The role of above-ground competition and nitrogen <i>vs</i>. phosphorus enrichment in seedling survival of common European plant species of semi-natural grasslands

<div><p>Anthropogenic activities have severely altered fluxes of nitrogen and phosphorus in ecosystems worldwide. In grasslands, subsequent negative effects are commonly attributed to competitive exclusion of plant species following increased above-ground biomass production. However, some studies have shown that this does not fully account for nutrient enrichment effects, questioning whether lowering competition by reducing grassland productivity through mowing or herbivory can mitigate the environmental impact of nutrient pollution. Furthermore, few studies so far discriminate between nitrogen and phosphorus pollution. We performed a full factorial experiment in greenhouse mesocosms combining nitrogen and phosphorus addition with two clipping regimes designed to relax above-ground competition. Next, we studied the survival and growth of seedlings of eight common European grassland species and found that five out of eight species showed higher survival under the clipping regime with the lowest above-ground competition. Phosphorus addition negatively affected seven plant species and nitrogen addition negatively affected four plant species. Importantly, the negative effects of nutrient addition and higher above-ground competition were independent of each other for all but one species. Our results suggest that at any given level of soil nutrients, relaxation of above-ground competition allows for higher seedling survival in grasslands. At the same time, even at low levels of above-ground competition, nutrient enrichment negatively affects survival as compared to nutrient-poor conditions. Therefore, although maintaining low above-ground competition appears essential for species’ recruitment, for instance through mowing or herbivory, these management efforts are likely to be insufficient and we conclude that environmental policies aimed to reduce both excess nitrogen and particularly phosphorus inputs are also necessary.</p></div

Seedling survival rate of eight common European grassland species across different nutrient addition treatments in experimental grassland mesocosms.

<p>Seedling survival is expressed as the % of surviving seedlings after 16 weeks of growth. Open circles represent short-clipped treatment, closed circles represent long-clipped treatment. Data are mean ± standard error of 5 trial individuals in 5 replicate mesocosms per treatment (8 trial individuals for <i>B</i>. <i>media</i>).</p

Results of the statistical models.

<p>The dependent variables were seedling survival, modeled as the survival success out of 5 trials with a binomial distribution in Generalized Linear Model, and seedling growth rate (mg day^-1) modeled with a normal distribution in a General Linear Model. We could not analyze growth rate of <i>C</i>. <i>rotundifolia</i>, <i>H</i>. <i>radicata</i>, <i>N</i>. <i>stricta</i> and <i>R</i>. <i>acetosa</i> as there were too few surviving seedlings in at least one treatment. Nitrogen addition, phosphorus addition and clipping treatment and their interaction factors were the explanatory variables.</p

NDVI

NDVI data per mesocosm and dat

biomass_1.2

biomass per mesocosm and harvest and species (or community if no species-specific values available