Ecological Application of ‘Herbchronology’: Comparative Stand Age Structure Analyses of the Invasive PlantBunias orientalisL.

Abstract

The age structures of stands of the herbaceous perennial forbsBunias orientalisandRumex crispuswere determined in different habitats in the vicinity of Würzburg (Unterfranken, Germany). Plant age was determined by counting annual rings in the secondary root xylem (herbchronology). In rarely-disturbed (unmown, not disrupted) habitats, stands ofB. orientalisshowed a high proportion of older (⩾4-year old) individuals, whereas comparable stands at frequently-disturbed habitats were dominated by 1- or 2-year old plants. Plant size was positively and linearly related to plant age at frequently-disturbed sites and plants in the reproductive stage were younger than at the rarely-disturbed sites. Thus, the age structure data indicate favourable effects of anthropogenic disturbance on stand development ofB. orientalis. At recently-disturbed, productive sites,B. orientalisshowed narrow age distributions for a few years after the disturbance event, apparently resulting from a strong decline in seedling establishment after canopy closure. Stand development of the co-occurring speciesR. crispuswas less affected by vegetation density at such productive sites, but seemed to suffer more from relatively dry soil conditions. Besides being markers of plant age, annual rings may be sensitive records of past growing conditions, as suggested forB. orientalisby the relationship between annual ring width and position in a gradient of light supply. These results suggest that age structure data obtained by ‘herbchronology’ can provide valuable supplementary information for studies on the population ecology of perennial dicotyledonous herbs with clearly-demarcated annual rings.