in southeast Sweden, within which both the age of the ex-arable fields and the soil conditions were fairly homogeneous. Effects of variation in the available time for colonization, and soil conditions, were therefore outside the scope of this study.
Section snippets
Field sites and species survey
The field study was carried out in or in the close vicinity of Nynäs nature reserve, c. 100 km south of Stockholm, Sweden (50°50′N, 17°24′E). Nynäs nature reserve contains a mosaic of species-rich semi-natural grasslands, arable fields, ex-arable fields and various types of forests, mainly coniferous or mixed deciduous-coniferous forests. The areas with the highest nature conservation interest are the species-rich grasslands (Eriksson and Eriksson, 1997, Cousins and Eriksson, 2001, Cousins and
Results
The density of species declined from species-rich semi-natural grasslands to ex-arable fields (Fig. 1, ANOVA, F3,87 = 51.75, P < 0.001). In the semi-natural grasslands, between 38 and 40 species were recorded per 0.5 m2. In the ex-arable fields, the species density declined outwards (although with a slight, but not significant tendency of an increase between 8 m and 10 m). Close to the semi-natural grassland, c. 30 species were recorded per 0.5 m2, and further out between 20 and 25 species per 0.5 m2
Discussion
The investigated ex-arable fields have been managed as pastures for less than 50 years, and during this time species that inhabit the adjacent species-rich semi-natural grasslands have had the opportunity to colonize these ex-arable fields. Among the seven sites used, 50 species were recorded in the ex-arable field margins, having a higher frequency on the semi-natural grasslands, and 21 species occurred on the semi-natural grasslands but were not recorded in the ex-arable fields. This could
Acknowledgements
We are grateful for comments on the manuscript by two anonymous reviewers. This study was supported by grants from Formas (The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning).
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