Shifts in plant community structure of a threatened sandy grassland over a 9‐yr period under experimentally induced nutrient regimes: is there a lag phase?

Abstract

Abstract Questions Does nutrient addition to nutrient‐poor pioneer grassland lead to altered successional pathways after a lag phase? Are there shifts in plant functional types with time after nutrient addition? Is phytodiversity negatively affected by a 9‐yr nutrient addition? Is succession affected by local seed availability? Location Upper Rhine valley, Germany. Methods A five‐fold replicated randomized block design was used for addition of phosphorus, organic carbon, nitrogen (low and high dose) or combined applications of high‐dose N with P ( NP ), potassium ( NPK ) or other essential nutrients ( NPKM ) for 9 yr. Seed limitation was assessed as local seed rain. Data were analysed by ordination ( DCA ) and linear mixed models. Results DCA revealed two successional pathways: one typical for sandy grassland, and another on plots with high‐dose N , which was accelerated and clearly separated from the typical one after a distinct ‘lag phase’ of about 5 yr. As a general trend, phytodiversity diminished on all plots during succession, but the decrease was significantly stronger on plots with high‐dose N , which had higher turnover ratios. Habitat‐typical species from pioneer stages (‘stress tolerators’ and ‘ruderals’) and Red Data species decreased with nutrient addition. There was an increase in cover of tall plants, geophytes and hemicryptophytes, ‘competitor/stress tolerator/ruderal’ strategists and competitive graminoids on plots with high‐dose N . Above‐ground phytomass production of phanerogams increased three‐fold, but was significantly lower for cryptogams. Litter accumulation increased five‐fold on high‐dose N plots. In contrast, low‐dose N and P plots only showed responses for legumes, predominantly facilitated by P . Seed rain consisted of autochthonous and allochthonous species, but most abundant species were non‐target species. Conclusions Community structure, plant strategies and general successional trajectories of the studied system changed on plots with high‐dose N after 5 yr. A threatened vegetation type was gradually replaced by one with competitive drivers of succession. These findings emphasize the importance of long‐term observations for studying nutrient effects.