Shrub encroachment of temperate grasslands: Effects on plant biodiversity and herbage production

In Europe, semi-natural grasslands are an important resource of biodiversity and are, therefore, of high conservation interest. They are often invaded by shrubs, and they are threatened by under-utilization or abandonment which leads to further shrub encroachment. A gradient analysis of shrub-invaded temperate grasslands (from shrub-free to pioneer forest) in Germany was carried out to study the effect of shrub encroachment on plant biodiversity, herbage mass and nutritive value of herbage, as well as the effect on seed bank-vegetation similarity. The latter was driven by the question of the role of the soil seed bank for ecological restoration (Bossuyt & Honnay 2008). Our hypotheses are: (1) An initial shrub invasion leads to enhanced habitat diversity and according to the habitat heterogeneity hypothesis (MacArthur & Wilson 1967) thereby to higher species diversity. (2) Shrub encroachment leads to a loss of similarity between vegetation and soil seed bank. (3) Herbage mass and variables describing nutritive value of herbage decrease with increasing shrub encroachment.The selected 30 plots (10 m x 10 m) covered a wide gradient of shrub encroachment. Canopy height of the shrubs was between 15 and 500 (1000) cm, and the canopy cover ranged from less than 1 % to nearly 100 %. We recorded a total of 203 vascular plant species in the vegetation. Number of species per plot ranged between 27 and 68 (mean = 44.5), maximum species number per square meter was 41. Highest species diversity was found at medium shrub invaded sites, which is in line with our hypothesis of a hump-back relation between shrub encroachment and plant biodiversity. Similarity between seed bank and aboveground vegetation (Jaccard s coefficient of community) ranged between 7.5 and 36.4 %. Highest similarity was found in intermediate shrub invaded grasslands. We have to reject our hypothesis of decreasing seed bank-vegetation similarity with increasing shrub encroachment, but we found evidence for decreasing similarity with changing vegetation composition from grassland to shrub and forest vegetation. The following plant functional types were positively correlated with Jaccard index: therophytes (life form), annual plants (life span), and proportion of forbs. We found a reduction of herbage mass of DM from 3570 to 210 kg ha-1 with increasing shrub encroachment. Metabolizable energy concentration of herbage ranged from 8.9 to 10.2 MJ kg-1 DM and crude protein concentration from 72 to 171 g kg-1 DM, both measures being positively correlated with shrub occurrence. Increasing shrub occurrence was associated with a decrease in water-soluble carbohydrates concentration (from 151 to 31 g kg-1 DM) and a reduction in the indicator forage value . The results are in line with our hypothesis of decreasing herbage mass, but are contradictory to the hypothesis of decreasing nutritive value of herbage with increasing shrub encroachment.Our results emphasise the importance of semi-natural grasslands as a resource of biodiversity. Conserving these grasslands is closely connected to continuous agricultural use. Our results indicate a potentially large agronomic value for shrub-encroached temperate grasslands and suggest its utilization for livestock production.