Differences in population structure require habitat-specific conservation strategies in the threatened steppe grassland plant Astragalus exscapus

Abstract

Analysing population structure can reveal insights in demographic processes and may allow the derivation of conservation strategies. We studied the population structure in relation to habitat characteristics and population size in the endangered steppe grassland perennial Astragalus exscapus. Three types of populations were distinguished based on the proportions of five plant stages (juveniles, vegetative adults as well as small, medium and large flowering adults). 14 of the 22 study populations belonged to a “stable” population type that mainly consisted of small and medium flowering adults. Six populations belonged to an “aged” type, with nearly 50% large flowering adults but also the highest proportion of juveniles, only found in dry steppe grasslands with open vegetation. The remaining two populations belonged to a “retrogressive” type that was dominated by vegetative adults and was only found in semi-dry steppe grasslands with dense vegetation cover. The proportion of large flowering adults was higher in dry than in semi-dry steppe grasslands whereas semi-dry steppe grasslands had higher proportions of either small flowering or vegetative adults. The number of juvenile plants was low in general but highest in habitats with low vegetation and high proportions of bare soil. Overall plant density was positively related to population size, but this relationship was steeper in semi-dry grassland populations. Small populations did not have less juveniles than large ones. Our results indicate a habitat-specific endangerment in A. exscapus. We conclude, that in dry steppe grasslands persistence is a successful strategy leading to large plants which reproduce much. However, severe droughts prevent juveniles from establishment in dry grasslands. In contrast, semi-dry steppe grasslands are less extreme, leading to higher plant densities, but makes these populations more vulnerable to competition with other plants through higher biomass production, if these sites are abandoned. Consequently, in dry steppe grasslands, a conservative, individual-based conservation strategy should be applied while in semi-dry grasslands proper management is most important.