Light interception principally drives the understory response to boxelder invasion in riparian forests

Since several decades, American boxelder (Acer negundo) is replacing white willow (Salix alba) riparian forests along southern European rivers. This study aims to evaluate the consequences of boxelder invasion on understory community in riparian areas. We determined the understory species richness, composition and biomass in boxelder and white willow stands located in three riparian forests, representative of three rivers with distinct hydrological regimes. We investigated correlation of these variables to soil moisture and particle size, main soil nutrient stocks, potential nitrification and denitrification, tree canopy cover and photosynthetic active radiation (PAR) at the ground level. A greenhouse experiment was then conducted to identify the causal factors responsible for changes in the understory. The effect of soil type, PAR level and water level on the growth and the biomass production of Urtica dioica were examined. A lower plant species richness and biomass, and a modification of community composition were observed for boxelder understory in all sites, regardless of their environmental characteristics. The strongest modification that follows boxelder invasion was the decline in U. dioica, the dominant species of the white willow forest understory. These differences were mainly correlated with a lower incident PAR under boxelder canopy. The greenhouse experiment identified PAR level as the main factor responsible for the changes in U. dioica stem number and biomass. Our results indicate that adult boxelder acts as an ecosystem engineer that decreases light availability. The opportunistic invasion by boxelder leads to important understory changes, which could alter riparian ecosystem functioning

Varieties of capitalism and job quality: the attainment of civic principles at work in the United States and Germany

This article explores how institutional differences matter to the quality of a nation’s jobs; job quality is conceived as a dimension of a national economy’s social performance and thus defined in accordance with civic principles. Focus is on the two archetypical varieties of capitalism, the United States and Germany. Using data from a 2009 telephone survey of U.S. and German workers, we find that the overall attainment of civic principles, as perceived by workers, is no different in Germany than in the United States, even though the German institutional environment should be more conducive to them. This is due to higher worker expectations in Germany and a tendency for employer practices to compensate for the weaker (liberal) institutional environment in the United States. Once these are controlled, German workers report substantially more positive outcomes. We find that institutional differences also matter in how various employer practices are adopted and hence have indirect as well as direct implications