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

Abstract

International audienceSince 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

Forecasting functional implications of global changes in riparian plant communities

“…A well‐studied example is the observed decline of Salicaceae, a family of plants that includes obligate and facultative phreatophytic species (plants that obtain water from a permanent ground supply or directly from the water table) of the genera Salix (willow) and Populus (cottonwood, poplar). In both North America and Europe, many species of Salicaceae are being replaced by both native and non‐native, drought‐tolerant species (Figure 2b; Friedman et al 2005; Bottollier‐Curtet et al 2011). A notable exception to the decline in Salicaceae is the introduction of non‐native Populus spp in European plantations and the invasion of Salix spp in areas of Australia, New Zealand, and South Africa (Figure 2b; WebTable 2; Read and Barmuta 1999).…”

“…The establishment and spread of Tamarix and E angustifolia is of less concern in Europe than in North America. Other drought‐tolerant species, such as Acer negundo (American boxelder), however, have replaced native phreatophytes along European rivers where flow regulation and groundwater pumping restrict recruitment of Salicaceae, especially Salix alba (white willow; Bottollier‐Curtet et al 2011).…”

Projected warming disrupts the synchrony of riparian seed release and snowmelt streamflow

“…Populus and Salix are dominant features of many riparian ecosystems (Karrenberg et al, 2002;Friedman et al, 2005), reducing their abundance alters these ecosystems substantially: reducing forest structural complexity, patch diversity, and understory plant diversity (Stromberg et al, 2010;Bottollier-Curtet et al, 2012), altering soil biogeochemistry (Pomeroy et al, 2000;Harner et al, 2009), and reducing habitat quality for diverse wildlife that rely on riparian Populus and Salix forests (Moskat & Fuisz, 1995;Matos et al, 2009;Perry et al, 2012).…”

Killing it Softly: Girdling as an Efficient Eco-friendly Method to Locally Remove InvasiveAcer negundo

“…Locally, this species may form monospecific stands (Lamarque et al 2012), reducing both native species richness and abundance by decreasing light availability (Saccone et al 2010, BottollierCurtet et al 2012. In France, A. negundo invasion leads to replacement of economically important trees such as Salix alba (white willow) (Bottollier-Curtet et al 2012) and Populus spp., to river bank collapses and reduces bird nesting (L. Degrave, Parc Naturel Régional des Landes de Gascogne and S. Buyle, Réserve Naturelle des Marais de Bruges, pers. comm.…”